When selecting a cable, the performance of the cable under fault conditions is an important consideration. It is important that calculations be carried out to ensure that any cable is able to withstand the effects of any potential fault or short circuit.  This note looks at how to do this.

The primary concern with cables under a fault condition is the heat generated, and any potential negative effect this may have on the cable insulation. 

Calculation of fault rating is based on the principle that the protective device will isolate the fault in a time limit such that the permitted temperature rise within the cable will not be exceeded.

In addition to the direct heating effect of fault currents, other considerations include:

• electro-mechanical stress and fault levels large enough to cause cable failure
• performance of joint and terminations under fault conditions