Inrush current,
input surge current or
switch-on surge is the maximum, instantaneous input
current drawn by an electrical device when first turned on. Alternating current
electric motors and
transformers may draw several 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 overcurrent protection devices such as
fuses and
circuit breakers is made more complicated when high inrush currents must be tolerated. The overcurrent protection must react quickly to overload or
open circuit but must not interrupt the circuit when the (usually harmless) inrush current flows.