What is a surge arrester?

A surge arrester is a device to protect electrical equipment from over-voltage transients caused by external or internal events.

Now let us understand these terms in the definition:

Transients: sudden change which lasts for a short time in other words a momentary variation in current, voltage or frequency.

External events: In electrical power system, the most common surge inducing event is lightning.

Internal events: switching events, for example sudden switching of loads, switching of reactive power sources( like inductor or capacitor banks), short-circuits and sparking etc.

What does the surge arrester do?

It basically diverts the high voltage current directly to the insulation or to the ground to avoid damaging the system. It does not absorb all of the high voltage that passes through it. It simply diverts it to the ground or clamps it to minimize the voltage that passes through it using MOV or Metal Oxide Varistor. Surge arrestors are NOT generally designed to offer protection against a direct lightning strike, but rather against electrical transients which might occur due to lightning in the vicinity of the line or conductor.