Inrush current
The current peak at switch-on, up to 8 to 15 times the rated current.
Definition
The inrush current is the very high current peak a transformer draws at the moment it is switched on. It can reach 8 to 15 times the rated current for a few milliseconds, before decaying over a few dozen cycles.
The phenomenon comes from transient saturation of the core: depending on the exact instant of switch-on and the residual magnetism, the flux can double and drive the core into deep saturation, where it barely opposes the current. It is a transient magnetising current, not a fault.
It must absolutely be taken into account when choosing upstream protections, otherwise nuisance tripping occurs at every start-up.
The ABL tip
If your protections trip at switch-on with no real fault, the inrush current is to blame. ABL Transfo advises you on tripping curves (type D, time-delayed) or suitable inrush-limiting devices.
