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Threshold current of the fuse link. Fuses

Protection devices are designed to ensure the safety of the operation of electrical networks, machines, electrical installations in the event of emergency conditions (short circuits, overloads). However, if installed and used incorrectly, they themselves can cause an accident, fire and explosion, because During their operation, electrical sparks and arcs occur.

The most common protection devices are:

fusible circuit breakers;

air circuit breakers;

thermal relay;

devices protective shutdown.

Fuse is a device in which, when a current exceeds the permissible value, the fuse-link melts and the electrical circuit opens. Fuses are single-use protective devices.

Compound:

A) fusible insert;

b) contact device;

V) frame(cartridge);

d) and sometimes filler(talc, quartz sand, etc.) to improve arc extinction and visual response rate.

Principle The action of fuses is based on the fact that the current passing through the fuse-link generates heat in accordance with the equality where I is the current passing through the fuse-link, R is the resistance of the fuse-link, t is the time of passage of the current: at a certain value of the current I and time t, heat is released enough to melt the fuse link and open the electrical circuit. This provides protection against overload current and short circuit.

Fuse parameters

A) rated current of fuse link I n.vst . – the current for which it is designed for long-term operation and is indicated on it.

b) rated fuse current I n.pr . – current equal to the largest of In.in and which is indicated on the fuse. All current-carrying contact parts of the fuse are designed for this current;

V) Rated voltage U n.pr . – voltage corresponding to the highest voltage at which it is permitted to be used and is indicated on the fuse.

G) maximum breaking current at a given voltage I pr.pr . – the highest value of short-circuit current at which reliable operation is guaranteed (without destruction of the housing).

(3 min) Full shutdown time of the electrical circuit, the fuse is determined by the time the insert is heated to the melting temperature, the time of its melting and combustion that appears when the arc melts.

Dependence of the total shutdown time of the circuit fuse off. from relative overload current or short circuit I/In.in. called protective characteristic, i.e. off =f(I/ In.vst.).

The dependence of the period of time during which the temperature of an element of an electrical installation reaches the maximum permissible on the ratio of the actual current in it I to the rated current Iн is called thermal characteristics of this element, i.e. load=f(I/ In).

Comparison of the protective characteristics of fuses with the thermal characteristics of the protected elements allows us to evaluate

possibility of reliable protection. (Fig.1)

I/I N.VST and I/I h

(5 min) It can be seen that the insert with a protective characteristic A protects an element of an electrical installation with a thermal characteristic IN at any current ratio, and the insert with a protective characteristic WITH– only for multiplicities of more than 4.

We need to strive for the shutdown time to be as short as possible under the action of short-circuit currents. and have a delay during overload currents. It can be done:

Right select the material of the fuse link;

use metallurgical effect;

choose rational design.

Inserts from low-melting metals (tin, lead, zinc, aluminum) have low thermal conductivity, so they heat up slowly; they are convenient for protecting elements from overload currents.

Inserts from refractory metals ( copper, silver) have low heat capacity and high thermal conductivity, therefore they heat up quickly, give a shorter delay time during overloads, which worsens their protective characteristics. But they have a large maximum shutdown current, so they are convenient for protecting elements from short-circuit currents.

To reduce the melting point (so that they heat up more slowly), inserts with metallurgical effect, for which a ball of low-melting metal (tin, an alloy of tin with cadmium, etc.) is soldered in the middle of an insert made of a refractory metal.

At the point where the ball is soldered, the more refractory metal dissolves into the low-melting one. This insert has better protective characteristics during overload currents and a lower melting temperature (2-3 times lower than the melting temperature of the base metal).

From point of view design influences the protective characteristics length (for fuses with U = 120 – 500V, the optimal insertion length is 70mm) and insert form(inserts are made with several parallel branches; inserts with 2–4 short isthmuses are used).

When operating a household and industrial electrical network, there is always a risk of electrical injury or equipment damage. They can occur at any time when critical conditions appear. Protective devices can reduce such consequences. Their use significantly increases the safety of using electricity.

Electrical circuit protections operate on the basis of:

fuse;

mechanical circuit breaker.

Operating principle and fuse design

Two brilliant scientists, Joule and Lenz, simultaneously established the laws of mutual relationships between the amount of current passing in a conductor and the release of heat from it, revealing the dependence on the resistance of the circuit and the duration of the period of time.

Their findings made it possible to create the simplest protective structures based on the thermal effect of current on the metal wire. It uses a thin metal insert through which the full current of the circuit is passed.

At rated parameters for transmitting electricity, this “wire” reliably withstands the thermal load, and if its values exceed the norm, it burns out, breaking the circuit and relieving the voltage from consumers. To restore the functionality of the circuit, it is necessary to replace the burnt-out element: the fuse-link.

It is clearly visible on the designs of fuses for household television and radio equipment with glass, transparent insert housings.

Special metal pads are mounted at its ends, creating electrical contact when installed in the sockets. This principle is embodied in electrical plugs with fusible links, which for many decades protected our parents and older generations from damage in electrical wiring.

Automatic structures were developed using the same form, which were screwed into sockets instead of plugs. But they did not need to be replaced when triggered components. To restore power supply, simply push the button inside the case.

Old electrical connections to the apartment were protected in this way. Then, along with fuses, they began to appear.

The choice of fuse is based on:

rated current values of the fuse itself and its insert;

coefficients of minimum/maximum test current multiplicity;

limit switchable electric current and the possibility of interruption of transported power;

protective characteristics of the fuse link;

fuse rated voltage;

compliance with the principles of selectivity.

The fuses have a simple design. They are widely used in electrical installations, including high-voltage equipment up to 10 kV, for example, in the protection of voltage instrument transformers.

Operating principle and design of the circuit breaker

The purpose of a mechanical switching device called a circuit breaker is:

turning on, passing, turning off currents in normal circuit mode;

automatic removal of voltage from an electrical installation during emergency conditions, for example, metal short circuit currents. Circuit breakers operate in reusable short circuit and overload protection modes. The possibility of repeated use is considered their main difference from a fuse.

During the Soviet era, automatic circuit breakers of the AP-50, AK-50, AK-63, and AO-15 series were widely used in the energy sector.

In modern electrical diagrams Improved designs from foreign and domestic manufacturers are in use.

All of them are enclosed in dielectric housings and have common executive bodies that provide:

1. thermal tripping of the circuit when the permissible current value is slightly exceeded;

2. electromagnetic cut-off during sudden load surges;

3. arc suppression chambers;

4. contact systems.

In the case of heating by the energy of the generated heat, a bimetallic plate works, bending under the influence of temperature until the release mechanism is activated. This function depends on the amount of heat released and is extended over time until a certain point.

The cut-off operates as quickly as possible from the operation of the electromagnetic solenoid with the occurrence of an electric arc. To extinguish it, special measures are used.

Reinforced contacts are designed to withstand repeated breaks.

Operational differences between circuit breakers and fuses

The protective properties of both methods have been time-tested, and each method requires an analysis of specific operating conditions when assessing the cost of the structure, taking into account the duration and reliability of operation.

Circuit breakers simpler design, disable the circuit once, cheaper. They can relieve tension manually, but this is usually not very convenient. In addition, at slightly higher currents, they disconnect the load for a long time. This factor may cause increased fire danger.

Any fuse protects only one phase of the network.

Circuit breakers more complex, more expensive, more functional. But they are more accurately adjusted to the settings of the protected electrical circuit, selected according to the operating design current, taking into account the switched powers.

The casings of modern machines made of thermosets have increased resistance to thermal effects. They do not melt and are resistant to fire. For comparison, the polystyrene housing of old switches could withstand temperatures no higher than 70 degrees.

The design allows you to select models for simultaneous opening of one to four electrical circuits. If fuses are used in a three-phase circuit, they will remove voltage from the circuit with different time delays, which can become an additional reason for the development of an accident.

Fuses operate on current, without taking into account its characteristics. Circuit breakers are selected for the load and classified by letters:

A - electrical networks of increased length;

B - lighting of corridors and areas;

C - power and lighting systems with moderate starting currents;

D—predominant loads from turning on electric motors with high starting parameters;

K - induction furnaces and electric dryers;

Fuses are switching electrical products used to protect the electrical network from overcurrents and short circuit currents. The principle of operation of fuses is based on the destruction of specially designed current-carrying parts (fuse links) inside the device itself when a current flows through them, the value of which exceeds a certain value.

Fuse links are the main element of any fuse. After burning out (cutting off the current), they must be replaced. Inside the fuse link there is a fusible element (it is this that burns out), as well as an arc extinguishing device. The fuse link is most often made of a porcelain or fiber body and is attached to special conductive parts of the fuse. If the fuse is designed for low currents, then the fuse for it may not have a housing, i.e., be frameless.

The main characteristics of fuse ratings include: rated current, rated voltage, breaking capacity.

Fuse elements also include:

The fuse holder is a removable element, the main purpose of which is to hold the fuse;

Fuse contacts are the part of the fuse that provides electrical communication between the conductors and the fuse contacts;

The fuse striker is a special element whose task, when the fuse trips, is to influence other devices and contacts of the fuse itself.

All fuses are divided into several dozen types:

According to the design of fuse links, fuses are either dismountable or non-removable. With collapsible fuses, you can replace the fuse link after it burns out; with non-removable fuses, this cannot be done;

Presence of filler. There are fuses with and without filler;

Designs for manufacturing fuse links. There are fuses with blade, bolt and flange contacts;

Fuses for the fuse-link body are divided into tubular and prismatic. In the first type of fuses, the fuse link has a cylindrical shape, in the second type it has the shape of a rectangular parallelepiped;

Type of fuse links depending on the range of tripping currents. There are fuses with a breaking capacity in the full range of shutdown currents - g and with a breaking capacity in part of the range of shutdown currents - a;

Speed. There are slow-acting fuses (used in most cases in transformers, cables, electrical machines) and high-speed fuses (used in semiconductor devices);

Fuse base designs can be with a calibrated base (in such fuses it will not be possible to install a fuse link designed to work with a rated current greater than the fuse itself) and with an uncalibrated base (in such fuses it is possible to install a fuse link whose rated current is greater than the rated current the fuse itself);

Voltage fuses are divided into low-voltage and high-voltage;

Number of poles. There are one-, two-, three-pole fuses;

The presence and absence of free contacts. There are fuses with and without free contacts;

Depending on the presence of a striker and an indicator, there are fuses - without a striker and without an indicator, with an indicator without a striker, with a striker without an indicator, with an indicator and a striker;

By the method of fastening the conductors, fuses are divided into fuses with front connection, rear connection, universal (both rear and front);

Installation method. There are fuses on their own base and without it.

Historically, the mechanical design of fuse boxes and their overall and connection dimensions have varied from country to country. There are four main national standards for fuse mounting sizes: North American, German, British and French. There are also a number of fuse housings that are the same from country to country and are not national standards. Most often, such cases refer to the standards of the manufacturer that developed a specific type of device, which turned out to be successful and gained a foothold in the market. In recent decades, as part of the globalization of the economy, manufacturers have gradually joined the international system of fuse housing standards to simplify the conditions for the interchangeability of devices. When choosing, you should try to use fuses of international standards: IEC 60127, IEC 60269, IEC 60282, IEC 60470, IEC60549, IEC 60644.

It should be noted that according to the type of fuse-links, depending on the range of shutdown currents and operating speed, fuses are divided into usage classes. In this case, the first letter indicates the functional class, and the second indicates the object to be protected:

1st letter:

a - protection with breaking capacity in part of the range (accompanied fuses): fuse links capable of at least long-term passing currents not exceeding the rated current specified for them, and disconnecting currents of a certain multiple relative to the rated current up to the rated breaking capacity;

g - protection with breaking capacity over the entire range (general purpose fuses): fuse links capable of at least continuously passing currents not exceeding the rated current specified for them, and disconnecting currents from the minimum melting current to the rated breaking capacity.

