Where do the short circuit consequences appear? What is short circuit and network overload? Limit values ​​of short-circuit currents

Short circuit (SC)- this is the occurrence of electrical contact between different phases, phase and neutral working or protective wire. Online with solidly grounded neutral A short circuit can be considered a contact between a phase conductor and the ground.

The causes of a short circuit may be:

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

With time insulation ages and loses its properties. This applies equally to cables, to electric motor windings, and to insulators. Insulating surfaces are also subject to this property: the housings of circuit breakers and fuses. The deterioration of the properties of insulators is affected by the environment in which they operate: the degree of contamination, the presence of moisture, dust, and aggressive gases. As soon as a small conductive area appears, it begins to heat up and grow until the current through it reaches a critical value. It will increase like an avalanche, heat up and char the surface along which it flows. From this moment on, the area with weakened insulation becomes the site of a short circuit.

Example foreign objects on live parts are trees falling on power lines. They themselves create contact between the ground and the phase conductors; additionally, the wires break or short circuit to each other.

Wear of electric motor bearings can also lead to a short circuit. When the rotor rotates, its windings cling to internal parts or stator winding. The insulation is damaged and a short circuit occurs. Cables laid in the ground are inevitably subject to mechanical deformation. Vehicles pass over them, and as the seasons change, soil movements test their strength.

Inattention, carelessness, failure to comply with safety rules can also lead to a short circuit. This further harms the health of workers.

Overvoltage by themselves are not causes of short circuits. 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 case of short circuit, all power electrical network focuses on small area. If cables, wires and switching devices did not have their own resistance, 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 purpose, data on the resistance (active and reactive) of all electrical equipment installed along the short circuit path is used. The current is counted for the point furthest from the source to check whether the protection will turn it off.

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

• instead of modular switches with characteristic “C” (cutoff ratio 5-10), “B” (cutoff ratio 3-5) is used;
• increase the cross-section of power cables.

Effect of a short circuit on electrical equipment

Short circuit – emergency operation mode for the electrical network. When it occurs, it has two effects on electrical equipment simultaneously:

• electrodynamic;
• thermal.

According to the laws of physics, when current passes through two conductors located nearby, 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 strength increases.

This is the principle on which it happens electrodynamic effect of short-circuit current on tires, wires, windings of electrical machines. At substations and other power facilities, where fault current values ​​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 may occur somewhere to the side.

Thermal impact based on heating conductors as they pass through them electric current. In this case, the temperature sometimes rises so much that the wires or busbars melt.

IN living conditions The thermal effect of a short circuit is more clearly expressed; the dynamic effect can be ignored due to the small current values.

Network congestion

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

The causes of overload are:

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

The rapid electrification of residential buildings requires a more careful analysis of the electrical installation (electrical 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 household 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 flammable outer sheath or insulation must be protected from overload.
In addition, the following must be protected from indoor network overload:
lighting networks in residential and public buildings, in retail premises, service premises industrial enterprises, including networks for household and portable electrical receivers (irons, kettles, stoves, room refrigerators, vacuum cleaners, washing and sewing machines etc.), as well as in fire hazardous areas.

3.1.11
In networks protected from overloads (see 3.1.10), conductors should be selected according to the rated current, and the condition must be ensured that in relation to the long-term permissible current loads given in the tables of Chapter. 1.3, the protection devices had a multiplicity of no more than:
80% for the rated current of a fuse link or the setting current of a circuit breaker that has only a maximum instantaneous release (cut-off) - for conductors with polyvinyl chloride, rubber and similar thermal characteristics insulation; for conductors laid in non-explosive areas production premises industrial enterprises, 100% allowed;
100% for the rated current of a circuit breaker release with a non-adjustable inverse current characteristic (regardless of the presence or absence of a cut-off) - for conductors of all brands.

Rice. 1. Typical power supply diagram for a residential building

Power supply diagram

Let's consider a typical diagram (Fig. 1), where the source of power supply is, as a rule, a separate substation with a 10(6)/0.4/0.23 kV distribution board. At the entrance to the building there is an ASU-0.4/0.23 kV. The next stage is the floor group distribution panel, and the last stage is the apartment one. The above distribution devices connected to each other by conductors, the minimum permissible cross-sections of which are specified in the requirements of the PUE. Rated currents of devices that protect wires and cables from currents short circuits and from overload, are selected in accordance with the requirements of the PUE.

