What are overcurrents?

An overcurrent is a situation where a circuit experiences a current that is higher than the normal operating current. This is either an overload or a short circuit (fault) current.

What is an overload?

An overload current is an excessive current relative to the normal operating current, but one which is confined to the normal conductive paths of the circuit. Overloads are often between one and six times the normal current level. They are usually caused by harmless temporary surges in current that occur when motors are started up or transformers are energised. Such overloads (transients) are very brief in duration and any rise in temperature is trivial and has no harmful effects on the circuit components.

Continuous overloads can result from defective motors, overloaded equipment or too many loads on the one circuit. Such sustained overloads are destructive and must be cut off by protective devices (fuses) before they damage the circuit.

What is a short circuit?

Short circuit currents are also commonly known as fault currents and occur when a circuit is given an opportunity to flow through a shorter than normal path. Whereas overload currents occur at rather modest levels, the short circuit or fault current can be many hundreds of times larger than the normal operating current.

If not cut off quickly, damage and destruction can become rampant.

Engineers need to know the potential fault current when designing circuits. This is the value of the highest possible current in the event of a short circuit.