2nd letter:

G - protection of cables and wires;

M - protection of switching devices/motors;

R - protection of semiconductors/thyristors;

L - protection of cables and wires (in accordance with the old, no longer valid DIN VDE standard);

Tr - transformer protection.

A general view of the time-current characteristics of fuses of the main categories of use is shown in Figure 2.1.

Fuse links with the following usage classes provide:

gG (DIN VDE/IEC) - protection of cables and wires over the entire range;

aM (DIN VDE/IEC) - protection of switching devices in part of the range;

aR (DIN VDE/IEC) - protection of semiconductors in part of the range;

gR (DIN VDE/IEC) - protection of semiconductors over the entire range;

gS (DIN VDE/IEC) - protection of semiconductors, as well as cables and lines over the entire range.

Fuses with breaking capacity over the entire range (gG, gR, gS) reliably switch off both short-circuit currents and overloads.

Rice. 2.1.

Fuses with partial breaking capacity (aM, aR) serve exclusively for short-circuit protection.

To protect installations for voltages up to 1000 V, electric, tubular and open (plate) fuses are used.

The electrical fuse consists of a porcelain body and a plug with a fuse link. The supply line is connected to the fuse contact, the outgoing line to the screw thread. In the event of a short circuit or overload, the fuse link burns out and the current in the circuit stops. The following types of electrical fuses are used: Ts-14 for current up to 10 A and voltage 250 V with a rectangular base; Ts-27 for current up to 20 A and voltage 500 V with a rectangular or square base and Ts-33 for current up to 60 A and voltage 500 V with a rectangular or square base.

For example, electrical threaded fuses, PRS series, are designed to protect against overloads and short circuits of electrical equipment and networks. Rated voltage before

Keepers - 380 V AC at 50 or 60 Hz. Structurally, PRS fuses (Fig. 2.2) consist of a body, a fuse-link PVD, a head, a base, a cover, and a central contact.

PRS fuses are produced for rated fuse-link currents from 6 to 100 A. The designation of the fuse indicates what connection it is: PRS-6-P - 6 A fuse, front wire connection; PRS-6-Z - 6A fuse, rear wire connection.

Cylindrical fuses PTSU-6 and PTSU-20 with a threaded base Ts-27 and fuse-links for currents of 1, 2, 4, 6, 10, 15, 20 amperes are produced in a plastic case. PD fuses have a porcelain base, while PDS fuses have a base material of steatite. In domestic conditions, automatic plug fuses are used, where the protected circuit is restored by a button.

Tubular fuses are produced in the following types: PR-2, NPN and PN-2. The PR-2 fuse (dismountable fuse) is intended for installation in networks with voltages up to 500 V and for currents of 15, 60, 100, 200, 400, 600 and 1000 A.

In the fuse holder PR-2 (Fig. 2.3), the fuse link 5, attached with screws 6 to the contact blades 1, is placed in a fiber tube 4, onto which threaded bushings 3 are mounted. Brass caps 2 are screwed onto them, securing the contact knives, which fit into fixed spring contacts installed on the insulating plate.

Rice. 2.2.

Rice. 2.3.

Under the influence of an electric arc that occurs when a fuse blows, the inner surface of the fiber tube decomposes and gases are formed that help quickly extinguish the arc.

Closed fuses with fine-grained filler include fuses of the NPN, NPR, PN2, PN-R, and KP types. Fuses of the NPN type (filled, non-removable fuse) have a glass tube. The rest have porcelain pipes. NPN type fuses are cylindrical in shape, PN type are rectangular.

The NPN fuse set consists of: fuse link - 1 piece; contact bases - 2 pcs.

NPN fuses are manufactured for voltages up to 500 V and currents from 15 to 60 A, fuses PN2 (bulk fuse, collapsible) - for voltages up to 500 V and currents from 10 to 600 A. Bulk fuses have fuse links made of several parallel copper or silver-plated wires are placed in a closed porcelain cartridge filled with quartz sand. Quartz sand promotes intensive cooling and deionization of gases produced during arc combustion. Since the tubes are closed, splashes of molten metal from the fuse links and ionized gases are not emitted outside. This reduces fire hazards and increases the safety of fuse servicing. Fuses with filler, like PR type fuses, are current-limiting.

Open plate fuses consist of copper or brass plates - tips into which calibrated copper wires are soldered. The tips are connected to the contacts on the insulators using bolts.

NPR type fuses are a closed, collapsible (porcelain) cartridge filled with quartz sand for rated currents up to 400 A.

PD fuses (PDS) - 1, 2, 3, 4, 5 - with filler for installation directly on busbars for currents from 10 to 600 A.

To protect the power valves of semiconductor converters of medium and high power For external and internal short circuits, high-speed fuses are widely used, which are the cheapest means of protection. They consist of contact blades and a silver foil fusible link placed in a closed porcelain socket.

The fuse link of such fuses has narrow calibrated isthmuses, which are equipped with radiators made of a ceramic material that conducts heat well, through which heat is transferred to the fuse body. These radiators also serve as arc-extinguishing chambers with a narrow slot, which significantly improves the extinction of the arc that occurs in the isthmus region. A signal cartridge is installed parallel to the fuse-link, the blinker of which signals the melting of the fuse-link and, acting on the microswitch, closes the signal contacts.

For a long time, the industry produced two types of high-speed fuses designed to protect converters with power semiconductor valves from short-circuit currents:

1) fuses of the PNB-5 type (Fig. 2.4, a) for operation in circuits with a rated voltage of up to 660 V DC and AC for rated currents of 40, 63, 100, 160, 250, 315, 400, 500 and 630 A;

2) PBV type fuses for operation in alternating current circuits with a frequency of 50 Hz with a rated voltage of 380 V for rated currents from 63 to 630 A.

Rice. 2.4.

Currently, the industry produces fuses of the PNB-7 type (Fig. 2.4, b) for a rated current of 1000 A and for a rated electrical circuit voltage of 690 V AC. The fusible elements of the PNB-7 fuse are made of pure silver (speed and durability). The contacts (terminals) of the fuse are made of electrotechnical copper with galvanic coating (high conductivity and durability).

The fuse housing is made of high-strength ultra-porcelain. The design of the fuse allows the use of additional devices - trip indicator, free contact.

Structure symbol fuses PNB7-400/100-X1-X2:

PNB-7 - series designation;

400 - rated voltage, V;

100 - rated current;

X1 - symbol of the type of installation and type of connection of conductors to the terminals: 2 - on its own insulating base with base contacts; 5 - on the bases of complete devices with base contacts; 8 - without base, without contacts (fuse link);

X2 - symbol for the presence of an operation indicator: 0 - without alarm; 1 - with striker and free contact; 2 - with operation indicator; 3 - with striker.

Industrial fuses of the PP series are designed to protect electrical equipment industrial installations and electrical circuits from overloads and short circuits.

Fuses of this series are produced in the following main types: PP17, PP32, PP57, PP60S. Fuses are manufactured with a trip indicator, with a trip indicator and free contact, or without signaling. Depending on the type, fuses are designed for voltages up to 690 V and rated currents from 20 A to 1000 A. Design features allow the installation of free contacts, normally open or closed, as well as the installation method - on their own base, on the base of complete devices, on conductors of complete devices .

Designation structure for fuses of types PP17 and PP32 - Х1Х2 - Х3 - Х4 - ХХХХ:

1) X1X2 - size designation (rated current, A): 31 -100A; 35 - 250A; 37 - 400A; 39 - 630A.

2) X3 - symbol of the type of installation and type of connection: 2 - on its own base, 5 - on the base of complete devices, 7 - on conductors of complete devices (bolt connection), 8 - without a base (fuse link), 9 - without a base ( The fuse link is unified in size with fuses PN2-100 and PN2-250).

3) X4 - symbol for the presence of an operation indicator, striker, free contact: 0 - without signaling, 1 - with striker and free contact, 2 - with operation indicator, 3 - with striker.

4) ХХХХ - climatic version: UHL, T and placement category 2, 3.

Currently, semiconductor converters are equipped with fuses of the PP57 (Fig. 2.5, a) and PP60S (Fig. 2.5, b) series.

Rice. 2.5.

The first ones are for protection. converting units for internal short circuits of alternating and direct current at voltages of 220 - 2000 V for currents of 100, 250, 400, 630 and 800 A. The second - for internal short circuits of alternating current at voltages of 690 V for currents of 400, 630, 800 and 1000 A.

Designation structure for fuses type PP57 - ABCD - EF:

Letters PP - fuse;

The two-digit number 57 is the conditional series number;

A - two-digit number - symbol of the rated current of the fuse;

B - number - symbol of the rated voltage of the fuse;

C - number - symbol according to the installation method and type of connection of conductors to the fuse terminals (for example, 7 - on the conductors of the converter device - bolted with angled terminals);

D - number - symbol for the presence of an operation indicator and an auxiliary circuit contact:

0 - without operation indicator, without auxiliary contact

1 - with operation indicator, with auxiliary contact

2 - with operation indicator, without auxiliary circuit contact;

E - letter - symbol of climatic version;

An example of a fuse symbol: PP57-37971-UZ.

PPN fuses are intended to protect cable lines and industrial electrical installations from overload and short circuit currents. The fuses are used in AC electrical networks with a frequency of 50 Hz and a voltage of up to 660 V and are installed in low-voltage complete devices, for example, in ShchO-70 distribution panels, VRU1 input-distribution devices, ShRS1 power distribution cabinets, etc.

Advantages of PPN fuses:

1) the fuse body and the base of the holder are made of ceramics;

2) the fuse and holder contacts are made of electrical copper;

3) the fuse housing is filled with fine quartz sand;

4) overall dimensions of fuses are ~15% smaller than PN-2 fuses;

5) power losses are ~40% less than those of PN-2 fuses;

6) presence of an operation indicator;

7) fuses are mounted and removed using a universal puller.

The design features of the PPN series fuses are shown in Fig. 2.6.

Fuses of the PPNI series (Fig. 2.7) for general use are designed to protect industrial electrical installations and cable lines from overload and short circuit and are available for rated currents from 2 to 630 A.

Used in single-phase and three-phase networks with voltages up to 660 V, frequency 50 Hz. Areas of application of PPNI fuses: input distribution devices (IDU); cabinets and distribution points (ShRS, ShR, PR); equipment of transformer substations (KSO, ShchO); low voltage cabinets (ShR-NN); control cabinets and boxes.

Rice. 2.6.

Due to the use of high-quality modern materials and a new design, PPNI fuses have reduced power losses compared to PN-2 fuses. The data presented in Table 2.1 shows the efficiency of PPNI fuses compared to PN-2.

Rice. 2.7.

The fuse and holder contacts are made of electrical copper with galvanic coating with a tin-bismuth alloy, which prevents their oxidation during operation.

The base of the holder (insulator) is made of reinforced thermosetting plastic, resistant to corrosion, mechanical stress, temperature changes and dynamic shocks that occur during short circuits up to 120 kA.

The fuse-link contacts are knife-shaped (sharpened), which allows them to be installed in holders with less effort.

All dimensions of PPNI fuse-links can be conveniently installed or dismantled using the universal removal handle RS-1, the insulation of which can withstand voltages up to 1000 V.

For fast and effective arc extinguishing, the fuse body is filled with highly chemically purified quartz sand.

The fusible element is made of phosphor bronze (an alloy of copper and zinc with the addition of phosphorus) and is reliably connected spot welding with fuse terminals.

The design of the fuse link has a special indicator, made in the form of a retractable rod, which allows you to visually determine tripped fuses.

PPNI fuses with a breaking capacity over the entire “gG” range operate reliably both under short-circuit currents and overloads.

The design, technical parameters, overall and installation dimensions of fuse-links and PPNI holders comply with modern IEC and GOST standards, and, therefore, these fuses can replace other domestic and imported fuses.