Conditions for electrical fires

The question arises whether a short circuit can cause a fire in the electrical wiring if the above and others are done. PUE requirements? When considering this issue, it is necessary to pay attention to the fact that electrical wiring ignites when the conductor reaches a certain temperature, depending on the type of cable insulation. Currently widely used, in which this temperature is equal to: Q = 350 O C.
The change in conductor temperature during short circuit current flow is described by the formulas 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 wires you can use the following formula:

where Q con. and Q start – respectively, the final and initial temperatures of the current-carrying core of the conductor, O C;
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 general, 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 attenuation (attenuation time constant T 0.003 s). As a result of integration over the time interval of action of the protective equipment (0 - 0.02 s) we obtain:

where I d – effective value periodic component of 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 fire of the conductor will not occur depend on its cross-section and the short-circuit shutdown time.

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

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

Limit values ​​of short circuit currents and minimum permissible values ​​of short circuit currents

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

I kzmin. = I nom · 2 · 5,

where I nom – rated current of the machine;
2 – reliability factor;
5 – multiple of the cut-off current.

To determine the maximum permissible values ​​of short-circuit currents at which a fire will not yet occur in the electrical wiring, we use formulas (1) and (2).
Let us take the initial temperature of the conductor Q init. = 30 O C. As the final value, it is required to accept one in which the insulation of the electrical wiring does not yet lose its properties and allows for 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. = 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 operating time of circuit breakers in the initial cut-off zone (instantaneous tripping time) should be less than 0.1 s. However, from the catalog time-current characteristics of the machines it follows that in fact the response time of the switches is much less. Thus, for circuit breakers such as LSN and C 60a, this time does not exceed 20 ms, and at large multiples of the short circuit current it is even less (Fig. 2a and 2b). With a shutdown 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:

Specifying the regulated PUE minimum permissible cross-section values 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.

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

It should be emphasized once again that the maximum permissible 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 permissible 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

In most cases, an overload of the electrical network in the residential sector can occur when additional heating electrical appliances are used 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 the conductors. This is due to the fact that reliable operation of fuses occurs at currents exceeding 1.6I nom, and for automatic circuit breakers - 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 continuous permissible current conductor, then the latter can work for a long time with a load of 145% I admissible, while its temperature can reach:

Q r = Q o + (Q d – Q r) · (I pre / I r) 2 = 30 + (65 – 25) 1.45 2 = 147 O C.

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

Q con. = 147 · e k + 228 (e k – 1) = 394 O C, where k = 0.506.

Based on the above calculations and analysis, the conclusion suggests itself that to eliminate possible excess permissible temperatures electrical wiring during overloads and short circuits rated currents protective equipment should be selected slightly lower than required by the PUE, as, for example, for circuit breakers: I nom.aut. 80% I add.
Let's reverse Special attention the fact that the current requirements of the PUE do not oblige testing of conductors up to 1 kV for thermal resistance to short-circuit currents. However, in relation to residential, public, administrative and household premises This is difficult to agree with, given the possible dire consequences.

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

Short-circuit currents in power supply systems with voltages up to 1 kV are calculated according to the methodology set out in GOST 2824993. The calculation turns out to be more complex 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 arc resistance;
• lack of accurate data on the zero-sequence resistance of some elements of the power supply system (cables with a non-conducting sheath, power transformers with a winding connection diagram Y/Yn, Y/Zn).

However, this is a separate topic for discussion.
As shown, when installing transformers with a capacity of 630 kVA or more at substations, the consumer's short-circuit currents may exceed those indicated in table. 1 maximum allowed values. In order to limit short-circuit currents in the electrical network of a residential premises, supply transformers with winding connection diagrams Y/Yn can be used. Such transformers have increased zero-sequence resistances, which reduce single-phase short-circuit currents. In some cases, it is necessary to increase the cross-section of the conductors internal wiring compared to what is required by the conditions permissible load and the minimum permissible values ​​specified in the PUE.

From all of the above, it follows that even if current regulatory requirements are met, as a result of a short circuit, conditions for fire may be created in certain sections of the electrical wiring of residential buildings. However, in this case, the short circuit itself would be incorrectly classified as the cause of the fire. The true causes of the fire are either incorrect technical solutions, or insufficient reliability and performance of the protective equipment used, or exceeding regulatory period operation of electrical equipment, etc.

CONCLUSIONS

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

Literature used in the article

1. Rules for Electrical Installations, 6th and 7th ed.
2. Technical circular No. Ts0298(e) of the Department of Development Strategy and Scientific and Technical Policy of RAO UES of Russia.
3. GOST R 5034599. Circuit breakers for overcurrent protection for household 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.