Calculation of short-circuit currents

Protection

Electrical installations almost always require protection against short-circuits wherever there is an electrical discontinuity. This most often corresponds to points where there is a change in conductor cross-section. The short-circuit current must be calculated at each level in the installation in view of determining the characteristics of the equipment required to withstand or break the fault current.

The flow chart in Figure 1 indicates the procedure for determining the various short circuit currents and the resulting parameters for the different protection devices of a low-voltage installation.

In order to correctly select and adjust the protection devices, the graphs in Figures 2, 3 and 4 are used. Two values of the short-circuit current must be evaluated:

The maximum short-circuit current, used to determine

• The breaking capacity of the circuit breakers
• The making capacity of the circuit breakers
• The electrodynamic withstand capacity of the wiring system and switchgear

The maximum short-circuit current corresponds to a short-circuit in the immediate vicinity of the downstream terminals of the protection device. It must be calculated accurately and used with a safety margin.

The minimum short-circuit current, essential

when selecting the time-current curve for circuit breakers and fuses, in particular when:

• Cables are long and/or the source impedance is relatively high (generators, UPSs)
• Protection of life depends on circuit breaker or fuse operation, essentially the case for TN and IT electrical systems

Note that the minimum short-circuit current corresponds to a short-circuit at the end of the protected line, generally phase-to-earth for LV and phase-to-phase for HV (neutral not distributed), under the least severe operating conditions (fault at the end of a feeder and not just downstream from a protection device, one transformer in service when two can be connected, etc.).

AUTHOR: Schneider Electric experts | Benoît de METZ-NOBLAT, Frédéric DUMAS, Christophe POULAIN

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1. visakh.sukumaran
Feb 26, 2013

• Edvard
Feb 27, 2013

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2. boedyzoldik
Oct 02, 2012

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