Installations have always required protection from the effects of unwanted over-voltages, the most obvious sources being lightning and switching. Early designs using generous clearances, heavy insulation and thermionic valve electronics presented few problems.
Modern sophisticated designs in both electronics and power with high component densities and low withstand semiconductors, demand an effective method of over-voltage protection.
Over-voltage protection increases the reliability of the equipment protected and thereby reduces repair costs and costly 'out-of-service' conditions.
Transients can cause breakdown or malfunction of semiconductors and insulations if not suppressed to a safe level.
Varistors give an all round standard of protection that other methods cannot claim. There are many ways to suppress transients - the traditional CR circuit - spark gaps - diodes - crowbars - silicon carbide varistors etc., but none can combine the cumulative advantages of Metal Oxide Varistors (MOVs)