Introduction

It is very important that appliances and similar kinds of equipment have reliable protection against transient voltage conditions, because it is common for these products to suffer significant damage when a transient voltage appears on the ac voltage line. Transients can arise internally, from normal circuit operations, or externally from the environment. The latter is particularly frustrating because the transient characteristics are undefined. A statistical description can apply though but greater or smaller stresses are possible. Long-duration high-voltage transients are much less probable than those of lower amplitude and higher frequency.

The natural frequencies and impedance of indoor ac wiring result in damped oscillatory surges with typical frequencies ranging from 30 kHz to 1.5 MHz. Surge amplitude depends on both the wiring and the source of the surge energy. Disturbances die out further away from the source. Spark-over (6.0 kV in indoor ac wiring) sets the maximum voltage when transient suppressors are not present. Transients closer to the service entrance, or in heavy wiring, have higher amplitudes, longer durations, and more damping because of the lower inductance at those locations.

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