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MOV fire risk
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Kleinstein:
MOVs get increasing leakage when getting old. So they may slowly run hotter and hotter and thus can overheat without blowing a normal mains fuse. The MOV is usually not flamable (after all it is metal oxide), but the hot PCB and neighboring parts may catch fire.

There are MOVs that aready come with a thermal fuse, e.g. Little fuse TMOV series.
SeanB:
I have had one or three Zinc oxide MOV units, NOS GE ones, in the red dipped package, die short circuit while in storage, but in general with 230VAC mains they will all ineviatably fail as short, so best is to make sure they are fused correctly. But as all of those I have used failed safely, small one blowing apart, bigger ones drawing enough current to trip a breaker rather rapidly, often with no apparent damage to the MOV, other than it having a low varying DC resistance, in the order of 10's of ohms, with 1V applied from a multimeter, as opposed to being well over 10M normally. Seen also units with fuses in the path to the MOV units, either as PC board traces, or a proper ceramic fuse soldered into the board, or for the one, with a 20A ATO automotive fuse soldered into a slot in the board. Last one made by Clearline, guess they were happy using a 20A 32VDC rated fuse well past the warranted ratings.

I have plenty in use, often simply mounted in a connection block to an external LED light, to keep the surges down, to keep the lamp life as long as possible, even if the lights are rated for 3kV surge, extra always helps. they still all fail after a while, some soon, some after a year or two, and a few that still work after 4 years.
NiHaoMike:

--- Quote from: Siwastaja on December 11, 2023, 07:28:07 am ---But MOVs have additional energy limitation, which is non-resetting over the lifetime. Every clamped peak, even benign ones which are not even close to exceeding said repetitive limit (and thus not even close to blowing the fuse), wears out the MOV, until it one day fails short from such small event.

--- End quote ---
It helps to have a large film capacitor, on the order of 5-10uF, in parallel to shunt the small switching transients. That, of course, can only be applied to the L-N or L-L MOVs, using one to ground would cause too much leakage current.
CosteC:
Nobody mentioned that while in EU nominal voltage is 230 VAC, the realistic voltages in in many places of network reach 250 V due to photovoltaic and other prosumers trying to sell energy. If MOV is rated for 250 VAC it will leak a lot and age quickly. I am not sure how this phenomenon widespread in USA with its 120 VAC system.

Other aspect is common issue - fuses rated "as high as possible" not "sensible". Lack of inrush current protection forces high fuse ratings, this does not work well when MOV will activate or there is failure of some primary side semiconductor. As usual cheap designs done quickly are not good designs and may caught fire.
T3sl4co1l:

--- Quote from: CosteC on December 19, 2023, 02:28:23 pm ---Nobody mentioned that while in EU nominal voltage is 230 VAC, the realistic voltages in in many places of network reach 250 V due to photovoltaic and other prosumers trying to sell energy. If MOV is rated for 250 VAC it will leak a lot and age quickly. I am not sure how this phenomenon widespread in USA with its 120 VAC system.

--- End quote ---

Very rarely, it can be much worse: since the supply is a 240VCT transformer, if the neutral comes loose for some reason (say, improper, faulty, or damaged wiring), more load on one side than the other will shift the neutral voltage, browning out one side and overvolting the other.  UL 1449 for example covers this.

Tim
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