A transformer switching onto a line can sometimes cause a circuit breaker to trip or a fuse to blow. This phenomenon occurs even if the transformer presents no load, such as when the secondary is open. The problem arises because of the heavy magnetizing inrush current in the transformer. The amplitude of the current de-pends on the instant on the ac waveform at which the transformer becomes energized. The inrush current is at its maximum value if the transformer becomes energized when the ac waveform goes through its zero-crossing point. A similar situation exists when a capacitor in a power-factor-improvement bank switches onto the line. In this case, the in-rush current is at its maximum when the ac waveform goes through its peak value. Normally, a mechanical contactor effects the switching with-out any control of the instant of switching.
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