Selection of fuse links

Fuses are installed on all branches if the cross-section of the wire on the branch is smaller than the cross-section of the wire in the main line, at the inputs and in the head sections of the network in input distribution devices, power distribution cabinets and power boxes complete with switches or on separate panels. For selectivity of action, it is necessary that each subsequent fuse in the direction of the current source has

the rated current of the fuse link is at least one step higher than the previous one.

To calculate the protection of networks and equipment using fuses, the following data is required:

Rated fuse voltage;

Maximum short circuit current switched off by fuse;

Rated fuse current;

Rated current of fuse link;

Protective characteristic of the fuse.

The rated voltage of the fuse (Unom, pr) is called

the voltage indicated on it for continuous operation at which it is intended. The actual mains voltage (Uc) should not exceed the rated fuse voltage by more than 10%:

Uс ≤ 1.1 Unom,pr (2.1)

The rated current of a fuse (Inom, pr) is the current indicated on it, equal to the largest of the rated currents of the fuse links (Imax nom, PV) intended for this fuse. This is the maximum long-term current passed by the fuse under the condition of heating its parts, except for inserts.

Inom,pr = Imax nom,PV (2.2)

The maximum switchable current (breaking capacity) of a fuse (Imax,pr) is the largest value (effective) of the periodic component of the current that is switched off by the fuse without destruction and dangerous emission of flame or combustion products of an electric arc. This fuse size for each type may vary depending on the voltage, rated current of the fuse, the value of cosph in the disconnected circuit and other conditions.

The rated current of a fuse link (Inom, PV) is the current indicated on it for continuous operation at which it is intended. In practice, this is the maximum long-term current passed by the insert (Imax, PB), according to the condition of the permissible heating of the insert itself.

Inom,PV = Imax,PV (2.3)

Usually, in addition to the rated current of the insert, two more values of the so-called test currents are indicated, by which the inserts are calibrated. The lower value of the test current, the fuse-link must withstand a certain time, usually 1 hour, without melting; at the upper value of the test current, the insert should burn out in no more than a certain time, usually also 1 hour.

The main data for determining the burnout time of the insert, and, consequently, the selectivity of fuses connected in series, are their protective characteristics.

The protective characteristic of a fuse is the dependence of the total shutdown time (the sum of the melting time of the insert and the arc burning time) on the value of the switched off current.

Protective characteristics are usually given in the form of a graph, in rectangular coordinates. Time is plotted along the vertical coordinate axis, and the multiplicity of the current switched off by the fuse to the rated current of the insert, or the switched current, is plotted along the horizontal axis.

The selectivity of fuse protection is ensured by selecting fuse links in such a way that if a short circuit occurs, for example, on a branch to an electrical receiver, the nearest fuse protecting this electrical receiver will trip, but the fuse protecting the head section of the network will not trip.

The selection of fuse links according to the selectivity condition should be made using the standard protective characteristics of the fuses, taking into account the possible spread of actual characteristics according to the manufacturer.

A typical time-current characteristic of a modern double-action fuse is shown in Figure 2.8.

With a rated current of 200 A, the fuse should operate indefinitely. The characteristic shows that as the current decreases, the response time in the region of low currents increases rapidly and the dependence curve should ideally asymptotically tend to the straight line I = 200 A, for time t = + ∞. In the area of operating overloads, that is, in the case when the current through the fuse is within the range of (1-5)⋅In, the response time of the fuse is quite long - it exceeds a few seconds (at a current of 1000A, the response time is 10 s).

This type of dependence allows the protected equipment to operate freely over the entire range of operating overload characteristics. With a further increase in current, the slope of the time-current characteristic (Fig. 2.8) quickly increases, and already with an eleven-fold overload, the response time is only 10 ms. A further increase in the overload current reduces the response time to an even greater extent, although not as quickly as in the area between five and ten times the overload. This is explained by the finite rate of arc extinction due to the finite heat capacity of the filler material, the finite heat of fusion of the fusible bridge material, and the certain mass of the melting and evaporating bridge metal. With a further increase in current (more than 15-20 times the rated value), the response time of the fuse element can be 0.02-0.5 ms, depending on the type and design of the fuse.

Rice. 2.8.

With a rated current of 200 A, the fuse should operate indefinitely. The characteristic shows that as the current decreases, the response time in the region of low currents quickly increases, and the dependence curve should ideally asymptotically tend to the straight line I = 200 A, for time t = + ∞. In the area of operational overloads, i.e. in the case when the current through the fuse is within the range of (1-5)⋅In, the response time of the fuse is quite long - it exceeds a few seconds (at a current of 1000 A, the response time is 10 s).

Siemens produces a wide range of fuses (combinations gG, gM, aM, gR, aR, gTr, gF, gFF), six standard sizes - 000(00С), 00, 1, 2, 3, 4а (designations according to IEC) for rated currents from 2 to 1600 A and voltages (~ 400V, 500V and 690V; - 250V, 440V) with the most commonly used knife type (NH) contacts in practice, predominantly in a vertical installation position.

NH type fuses have high breaking capacity and stable characteristics. The use of NH type fuses allows for selectivity of protection during short circuit.

Knife-type fuses NH (analogue of PPN) are intended for installation in contact holders PBS, PBD, in PVR series APC and RBK, as well as in load switches type RAB. It is possible to use these fuses in protective devices designed for the use of domestic PPN-type inserts.

NH type fuses are arc extinguishing fuses in a closed volume. The fusible link is stamped from zinc, which is a low-melting and corrosion-resistant metal. The shape of the fuse-link makes it possible to obtain a favorable time-current (protective) characteristic. The insert is located in a sealed insulating ceramic housing. Filler - quartz sand with a SiO content of at least 98%, with grains (0.2-0.4)⋅10 -3 m and humidity not higher than 3%.

When disconnected, the narrowed isthmuses of the fuse-link burn out, after which the resulting arc is extinguished due to the current-limiting effect that occurs when the narrowed sections of the fuse-link burn out. The average arc extinction time is 0.004 s.

The time-current characteristics of NH type fuses for use class gG are shown in Figure 2.9.

2 10 100 1 000 10 000 100 000

Expected short-circuit current IP, A

Rice. 2.9.

NH type fuses operate silently, with virtually no emission of flame or gases, which allows them to be installed at close distances from each other.

Another important characteristic of a fuse as a protective device is the so-called protective indicator, called I 2 ⋅t in foreign sources. For a protected electrical circuit, the protective indicator is the amount of heat generated in the circuit from the moment an emergency occurs until the circuit is completely turned off by the protective device. The value of the protective indicator specific device, in fact, determines the limit of its resistance to thermal destruction in emergency modes. When calculating the value of the protective index, the effective value of the current in the circuit is used.

For example, the effective value of the current flowing through the fuse can be calculated for commonly used AC rectifier circuits from the (smoothed) direct current Id or from the phase current IL, the values of which are given in Table 2.2.

During a short circuit, the fuse current (Fig. 2.10) increases during the melting time tS to the short circuit current IC (melting current peak).

Effective value of current flowing through the fuse

AC Rectifier Circuit	Effective value of phase current (phase fuse)	Branch current rms value (fuse in branch)
Single-pulse with midpoint
Two-pulse with midpoint
Three-pulse with midpoint
Six-pulse with midpoint
Double three-phase half wave
with midpoint (parallel)
Two-pulse bridge circuit
Six-pulse bridge circuit
Single-phase bidirectional circuit

During the arc extinguishing time tL, an electric arc is formed and the short circuit current is extinguished (Fig. 2.10).

The integral of the quadratic value of the current (∫l 2 dt) over the entire operating time (tS + tL), briefly called the total Joule integral, determines the heat that is supplied to the semiconductor element to be protected during the opening process.

To achieve a sufficient protective effect, the total Joule integral of the fuse insert must be less than the value of I 2 ⋅t (ultimate load integral) of the semiconductor element. Since the total Joule integral of the safety insert with increasing temperature, and, consequently, with increasing preload, practically decreases in the same way as the value of I 2 ⋅t of the semiconductor element, it is enough to compare the values of I 2 ⋅t in an unloaded (cold) ) condition.

Rice. 2.10.

The total Joule integral (I 2 ⋅tA) is the sum of the melting integral (I 2 ⋅tS) and the arc integral (I 2 ⋅tL). In general, the value of the total Joule integral of a semiconductor device must be greater than or equal to the value of the protective indicator of the fuse:

((∫I 2 t) (semiconductor, t = 25 °C, tP = 10 ms) ≥ ((∫I 2 ⋅tA) (fuse link).

The melting integral I 2 ⋅tS can be calculated for any time values, based on pairs of values of the time-current characteristic of the fuse insert.

As the melting time decreases, the melting integral tends to a lower limit value, at which during the melting process practically no heat is removed from the bridges of the melting conductor into the surrounding space. The melting integrals specified in the selection and ordering data and in the characteristics correspond to a melting time tS = 1 ms.

While the melting integral I 2 ⋅tS is a property of the fuse link, the arc integral I 2 ⋅tL depends on the characteristics of the electrical circuit, namely:

From recovery voltage UW;

From the power factor cosф of the short-circuited circuit;

From the expected current IP// (current at the installation location of the fuse link if it is short-circuited).

The maximum arc integral is achieved for each type of fuse at a current from 10⋅IP to 30⋅IP.

When protecting networks with fuses of types PN, NPN and NPR with given protective characteristics, the selectivity of the protection action will be carried out if between the rated current of the fuse-link protecting the head section of the network (Inom G, PV) and the rated current of the fuse-link at the branch to the consumer (Inom O , PV) certain ratios are maintained.

For example, at small fuse-link overload currents (about 180-250%), selectivity will be maintained if Inom G, PV > Inom O, PV by at least one step of the standard scale of rated currents of fuse-links.

In the event of a short circuit, selectivity of protection with NPN type fuses will be ensured if the following ratios are maintained:

I(3)SC / Inom O, PV ≤ …50; 100; 200;

Inom G, PV / Inom O, PV…2.0; 2.5; 3.3,

where I(3)SC is the three-phase short circuit current of the branch, A.

The relationships between the rated currents of fuse links Inom G, PV and Inom O, PV for fuses of the PN2 type, ensuring reliable selectivity, are given in Table 2.3.

If the protective characteristics of fuse links are unknown, a method of checking the selectivity in relation to the cross-sections of the inserts, adjusted for the material of the insert and the design of the fuse, is recommended. In this case, the cross-sections of the fuse links of the fuses connected in series (SK and SH) are determined; the ratio SP/SK is calculated and compared with the value SP/SK = a, which ensures selectivity.

SK - cross-section of the fuse insert installed closer to the short circuit; SP - cross-section of the fuse insert installed closer to the power source.

The value of a is determined from Table 2.4; if the calculated value Sn/SK ≥ a, then selectivity is ensured.

The main condition determining the choice of fuses for protecting squirrel-cage asynchronous motors is detuning from the starting current.

Rated current of the smaller fuse-link Inom O, PV A	Rated current of the larger fuse-link Inom G, PV, A, with the ratio I(3)SC / Inom O, PV
Rated current of the smaller fuse-link Inom O, PV A		100 or more

Note. 1(3) Short circuit - short circuit current at the beginning of the protected section of the network.

The detuning of fuse-links from starting currents is carried out according to time: the start of the electric motor must be completely completed before the insert melts under the influence of the starting current.

Operating experience has established a rule: for reliable operation of inserts, the starting current should not exceed half the current, which can melt the insert during the start-up.

All electric motors are divided into two groups according to start time and frequency. Motors with easy starting are considered to be motors of fans, pumps, metal-cutting machines, etc., the start of which ends in 3-5 s; these motors are started rarely, less than 15 times in 1 hour.

Engines with heavy starting include engines of cranes, centrifuges, ball mills, the start of which lasts more than 10 s, as well as engines that are started very often - more than 15 times in 1 hour. This category also includes engines with easier starting conditions, but especially responsible ones, for whom false burnout of the insert during startup is completely unacceptable.

