Editing Varistor (section)

=== Electrical characteristics ===
A varistor remains non-conductive as a [[shunt (electrical)|shunt]]-mode device during normal operation when the voltage across it remains well below its "clamping voltage", thus varistors are typically used for suppressing line voltage surges. Varistors can fail for either of two reasons.

A catastrophic failure occurs from not successfully limiting a very large surge from an event like a [[lightning]] strike, where the energy involved is many orders of magnitude greater than the varistor can handle. Follow-through current resulting from a strike may melt, burn, or even vaporize the varistor.  This [[thermal runaway]] is due to a lack of conformity in individual grain-boundary junctions, which leads to the failure of dominant current paths under thermal stress when the energy in a [[Transient (oscillation)|transient]] pulse (normally measured in [[joule]]s) is too high (i.e. significantly exceeds the manufacture's "Absolute Maximum Ratings"). The probability of catastrophic failure can be reduced by increasing the rating, or using specially selected MOVs in parallel.<ref>{{cite web|url=https://www.littelfuse.com/~/media/electronics_technical/application_notes/varistors/littelfuse_the_abcs_of_movs_application_note.pdf|title=The ABCs of MOVs|date=1999|publisher=Littelfuse, Inc.|access-date=2022-08-09|url-status=live|archive-date=2022-05-14|archive-url=https://web.archive.org/web/20220514123022/https://www.littelfuse.com/~/media/electronics_technical/application_notes/varistors/littelfuse_the_abcs_of_movs_application_note.pdf}}</ref>

Cumulative degradation occurs as more surges happen.  For historical reasons, many MOVs have been incorrectly specified allowing frequent swells to also degrade capacity.<ref>{{cite web|url=https://www.nist.gov/pml/div684/upload/Lower_not_better.pdf |title=Lower not better |website=www.nist.gov |format=PDF |date= |access-date=2021-12-11}}</ref>  In this condition the varistor is not visibly damaged and outwardly appears functional (no catastrophic failure), but it no longer offers protection.<ref>{{cite web|url=https://www.research.usf.edu/dpl/content/data/PDF/05B127.pdf|title=MOV Failure Mode Identification|date=n.d.|publisher=University of South Florida|access-date=2022-08-09|url-status=live|archive-date=2021-02-25|archive-url=https://web.archive.org/web/20210225100556/https://www.research.usf.edu/dpl/content/data/PDF/05B127.pdf}}</ref> Eventually, it proceeds into a shorted circuit condition as the energy discharges create a conductive channel through the oxides.

The main parameter affecting varistor life expectancy is its energy (Joule) rating.  Increasing the energy rating raises the number of (defined maximum size) transient pulses that it can accommodate exponentially as well as the cumulative sum of energy from clamping lesser pulses.  As these pulses occur, the "clamping voltage" it provides during each event decreases, and a varistor is typically deemed to be functionally degraded when its "clamping voltage" has changed by 10%. Manufacturer's life-expectancy charts relate [[electric current|current]], severity, and number of transients to make failure predictions based on the total energy dissipated over the life of the part.

In consumer electronics, particularly [[surge protector]]s, the MOV varistor size employed is small enough that eventually failure is expected.<ref>{{cite web|url=http://www.circuitstoday.com/metal-oxide-varistor-mov|title=Metal Oxide Varistor (MOV) – Electronic Circuits and Diagram-Electronics Projects and Design|date=23 March 2011}}</ref> Other applications, such as power transmission, use VDRs of different construction in multiple configurations engineered for long life span.<ref>{{cite web|url=https://www.gegridsolutions.com/products/brochures/arrestertranquellpolyporceint.pdf|title=GE TRANQUELLTM Surge Arresters|date=2013|publisher=GE Grid Solutions|access-date=2022-08-09|url-status=live|archive-date=2021-11-23|archive-url=https://web.archive.org/web/20211123000903/https://www.gegridsolutions.com/products/brochures/arrestertranquellpolyporceint.pdf}}</ref>