A surge arrester is a protective device that is installed on overhead lines and at substations to safeguard electrical equipment from the damaging effects of overvoltages. These overvoltages, or surges, can be caused by two main phenomena:

• Lightning Strikes: A lightning strike, even if it doesn't hit the line directly, can induce a massive voltage spike on the conductors.

• Switching Surges: These are temporary overvoltages caused by switching operations, such as turning on or off a large transformer or a capacitor bank.

The surge arrester's primary function is to limit these overvoltages to a safe level. It does this by acting as a temporary path to ground for the excess energy, preventing it from damaging critical and expensive equipment like transformers, circuit breakers, and other sensitive components.