Editing Surge protector (section)

=== Metal oxide varistor ===
{{Main|Varistor}}
[[File:Varistoren.jpg|thumb|[[Metal-oxide varistor]]s]]
A metal-oxide varistor (MOV) consists of a bulk [[semiconductor]] material (typically [[sintered]] granular [[zinc oxide]]) that can conduct large currents when presented with a voltage above its rated voltage.<ref name="LittelfuseAN9767">{{cite web |title=AN9767 – Littelfuse Varistors: Basic Properties, Terminology and Theory |url=http://www.littelfuse.com/data/en/Application_Notes/Littelfuse_app-note_an9767.pdf |access-date=2011-03-29 |publisher=Littelfuse, Incorporated}}</ref> MOVs typically limit voltages to about 3 to 4 times the normal circuit voltage by diverting surge current elsewhere than the protected load. MOVs may be connected in parallel to increase current capability and life expectancy, providing they are ''matched sets''.{{efn|Unmatched MOVs have a tolerance of approximately ±10% on voltage ratings, which may not be sufficient.<ref name="Walaszczyk0" /> For more details on the effectiveness of parallel-connected MOVs, see {{slink||Joule rating}}}}

MOVs have finite life expectancy and degrade when exposed to a few large transients, or many small transients.<ref name="Brown2004">{{cite journal|last=Brown|first=Kenneth|title=Metal Oxide Varistor Degradation|journal=IAEI Magazine|date=March 2004|url=http://www.iaei.org/magazine/2004/03/metal-oxide-varistor-degradation/|access-date=2011-03-30|url-status=dead|archive-url=https://web.archive.org/web/20110719023317/http://www.iaei.org/magazine/2004/03/metal-oxide-varistor-degradation/|archive-date=2011-07-19}}</ref><ref name="Walaszczyk1">[http://www.littelfuse.com/data/en/Technical_Articles/Littelfuse_SizingMOVs_EC921.pdf Walaszczyk, et al. 2001 "Does Size Really Matter? An Exploration of ... Paralleling Multiple Lower Energy Movs".] See Figures 4 & 5 for Pulse Life Curves.</ref> Every time an MOV activates, its threshold voltage reduces slightly. After many spikes the threshold voltage can reduce enough to be near the protection voltage, either mains or data. At this point the MOV conducts more and more often, heats up and finally fails. In data circuits, the data channel becomes shorted and non-functional. In a power circuit, you may get a dramatic meltdown or even a fire if not protected by a fuse of some kind.<ref name="ABC">{{citation |title=Application Note 9311, The ABCs of MOVs |pages=10–48 |url=http://www.littelfuse.com/data/en/Application_Notes/an9311.pdf |access-date=18 January 2018 |section=Q. How does an MOV fail?}}.</ref>

Modern surge strips and house protectors have circuit breakers and temperature fuses to prevent serious consequences. A thermal fuse disconnects the MOV when it gets too hot. Only the MOV is disconnected leaving the rest of the circuit working but without surge protection. Often there is an LED light to indicate if the MOVs are still functioning. Older surge strips had no thermal fuse and relied on a 10 or 15 amp circuit breaker which usually blew only after the MOVs had smoked, burned, popped, melted and permanently shorted.

A failing MOV is a fire risk, which is a reason for the [[National Fire Protection Association]]'s (NFPA's)<ref>{{cite web |title=About NFPA |url=http://www.nfpa.org/categoryList.asp?categoryID=143&URL=About%20NFPA |url-status=dead |archive-url=https://web.archive.org/web/20120212162242/http://www.nfpa.org/categoryList.asp?categoryID=143&URL=About%20NFPA |archive-date=2012-02-12 |access-date=2012-02-07 |website=nfpa.org}}</ref> UL1449 standard in 1986< and subsequent revisions in 1998, 2009 and 2015. NFPA's primary concern is protection from fire.<ref>{{cite web |title=Investigating Surge Suppressor Fires |url=http://www.esdjournal.com/techpapr/Pharr/INVESTIGATING%20SURGE%20SUPPRESSOR%20FIRES.doc |url-status=dead |archive-url=https://web.archive.org/web/20070316234642/http://www.esdjournal.com/techpapr/Pharr/INVESTIGATING%20SURGE%20SUPPRESSOR%20FIRES.doc |archive-date=2007-03-16 |access-date=2007-06-20 |website=esdjournal.com}}</ref>

Therefore, all MOV-based protectors intended for long-term use should have an indicator that the protective components have failed, and this indication must be checked on a regular basis to ensure that protection is still functioning.<ref name="Testing">{{cite web |date=January 1998 |title=Application Note 9773 "Varistor Testing". See "Varistor Rating Assurance Tests" |url=http://www.littelfuse.com/data/en/Application_Notes/an9773.pdf |access-date=18 January 2018 |website=Littelfuse.com |pages=10–145 |quote=See definition of "end-of-lifetime".}}</ref>

Because of their good [[price–performance ratio]], MOVs are the most common protector component in low-cost basic AC power protectors.