Short circuit in socket. What is a short circuit and network overload

Short circuit (short circuit) is an electrical contact between different phases, phase and zero working or protective wire. Online with dead-earthed neutral a short circuit can be considered a contact between a phase conductor and earth.

Short circuits can be caused by:

• deterioration or damage to the insulation;
• hit foreign objects, conducting electric current, on current-carrying parts;
• mechanical damage or destruction electrical machines and devices;
• errors of workers during the installation or maintenance of electrical equipment;
• emergency modes of operation of the network associated with the occurrence of overvoltages or sudden current surges in it.

With time insulation ages and loses its properties. This applies equally to cables, and to the windings of electric motors, and to insulators. Insulating surfaces are also subject to this property: cases of automatic switches, fuses. The deterioration of the properties of insulators is affected by the environment in which they work: the degree of pollution, the presence of moisture, dust, corrosive gases. As soon as a small conductive section appears, it begins to heat up and grow until the current through it reaches a critical value. It will grow like an avalanche, heat up and char the surface over which it flows. From this point on, the area with weakened insulation becomes a short circuit.

An example foreign objects on live parts are the trees falling on the wires of power lines. They themselves create contact between the ground and the phase conductors, the wires are additionally broken or closed together.

Wear of electric motor bearings can also lead to a short circuit. During rotation, the rotor clings to its windings for internal details or stator winding. The insulation is damaged and a short circuit occurs. Cables laid in the ground are inevitably subjected to mechanical deformations. Vehicles pass over them, and when the seasons change, ground movements test them for strength.

Carelessness, inaccuracy, non-compliance with safety rules can also lead to short circuit. This further harms the health of workers.

Surge by themselves are not the causes of short circuit. They only accelerate their occurrence in areas with reduced insulation, where sooner or later a short circuit would still occur.

Calculation and measurement of short-circuit currents

In short circuit, all power electrical network focuses on small plot. If cables, wires and switching devices did not have their own resistances, the short circuit current would reach enormous values. But in fact, it is limited by the total resistance of the line from the power source (transformer at the substation, power system generators) to the short circuit point.

When designing electrical installations, the magnitude of this current must be calculated. For this, data on the resistances (active and reactive) of all electrical equipment installed on the short circuit path are used. The current is considered for the point farthest from the source to check if the protection will turn it off.

In operation or after installation, the short-circuit current is measured special devices: phase-zero loop meters. This is done in order to make sure that the calculations are correct or in places for which this calculation cannot be performed.

• instead of modular switches with characteristic "C" (cut-off ratio 5-10), "B" is used (multiplicity 3-5);
• increase the cross section of the supply cables.

The effect of a short circuit on electrical equipment

Short circuit - emergency operation for the electrical network. When it occurs, it has two effects on electrical equipment at the same time:

• electrodynamic;
• thermal.

According to the laws of physics, when current passes through two conductors located side by side, they interact with each other. Depending on the direction of the current, they either attract or repel. As the current increases and the distance decreases, the interaction force increases.

It is on this principle that electrodynamic effect of short circuit current for tires, wires, windings of electrical machines. At substations and other power facilities, where the fault currents reach tens and hundreds of thousands of amperes, after a short circuit, the equipment may become completely unusable due to mechanical damage. In this case, the short circuit itself can occur somewhere aside.

thermal effect based on the heating of conductors when passing through them electric current. In this case, the temperature sometimes rises so much that the wires or tires melt.

IN living conditions the thermal effect of the short circuit is more pronounced, the dynamic effect can be ignored due to the small values ​​of the currents.

Network congestion

This is also an emergency mode. All electrical equipment is designed for rated current, the excess of which is unacceptable. Otherwise contact systems switching devices, the strands of cables and wires begin to heat up. Overheating leads to melting or charring of the insulation, which soon leads to a fire or short circuit.

The reasons for overload are:

• connecting a load to a group line exceeding that for which its cable and circuit breaker are designed. This is either due to the connection of a powerful electrical receiver or the excess of the total power of a group of electrical receivers.
• malfunctions that occur in one of the electrical receivers. For example, a winding circuit in an electric motor, partial failure heating element in the heater.

