Inrush current
The brief, high current many loads draw at switch-on — and why it trips protection if not allowed for.
What it is
Inrush is the surge of current a load draws for a brief moment when first switched on — far higher than its normal running current. Motors, transformers, and capacitor-heavy electronics all have significant inrush.
Why it happens
A motor at standstill looks almost like a short to the supply until it starts turning, so it pulls heavy 'locked-rotor' current that falls away as it speeds up. Transformers and power supplies charge their cores/capacitors with a brief spike at switch-on.
Protection has to ride through this legitimate surge without tripping, then still protect against real faults — which is why breaker 'curves' and overload time-delays exist.
On site
If something trips only at switch-on but runs fine once going, suspect inrush versus protection. The fix is the right protection characteristic, a reduced-voltage starter (star-delta/soft start), or staggering several large loads — not upsizing protection to defeat it.
Related faults
Motor trips protection on start but runs fine if it gets going
Protection trips during the start/inrush, but on the rare occasion it gets running it's fine — pointing at starting current, settings, or load inertia rather than a running fault.
Equipment tripping the supply on startup surge
Switching on a piece of equipment trips an upstream breaker or causes a momentary dip — the inrush/startup surge is exceeding what the protection or supply can ride through.
Large motor start trips the distribution board
Starting a large three-phase motor (lift, pump, compressor, aircon) trips the board feeding it or dips the supply — an inrush/coordination issue rather than a running fault.
Related definitions
Induction motor
The workhorse AC motor — a rotating magnetic field in the stator drags the rotor around with it.
Soft starter
Reduces motor starting current by ramping the voltage up, then often hands over to a bypass contactor.
Star-delta starting
Starts a motor in star (lower current) then switches to delta (full power) once it's up to speed.
Circuit breaker (MCB)
Automatically disconnects a circuit on overload or short circuit, and can be reset rather than replaced.