From Electrical Installation Guide
|Knowing the levels of 3-phase symmetrical short-circuit currents (Isc) at different points in an installation is an essential feature of its design|
A knowledge of 3-phase symmetrical short-circuit current values (Isc) at strategic points of an installation is necessary in order to determine switchgear (fault current rating), cables (thermal withstand rating), protective devices (discriminative trip settings) and so on...
In the following notes a 3-phase short-circuit of zero impedance (the so-called bolted short-circuit) fed through a typical MV/LV distribution transformer will be examined. Except in very unusual circumstances, this type of fault is the most severe, and is certainly the simplest to calculate.
Short-circuit currents occurring in a network supplied from a generator and also in DC systems are dealt with in Chapter N.
The simplified calculations and practical rules which follow give conservative results of sufficient accuracy, in the large majority of cases, for installation design purposes.