| General > General Technical Chat |
| MOV fire risk |
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| dietert1:
As far as i remember MOVs also suffer from aging but i can't say whether it is just time or number of surges. Aging tends to increase leakage currents and the thermal fuse is meant to disconnect the circuit when it starts to consume power, heating up to some level. Of course the thermal fuse is far to slow in case of a nearby lightning strike. MOVs also have a significant capacitance, which may help for EMI suppression. Regards, Dieter |
| nctnico:
--- Quote from: coppice on December 10, 2023, 04:34:04 pm --- --- Quote from: madires on December 10, 2023, 04:28:37 pm ---Or alternatively MOV plus thermal fuse in heat-shrink tubing. --- End quote --- Those heat shrink sleeves are remarkably good at holding the bits together when a MOV pops. Without that sharp bits can punch through a thin plastic case. --- End quote --- Agreed. I have seen MOVs being covered by heat shrink tubing in many devices. |
| coppercone2:
I seen mov utterly detonate and disappear but they never seemed to cause a fire risk. Maybe its 240v vs 120. I also have alot of them around, i think they split up surges or something like that. maybe if there was just one little mov in the house it would be worse |
| Siwastaja:
--- Quote from: shapirus on December 10, 2023, 02:43:01 pm ---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? --- End quote --- You are correct, just missing one detail. Point 4, fuses "reset" nearly indefinitely as they cool down, hence the energy you refer to in point 3 is an integral which is also assumed to reset: as you explain, if the MOV and fuse are dimensioned together correctly, if the fuse does not blow, then the MOV does not blow up either, both cool down and reset for the next cycle. Fair enough, this is correct. 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. This of course does not change the fact that a correctly dimensioned fuse is needed to protect the MOV from fire, and usually, the components surrounding the MOV also need some protection, at least a physical gap. |
| madires:
The thermal fuses commonly used together with MOVs are fuses (stay open after blown) and not thermal switches. |
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