Section 1 General Rules

The choice of electrical apparatus and conductors according to the conditions of a short circuit. General requirements

Encyclopedia of radio electronics and electrical engineering / Rules for the installation of electrical installations (PUE)

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1.4.2. According to the short circuit mode, the following should be checked (for exceptions, see 1.4.3):

1. In electrical installations above 1 kV:

a) electrical apparatus, conductors, cables and other conductors, as well as supporting and supporting structures for them;

b) overhead power lines with a shock short-circuit current of 50 kA or more to prevent wires from lashing under the dynamic action of short-circuit currents.

In addition, for lines with split wires, the distances between the spacers of the split wires must be checked to prevent damage to the spacers and wires due to whipping.

Wires of overhead lines equipped with high-speed automatic reclosing devices should also be checked for thermal stability.

2. In electrical installations up to 1 kV - only switchboards, conductors and power cabinets. Current transformers in short circuit mode are not checked.

Devices that are designed to turn off short-circuit currents can, according to the conditions of their operation, include a short-circuited circuit, and must, in addition, be able to perform these operations at all possible short-circuit currents.

Resistant to short-circuit currents are those devices and conductors that, under the design conditions, withstand the effects of these currents without being subjected to electrical, mechanical and other damage or deformations that prevent their further normal operation.

1.4.3. According to the short circuit mode at a voltage above 1 kV, the following are not checked:

1. Apparatus and conductors protected by fuses with inserts for a rated current up to 60 A - according to electrodynamic resistance.

2. Apparatus and conductors protected by fuses, regardless of their rated current and type, according to thermal resistance.

A circuit is considered to be protected by a fuse if its breaking capacity is selected in accordance with the requirements of this Regulation and it is capable of breaking the smallest possible fault current in this circuit.

3. Conductors in circuits to individual electrical receivers, including shop transformers with a total capacity of up to 2,5 MVA and with a higher voltage of up to 20 kV, if the following conditions are met simultaneously:

a) in the electrical or technological part, the necessary degree of redundancy is provided, made so that the disconnection of these electrical receivers does not cause a breakdown in the technological process;

b) damage to the conductor during a short circuit cannot cause an explosion or fire;

c) it is possible to replace the conductor without significant difficulties.

4. Conductors to individual electrical receivers specified in clause 3, as well as to separate small distribution points, if such electrical receivers and distribution points are irresponsible for their purpose and if at least only the condition given in clause 3, b.

5. Current transformers in circuits up to 20 kV, supplying transformers or reacted lines, in cases where the choice of current transformers for short circuit conditions requires such an overestimation of the transformation ratios, in which the required accuracy class of the connected measuring instruments (for example, calculated meters) cannot be provided , while on the high voltage side in the circuits of power transformers it is recommended to avoid the use of current transformers that are not resistant to short-circuit current, and it is recommended to connect metering devices to current transformers on the low voltage side.

6. Wires of overhead lines (see also 1.4.2, clause 1, b).

7. Devices and busbars of circuits of voltage transformers when they are located in a separate chamber or behind an additional resistor built into the fuse or installed separately.

1.4.4. When choosing a design scheme for determining short-circuit currents, one should proceed from the conditions of long-term operation provided for a given electrical installation and not take into account short-term modifications of the circuit of this electrical installation that are not provided for long-term operation (for example, when switching). Repair and post-emergency modes of operation of the electrical installation do not apply to short-term changes in the scheme.

The calculation scheme should take into account the prospect of development of external networks and generating sources, with which the installation in question is electrically connected, for at least 5 years from the planned period of its commissioning.

In this case, it is permissible to calculate the short circuit currents approximately for the initial moment of the short circuit.

1.4.5. As the design type of short circuit should be taken:

1. To determine the electrodynamic resistance of devices and rigid tires with their supporting and supporting structures - a three-phase short circuit.

2. To determine the thermal resistance of devices and conductors - a three-phase short circuit; at the generator voltage of power plants - three-phase or two-phase, depending on which of them leads to more heating.

3. To select devices by switching capacity - according to the larger of the values ​​\uXNUMXb\uXNUMXbobtained for cases of three-phase and single-phase short circuits to earth (in networks with high earth fault currents); if the switch is characterized by two values ​​of switching capacity - three-phase and single-phase - respectively for both values.

1.4.6. The rated short-circuit current should be determined based on the fault condition at such a point of the circuit under consideration, during a short-circuit in which the devices and conductors of this circuit are in the most severe conditions (for exceptions, see 1.4.7 and 1.4.17, clause 3). It is permissible to disregard cases of simultaneous earth faults of different phases at two different points in the circuit.

1.4.7. On reacted lines in closed switchgears, conductors and devices located before the reactor and separated from the supply busbars (on branches from the lines - from the elements of the main circuit) by separating shelves, ceilings, etc., are recruited according to the short-circuit current behind the reactor, if the latter is located in the same building and the connection is made by busbars.

Bus branches from busbars to separating shelves and bushings in the latter must be selected based on the short circuit to the reactor.

1.4.8. When calculating the thermal resistance, the calculated time should be taken as the sum of times obtained from the addition of the main protection action time (taking into account the action of the automatic reclosure) installed at the circuit breaker closest to the fault location and the total opening time of this circuit breaker (including the arc burning time).

If there is a dead zone for the main protection (for current, voltage, resistance, etc.), thermal stability must be additionally checked based on the time of action of the protection that responds to damage in this zone, plus the total time of opening the circuit breaker. In this case, as the calculated short-circuit current, it should be taken that value of it, which corresponds to this place of damage.

Equipment and conductors used in circuits of generators with a capacity of 60 MW or more, as well as in circuits of generator-transformer blocks of the same power, must be checked for thermal stability, based on a short-circuit current passage time of 4 s.

See other articles Section Rules for the installation of electrical installations (PUE).

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