Sn/SK insert cross-section ratio ensuring selectivity

Metal fuse link	Metal fuse link,
fuse located	located closer to the short circuit.
closer to the power source
Fuse with filler




Fuse without filler

The selection of the rated current of the fuse link for detuning from the starting current is made according to the expression:

Inom,PV ≥ I start,DV / K, (2.4)

where Ipus, DV is the starting current of the motor, determined from the passport, catalogs or direct measurement; K is a coefficient determined by the starting conditions and is equal to 2.5 for engines with easy starting, and 1.6-2 for engines with heavy starting.

Since the insert heats up and oxidizes when starting the engine, the cross-section of the insert decreases, the condition of the contacts deteriorates, and it can falsely burn out during normal engine operation. An insert selected in accordance with (2.4) can also burn out when

Starting or self-starting of the engine is delayed compared to the estimated time.

Therefore, in all cases, it is advisable to measure the voltage at the motor inputs at the time of start-up and determine the start-up time.

To prevent the inserts from burning out during startup, which may result in the engine operating in two phases and causing damage, it is advisable in all cases where this is permissible due to sensitivity to short-circuit currents, to select inserts that are coarser than according to condition (2.1).

Each engine must be protected by its own separate protection device. A common device is allowed to protect several low-power motors only if the thermal stability of the starting devices and overload protection devices installed in the circuit of each motor is ensured.

Selection of fuses to protect lines supplying several asynchronous electric motors

Protection of lines supplying several motors must ensure both starting the motor with the highest starting current and self-starting of the motors, if it is permissible under safety conditions, technological process, etc.

When calculating protection, it is necessary to accurately determine which motors are switched off when the voltage drops or completely disappears, which remain switched on, and which are switched on again when voltage appears.

To reduce disruptions to the technological process, special circuits are used to turn on the holding electromagnet of the starter, which ensures immediate inclusion of the motor in the network when voltage is restored. Therefore, in the general case, the rated current of the fuse link, through which several self-starting motors are powered, is selected according to the expression:

Inom, PV ≥ ∑Ipus, DV / K, (2.5)

where ∑Ipus, DV is the sum of the starting currents of self-starting electric motors.

Selecting fuses to protect lines in the absence of self-starting electric motors

In this case, fuse links are selected according to the following ratio:

Inom, PV ≥ Imax, TL / K, (2.6)

where Imax, TL = Ipus, DV + Idolt, TL - maximum short-term line current; Ipus, DV - the starting current of an electric motor or a group of simultaneously switched on electric motors, when starting which the short-term line current reaches highest value; Idlit, TL - long-term calculated line current until the electric motor (or group of electric motors) is started - this is the total current consumed by all elements connected through a fuse, determined without taking into account the operating current of the started electric motor (or group of motors).

Selection of fuses to protect asynchronous electric motors from overload

Since the starting current is 5-7 times the rated current of the motor, the fuse-link selected according to expression (2.4) will have a rated current 2-3 times the rated current of the motor and, while withstanding this current for an unlimited time, cannot protect the motor from overload . To protect motors from overload, thermal relays are usually used, built into magnetic starters or circuit breakers.

If a magnetic starter is used to protect the motor from overload and control it, then when choosing fuse-links it is also necessary to take into account the condition of preventing damage to the contactors of the starter.

The fact is that during short circuits in the engine, the voltage on the holding electromagnet of the starter decreases, it falls off and breaks the short circuit current with its contacts, which, as a rule, are destroyed. To prevent this short circuit, the motors must be switched off by a fuse before the starter contacts open.

This condition is ensured if the shutdown time of the short circuit current by the fuse does not exceed 0.15-0.2 s; for this, the short circuit current must be 10-15 times greater than the rated current of the fuse insert protecting the electric motor, i.e.:

I(3) Short circuit / Inom, PV ≥ 10–15. (2.7)

Protection by fuses of networks up to 1000 V from overload

PUE 3.1.10 specifies networks with voltages up to 1000 V, which require, in addition to short circuit protection, overload protection. These include:

1. All networks laid openly with unprotected insulated wires with a flammable sheath, inside any premises.

2. All lighting networks, regardless of the design and method of laying wires or cables in residential and public buildings, in retail premises, in service and domestic premises of industrial enterprises, in fire hazardous industrial premises, all networks for powering household and portable electrical appliances.

3. All power networks in industrial enterprises, residential and public premises, if, due to the conditions of the technological process, long-term overload of wires and cables may occur.

4. All networks of all types in explosive premises and explosive outdoor (outside buildings) installations, regardless of the operating mode and purpose of the network.

The rated current of the fuse-link must be selected as low as possible, subject to the condition of reliable transmission of the maximum load current. Almost at a constant, without shocks, load, the rated current of the insert 1nom, PV is taken approximately equal to the maximum continuous load current Imax, TN, namely:

Inom, duty cycle ≥ Imax, TN. (2.8)

Based on the rated current of the insert, the permissible continuous load current 1dlit,TN for the conductor (laid under normal conditions) protected by the selected insert is determined:

kк⋅Inom, PV ≤ kп⋅Idlit, TN, (2.9)

where kk is a coefficient that takes into account the design of the conductors protected by the insert, equal to 1.25 according to PUE 3.1.10 for conductors with rubber and similar flammable insulation, laid in all rooms except non-explosive industrial ones. For any conductors laid in non-explosive industrial premises, and paper-insulated cables in any premises, kк = 1:

kп = kп1⋅kп2⋅kп3, (2-10)

where kп is a general correction factor corresponding to the case when the actual laying conditions differ from normal ones.

If the load is of the nature of shocks, for example, a crane electric motor, and the duration of the load is less than 10 minutes, then a correction factor kп1 is introduced. This coefficient is introduced for copper conductors with a cross-section of at least 6 mm2 and aluminum conductors with a cross-section of at least 10 mm2. The value kп1 is taken according to the expression

kп1 = 0.875/ √PV,

where PV is the on-time duration expressed in relative units, equal to the ratio of the on-time of the receiver, for example an electric motor, to the total cycle time of the intermittent mode. The kP1 coefficient is introduced if the duration of the switching on is no more than 4 minutes, and the break between switching on is at least 6 minutes. Otherwise, the load current value is taken as for the continuous mode.

If the ambient temperature differs from normal, a correction factor kP2 is introduced, determined from the PUE tables.

When laying more than one cable in one trench, a correction factor kP3 is introduced, which is also determined from the PUE tables.

In secondary switching circuits (operating current, instrumentation, voltage measuring transformers, etc.), fuse-links are selected according to short-circuit currents based on the condition:

I(3)SC / Inom,PV ≥ 10 (2.11)

Fuses are installed on distribution boards and power points. The fuse link is installed vertically. After tightening all the fasteners, check the contact between the contacts of the knife or cartridge cap and the jaws of the racks. The “springing” of the contact jaws of the racks when a knife or cartridge cap enters them should be noticeable to the eye. Fuse holders must not fall out of the contact posts when a force is applied to them, equal to for fuses rated for current: 40A - force 30N; 100A - 40N; 250A - 45N; 400A - 50N; 600A - 60N.

When switching on again, the fuses are checked to the following extent:

1. External inspection, cleaning, checking contact connections.

2. Checking the correct choice of the rated current of the fuse link.

In production conditions, reasons arise when it is necessary, in the absence of a standard fuse link, to replace it with a conductor whose properties will be equivalent to the fuse link.

Table 2.5 shows the cross-sectional area of various conductor materials suitable for use as a fuse link.

Selecting fuses to protect semiconductor elements

Fuses for protecting the semiconductor elements of the insert are selected according to the rated voltage, rated current, total Joule integral I2⋅tA and load cycle factor, taking into account other specified conditions.

The design voltage Uр of a fuse link is the voltage given as the effective value of the alternating voltage when generating ordering and design data, as well as indicated on the fuse link itself.

The design voltage of the fuse link is selected in such a way that it reliably switches off the voltage that initiates the short circuit. This voltage should not exceed the value of Uр +10%. In this case, it is also necessary to take into account the fact that the supply voltage Upc of the AC rectifier can increase by 10%. If in a short-circuited circuit two branches of the AC rectifier circuit are located in series, then if the short-circuit current is sufficiently large, one can count on uniform voltage distribution.

The value of the wire cross-section for the fuse link depending on the load current

Current value, A	Lead, mm2	Alloy, mm2: 75% - lead, 25% - tin	Iron, mm2

Straightening mode. For AC rectifiers that operate only in rectification mode, the supply voltage Uпc acts as the exciting voltage.

Invert mode. For AC rectifiers that also operate in inverting mode, the failure may be caused by the inverter stalling. In this case, the sum from the supply voltage acts as the exciting voltage Uin in the short-circuited circuit DC voltage(for example, the electromotive force of a DC machine) and the voltage of the three-phase current of the supply network. When selecting a fuse insert, this amount can be replaced by alternating voltage, the effective value of which corresponds to 1.8 times the value of the three-phase voltage of the supply network (Uin = 1.8 Upc). Fuse links must be designed in such a way that they reliably interrupt the voltage Uin.

The rated current, load capacity Ip of the fuse link is the current given in the selection and ordering data and characteristics, and also indicated on the fuse link as the effective value of the alternating current for the frequency range 45-62 Hz.

For operation of a fuse link with rated current, the normal operating conditions are:

Natural air cooling at ambient temperature +45°C;

The cross-sections of the connections are equal to the control cross-sections when operating in NH fuse bases and disconnectors;

The half-cycle current cut-off angle is 120°;

Constant load is maximum at rated current.

For operating conditions different from those listed above, the permissible operating current Ip of the fuse link is determined by the following formula:

Ip = ku ⋅ kq ⋅ kl ⋅ ki ⋅ kwl ⋅ Ip, (2.12)

where Ip is the calculated current of the fuse link;

ku - correction factor for ambient temperature;

kq - correction factor cross section accessions;

kl - correction factor for current cut-off angle;

ki is the correction factor for intensive air cooling;

kwl - load cycle coefficient.

The load cycle factor kwl is a reduction factor that can be used to determine the time-invariant load capacity of fuse links under any load cycle. Safety inserts have different load cycle coefficients due to their design. The characteristics of the fuse links indicate the corresponding load cycle factor kwl for > 10,000 load changes (1 hour "On", 1 hour "Off") over the expected service life of the fuse links.

With a uniform load (there are no load cycles and shutdowns), you can take the load cycle factor kwl = 1. For load cycles and shutdowns that last more than 5 minutes and occur more than once a week, you should choose the load cycle factor kwl specified in characteristics of individual safety links from manufacturers.

Residual coefficient - krw.

Preloading the safety insert reduces the permissible overload and melting time. Using the residual coefficient krw, it is possible to determine the time during which the fuse link, with a periodic or non-periodic load cycle in excess of the pre-calculated permissible load current Ip, can operate with any overload current Ila without losing its original properties over time.

The residual coefficient kRW depends on the preload V= Ieff/Ip - (the ratio of the effective value of the current Ieff flowing through the fuse during the load cycle to the permissible load current Ip), as well as on the overload frequency F. Graphically, this dependence is represented by two curves (Fig. 2.11): kRW1 = f (V), with F = frequent shock currents / load cycle currents > 1/week; kRW2 = f (V), with F = rare surge currents / load cycle currents

After determining the coefficient kRW1 (kRW2) graphically, the reduced permissible load duration tsc can be determined using the expression:

tsc = kRW1 (kRW2) ⋅ ts

The reduction in the melting time of the safety insert tsy during preload is determined from the calculated value of V using the given curve kR3 = f (V) (Fig. 2.11) according to the expression:

tsy = kR3 ⋅ ts

Rice. 2.11.

AC rectifiers often operate not with continuous loads, but with alternating loads, which may also briefly exceed the rated current of the AC rectifier.

For the case of variable load, four typical types of load are classified for the operating mode of fuse links that does not change over time:

Unknown variable load, but with a known maximum current (Fig. 2.13);

Variable load with a known load cycle (Fig. 2.14);

Random shock load from a preload with an unknown sequence of shock pulses (Fig. 2.15).

