Lightning Protection-Surge Arrestors
Large over voltages that develop suddenly on electric transmission and distribution system are referred to as "surges" or "transients". These are caused by lightning strikes or by circuit switching operations. Surge arrestor is a protective device for limiting surge voltages on equipment by discharging or bypassing surge current.
The surge arrestor which responds to over-voltages without any time delay is installed for protection of 33 kV switchgear, transformers, associated equipment and 11 kV and 33 kV lines. The rated voltage of arrestors for 33 kV should be 30 kV for use on 33 kV systems and with nominal discharge current rating of 10 kA. The rated voltage of lightning arrestors should be 9 kV (r.m.s.) for effectively earthed 11 kV system (coefficient of earth not exceeding 80 % as per IS: 4004) with all the transformer neutrals directly earthed. The nominal discharge current rating should be 5 kA.