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Electrical equipment such as transformers, generators, CTs, PTs and motors need to be protected against overvoltage. The surge arrester is used to protect these devices from lightning and switching voltages. Modern arrester uses metal oxide rheostats as active components and provides excellent overvoltage protection for devices connected to power systems.

Electrical equipments connected in power system are exposed to many stresses, and one of major concern is protecting them from transient over voltages. Transients over voltages are caused by lightning discharges and switching operations.

 A surge arrester is a protective device connected in parallel with system equipment to be protected. Overvoltages at the protected equipment are limited by the arrester that conducts energy associated with surge to ground and protect the equipment. The highly non-linear characteristics of an arrester allow the arrester to limit the voltage across its terminal nearly a constant value over a wide range of arrester current. The voltage across the equipment to be protected is almost same as voltage across arrester (Unless there is large separation distance between surge Arrester and equipment to be protected & large lead lengths).

 During conduction of surge current, arrester exhibits very low impedance & forms a voltage divider to applied surge voltage in conjunction with line surge impedance. During the time arrester is in conduction, a large percentage of surge voltage appears across line surge impedance and not across equipment to be protected.

By properly using the arrester, the insulation of the equipment is not affected by the damage voltage, thus eliminating the chance of insulation failure. It is important to correctly select the surge arrester parameters to enable them to perform the required protection functions without causing any nuisance in the system.

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