Determining the required rated current IP of the fuse link for each of the four types of load is carried out in two stages:

1. Determination of the design current IP based on the effective value Ieff of the load current:

IP > Ieff ⋅(1/ ku ⋅ kq ⋅ kl ⋅ ki ⋅ k). (2.13)

2. Checking the permissible duration of overload by current blocks that exceed the permissible operating current of the IP/ fuse, using the expression:

kRW ⋅ ts ≥ tk, (2.14)

where tK is the duration of the overload.

If the resulting overload duration is shorter than the corresponding required overload duration, select a fuse link with a higher rated current Ip (taking into account the rated voltage Up and the permissible total Joule integral) and repeat the test.

Fuse selection example

ELECTROSPETS

Fuse material

Fuse links are made of copper, zinc, lead or silver. The main technical data of these materials in terms of their applicability for fuse links are given in Table. 1.

Table 1.

In today's most advanced fuses, preference is given to copper inserts with a tin solvent. Zinc inserts are also widespread. Copper fuse inserts are the most convenient, simple and cheap. Improving their characteristics is achieved by fusing a tin ball in a certain place, approximately in the middle of the insert. Such inserts are used, for example, in the mentioned series of bulk fuses PN2. Tin melts at a temperature of 232° C, significantly lower than the melting point of copper, and dissolves the copper of the insert at the point of contact with it. The arc that appears in this case already melts the entire insert and is extinguished. The current circuit turns off.
Thus, fusing a tin ball results in the following.
Firstly, copper inserts begin to react with a time delay to such small overloads, to which they would not react at all in the absence of a solvent. For example, a copper wire with a diameter of 0.25 mm with a solvent melted at a temperature of 280 ° C in 120 minutes.
Secondly, at the same sufficiently high temperature (i.e., under the same load), inserts with a solvent react much faster than inserts without a solvent. For example, a copper wire with a diameter of 0.25 mm without a solvent at an average temperature of 1000 ° C melted in 120 minutes, and the same wire, but with a solvent at an average temperature of only 650 ° C, melted in just 4 minutes.
The use of a tin solvent makes it possible to have reliable and cheap copper inserts that operate at a relatively low operating temperature, have a relatively small volume and weight of metal (which favors the switching ability of the fuse) and at the same time have greater speed at high overloads and react with a time delay to relatively small overloads. The ratio Ip og:Iv for such inserts is relatively small (no more than 1.45), which facilitates the selection of conductors protected by such fuse-links from overloads.
Zinc is often used to make fuse links. In particular, such inserts are used in the mentioned series of PR2 fuses. Zinc inserts are more resistant to corrosion. Therefore, despite the relatively low melting point, for them, generally speaking, it would be possible to allow the same maximum operating temperature as for (copper 250°C) and design inserts with a smaller cross-section. However, the electrical resistance of zinc is approximately 3.4 times greater than that of copper. To maintain the same temperature, it is necessary to reduce energy losses in it, accordingly increasing its cross-section. The insert turns out to be much more massive. This, other things being equal, leads to a decrease in the switching capacity of the fuse. In addition, with a massive insert with a temperature of 250°C, it would not be possible to maintain the temperature of the cartridge and contacts at an acceptable level in the same dimensions. All this makes it necessary to reduce the maximum temperature of zinc inserts to 200°C, and therefore further increase the cross-section of the insert. As a result, fuses with zinc inserts of the same dimensions have significantly less resistance to short-circuit currents than fuses with copper inserts and tin solvents.
When there is a great need, a number of enterprises produce fuse links in their own electrical repair shops. At the same time, the materials from which the fuse-link elements are made must be carefully calibrated and at least 10% of the finished fuse-links must be selectively tested for minimum and maximum currents.
The minimum current is taken at which the fuse-link should not burn out in less than 1 hour. Usually this current is equal to 1.3-1.5 of its rated current, i.e. Imin = (l.3-1.5)In.
The maximum current is taken at which the fuse link must burn out in less than 1 hour; it is usually (l.6-2.l)In.
The manufactured fuse inserts must meet the requirements of the relevant GOSTs in terms of their qualities, characteristics and rated currents.
It is unacceptable to use home-made inserts, since at best they protect the installation only from short-circuit currents. To fasten the zinc fuse link, a steel washer of increased diameter and a spring washer must be used. In the absence of these washers, the zinc is gradually squeezed out from under the contact bolt and weakens the contact. A copper insert cannot be installed in a PR fuse holder without a tin solvent, since at the high melting temperature of the copper insert, the fiber cartridge is quickly destroyed.

Burnt-out fuse links should be replaced with spare factory-calibrated ones. If there are none, they can be temporarily replaced with pre-prepared wires designed for a certain current. The diameters and materials of the wires are given in Table 2.

Table 2.

The body of fuse links is made of high-strength varieties of special ceramics (porcelain, steatite or corundum-mullite ceramics) to ensure their high breaking capacity. Some foreign companies (USA, Japan) make fuse housings from fiberglass impregnated with silicone resin. Analysis of the mechanical resin barrels confirms that they can be used to make fuse housings. The tensile strength of enclosures manufactured in this way is higher than that of similar sized ceramic enclosures with steel roofs. The main factor preventing the use of resins is their aging at elevated temperatures. At a body temperature not exceeding 30 0 C, no aging is detected, but at higher temperatures the mechanical and electrical properties of the resins deteriorate over time. Due to the fact that significant overheating of the fuse body is possible both in the nominal mode (up to 120 0 C) and in the field of current overloads, the use of insulating resins for the manufacture of housings and other structural elements of fuses will become possible only after the creation of casting resins with a sufficiently large thermal resistance in various modes operation of fuses.

The Fritz Driescher company (Germany) manufactured fuses with a spherical body made of epoxy resin, which greatly simplified the mass production of fuses. To increase mechanical strength, fibrous material is added to the epoxy resin. This fuse does not have threaded connections. These fuses are waterproof. But such fuses are designed only to cut off large short-circuit currents, since at low current overloads unacceptable overheating of the resin housing occurs.

For fuse housings with low rated currents, special glass is usually used.

DESIGN OF FUSABLE ELEMENTS.

All types of fusible elements can be divided into two groups: a cross-section of the fusible element that is constant along the length and a variable one. Constant-section fusible elements are usually made of wire, and variable-section fusible elements are usually made of metal foil or thin metal film.

The ratio of the cross-section of the wide part of the fuse element to the cross-section of the narrow isthmus determines the type of protective characteristic. For example, fast-blow fuses typically use fusible elements with a ratio greater than five. Characteristics for slow-blow and normal-acting fuses are obtained with a ratio of less than five.

Fuse elements with a constant cross-section usually have a current density much lower than that of fusible elements with a variable cross-section. When triggered, fuses with fuse elements of constant cross-section have large values of the melting current and melting integral, large overvoltages, but the arc burning duration and the ratio of the maximum value of the transmitted current to the melting current in these fuses are significantly less.

With an increase in the rated voltage of the fuse in fusible elements of variable cross-section, the number of series-connected narrow isthmuses increases, which is necessary so that when the fuses operate, a separate arc lights up on each isthmus. As a result of an increase in the number of sequentially burning arcs, the voltage at the fuse increases more rapidly than in cases where the fuse element has only one narrow isthmus.

The creation of several relatively narrow parallel channels for the combustion of an electric arc improves the conditions for its extinguishing by using more filler materials and reducing the current in each of the parallel arcs, therefore, when designing, it is preferable to divide the fusible elements into a number of parallel branches. The number of parallel branches is limited by the technological difficulties of manufacturing narrow isthmuses of small sizes.

The temperature of fusible elements in different operating modes of fuses varies within significant limits. As a result, a greater or lesser elongation of the fusible element occurs. Some variation in the sizes of fuse-link housings also leads to variation in the lengths of fuse elements from fuse to fuse, therefore, several bends along the length are provided in the fuse elements, compensating for the difference in the lengths of the body and the fuse element as a result of the influence of various factors.

The quality of fuses largely depends on the transient values electrical resistance. As studies have shown, if the contact connection of the fuse element with the contacts of the fuse-link is poor, the transition resistance can reach 50% of the electrical resistance of the fuse element. Because of this, the fuses overheat in nominal operating mode and their service life is reduced. In addition, if the contact connection is poor, the reproducibility of test results from one sample to another is impaired. All fusible elements of fuses with high rated currents are connected to the contact terminals by welding, ensuring good quality of the contact connection. For fuses with low rated currents, soft soldering is sometimes used, but more often mechanical crimping is used. In dismountable fuses, the fuse element is connected to the terminals of the fuse link with a bolt clamp.

DESIGN OF FUSE LINKS ACTIVATION INDICATORS

The fusible elements of modern fuses are located inside an opaque housing, and the condition of the fusible element cannot be visually determined. It is especially important to have an understanding of the condition of the fuse element for fuses with high current ratings due to the significant difficulties associated with installing and removing the fuse. In this regard, various types of indicators are used that indicate whether the fuse element has blown.

There are a large number of patents on signage designs. The most widely used is the actuation indicator, which uses the same principle as the main fusible element - melting under the influence of supercurrent. To create such an indicator, a thin metal wire with sufficient mechanical tensile strength is electrically connected parallel to the main fusible element. When overcurrent flows through the fuse, the main fuse element and the indicator wire burn out. The trigger indicator wire is tightly fixed on one side, and on the other it is connected to a pin, which is pulled into a special hole using a spring. The trigger wire is in quartz sand. Its length is usually approximately equal to the length of the fuse element, which is necessary for reliable extinguishing of the arc at the rated fuse voltage.

Trigger indicators of this type are manufactured in two types: autonomous - in the form of a small fuse-link with a high-resistance fuse element and filler, installed in its own housing outside the fuse-link and built into the body of the fuse-link. Autonomous trip indicators are sometimes mounted directly on the fuse link, and sometimes they are installed completely away from the fuse, having only an electrical connection with it. The latter is typical for fuses from English Electric (Great Britain).

After the indicator wire burns out, a spring is released, which pushes out a pin, painted in a bright color and which is a visual indicator that the fuse has blown. Sometimes the pin also serves as a striker, acting on the auxiliary contacts of the fuse. As a result, the signal that the fuse has tripped is transmitted to the appropriate controls.

Depending on the ratio of electrical resistances and thermophysical parameters of the main fusible element and the indicator, three different cases can be observed when the fuse is triggered:

1) initial melting of the main fusible element, burning of an arc on it. The active resistance of the pointer shunts the arc of the main fuse element, helping to reduce the rate of voltage rise across the gap and reduce the voltage peak;

2) initial melting of the pointer wire, and then melting of the main fuse element. Due to the fact that the main fusible element has a low active resistance, it will bridge the gap formed after the melting of the indicator wire and prevent the arc from burning in the indicator for any long time;

3) almost simultaneous melting of the main fusible element and the trigger wire. The arc burning on the pointer can occur until the end of the arc burning on the main fusible element in some cases, and in others, the arc burning on the pointer will stop much earlier than in the main fusible element

Unfortunately, pointers of this type are unstable. At low voltages and low current overloads, the wire burns out at small area. If this area is located at a large distance from the spring and if the packing density of the sand filler in the indicator body is large, the frictional forces of the wire on the sand filler may exceed the elastic force of the spring and the operation indicator may not work. The disadvantage of these indicators is also that in case of accidental mechanical breakage of the fuse element during the assembly process or for some other reason, the operation indicator does not show the actual state of the fuse without turning on the voltage.

Gas-discharge lamps and LEDs connected parallel to the fuse link are also used as visual indicators of operation. But the cost of such response indicators is higher, and their operational reliability is lower than that of the operation indicators described above.

CLOSED FUSES

Closed fuses are usually made in the form of a fiber tube, closed at the ends with brass caps. There are fusible inserts inside the tube. The electric arc formed during combustion of the insert burns in a closed volume. When the arc burns, the walls release gas, the pressure in the tube increases, and the arc goes out.

