Inrush current flows at switch-on, while a magnetic core or capacitance is far from steady state. A transformer energised near a voltage zero can saturate its core and draw 8 to 12 times rated current, decaying over hundreds of milliseconds to seconds and rich in second harmonic. Motors pull six to eight times full-load current until they accelerate, and LED drivers and power supplies spike as their input capacitors charge. Protection has to ride through it without nuisance tripping.
Transformer inrush keeps tripping the incomer on instantaneous, raise the high-set or turn on second-harmonic blocking.
Also heard as
- inrush
- switch-on surge
Related terms
Differential protection
Scheme that compares current entering and leaving a protected zone and trips when the two disagree.
Nuisance trip
Protective device opening with no real fault present, usually from bad settings, leakage or transients.
Black start
Restarting a power system from total shutdown using generators that can start without any outside supply.