Different categories of lightning arrester now in use are rod gap arrester, sphere gap lightning arrester, horn gap lightning arrester, expulsion-type arrester, impulsive protective gap with electrolyte lightning arrester, electrolytic-type arrester, lead oxide-type arrester, pellet-type of lead peroxide arrester, thyrite lightning arrester and valve-type arrester. Rod gap arrester consists of two bent rods, one connected to line circuit and the other to earth. In sphere gap lightning arrester, air gap is provided by two similar spheres. In horn gap arrester, two bent metal rods in the form of horns, mounted in a vertical plane. Expulsion-type arrester which is also known as protector tube, has two electrodes at two ends of the tube. These electrodes are composed of a fiber tube which can produce a gas when the arc is generated. In impulsive arrester with protective gap, two gaps namely spark gap and secondary gap exists. In electrolytic type, an electrolyte, in lead oxide arresters, lead oxide and in pellet type lead oxide, pellets of lead oxide is used. Thyrite, a type of clay mixed with carborandom is used in thyrite lightning arrester. A divided spark gap in series with a non-linear resistor is used in valve-type lightning arrester.
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