Closed fuses of the PR-2 series (collapsible) have rated currents from 100A to 1000 A, the maximum switchable currents at a voltage of 380V and cosj³0.4 range from 6 kA to 20 kA. The insertions are mainly with isthmuses.

FUSES WITH FILLER (FILLING)

Fuse-links are placed in a medium of fine-grained solid filler (for example: chalk, quartz sand), placed in a porcelain or plastic case. The electric arc that occurs during melting of the inserts comes into close contact with the small grains of the filler, is intensively cooled, deionized, and therefore quickly extinguished.

Backfill fuses of the PN-2 series have rated currents from 100 A to 600 A, the maximum breaking current at a voltage of 500 V () is in the range from 25 kA to 50 kA. PP31 series for rated currents from 63 A to 1000 A, maximum shutdown current up to 100 kA at a voltage of 660 V.

In such fuses, parallel inserts are used, which makes it possible to obtain a larger cooling surface with the same total cross-section of the inserts.

BLOOD FUSES

Characteristics on the site b-c is ensured by a normal insert of an enlarged cross-section, and in the area a-b another element.

IP series for voltage 30 V and currents from 5 A to 250 A.

LIQUID METAL– current up to 250 kA at a voltage of 450 V AC. Fuses operate repeatedly with high current limitation. (Consider the device yourself; Chunikhin, pp. 514-515).

FAST ACTING FOR SEMICONDUCTOR DEVICE PROTECTION. PP-57 for rated currents (40-800) A, PP-59 for rated currents (250-2000) A. Rated voltages are up to 1250 V AC and 1050 V DC.

FUSE-SWITCH BLOCK. BPV rated current up to 350 A at alternating voltage up to 550 V.

FUSE SELECTION

Fuses choose

1. according to start-up conditions and long-term operation;

2. according to the selectivity condition.

1 During long-term operation, the heating temperature of the fuse should not exceed permissible values. In this case, the stability of the time-current characteristics of the fuse is ensured. To fulfill this requirement, it is necessary that the cartridge and fuse-link be selected for a rated current equal to or slightly greater than the rated current of the protected installation.

The fuse should not turn off the installation during overloads that are operational (for example, the starting current asynchronous motor with a squirrel-cage rotor it can reach seven times the rated current. As acceleration occurs, the starting current drops to a value equal to the rated motor current. The duration of the start depends on the nature of the load).

For motors with easy starting conditions (motors of pumps, fans, machine tools)

,those. The rated current of the insert is selected based on the starting load current.

For severe starting conditions, when the engine turns slowly (centrifuge drive, cranes, crushers), or in intermittent mode, when starts occur with high frequency, inserts are selected with an even larger margin

where is the calculated rated current of the line, equal to .

Difference is taken for the engine with the largest value.

For welding transformers, the fuse selection conditions are as follows: ,where PV is the duration of switching on.

2 Selection of fuses based on selectivity conditions.

Several fuses are usually installed between the energy source and the consumer, which should disconnect the damaged areas as selectively as possible.

A fuse that passes a higher rated current has an insert with a larger cross-section than a fuse installed at one of the consumers.

In the event of a short circuit, it is necessary that the fault be switched off by a fuse located at the fault location. All other fuses located closer to the source should remain operational. Such consistency in the operation of fuses is called selectivity or selectivity. To ensure selectivity, the total operating time () of the fuse must be less than the time it takes for the fuse to heat up to the melting temperature of its insert, i.e. t pl1 ³t p2. To ensure selectivity, the shortest actual operating time of the fuse (for a larger current) must be greater than the longest response time of the fuse (for a lower rated current): , where and is the response time of the fuse for higher and lower rated currents corresponding to the rated characteristic.

Due to manufacturing tolerances, the fuse response time may deviate from the nominal value by . Then the above inequality can be written in the form .Multipliers of 0.5 and 1.5 take into account that the fuse is taken with a negative response time tolerance, and the fuse is taken with a positive tolerance. As a result we get necessary condition selectivity: ,those. for selective operation, the response time of a fuse with a higher current should be 3 times longer than that of a fuse with a lower current. For fuses of the same type, to check selectivity, it is enough to check the insert with a lower rated current at the highest current.

For different types of fuses, selectivity is checked over the entire range of currents: from a 3-phase short circuit at the end of the protected section to the rated current of the fuse link.

10 CIRCUIT BREAKERS (CIRCUIT BREAKERS)

Circuit breakers, as a rule, are intended to disconnect a damaged section of the network when an emergency mode occurs in it (short circuit, overload current, low voltage). Thermal and electrodynamic (during a short circuit) effects of increased currents can lead to failure of electrical equipment. Under reduced voltage conditions, if the mechanical load torque on the shaft remains unchanged, increased current will also flow through the running motors.

The machine, unlike a contactor, has a unit of protection elements that automatically detects the appearance of abnormal conditions in the network and gives a signal to shut down. If the contactor is designed only to cut off overload currents that reach several thousand amperes, then the machine must turn off short-circuit currents that reach many tens and even hundreds of kiloamps. In addition, the machine rarely turns off the electrical circuit, while the contactor is intended for frequent operational switching of rated load currents.

There are several types of machines: universal(work on direct and alternating current), installation(intended for installation in publicly accessible areas and are made according to the type of installation products), fast-acting DC and cancellations magnetic field powerful generators.

Figure - Structural diagram of the machine

The figure shows a schematic design diagram of a universal machine in a simplified representation. The machine switches the electrical circuit connected to terminals A and B. In this position, the machine is turned off and the power electrical circuit is open. To turn on the machine, you need to manually rotate handle 3 clockwise. A force is created that, by moving levers 4 and 5 to the right, will rotate the main load-bearing part 6 of the machine around the fixed axis O clockwise. First, the arc extinguishing contacts 8 and 10, and then the main contacts 7 and 11 of the machine are closed and switched on. After this, the entire system remains in the extreme right position, fixed by a special latch, and is held by it (not shown in the figure).

Trip spring 2 is charged when the machine is turned on. When a shutdown command is given, it turns off the machine. When a short circuit current flows through the coil of electromagnetic release 1, an electromagnetic force is created on its armature, moving levers 4 and 5 upward beyond the dead center, as a result of which the machine is automatically turned off by spring 2. In this case, the contacts open, and the arc arising on them is blown into the arc-extinguishing chamber 9 and extinguished in it.

The system of levers 4 and 5 performs the functions of a free release mechanism, which in real machines has a more complex structure. The free release mechanism allows the machine to turn off at any time, including during the switching process, when the turning force acts on the moving system of the machine. If levers 4 and 5 are moved upward beyond the dead center, then the rigid connection between the drive and moving systems is broken. The dead center corresponds to the position of the levers when the straight lines and connecting the axes of rotation coincide in direction with each other. The machine is immediately switched off due to the action of the return spring 2, regardless of whether the turning force acts on the drive system of the machine or not.

The free release mechanism prevents the possibility of successive “off-on-on” cycles of the machine (“jumping of the machine”) when it is possible to turn it on due to a short circuit existing in the circuit. Let's imagine that when the contacts of a switched-on machine come into contact, a short-circuit current will pass through the circuit. In this case, the maximum release 1 will operate and move the levers of the free release mechanism 4 and 5 up beyond the dead center. The machine will turn off and will not turn on again, since the mechanical connection between the turning force and the moving system of the machine is broken. If there were no free release mechanism, then after the automatic shutdown of the machine, it would be immediately re-engaged under the influence of the force of the switching device, which by this time could not have been removed. There would be multiple shutdowns and switch-ons of the machine in heavy short-circuit mode, quickly following each other, which could lead to the destruction of the machine.

When the machine is turned off, the main contacts 7 and 11 are the first to open, and all the current will go into a parallel circuit of arc-extinguishing contacts 8 and 10 with linings made of arc-resistant material. An arc should not occur on the main contacts so that these contacts do not burn. The arcing contacts open when the main contacts are separated by a significant distance. An electric arc appears on them, which is blown upward and extinguished in the arc-extinguishing chamber 9.

When the machine is turned on, the arcing contacts close first, and then the main ones. An electric arc that is possible due to vibration of the contacts occurs and is extinguished only at the arc extinguishing contacts.

High-speed machines are intended to protect direct current installations (transport, converter). Their own response time is a fraction of a millisecond, while that of conventional machines is tenths of a second.

The rapid opening of contacts when an emergency occurs in the network determines the characteristic feature of these machines. The resistance of the electric arc that appears early on the contacts, connected in series to the disconnected circuit, limits the short circuit current, preventing it from increasing to a steady value. The speed of the device is achieved by using polarized electromagnetic devices in the drive, intensive arc extinguishing devices, magnetic systems in which changing magnetic fluxes do not engage with closed windings and pass through the laminated part of the magnetic circuits (combat the retarding effect of eddy currents), etc., as well as maximum simplification of the kinematic diagram of the device and the elimination of intermediate links between the measuring element (release) and the contacts.

AUTOMATIC RELEASES

The releases in automatic machines are measuring elements. They control the value of the corresponding parameter of the protected circuit and give a signal to turn off the machine when it reaches a specified value, called setting(operation current, operation voltage, etc.). The releases provide the ability to regulate the setting within fairly wide limits. This is necessary to implement selective(selective) protection of the electrical network in which the machine is connected.

Selectivity of protection is achieved primarily due to different response times of the previous and subsequent protection stages. The difference in the response time of these stages is called step of selectivity in time. There is also current selectivity stage.

In a branched network, an increase in the time delay from one protection stage to another can lead to an unacceptably large value of this delay at the last stages of protection. Prolonged flow of a large short circuit current (10 kA) can lead to unacceptable heating of the wires in the circuit. Therefore, at high currents, it is advisable to instantly turn off the circuit breaker (located close to the place of the circuit) using a current cut-off release.

In addition to the electromagnetic current, a thermal release can respond to the current value, the structure of which is similar to a thermal relay. This release is not used for protection against short-circuit currents, since it creates unacceptably high time delays, however, it allows one to obtain long time delays necessary under operating conditions for overload currents. Thermal releases have disadvantages: their protective characteristics (dependence of response time on current) are unstable and change with ambient temperature; the time it takes to return the release to its original position after tripping is long.

The machines also use undervoltage releases, which issue a command to turn off the machine when the voltage drops below a predetermined level. Such releases are usually built on the electromagnetic principle. When the voltage drops below a predetermined level, the electromagnetic force is less than the force of the return spring. The armature of the electromagnet is released and, through an intermediate link (roller), acts on the latch of the machine, as a result of which the latter turns off.

Unlike electromagnetic releases, semiconductor releases, which have been widely used recently, do not have such a large number of moving mechanical elements. But their main advantages lie in improved performance characteristics: wide ranges of regulation of currents and response times, which makes it possible to unify products and produce a smaller range of products, finer and more precise adjustment of response times at high short-circuit currents, etc. The measuring elements of such releases use current transformers, and one of their main units is a time delay unit. They also include an output relay that transmits a signal to the tripping electromagnet. The time delay in such releases is carried out through the use of RC circuits in transistor control circuits and the use of magnetic storage devices and contactless pulse counters.

ARCLESS CONTACT DEVICES

An AC circuit can be switched off without arcing if the contacts are opened at a sufficient speed just before the current passes zero. At this time, the electromagnetic energy stored in the circuit approaches zero.

Figure Half-wave current

The figure shows a half-wave of alternating current. If point A corresponds to the moment of opening the contacts and forming an arc, then the arc in this half-cycle will burn for a period of time. During this time, an amount of electricity determined by the area will pass through it, and the energy released in the arc will be relatively large. When the contacts of the device open immediately before the current passes through zero (point B), significantly less energy will be released in the arc, since its lifetime and instantaneous current values will be significantly less. When the contacts of the device diverge before the current passes through zero, the amount of electricity in the gas discharge stage is determined by the area and the arc column does not have time to accumulate a significant reserve of thermal energy in its volume. This heat quickly dissipates near the current zero crossing, and the recovering strength of the intercontact gap acquires high values and rapidly increases with time. Conditions are created under which the arc goes out before it has time to develop. Disconnection of the alternating current circuit becomes practically arc-free. Disconnecting devices with a fixed moment of contact divergence immediately before the zero value of the alternating current are usually called synchronous switches.