SHORT CIRCUIT IN ELECTRICAL WIRING
Possible causes of fire

Vladimir Fishman, Chief Specialist, group of companies "ElectroshieldTMSamara", branch "EnergosetproektNNSESH", Nizhny Novgorod

If earlier the main cause of fires in residential buildings was considered to be “careless handling of fire”, now more and more often they are called a “short circuit in the wiring”. The rapid electrification of the residential sector makes it necessary to carefully analyze the home electrical installation (wiring, electrical appliances, protective and switching equipment) from the point of view of the risk of fire.
Vladimir Semenovich Fishman has already talked about the features of calculating short-circuit processes in low-voltage networks (“News of Electrotechnics” No. 2 (32) 2005, No. 3 (33) 2005). Today he considers the conditions under which a short circuit can actually cause a fire.

Regulatory requirements

According to the PUE, electrical networks with voltage up to 1 kV of residential and public buildings must be protected from short circuit currents and overload currents. Here are some excerpts from the PUE:
clause 3.1.10. “Indoor networks made with openly laid conductors with a combustible outer sheath or insulation must be protected from overload.
In addition, the following must be protected from network overload indoors:

• lighting networks in residential and public buildings, in commercial premises, service premises industrial enterprises, including networks for household and portable electrical appliances (irons, kettles, stoves, room refrigerators, vacuum cleaners, washing and sewing machines etc.), as well as in fire hazardous areas.”

• 80% for the rated current of the fusible link or the setting current of the circuit breaker, which has only the maximum instantaneous release (cut-off), - for conductors with PVC, rubber and similar insulation in terms of thermal characteristics; for conductors laid in non-explosive industrial premises industrial enterprises, 100% is allowed;
• 100% for the rated current of the circuit breaker release with a non-adjustable inverse current-dependent characteristic (regardless of the presence or absence of a cutoff) - for conductors of all brands.

POWER SUPPLY DIAGRAM

Consider a typical power supply scheme for a residential building (Fig. 1). The power source is, as a rule, a separate substation with its own switchboard 10 (6) / 0.4 / 0.23 kV. At the entrance to the building there is an input distribution device - VRU0.4 / 0.23 kV. The next step is a storey group switchboard (MSB), the last step is an apartment switchboard (KRShch). The mentioned switchgears are interconnected by conductors, the minimum allowable sections of which are indicated in the PUE. The rated currents of the devices that protect cables and wires both from short-circuit currents and from overload are selected in accordance with the EMP.

WIRE FIRE CONDITIONS

The question arises whether, when the above and other requirements of the PUE are met, an electrical wiring fire can occur during a short circuit (short circuit)? It is believed that the ignition of electrical wiring occurs when the conductor reaches a certain temperature, depending on the type of cable insulation. So, for cables with PVC insulation, widely used at present, this temperature is equal to: Q = 350 O С.
The change in the temperature of the conductor during the flow of a short-circuit current is described by the formulas given in. Taking into account some features, in particular, the short duration of the short-circuit current flow (which will be discussed below), in the cases under consideration for conductors with copper conductors you can use the following formula:

(1a)

In the general case, the short-circuit current contains periodic and aperiodic components, i.e.:

However, as the analysis shows, the influence of the aperiodic component in this case is small due to its rapid decay (decay time constant T 0.003 s). As a result of integration on the time interval of the protective equipment (0 - 0.02 s) we get:

where I d - effective value periodic component of the short circuit current.
Then formula (1a) will take the form:

(4)

From the above formulas, it can be seen that the limiting values ​​​​of short-circuit currents at which ignition of the conductor does not occur depend on its cross section and the short-circuit off time.

LIMIT VALUES OF SC CURRENTS

Minimum allowable values ​​of short circuit currents

Analyzing the protective time-current characteristics of circuit breakers (Fig. 2), we see two areas: the cut-off operation area, designed to turn off short-circuit currents, and the operation area of ​​thermal releases, designed to protect against overload.
The cutoff action time is measured in hundredths and thousandths of a second, and the overload protection action time is from several seconds to several minutes. Obviously, short circuits should be disconnected as quickly as possible, i.e. cut-off circuit breaker. If the short circuit is switched off by slow-acting thermal protection, then the burning arc will inevitably damage neighboring conductors, on which, as a result, short circuits will also occur. In this case, a fire is inevitable.
Based on the sensitivity requirements, it is possible to determine the minimum values ​​of short-circuit currents at which the cutoff of automatic switches will reliably operate:

I kzmin. = I nom 2 5,

where I nom - rated current of the machine;
2 – reliability factor;
5 - the multiplicity of the cutoff actuation current.

