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Electrical insulation, as seen in household wiring, is crucial for safety, preventing electric shock and short circuits by covering conductive materials with insulating materials such as plastic or rubber.

The strength of equipment insulation varies with time, but it can withstand high transient overvoltages, provided they are short-lived.

For testing insulation strength, a standard impulse voltage wave is used. This wave shape is defined by the time it takes for the voltage to reach its peak and decay to half its peak value.

The peak value of this defines the Basic Insulation Level, or BIL. Equipment meeting standard BILs can endure repeated applications of the standard waveform without insulation failure.

Some equipment, like overhead transmission line insulation, is self-restoring after a short circuit, whereas internal transformer insulation requires repair or replacement upon failure.

Protective devices, connected in parallel with the equipment from each phase to the ground, safeguard the equipment against overvoltages exceeding its BIL, providing a protection margin.

A simple example of a protective device is the rod gap, designed to spark over at specified overvoltages.