Tuesday, July 31, 2007

Light Arrester

In telegraphy and telephony a lightning arrester is to be found where wires enter a structure, preventing harm to electronic instruments within and ensuring the protection of individuals near them. Lightning arresters, also called rush protector, are devices which are linked between each electrical conductor in a power and transportation systems and the earth. These provide a short circuit to the earth that is interrupted by a non-conductor over which lightning jumps. Its function is to limit the rise in voltage when a connections or power line is struck by lightning.

The non-conducting substance may consist of a semi-conducting material like silicon carbide or zinc oxide, or a spark gap. Primitive varieties of such flash gaps are simply open to the air, but more modern varieties are filled with dry gas and provided with a little amount of radioactive material to support the gas to ionize when the voltage across the gap reaches a particular level. Other designs of lightning arresters use a glow-discharge tube associated between the protected conductor and ground, or any one of a many of voltage-activated solid-state switches called varistors or MOV's. Lightning arresters built for substation use are consisting of a porcelain tube several feet in length and several inches in diameter, impressive devices, fill with disks of zinc oxide. A safety port is full on the side of the device to vent the occasional internal explosion without shocking the porcelain cylinder.