The main difficulty in creating synchronous switches is to achieve the required accuracy of operation of the device immediately before current zero and to separate the contacts to the required insulating distance in a very short time before the current passes through zero. To overcome these difficulties, the current pause is artificially extended to one half-cycle (c at) using diodes.

CONTROL DEVICES AND NON-AUTOMATIC SWITCHES

Command devices include travel and limit switches, control buttons, multi-circuit devices - control keys and command controllers, numerous pairs of contacts of which are switched in a certain sequence when the handle is turned from one position to another.

Travel and limit switches carry out switching of control and automation circuits on a given section of the path traversed by the controlled mechanism. Limit switches are installed, for example, in the mechanisms of lifting and transport devices, in the supports of metal-cutting machines. In the first case, they limit the height of lifting loads, in the second - the stroke of the caliper, giving a signal at the end of the controlled stroke of the mechanism to turn off the engines (and in lifts, also a signal to activate the brake electromagnet).

Command controller– a multi-position device that controls contactor coils, the main contacts of which are included in the power circuits of electrical machines, transformers and resistors. A controller is also a multi-position device designed to control electrical machines and transformers by directly switching the power circuits of machine windings, transformers, and resistors. With the help of controllers (and command controllers), motors can be started, speed controlled, reversed and stopped.

Batch switches– closed type devices. The arc arises and is extinguished in a limited volume, as a result the pressure in this volume increases. As pressure increases, arc resistance and arc voltage increase. Physically, this is explained by the fact that with increasing pressure, the distances at which elementary gas particles interact decrease. This leads, firstly, to an increase in the intensity of heat exchange between gas particles and improved conditions for heat transfer from the arc and, secondly, to a decrease in the mean free path of electrons in the gas. All other things being equal, this reduces the intensity of ionization processes, since an electron with a shorter mean free path is able to acquire less energy when moving in an electric field. This leads to an increase in arc resistance and voltage.

11 ELECTROMECHANICAL SWITCHING DEVICES

CONTACTORS AND MAGNETIC STARTERS

A contactor is a two-position self-resetting device, designed for frequent switching of currents not exceeding overload currents, and driven by a drive. This device has two switching positions corresponding to its on and off states. The electromagnetic drive is most widely used in contactors. The return of the contactor to the off state (self-return) occurs under the action of the return spring, the mass of the moving system, or the combined action of these factors.

Actuator is a switching device designed to start, stop and protect electric motors without removing or introducing resistors into their circuits. Starters protect electric motors from overload currents. A common element of such protection is a thermal relay built into the starter.

Overload currents for contactors and starters do not exceed (8-20) times the overload in relation to the rated current. For the starting mode of phase rotor motors and countercurrent braking, (2.5-4) times the overload currents are typical. The starting currents of electric motors with a squirrel-cage rotor reach (6-10) times overload compared to the rated current.

The electromagnetic drive of contactors and starters, with appropriate selection of parameters, can perform the functions of protecting electrical equipment from undervoltage. If the electromagnetic force developed by the drive, when the voltage in the network decreases, is insufficient to keep the device in the on state, then it will spontaneously turn off and thus provide protection against voltage drop. As is known, a decrease in voltage in the supply network causes overload currents to flow through the windings of electric motors if the mechanical load on them remains unchanged.

Contactors are designed for switching power circuits of electric motors and other powerful consumers. Depending on the type of switched current of the main circuit, direct and alternating current contactors are distinguished. They have main contacts equipped with an arc extinguishing system, an electromagnetic drive and auxiliary contacts. As a rule, the type of current in the control circuit that powers the electromagnetic drive coincides with the type of current in the main circuit. However, there are cases where the coils of AC contactors are powered by a DC circuit.

Figure 1 - Contactor design diagram

In Fig. 1 shows a design diagram of a contactor that disconnects the motor circuit. In this case, there is no voltage on the coil 12 and its moving system, under the action of the return spring 10, which creates the force F in, will return to its normal state. The arc D that occurs when the main contacts diverge is extinguished in the arc-extinguishing chamber 5.

Rapid movement of the arc from the contacts to the chamber is ensured by the system magnetic blast. The main current circuit includes a series coil 1, which is placed on a steel core 2. Steel plates - poles 3, located on the sides of the core 2, bring the magnetic field created by coil 1 to the arc burning zone in the chamber. The interaction of this field with the arc current leads to the appearance of forces that move the arc into the chamber.

The contactor will turn on the circuit with current I 0 if voltage is applied U per reel 12 drive electromagnet. The flow F, created by the current flowing through the electromagnet coil, will develop a traction force and attract the armature 9 electromagnet to the core, overcoming the forces F in countering return 10 And F k contact 8 springs

The electromagnet core ends in a pole piece 11, the cross-section of which is greater than the cross-section of the core itself. By installing a pole piece, a slight increase in the force created by the electromagnet is achieved, as well as a modification of the traction characteristics of the electromagnet (the dependence of the electromagnetic force on the size of the air gap).

Contact Contacts 4 And 6 with each other and the closure of the circuit when the contactor is turned on will occur before the electromagnet armature is completely attracted to the pole. As the armature moves, the movable contact 6 will seem to “fall through”, resting its upper part on the stationary contact 4. It will rotate at some angle around the point A and will cause additional compression of the contact spring 8. will appear failure of contacts, by which is meant the amount of displacement of the movable contact at the level of the point of its contact with the fixed contact in the event that the fixed one is removed.

The failure of the contacts ensures reliable closure of the circuit when the thickness of the contacts decreases due to burnout of their material underneath. by the action of an electric arc. The magnitude of the dip determines the supply of contact material for wear during contactor operation.

After contact, the moving contact rolls over the stationary one. The contact spring creates a certain pressure in the contacts, so when rolling, the destruction of oxide films and other chemical compounds that may appear on the surface of the contacts occurs. The contact points of contact during rolling move to new places on the contact surface that were not exposed to the arc and are therefore “cleaner”. All this reduces the contact resistance of the contacts and improves their operating conditions. At the same time, rolling increases the mechanical wear of the contacts (the contacts wear out).

At the moment of contact, the moving contact 6 immediately exerts on fixed contact 4 pressure due to pre-tensioning of the contact spring 8. As a result, the contact resistance of the contacts at the moment of contact will be small and the contact pad will not heat up to a significant temperature when turned on. In addition, the pre-contact pressure generated by the spring 8, allows you to reduce vibration(rebounds) of the moving contact when it hits a fixed contact. All this protects the contacts from welding when the electrical circuit is turned on. The contacts have contact pads, made of a special material, such as silver, to improve the conditions for long-term passage of current through closed contacts in the on state. Sometimes linings made of arc-resistant material are used to reduce wear of contacts under the influence of an electric arc (metal-ceramics “silver-cadmium oxide”, etc.). Flexible connection 7 (for supplying current to the moving contact) is made of copper foil (tape) or thin wire.

Contact solution is the distance between the moving and fixed contacts when the contactor is off. The contact gap usually ranges from 1 to 20 mm. The lower the contact opening, the smaller the armature stroke of the driving electromagnet. This leads to a decrease in the working air gap in the electromagnet, magnetic resistance, magnetizing force, power of the electromagnet coil and its dimensions. The minimum value of the contact opening is determined by: technological and operational conditions, the possibility of the formation of a metal bridge between the contacts when the current circuit is broken, the conditions for eliminating the possibility of contact closure when the moving system rebounds from the stop when the device is turned off. The contact solution must also be sufficient to ensure conditions for reliable arc extinguishing at low currents.

Figure 2 - Linear starter

Shown in Fig. 1 diagram of a rotary contactor is quite typical. Typically, such contactors are intended for heavy duty operation (high frequency cycles of switching operations, inductive circuits) at relatively high rated current values (tens and hundreds of amperes). Another common type of contactors and starters is linear; it is designed primarily for lower rated currents (tens of amperes) and lighter operating conditions. The linear starter (Fig. 2) has bridge contacts 2 And 3, from which the arc is blown into the arc-extinguishing chambers 1. Force F k contact spring creates pressure in closed contacts, return spring F p returns the moving system of the device to the off state when the voltage is removed from the coil. The device is turned on by an electromagnet when voltage is applied to its coil 5. Short-circuited turns are installed on the poles of the AC electromagnet 4, eliminating vibration of the armature in the on position of the device.

Unlike a DC contactor, in an AC contactor, to reduce eddy current losses, laminated magnetic cores and short-circuited turns on the poles are used to eliminate armature vibration. AC contactors are often made three-pole, DC contactors are single-pole and two-pole. Slotted chambers are more often used as an arc extinguishing device in direct current contactors, and an arc extinguishing grid is more often used in alternating current contactors.

Chambers with arc extinguishing grids are also used to extinguish the arc. The arcing grid is a package of thin metal plates 5 (Fig. 1). Under the influence of electrodynamic forces created by the magnetic blast system, the electric arc hits the grid and breaks into a series of short arcs. The plates intensively remove heat from the arc and extinguish it, but the plates of the arc extinguishing grid have significant thermal inertia - with a high frequency of switching on, they overheat and the efficiency of arc extinguishing decreases.

Powerful AC contactors have main contacts equipped with an arc extinguishing system - magnetic blast and an arc extinguishing chamber with a narrow slot or arc extinguishing grid, just like DC contactors. The design difference is that AC contactors are multi-pole; They usually have three main make contacts. All three contact units operate from a common valve-type electromagnetic drive, which turns the contactor shaft with moving contacts installed on it. Bridge-type auxiliary contacts are installed on the same shaft. Contactors have fairly large overall dimensions. They are used to control electric motors of significant power.

To increase service life, the design of contactors allows changing contacts.

There are combined AC contactors in which two thyristors are connected in parallel to the main normally open contacts. In the on position, current flows through the main contacts, since the thyristors are in the closed state and do not conduct current. When the contacts open, the control circuit opens the thyristors, which bypass the circuit of the main contacts and unload them from the shutdown current, preventing the occurrence of an electric arc. Since thyristors operate in short-term mode, their rated power is low and they do not require cooling radiators.

Our industry produces combined contactors of the KT64 and KT65 types with rated currents exceeding 100 A, made on the basis of the widely used KT6000 contactors and equipped with an additional semiconductor block.

The switching wear resistance of combined contactors in normal switching mode is at least 5 million cycles, and the switching wear resistance of semiconductor blocks is approximately 6 times higher. This allows them to be reused in control systems.

To control low-power AC electric motors, forward contactors with bridge contact units are used. Double circuit breaking and simplified conditions for extinguishing an alternating current arc make it possible to do without special arc extinguishing chambers, which significantly reduces the overall dimensions of contactors.

Forward contactors are usually produced by industry in a three-pole design. In this case, the main closing contacts are separated by plastic jumpers 1.

Along with low-current reed switches, sealed power magnetically controlled contacts (gersikons) have been created that are capable of switching currents of several tens of amperes. On this basis, contactors were developed to control asynchronous electric motors with power up to 1.1 kW. Gersikons are characterized by an increased contact opening (up to 1.5 mm) and increased contact pressure. To create a significant force of electromagnetic attraction, a special magnetic circuit is used.

The scope of application of electromagnetic contactors is quite wide. In mechanical engineering, AC contactors are most often used to control asynchronous electric motors. In this case they are called magnetic starters. A magnetic starter is the simplest set of devices for remote control electric motors and, in addition to the contactor itself, often has a push-button station and protection devices.

