Overcurrent Protection

When voltage is applied to the input winding of a transformer, there can be a brief period of inrush current until the transformer core stabilizes. Inrush lasts approximately six power cycles, or about 0.1 seconds. The magnitude of the inrush varies depending on when the switch closes on the power wave, so that inrush can be anywhere from zero to greater than the full load current rating of the transformer.

In addition, the impedance of the supply system can influence the amount of inrush current the transformer can draw. To avoid tripping circuit breakers or blowing fuses on the primary side of the transformer during energizing, careful coordination of fuse sizes or circuit breaker handle ratings and magnetic trip settings is essential. This coordination requires information about maximum possible inrush to be expected from the particular transformer in question.

In order to provide optimal coordination and prevent possible inrush nuisance tripping, the primary overcurrent protection should be adjusted based on the maximum inrush current. This will result in the primary overcurrent protection exceeding the 125% allowance in the NEC for primary-only protection, and secondary protection will be required.