Inrush current or input surge current refers to the maximum, instantaneous input current drawn by an electrical device when first turned on. For example, incandescent light bulbs have high inrush currents until their filaments warm up and their resistance increases. 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 feature high inrush currents relative to their steady state currents. This is typically 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 short circuit but must not interrupt the circuit when the (usually harmless) inrush current flows.
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Inrush current can be reduced by thermistor NTC type, a resistor with negative resistance coefficient. Specific NTC for transient protection have few hundried ohm at ambient temperature and less tha 1 ohm at working temperature.