Overview

In the previous chapter, we saw how to calculate the symmetrical fault current. This is important from the rms heating point of view, but for distribution switchgear engineers, the asymmetrical current is of much greater importance for a number of reasons. The rate of rise of current is higher under symmetrical fault conditions but the peak current of a fully asymmetrical fault current will induce the maximum electromagnetic force, and therefore stress, on conducting components. In addition, the total contact loading, which is the sum of electromagnetic and spring loading, must be sufficient to prevent contact burning. A fully asymmetrical current, as it is offset, will consist of major and minor loops. The time between current zeros in a major loop will therefore be greater than that implied by the power frequency of the system. This will induce greater stress on the interrupting system being used and, therefore, must be proven by test.

Medium-and high-voltage transmission and distribution of electricity takes place within a three-phase system. The voltage of each phase being displaced from the others by 120 electrical degrees. This means that the initiation of a three-phase fault will always take place with a finite value of prospective short-circuit current in at least two phases. In practice, the probability will be that all three phases will have some measure of finite prospective current. However, in an inductive circuit, an instantaneous value of prospective current cannot flow as the back e.m.f. of the circuit will provide an equal and...