3.7.1 -  Introduction

The overload currents considered in the previous section are never likely to be more than two to three times the normal rated current. Fault currents, on the other hand, can well be several hundreds, or even several thousands of times normal. In the event of a short circuit or an earth fault causing such current, the circuit must be broken before the cables are damaged by high temperatures or by electromechanical stresses. The latter stresses will be due to the force on a current carrying conductor which is subject to the magnetic field set up by adjacent conductors. This force is proportional to the current, and to the magnetic field strength. Since the field strength also depends on the current, force is proportional to the square of the current. If the current is one thousand times normal, force will be one million times greater than usual! Fault protection must not only be able to break such currents, but to do so before damage results. Abrasion of cable insulation by movement is usually prevented by normal fixings or by being enclosed in conduit or trunking. Support must he provided to cables in busbar chambers.