Maximum allowable values ​​of short circuit currents

To determine the maximum allowable values ​​of short-circuit currents, at which the ignition of the wiring will not yet occur, we use formulas (1) and (2).
We accept the initial temperature of the conductor Q beg. \u003d 30 O С. As the final one, one should take one in which the insulation of the electrical wiring does not yet lose its properties and allows further operation. For cables and wires with plastic insulated this temperature lies in the range of 160 - 250 ° C. Let's take the average value of Q con. \u003d 200 O C:

An important role is played by the response time of the electromagnetic releases of the machine during a short circuit. GOST R 5034599, as well as similar foreign documents, unfortunately, contain only the requirement that the time of action of circuit breakers in the initial cut-off zone (“instantaneous trip time”) should be less than 0.1 s.
However, from the catalog time-current characteristics of the automata, it follows that in fact the operating time of the switches is much less. So, for automatic machines of the LSN and C 60a types, this time does not exceed 20 ms, and at high multiples of the short-circuit current, it is even less (Fig. 2a and 2b). With an off time of 20 ms, the maximum permissible value of the short-circuit current for a copper conductor with a cross section of 1.5 mm 2 will be:

Given the regulated PUE minimum allowable values ​​of the sections copper conductors at different stages of the power supply system (Table 7.1.1), you can similarly determine the maximum and minimum current values ​​​​at other stages of the power supply system. The calculation results are given in Table. 1.
It should be emphasized again that the maximum allowable values ​​of the short-circuit current largely depend on the speed of the circuit breaker during a short circuit.

If it is necessary to solve another problem - determining the minimum allowable cross-section of a cable or wire for a given short-circuit current and its disconnection time, then you can use the formula:

EFFECT OF CONDUCTOR OVERLOAD

An overload of the electrical network in everyday life can occur, in particular, when using additional electrical heating appliances in the cold season, in the event of an accident in the water heating system, etc.
Despite the fact that, according to the PUE, the internal electrical networks of residential and administrative buildings must be protected against overload, yet protective devices allow some overload of conductors. This is due to the fact that the reliable operation of the fuses occurs at currents exceeding 1.6I nom, and automatic machines - 1.45I nom.
Therefore, if, for example, the machine is selected in accordance with the requirements of the PUE, i.e. its rated current is continuous admissible current conductor, the latter can work for a long time with a load of 145% I add. At the same time, its temperature can reach:

Q p \u003d Q o + (Q d - Q p) (I prev / I p) 2 \u003d 30 + (65 - 25) 1.45 2 \u003d 147 O С.

This value is greater than the long-term allowable temperature for cables with plastic insulation, specified in the PUE and equal to 65 ° C.
If a short circuit occurs during a prolonged overload, the temperature of the conductor will exceed the maximum allowable value of 350 O С and will be for S = 1.5 mm 2 at I short circuit = 1550 A (1):

Q con. \u003d 147 e k + 228 (e k - 1) \u003d 394 O С, where k \u003d 0.506.

Based on the foregoing, the conclusion suggests itself that in order to exclude a possible excess allowable temperatures electrical wiring in case of overloads and short circuits rated currents protective equipment should be chosen somewhat lower than required by the PUE, as, for example, for circuit breakers: I nom.aut. 80% I add.
Let us pay attention to the fact that the current PUEs do not require checking conductors up to 1 kV for thermal resistance to short-circuit currents. However, with regard to residential and administrative premises, it is difficult to agree with this, given the possible serious consequences.

REAL VALUES OF SC CURRENTS IN THE POWER SUPPLY OF BUILDINGS

Short-circuit currents in the power supply system with voltage up to 1 kV are calculated according to the methodology set forth in GOST 2824993. The calculation turns out to be more complicated than for networks with a voltage of 6–35 kV, which is explained by a number of circumstances:

• the need to take into account not only reactive, but also active resistances circuit elements;
• the need to take into account the resistance of contact connections;
• the need to take into account the increase in active resistance of the conductor with increasing temperature;
• the need to take into account the resistance of the arc;
• the lack of accurate data on the zero-sequence resistance of some elements of the power supply system (cables with a non-conductive sheath, power transformers with a Y / Yn, Y / Zn winding connection scheme).

