Power-on rush

The inrush current is a brief, significantly increased current that flows when an electrical device or system is switched on. It only occurs for a very short period of time, but can reach a multiple of the normal operating current.

Causes of the inrush current include the charging of capacitors, the switching on of transformers and magnetic saturation effects in the iron core. With transformers in particular, the inrush current can depend heavily on the switch-on phase of the AC voltage.

The inrush current subsides after a few milliseconds to seconds and becomes the normal operating current. Nevertheless, it can affect electrical components such as fuses or circuit breakers and, in unfavorable cases, lead to unwanted shutdowns.

Various measures are used to limit the inrush current, such as inrush current limiters.