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MOV fire risk

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miegapele:
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?

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

jonpaul:
Bonjour,

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:

"JON!! THERE IS FIRE!"

The MOV exploded and flamed dqwn towars the floor.

The carpet caught fire.

Since that day I havwe never used a MOV.

Jon

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.

shapirus:

--- 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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