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MOV fire risk

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I would like to ask what is general consensus, about MOVS in products and fire risk from them.

I stumbled upon this post there MOV exploded even without triggering fuse, and fuses are quite frequently used with MOVS in surge protection strips, like for example here .
However, it seems MOVs now appear in regular products and even without a thermal fuse. example IKEA here (but not in older generation here ), Samsung 65W here , even cheap Chinese smart plugs have them. However, Apple teardowns I have seen, seems to still omit them.

So, that's the current trends regarding use of MOVs in such devices, is it OK from the fire perspective?

Of course Apple sees easy money there, they will just argue it was a mains surge...
Please buy a new expensive thing from us!


See GE original MOV handbook.

Limited life and fire risk are known.

In 1983 we had a 20 mm GE MOV on a wall plug transient protectopr.

About 20m away a 12 KW cinema lamp had a fault and very high transients were on the 240B mains.

My engineer in the adjacent lab shouts:


The MOV exploded and flamed dqwn towars the floor.

The carpet caught fire.

Since that day I havwe never used a MOV.


Stray Electron:
   I found a heap of very nice and well-made COMMERCIAL quality surge suppressors in some surplus a while back.  They were mounted in steel boxes with a lot more room than necessary and the boxes were then filled with dry sand.  Presumably to absorb any blast from exploding MOVS. I just grabbed one out of the pile and I don't see a PN on any of the MOVs but they're in groups of 5 each and there are 7 groups of them and each MOV is larger in o.d. than an American quarter. There are five large wires (probably #8 gauge) and three small wires (for monitoring) coming out of each one and they appear to have been made for use on a 3 phase Y-connected circuit. 

  MOV tend to fail shorted so they all need to be fused IMO and packing them with sand and in a sturdy flame proof enclosure is a GOOD idea. These a bit bit weird in they way that they're fused, each incoming power lead goes through a 1 Ohm 1% resister (probably a fusible resistor) and then has a small 22? gauge bare wire that connects to a tent shaped insulator with 9 other small wires attached and each of the other wires connects to a group of MOVs. So it looks like the 1 Ohm resistors and the small bare wires are intended to serve as fuses.   I picked up about 60 of these of various sizes and we tested a bunch of them and they seems to be designed to operate on 120, 240, 277 and 480 volt circuits and both Delta and Y configurations.


--- Quote from: Stray Electron on December 10, 2023, 02:33:31 pm ---MOV tend to fail shorted so they all need to be fused IMO

--- End quote ---
Shorting the circuit is their actual purpose, is it not?

1) the MOV begins to conduct once voltage across its terminals exceeds a certain value;
2) current flowing through the fuse installed before the MOV increases rapidly and blows the fuse;
3) the MOV is rated for a specific amount of energy that it can absorb before blowing up (which determines its size -- or, rather, its size determines the energy);
4) if the fuse blows before that energy is exceeded, then the circuit designer used proper values and every part has done its job properly. Otherwise, it's bad design.

Am I wrong?


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