Inrush current

Inrush current/Input surge current/Switch-on surge is the maximal instantaneous input current drawn by an electrical device if first turned on.

• maximal: max value in one specific set; whereas "maximum" means the biggest in any set, with nothing being higher 

• instantaneous: even faster than "instantly"
   

AC motors and transformers may draw many times their normal full-load current when first energized, for a few cycles of the input waveform. Power converters also often have inrush currents much higher than their steady-state currents, due to the charging current of the input capacitance. The selection of over-current-protection devices such as fuses and circuit breakers is made more complicated when high inrush currents must be tolerated. The over-current protection must react quickly to overload or short-circuit faults but must not interrupt the circuit when the (usually harmless) inrush current flows.