When an unloaded transformer is energised, the magnetic flux demanded by the applied voltage can add to the flux left remanent in the core. If the two coincide, the core saturates during the first half-cycles and the winding draws a current that can reach eight to twelve times the rated value.
That surge is not a fault, but it has consequences: it trips protection that cannot tell it from a short circuit, it causes voltage dips on the network, and it subjects the winding to mechanical stresses that accumulate operation after operation. On motors the origin is different — a stalled rotor — but the effect on the network is similar.
It is controlled in several ways: by demagnetising the core before energising, by closing the breaker at the favourable instant of the waveform with controlled switching, or by inserting a pre-insertion resistor. Differential protection tells it from a fault by the second-harmonic content, which is high in inrush current and low in a short circuit.