However, this is a separate topic for discussion.
As calculations show, when transformers with a capacity of 630 kVA or more are installed at substations, the short-circuit currents at the consumer may exceed those indicated in Table. 1 is the maximum allowed values. In order to limit short-circuit currents in the residential electrical network, it is possible to use supply transformers with Y / Yn winding connection schemes. Such transformers have increased zero-sequence resistances that reduce single-phase short circuit currents. In some cases, it is necessary to go to increase the cross section of the conductors internal wiring compared to the required conditions permissible load and the minimum allowable values ​​specified in the PUE.
From all of the above, it follows that even if the current regulatory requirements are met, as a result of a short circuit in certain sections of the electrical wiring of residential buildings, conditions for ignition can be created. However, in this case, the short circuit itself would be wrong to qualify as the cause of the fire. The true causes of the fire are either incorrect technical solutions, or insufficient reliability and speed of the applied protective equipment, or exceeding normative term operation of electrical equipment, etc.

Tab. 1. Limit values short-circuit current at various stages of the power supply system

CONCLUSIONS

1. As a result short circuits, with significant values ​​of short-circuit current and insufficient speed of protective equipment, there is a real danger of fire or a serious deterioration in the condition of the insulation of the internal electrical wiring of buildings.
2. Given the special fire hazard, it is advisable to introduce regulatory requirement on the verification of the thermal resistance of electrical wiring in residential buildings.
3. To avoid overloading the internal wiring, the rated currents of the protective devices must be chosen below the long-term permissible currents of the protected conductors.
4. When choosing protective devices Special attention should be given to reliable circuit breakers with guaranteed speed in the instantaneous trip zone of 0.02 s or less.

LITERATURE

1. Electrical Installation Rules, 6th and 7th ed.
2. Technical circular No. Ts0298(e) of the Department for Development Strategy and Science and Technology Policy of RAO UES of Russia.
3. GOST R 5034599. Circuit breakers for protection against overcurrents for domestic and similar purposes.
4. GOST 2824993. Short circuit currents in electrical installations. Calculation methods in electrical installations alternating current voltage up to 1 kV.
5. Fedorovskaya A.I., Fishman V.S. Power transformers 10(6)/0.4 kV. Areas of use various schemes winding connections // News of Electrical Engineering. - 2006. - No. 5.

The rapid electrification of residential buildings requires a more careful analysis of the electrical installation (wiring, electrical appliances, protective and switching equipment) from the point of view of the risk of fire. In this article, we will consider the conditions under which a short circuit can actually cause a fire.

Regulatory requirements

In accordance with the PUE, the electrical network with voltage up to 1 kV in residential, public, administrative and domestic buildings must be protected from short circuit currents and overload currents.

PUE-7
3.1.10
Indoor networks made with openly laid conductors with a combustible outer sheath or insulation must be protected from overload.
In addition, the following must be protected from network overload indoors:
lighting networks in residential and public buildings, in commercial premises, utility rooms of industrial enterprises, including networks for household and portable electrical receivers (irons, kettles, stoves, room refrigerators, vacuum cleaners, washing and sewing machines, etc.), and also in fire hazardous areas.

3.1.11
In networks protected against overloads (see 3.1.10), conductors should be selected according to the rated current, while the condition must be ensured that, in relation to the long-term permissible current loads given in the tables of Ch. 1.3, protective devices had a multiplicity of not more than:
80% for the rated current of the fusible link or the setting current of the circuit breaker, which has only the maximum instantaneous release (cut-off), - for conductors with PVC, rubber and similar insulation in terms of thermal characteristics; for conductors laid in non-explosive industrial premises of industrial enterprises, 100% is allowed;
100% for the rated current of the circuit breaker release with a non-adjustable inverse current characteristic (regardless of the presence or absence of a cutoff) - for conductors of all brands.

Rice. 1. Typical power supply scheme of a residential building

Power supply scheme

Consider a typical circuit (Fig. 1), where, as a rule, a separate substation with a switchboard 10(6)/0.4/0.23 kV serves as the source of power supply. At the entrance to the building ASU-0.4 / 0.23 kV. The next step is a floor group switchboard, and the last step is an apartment one. The above switchgears connected to each other by conductors, the minimum allowable sections of which are specified in the requirements of the PUE. The rated currents of devices that protect wires and cables from short circuit currents and from overload are selected in accordance with the requirements of the PUE.

