by ssl-3 | 2026-03-14 08:43:56+00:00
> Not only should you get rid of them, but also they are a fire hazard.

Are they not a fire hazard even when new? MOVs do tend to degrade with use (especially after they've gone conductive to snuff one or more surges). But AFAICT we can't really know, without potentially-destructive testing, whether a given MOV is in good shape -- whether installed last week, last year, or 30 years ago.

> Also, do not accidentally plug surge protectors into each other, metal oxide varistors can star fires _without_ meaningful surge conditions when you do so.

What is the mechanism that increases risk for MOV-sourced fires in this arrangement?

I've also noticed that many of the power supplies I've taken apart (for very pedestrian consumer goods) have internal MOVs on their line input. Whatever the mechanism is that increases risk, isn't using one external surge protector already doing that in these instances?

> I prefer to buy products without MOVs entirely due to the risk, with the exception of one, Tripp Lite Isobars; but I prefer to use series mode protectors such as Brickwall or SurgeX.

I prefer to avoid MOVs, too. Broadly-speaking, diodes seem like a better way to do it. (Transtector is another reputable brand that uses diodes.)

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That all said, I've noticed over the years that problems with dead (presumed-to-be-hit-by-a-power-surge) electronics tend to follow particular structures. And the reason for this seems related to grounding more than it is anything else.

So when I find someone (a friend, a client, maybe someone online that I'm trying to help) complaining about repeated damage, I often ask about grounding. Almost always, it turns out that they've got multiple grounding points for the electronics: The electric service has one ground rod, and the telephone/cable feet/satellite/whatever is connected to some other ground.

This might be a dedicated rod, maybe a metal pipe; whatever it is, it is distinct from the main service ground. It happens all the time. (It is worth noting that the NEC prohibits this kind of configuration unless extraordinary effort is put forth. See 800.100(d), for example.)

The way that MOVs -- and avalanche diodes alike -- behave combines with the fact that the earth is an imperfect conductor, such that having multiple ground points promotes dynamic ground loops that can provide quite large potential -through- the electronics that we seek to protect.

The problem appears suddenly, and repetitiously. Everything is fine, and then ZANG: The cable modem gets smoked along with the router it is connected to. So the modem goes back to Spectrum or wherever to get swapped, and the router gets replaced again, until the next time: ZANG.

TV connected to satellite receiver, with coax incorrectly grounded? ZANG. Over and over again.

I'd see it all the time when I was a kid back in the BBS days: The phone line was grounded improperly, and computer was the only thing that connected to both electricity and the telephone line. Some folks would go through several modems over the course of a summer, which was very expensive -- while most people had no problems at all. Next-door neighbors would have completely different failure rates.

Structures with correct grounding tend to do very well at avoiding these issues, and I've fixed these conditions in subsequent years more times than I can count.

(A coworker installed a phone system at a business once, wherein he made extensive use of Ditek surge suppressors -- on the incoming POTS lines, and on the power inputs. It blew up one day. So he called Ditek to try to get at least the cost of the phone system hardware covered. They asked him to draw up a map of how the building was grounded and send that over, so that's exactly what he did. When they saw his map, they very quickly identified a ground loop and denied the claim.)