Editing Varistor (section)

== Hazards ==
While a MOV is designed to conduct significant power for very short durations (about 8 to 20 microseconds), such as caused by lightning strikes, it typically does not have the capacity to conduct sustained energy. Under normal utility voltage conditions, this is not a problem. However, certain types of faults on the utility power grid can result in sustained over-voltage conditions. Examples include a loss of a neutral conductor or shorted lines on the high voltage system. Application of sustained over-voltage to a MOV can cause high dissipation, potentially resulting in the MOV device catching fire. The [[National Fire Protection Association]] (NFPA) has documented many cases of catastrophic fires that have been caused by MOV devices in surge suppressors, and has issued bulletins on the issue.<ref>{{cite web|url=https://www.nfpa.org/-/media/Files/News-and-Research/Fire-statistics-and-reports/Electrical/RFDataAssessmentforElectricalSurgeProtectionDevices.ashx?la=en|title=Data Assessment for Electrical Surge Protection Devices|date=2014|publisher=Fire Protection Research Foundation|access-date=2022-08-09|url-status=live|archive-date=2021-08-18|archive-url=https://web.archive.org/web/20210818203156/nfpa.org/-/media/Files/News-and-Research/Fire-statistics-and-reports/Electrical/RFDataAssessmentforElectricalSurgeProtectionDevices.ashx?la=en}}</ref>

[[File:Varistorfail full.jpg|thumb|A 130 volt, 150 J MOV that has undergone catastrophic failure, apparently as a result of a lightning strike, showing evidence of heat and smoke. The 3 amp fast-blow fuse immediately in front of the varistor blew during the same event.]]
A series connected thermal fuse is one solution to catastrophic MOV failure.  Varistors with internal thermal protection are also available.

There are several issues to be noted regarding behavior of [[transient voltage surge suppressor]]s (TVSS) incorporating MOVs under over-voltage conditions. Depending on the level of conducted current, dissipated heat may be insufficient to cause failure, but may degrade the MOV device and reduce its life expectancy. If excessive current is conducted by a MOV, it may fail catastrophically to an open circuit condition, keeping the load connected but now without any surge protection.  A user may have no indication that the surge suppressor has failed. 

Under the right conditions of over-voltage and line impedance, it may be possible to cause the MOV to burst into flames,<ref>{{cite web|url=http://www.circuitbreakersblog.com/tag/metal-oxide-varistors/ |title=Metal Oxide Varistors &#124; Circuit Breakers Blog – Expert Safety and Usage Information |publisher=Circuit Breakers Blog |access-date=2013-01-14}}</ref> the root cause of many fires<ref>{{cite web|url=http://www.esdjournal.com/techpapr/Pharr/INVESTIGATING%20SURGE%20SUPPRESSOR%20FIRES.doc|title=Surge Suppressor Fires|publisher=ESD Journal|last=Pharr|first=Jim|access-date=2022-08-09}}</ref> which is the main reason for NFPA's concern resulting in UL1449 in 1986 and subsequent revisions in 1998 and 2009. Properly designed TVSS devices must not fail catastrophically, instead resulting in the opening of a thermal fuse or something equivalent that only disconnects MOV devices.