Electrical fire conditions

The question arises whether, in the event of a short circuit, electrical wiring can ignite if the above and others are fulfilled. PUE requirements? Considering this issue, it is necessary to pay attention to the fact that the ignition of electrical wiring occurs when the conductor reaches a certain temperature, depending on the type of cable insulation. It is currently widely used, in which this temperature is equal to: Q \u003d 350 ° C.
The change in temperature of the conductor during the flow of a short circuit current is described by the formulas that are given in. Taking into account some features, namely the short duration of the short-circuit current, which will be discussed below, in the cases under consideration for conductors with copper conductors, the following formula can be used:

where Q con. and Q beg. - respectively, the final and initial temperatures of the current-carrying core of the conductor, О С;
k - exponent:

(1a)

where t is the short circuit current flow time, s;
S - conductor cross section, mm 2;
- Joule integral or thermal impulse, kA 2 / s.

In the general case, the short-circuit current contains periodic and aperiodic components, i.e.:

However, as the analysis shows, the influence of the aperiodic component in this case is small due to its rapid decay (decay time constant T 0.003 s). As a result of integration over the time interval of the protective equipment (0 - 0.02 s), we get:

where I d is the effective value of the periodic component of the short circuit current.
Then formula (1a) will take the form:

(4)

From the above formulas, we see that the limiting values ​​​​of short-circuit currents at which the ignition of the conductor does not occur depend on its cross section and the short-circuit disconnection time.

Rice. 2(a). Time-current characteristics of circuit breakers type LSN

Rice. 2(b). Time-current characteristics of circuit breakers type C 60a Merlin Gerin

Limiting values ​​of short-circuit currents and minimum allowable values ​​of short-circuit currents

When analyzing the protective time-current characteristics of circuit breakers (Fig. 2), we observe two areas: cut-off operation, designed to cut off short-circuit currents, and operation of thermal releases, designed to protect against overload. The cutoff action time is measured in hundredths and even thousandths of a second, and the overload protection action time is measured from several seconds to several minutes. It is clear that short circuits should be cut off by cutting off the circuit breaker as quickly as possible. If the short circuit is disconnected more slowly than the acting thermal protection, then damage to neighboring conductors by a burning arc will inevitably occur, on which, as a result, short circuits will also occur. In this case, the occurrence of a fire is inevitable.
Based on the sensitivity requirements, it is possible to determine the minimum values ​​of short-circuit currents at which the cutoff of automatic switches will reliably operate:

I kzmin. = I nom 2 5,

where I nom - rated current of the machine;
2 – reliability factor;
5 - the multiplicity of the cutoff actuation current.

To determine the maximum allowable values ​​of short-circuit currents, at which ignition will not yet occur in the wiring, we use formulas (1) and (2).
We accept the initial temperature of the conductor Q beg. \u003d 30 O С. As the final one, it is required to take one in which the insulation of the electrical wiring does not yet lose its properties and allows further operation. For cables and wires with plastic insulation, this temperature is in the range of 160 - 250 ° C. Let's take the average value of Q con. \u003d 200 O C:

An important role is played by the response time of the electromagnetic releases of the machine during a short circuit. GOST R 5034599, as well as similar foreign documents, unfortunately, contain only the requirement that the time of action of circuit breakers in the initial cut-off zone (instantaneous trip time) should be less than 0.1 s. However, from the catalog time-current characteristics of the automata, it follows that in fact the operating time of the switches is much less. So, for automatic machines of the LSN and C 60a types, this time does not exceed 20 ms, and even less at high multiples of the short-circuit current (Fig. 2a and 2b). With an off time of 20 ms, the maximum permissible value of the short-circuit current for a copper conductor with a cross section of 1.5 mm 2 will be:

Setting the minimum allowable cross-sections of copper conductors at different stages of the power supply system (Table 7.1.1) regulated by the PUE, it is possible to determine the maximum and minimum current values ​​​​at other stages of the power supply system in a similar way. The calculation results are given in Table. 1.

Tab. 1. Boundary values ​​of the short-circuit current at various stages of the power supply system

It should be emphasized again that the maximum allowable values ​​of the short-circuit current largely depend on the speed of the circuit breaker during a short circuit.

