Over Voltage Protection

It happens from time to time that very large voltage spikes (lightning; switching of large loads) are superim- posed on the mains. Although these spikes are of very short duration, they may have disastrous consequences for mains-operated equipment. A mains power supply can be effectively pro- tected from such spikes with the aid of varistors. These components can handle, but only for a few microsec- onds, currents of thousands of amperes. In the proposed protection circuit, three varistors are used: one between L(ive) and N(eutral); one between L en E(arth); and one between N and E. The varistors are preceded by fuses, so that only the equipment connected via the circuit is protected. If these fuses were omitted, the entire household supply would be protected with the risk that one of the main fuses blows during an 10A over-voltage. The circuit is best built into a small man- made-fibre enclosure with integral plug and socket. The mains-carrying bare wires should be kept separated...