Figure 1 (a, b) shows, respectively, the installation and circuit diagrams of the connections of an irreversible magnetic starter. On the wiring diagram, the boundaries of one device are outlined with a dashed line. It is convenient for installing equipment and troubleshooting. These diagrams are difficult to read because they contain many intersecting lines.

Figure 1 - Irreversible starter circuits

In the circuit diagram, all elements of one device have the same alphanumeric designations. This allows you to avoid linking conventional images of the contactor coil and contacts together, achieving the greatest simplicity and clarity of the circuit.

The irreversible magnetic starter has a KM contactor with three main make contacts (L1-S1, L2-S2, L3-S3) and one auxiliary make contact (3-5).

The main circuits through which the electric motor current flows are usually depicted with thick lines, and the power circuits of the contactor coil (or control circuit) with the highest current are depicted with thin lines.

To turn on the electric motor M, you must briefly press the SB2 “Start” button. In this case, current will flow through the circuit of the contactor coil, and the armature will be attracted to the core. This will close the main contacts in the motor power supply circuit. At the same time, auxiliary contact 3 – 5 will close,

which will create a parallel circuit to power the contactor coil. If you now release the Start button, the contactor coil will be switched on through its own auxiliary contact. This type of circuit is called a self-locking circuit. It provides so-called zero motor protection. If during operation of the electric motor the mains voltage disappears or decreases significantly (usually by more than 40% of the nominal value), the contactor is switched off and its auxiliary contact opens. After the voltage is restored, to turn on the electric motor, you must press the “Start” button again. Zero protection prevents unexpected, spontaneous starting of the electric motor, which can lead to an accident.

Manual control devices (switches, limit switches) do not have zero protection, therefore contactor control is usually used in machine drive control systems.

To turn off the electric motor, just press the SB1 “Stop” button. This opens the self-supply circuit and turns off the contactor coil.

In the case when it is necessary to use two directions of rotation of the electric motor, a reversible magnetic starter is used, the circuit diagram of which is shown in Figure 2, a. To change the direction of rotation of an asynchronous electric motor, it is necessary to change the phase sequence of the stator winding. The reversible magnetic starter uses two contactors: KM1 and KM2. It can be seen from the diagram that if both contactors are accidentally switched on simultaneously, a short circuit will occur in the main current circuit. To prevent this, the circuit is equipped with a lock. If, after pressing the SB3 “Forward” button and turning on the KM1 contactor, press the SB2 “Back” button, the opening contact of this button will turn off the KM1 contactor coil, and the closing contact will supply power to the KM2 contactor coil. The motor will reverse.

Figure 2 - Reversing starter circuits

A similar diagram of the control circuit of a reversing starter with interlocking on auxiliary break contacts is shown in Figure 2, b. In this scheme, turning on one of the contactors, for example KM1, opens the power circuit of the coil of the other contactor KM2. To reverse, you must first press the SB1 “Stop” button and turn off the KM1 contactor. For reliable operation of the circuit, it is necessary that the main contacts of the KM1 contactor open before the closing of the breaking auxiliary contacts in the KM2 contactor circuit occurs. This is achieved by appropriate adjustment of the position of the auxiliary contacts along the armature.

In serial magnetic starters, double blocking is often used according to the above principles. In addition, reversible magnetic starters can have a mechanical interlock with a changeover lever that prevents the simultaneous operation of the contactor electromagnets. In this case, both contactors must be installed on a common base.

Open magnetic starters are mounted in electrical equipment cabinets. Dust-proof and dust-splash-proof starters are equipped with a casing and mounted on a wall or rack as a separate device.

Electromagnetic contactors choose according to the rated current of the electric motor, taking into account operating conditions. GOST 11206-77 establishes several categories of AC and DC contactors. AC contactors of categories AC-2, AC-3 and AC-4 are designed for switching power circuits of asynchronous electric motors. Contactors of category AC-2 are used for starting and stopping electric motors with a wound rotor. They operate in the lightest mode since these motors are usually started using a rotor rheostat. Categories AC-3 and AC-4 provide direct starting of electric motors with a squirrel-cage rotor and must be designed for a six-fold boost of starting current. Category AC-3 provides for turning off the rotating asynchronous electric motor. Contactors of category AC-4 are designed for countercurrent braking of electric motors with a squirrel-cage rotor or disconnecting stationary electric motors and operate in the most difficult conditions.

Contactors designed to operate in AC-3 mode can be used in conditions corresponding to AC-4 category, but the rated current of the contactor is reduced by 1.5-3 times. Similar application categories are provided for DC contactors.

Contactors of category DS-1 are used for switching low-inductive loads. Categories DS-2 and DS-3 are designed to control DC electric motors with parallel excitation and allow switching current equal to. Categories DS-4 and DS-5 are used to control DC electric motors with sequential excitation.

These categories define the normal switching mode in which the contactor can operate continuously for a long time. In addition, a mode of rare (random) switching is distinguished, when the switching capacity of the contactor can be increased by approximately 1.5 times.

If an asynchronous electric motor operates in intermittent mode, then the contactor is selected based on the value of the rms current. The choice of contactor is influenced by the degree of protection of the contactor. Protected contactors have worse cooling conditions, and their rated current is reduced by approximately 10% compared to open contactors.

CONTACT - ARC SYSTEMS OF CONTACTORS

Contactors usually use lever (Fig. 1, a) and bridge (Fig. 1, b) contacts. In lever contacts, when disconnected, one gap is formed (one arc), in bridge contacts - two (two arcs). Therefore, other things being equal, the possibilities for disconnecting electrical circuits for devices with bridge contacts are higher than for devices with lever (finger) contacts.

Figure 1 – Lever and bridge contacts

Bridge contacts, compared to lever contacts, have the disadvantage that in the closed state two contact current transitions are created in them, in each of which a reliable touch must be created. Therefore, the force of the contact spring must be doubled (compared to lever contacts), which ultimately increases the power of the electromagnetic drive of the contactor.

In AC contactors for interrupted currents up to 100 A at a network voltage of up to 100-200 V, arc extinguishing chambers can not be used, since the arc is extinguished by stretching it in atmospheric air(open gap). To prevent overlap of electric arcs at adjacent poles, insulating partitions are used. Contactors with an open arc break also exist on direct current, but the interrupted currents for them are significantly lower.

At high values of interrupted currents and voltages, the devices are equipped with arc extinguishing chambers, of which the most common slit cameras And arc suppression grids. The slot chamber (Fig. 2, a) forms a narrow gap (slot) inside between the walls made of arc-resistant insulating material (asbestos cement, etc.). An electric arc 1 is driven into it and there it is extinguished due to enhanced heat removal in close contact with the walls.

The arc extinguishing grid (Fig. 2, b) is a package of thin (mm) metal plates 2 onto which an arc is blown. The plates act as radiators that intensively remove heat from the arc column and help extinguish it.

The most important characteristic of the arc chute is the volt-ampere characteristic. Using it, you can calculate the processes of arc extinction when the circuit is turned off.

Figure 2 – Arc chambers

As operating experience has shown, the arc extinguishing grid is unsuitable for frequent circuit outages at relatively high currents. With a high frequency of shutdowns, its plates heat up to high temperatures and do not have time to cool down. They are unable to cool the arc column, and the grid fails to function. For the regime of frequent circuit outages, slotted arc chutes are more suitable. , m, between plates 3 in Fig. 3, a) in accordance with the law of total current for a uniform field (HL=Iw), field strength (A/m)

Substituting this value into (*), we get:

where is the number of turns of the coil.

Since in a system with a series magnetic blowing coil the force is proportional to the square of the current, it is advisable to use this type of blowing in contactors designed for relatively large rated currents. To reduce copper consumption for the manufacture of a coil, the cross-section of which should be selected according to the rated current of the contactor, it is desirable to have as few coil turns as possible. However, this number of turns must ensure such a magnetic field strength in the zone of its interaction with the arc current, which will create conditions for reliable extinguishing of the arc in a given range of switched currents. It is usually measured in units at rated currents of hundreds of amperes, and at currents of tens of amperes it reaches ten and higher.

The advantage of series magnetic blast coil systems is that the direction of the force is independent of the direction of the current. This allows the specified system to be used not only on direct, but also on alternating current. However, on alternating current, due to the appearance of eddy currents in the magnetic circuit, a phase shift may occur between the arc current and the resulting magnetic field strength in the arc burning zone, which can cause the arc to be thrown back into the chamber.

The disadvantage of a system with a serial magnetic blast coil is the low magnetic field strength it creates at small switched currents. Therefore, the parameters of this system must be chosen so as to ensure in the region of these currents the maximum possible magnetic field strength in the arc burning zone, without resorting to a significant increase in the number of turns of the magnetic blast coil, so as not to cause unnecessary consumption of copper for its manufacture. At low currents, the magnetic circuit of this system should not become saturated. Then almost the entire magnetizing force of the coil is compensated by the drop in the magnetic potential in the air gap and the magnetic field strength in it will be the maximum possible. At high currents, on the contrary, it is advisable to bring the magnetic circuit into saturation when its magnetic resistance becomes large. This will reduce the magnetic field strength in the area where the arc is located, reduce the strength and intensity of arc extinguishing, and reduce overvoltages during its extinguishing.

There is a system with a parallel magnetic blast coil, when coil 1 (see Fig. 3), containing hundreds of turns of thin wire and designed for the full voltage of the power source, creates a magnetic field strength (A/m) in the arc burning zone

Electrodynamic force acting on the arc (N) (see Fig. 3, b)

Where

In this system, the force acting on the arc is proportional to the first power of the current. Therefore, it turns out to be more appropriate for contactors with low currents (up to approximately 50 A).

A contactor with a parallel coil of magnetic blast reacts to the direction of the current. If the direction of the magnetic field remains unchanged, and the current changes its direction, then the force will be directed in the opposite direction. The arc will not move into the arc extinguishing chamber, but in the opposite direction - onto the magnetic blast coil, which can lead to an accident in the contactor. This is a drawback of the system under consideration. The disadvantage of this system is also the need to increase the level of insulation of the coil based on the full network voltage. A decrease in network voltage leads to a decrease in the magnetizing force of the coil and a weakening of the intensity of the magnetic blast, which reduces the reliability of arc extinguishing.

In a magnetic blowing system, a permanent magnet can be used instead of a voltage coil. The properties of such a system are similar to a system with a parallel magnetic blast coil. Replacing the voltage coil with a permanent magnet will eliminate the consumption of copper and insulating materials that would be required to create the coil. At the same time, the properties of the permanent magnet in the system should not be violated during operation.

Systems with a parallel magnetic blast coil and permanent magnets on alternating current are not used, since it is practically impossible to match the direction of the magnetic flux with the direction of the arc current in order to obtain the same direction of force at any time.

With an increase in the magnetic blast field strength, the conditions for the arc coming off the contacts onto the arc extinguishing horns improve and its entry into the chamber becomes easier. Therefore, with growth, the wear of contacts from the thermal effects of the arc also decreases, but up to a certain limit.

High field strengths create significant forces that act on the arc and eject molten metal bridges from the intercontact gap into the atmosphere. This increases contact wear. At optimal field strength, contact wear is minimal.

Contact wear is an important technical factor. Therefore, serious measures are taken, such as reducing vibration of the contacts when the device is turned on, to reduce wear and increase the service life of the contacts.

An important characteristic of an AC arc extinguishing device is the growth pattern recoverable strength intercontact gap after the current passes through zero.

12 RELAYS. INTEGRATED CIRCUITS – TECHNICAL BASIS FOR CREATION OF RELAY PROTECTION EQUIPMENT

Relay protection of any electrical installation contains three main parts: measuring, logical and output. The measuring part includes measuring and triggering protection elements, which act on the logical part when the electrical parameters (current, voltage, power, resistance) deviate from the values preset for the protected object.

The logical part consists of separate switching elements and time delay elements, which, upon a certain action (activation) of the measuring and triggering elements, in accordance with the launch program embedded in the logical part

Threshold current of the fuse link. Fuses

Fuse parameters

Fuse material

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