If it is necessary to determine the minimum allowable cross-section of a cable or wire for a given short-circuit current and its disconnection time, then you can use the formula:

Influence of conductor overload

In most cases, an overload of the electrical network in the residential sector can occur when using additional electrical heating appliances during the cold season, during accidents in the water heating system, etc. Despite the fact that the internal electrical networks of residential, public, administrative and domestic buildings must be protected from overload, in accordance with the requirements of the PUE, however, protective devices allow some overload of conductors. This is due to the fact that the reliable operation of the fuses occurs at currents exceeding 1.6I nom, and automatic machines - 1.45I nom.
If, for example, the machine is selected based on the requirements of the PUE, i.e. its rated current is equal to the long-term permissible current of the conductor, then the latter can work for a long time with a load of 145% I add., while its temperature can reach:

Q p \u003d Q o + (Q d - Q p) (I prev / I p) 2 \u003d 30 + (65 - 25) 1.45 2 \u003d 147 O С.

This value is greater than the long-term allowable temperature for cables with plastic insulation, specified not only in the PUE and equal to 65 ° C, but also more than that specified in GOST R 53769-2010 and equal to 70 ° C.
In the event of a short circuit during a prolonged overload, the temperature of the conductor will exceed the maximum allowable value of 350 O C and will be for S = 1.5 mm 2 at I short circuit = 1550 A (1):

Q con. \u003d 147 e k + 228 (e k - 1) \u003d 394 O С, where k \u003d 0.506.

Based on the above calculations and analysis, the conclusion is that in order to exclude the possible excess of the permissible wiring temperatures during overloads and short circuits, the rated currents of the protective equipment should be chosen somewhat lower than required by the PUE, as, for example, for circuit breakers: I nom.aut. 80% I add.
Let us pay special attention to the fact that the current requirements of the PUE do not oblige to check conductors up to 1 kV for thermal resistance to short-circuit currents. However, with respect to residential, public, administrative and household premises it is difficult to agree with this, given the possible severe consequences.

Real values ​​of short-circuit currents in the power supply circuit of buildings

Short-circuit currents in the power supply system with voltage up to 1 kV are calculated according to the methodology set forth in GOST 2824993. The calculation turns out to be more complicated than for networks with a voltage of 6–35 kV, which is explained by a number of circumstances:

• the need to take into account not only reactive, but also active resistances of circuit elements;
• the need to take into account the resistance of contact connections;
• the need to take into account the increase in active resistance of the conductor with increasing temperature;
• the need to take into account the resistance of the arc;
• the lack of accurate data on the zero-sequence resistance of some elements of the power supply system (cables with a non-conductive sheath, power transformers with a Y / Yn, Y / Zn winding connection scheme).

However, this is a separate topic for discussion.
As shown, when transformers with a capacity of 630 kVA or more are installed at substations, the short-circuit currents at the consumer may exceed those indicated in Table. 1 is the maximum allowed values. In order to limit short-circuit currents in the residential electrical network, it is possible to use supply transformers with Y / Yn winding connection schemes. Such transformers have increased zero-sequence resistances that reduce single-phase short circuit currents. In some cases, it is necessary to go for an increase in the cross section of the conductors of the internal electrical wiring in comparison with the required one under the conditions of the permissible load and the minimum allowable values ​​\u200b\u200bspecified in the PUE.

From all of the above, it follows that even if the current regulatory requirements are met, as a result of a short circuit in certain sections of the electrical wiring of residential buildings, conditions for ignition can be created. However, in this case, the short circuit itself would be wrong to qualify as the cause of the fire. The true causes of the fire are either incorrect technical solutions, or insufficient reliability and speed of the applied protective equipment, or exceeding the standard service life of electrical equipment, etc.

CONCLUSIONS

1. As a result of short circuits, with significant short-circuit currents and insufficient speed of protective equipment, there is a real danger of fire or serious deterioration of the insulation of the internal electrical wiring of buildings.
2. Given the particular risk of fire, it is advisable to introduce a regulatory requirement to perform a thermal resistance test of electrical wiring in residential buildings.
3. To avoid overloading the internal wiring, the rated currents of the protective devices must be chosen below the long-term permissible currents of the protected conductors.
4. When choosing protective devices, special attention should be paid to reliable automatic switches with guaranteed speed in the zone of instantaneous tripping of 0.02 s or less.

Literature used in the article

1. Electrical Installation Rules, 6th and 7th ed.
2. Technical circular No. Ts0298(e) of the Department for Development Strategy and Science and Technology Policy of RAO UES of Russia.
3. GOST R 5034599. Circuit breakers for overcurrent protection for domestic and similar purposes.
4. GOST 2824993. Short circuit currents in electrical installations. Methods of calculation in electrical installations of alternating current with voltage up to 1 kV.
5. Fedorovskaya A.I., Fishman V.S. Power transformers 10(6)/0.4 kV.