A surge protective device (SPD) was designed to. the protect electrical systems and equipment from surge events by limiting transient voltages and diverting surge current flow. Surges can originate externally, most intensely by lightning, and internally by the switching of the electrical loads.

Surge Protective Devices (SPD) were used to protect the electrical installation, which consists of the consumer unit, wiring, and accessories, from the electrical power supply surges known as transient overvoltages.

Surge current supply capacity was the maximum amount of surge current supply that a surge protector could pass for a single surge event. This level was used to indicate the protection capacity of the particular surge protector.

In general, a surge is a transient wave of current diagram voltage or power supply in an electric circuit. In power systems in particular – and this is likely the most common context that we relate surges to – a surge, or transient, was a subcycle overvoltage with a duration of less than a half-cycle of the normal by voltage waveform.

Protection against transient overvoltages should be provided for installations such as hotels, banks, industrial plants, and the like or parts thereof where a power supply interruption was deemed intolerable.