Use of a metal-oxide varistor with a series spark gap across the mains

Protection of electronic equipment from damage and upset by transient overvoltages on the AC supply mains is an important problem in electromagnetic compatibility; metal oxide varistors are commonly used for this purpose. To obtain the lowest clamping voltage during a surge, varistors with a nominal conduction current of 1 mA DC at 200 V are being used on mains with a nominal voltage of 120 V RMS. The author discusses the use of a series spark gap with a metal oxide varistor to achieve both a small clamping voltage and a long life time for the surge-protective circuit. A metal oxide varistor with a nominal conduction current of 1 mA DC at 100 V, in series with a 250 V DC spark gap, may be suitable for connection to the mains with a nominal voltage of 120 V RMS. Extinguishing follow current in the gap and attenuating the remnant that propagates downstream prior to the conduction of the spark gap are discussed in detail