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.