What a surge is

A transient overvoltage lasting microseconds to milliseconds, caused by a lightning strike, by switching of large loads, or by a fault elsewhere on the network. The voltage peak, not the duration, is what damages insulation and electronic equipment.

The three types

Type 1 — installed at the main incomer where a direct lightning strike or the partial lightning current is expected.

Type 2 — installed at distribution boards, handling switching and induced surges.

Type 3 — installed close to sensitive equipment, for the residual surge that reaches that point.

Coordination is the whole point

A single device does not protect an installation. Surges are reduced in stages: a Type 1 device at the incomer limits the surge to a level a Type 2 device can handle, which in turn limits it for a Type 3 device at the equipment. The distances between devices and the cable lengths between them determine whether the coordination actually works.

The earthing connection is critical

A surge protection device works by diverting current to earth. If the earthing conductor is long, or the earth resistance is high, the voltage rise during the surge defeats the purpose. The earthing system and the surge protection device have to be designed together.

Indicators and replacement

Devices carry an end-of-life indicator. A device that has absorbed its rated surges is no longer protection. Checking those indicators is part of routine maintenance.