Determination of short-circuit currents for the selection of devices and conductors

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Section 1 General Rules

The choice of electrical apparatus and conductors according to the conditions of a short circuit. Determination of short-circuit currents for the selection of devices and conductors

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

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1.4.9. In electrical installations up to 1 kV and above, when determining short-circuit currents, for selecting devices and conductors and determining the impact on supporting structures, one should proceed from the following:

1. All sources involved in the supply of the short circuit point under consideration operate simultaneously with the rated load.

2. All synchronous machines have automatic voltage regulators and excitation forcing devices.

3. A short circuit occurs at a point in time at which the short circuit current will have the greatest value.

4. The electromotive forces of all power supplies are in phase.

5. The rated voltage of each step is assumed to be 5% higher than the rated voltage of the network.

6. The effect on short circuit currents of synchronous compensators, synchronous and asynchronous motors connected to this network should be taken into account. The influence of asynchronous electric motors on short-circuit currents is not taken into account when the electric motor power is up to 100 kW per unit, if the electric motors are separated from the short circuit place by one transformation stage, and also at any power, if they are separated from the short circuit place by two or more transformation stages, or if the current from them can to enter the short circuit site only through those elements through which the main short circuit current passes from the network and which have significant resistance (lines, transformers, etc.).

1.4.10. In electrical installations above 1 kV, the inductive resistances of electrical machines, power transformers and autotransformers, reactors, overhead and cable lines, as well as current conductors should be taken as design resistances. Active resistance should be taken into account only for overhead lines with wires of small cross sections and steel wires, as well as for long cable networks of small cross sections with high active resistance.

1.4.11. In electrical installations up to 1 kV, the inductive and active resistances of all circuit elements, including the active resistances of the circuit transition contacts, should be taken as design resistances. It is permissible to neglect the resistances of one type (active or inductive), if in this case the total resistance of the circuit decreases by no more than 10%.

1.4.12. In the case of supplying electrical networks up to 1 kV from step-down transformers, when calculating short-circuit currents, one should proceed from the condition that the voltage supplied to the transformer is unchanged and equal to its rated voltage.

1.4.13. Elements of a circuit protected by a fuse with a current-limiting action should be checked for electrodynamic resistance according to the highest instantaneous value of the short-circuit current passed by the fuse.

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

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