What is a Short Circuit

A short circuit is an electrical circuit that allows a current to travel along an unintended path, often where essentially no (or a very low) electrical impedance is encountered.

Impedance is the measure of opposition that a circuit presents to a current when a voltage is applied. This unintended or abnormal path of negligible impedance can be between live conductors, or between a live conductor and an earth, which have a difference in potential under normal operating conditions.

When electric current flows in a conductor, it creates a magnetic field. In the case of alternating current the magnetic field varies with that current. This magnetic field affects adjacent conductors in two ways: the first is to induce eddy currents, and the second is to induce an electromagnetic field.

Under Short circuit conditions the magnetic fields around the conductors will generate mechanical forces between those conductors. These forces may be considerable and will be greater the closer together the conductors are.

Whilst direct current creates a field, this field is steady and its main effect is to magnetise nearby susceptible objects.

Eddy Currents (Foucault)

Eddy currents are electric currents induced in conductors when a conductor is exposed to a changing magnetic field.

Eddy currents are induced circumferentially around the current carrying conductors. For this reason, the use of steel wire or steel tape armour is not permitted in single core cables used in a.c. circuits. Similarly it is strongly recommended that cast iron or ferromagnetic cable cleats are not used in conjunction with individual single core cables deployed in a.c. circuits.