Einschaltstrom

Einschaltstrom (inrush current) is the transient surge of current drawn by an electrical device when it is first connected to a power supply. This peak current can greatly exceed the steady-state running current and lasts only briefly as the circuit reaches its operating state.

Causes of Einschaltstrom include charging of input capacitors in power supplies and DC links, magnetization of transformers, and the energizing of inductive loads such as motors and solenoids. Capacitive loads and large capacitor banks can also produce very high inrush when uncharged. Residual magnetization or switching with non-sinusoidal waveforms can amplify the effect.

Inrush current can cause voltage dips in the supply, nuisance tripping of fuses or circuit breakers, and increased electrical and mechanical stress on components and wiring. It can affect the reliability of power distribution and the life of switching devices.

It is typically characterized by a peak current measured at switch-on, often expressed as a multiple of the device's nominal running current. For small electronic power supplies the inrush may be about 2–3 times; for larger transformers or capacitor banks it can exceed 5–10 times; motor start currents are often several times higher than running current.

Mitigation methods include inrush-limiting devices such as NTC thermistors, pre-charge resistors, soft-start circuits, and controlled ramping using power electronics. Other approaches include autotransformers, using slow-contactors with pre-charge, or staggering the energization of multiple loads to reduce simultaneous inrush.

Designers consider Einschaltstrom in electrical installations to minimize voltage dips and avoid unnecessary tripping, selecting components and protective devices with adequate inrush ratings.