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 depends 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.
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