Over-Voltage Protaction

28 EE January 1989 OVER PROTECTION The use of some form of over-voltage protection in electronic equipment is often not contemplated until a huge voltage surge has caused considerable damage. Since it is invariably better to be safe than sorry, this article looks at two over-voltage protection devices (surge arresters), the varistor and the gas-filled conductor, and discusses their operation and application. A Over-voltage is basically any voltage that exceeds the nominal value plus the stated tolerance. Over-voltage can be generated by various sources, of which lightning is probably the best known. A thunderstorm, even while it is several miles away, can give rise to voltage peaks of considerable amplitude on the mains network. The switching on and off of relatively heavy loads connected to long wires or conductors, which form a con- siderable self-inductance, also causes such peaks. The mains network is, how- ever, not the only large self-inductance A 1 mm4..•• ■■■=mmm...