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Metal Oxide Varistors

January 15th Fact-of-the-Day

Most surge protectors installed inside homes are a type known as Transient Voltage Surge Suppressors (TVSS). TVSS's that provide both Line-to-Line (L-L) and Line-to-Ground (L-G) protection have voltage clamping devices of some kind connected across the AC line and from one or both lines to ground. A variety of voltage-clamping devices are used, but Metal Oxide Varistors (MOV's) are most common. Clamping voltages vary, but in home applications MOV's typically do not conduct significantly until the voltage across them reaches about 333 volts. At that voltage they essentially short-circuit to prevent further voltage increase unless they burn out and fail. As you might expect, they become extremely hot doing their line-short-circuit job and may sacrifice themselves to clamp a single large surge or may remain functional after several small surges. ©2005 Martek International All rights reserved.

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