movs catching fire

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movs catching fire

Postby arhi » Fri Dec 21, 2012 6:22 am

A friend had an accident, his extension cord (the one with surge protection) caught fire...

new: http://imageshack.us/photo/my-images/528/sweex.jpg/
burned: http://imageshack.us/photo/my-images/90/dscn3430e.jpg/

anyhow, going further we discovered that this is a "normal" behavior for surge protectors as mov's catch fire lightly and they are the key element in the surge protector ... if you have a very short peak trough it it will work ok but a longer peak (like for e.g. you get overvoltage from 230 to 270V or 200V instead of your 110) it will catch on fire ?!?!?!? not really protecting you from anything :(


things to see:
http://youtu.be/j53qtYc5ZeE?t=3m19s
http://www.davesieg.com/?p=226
http://www.sfowler.com/investigations/S ... ectors.htm

so what's your take on this?
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Re: movs catching fire

Postby bearmos » Fri Dec 21, 2012 9:34 am

wow....hopefully not too much damage was done.

Sounds like the surge protector wasn't UL approved - they require MOV's to not fail catastrophically, or to include a series thermal fuse.

Evidently this is a common problem:
http://en.wikipedia.org/wiki/Varistor#Hazards
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Re: movs catching fire

Postby bearmos » Fri Dec 21, 2012 9:37 am

I had always assumed that the MOV conducting for a period of time would trip the breaker or blow a fuse. It seems like the current protection and MOV should be spec'ed together for this purpose? Honestly, I haven't run across this since I use universal input supplies that require a fuse and maybe some filtering at the connector.
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Re: movs catching fire

Postby arhi » Fri Dec 21, 2012 10:06 am

that's the problem, looks like mov catches fire before fuse has time to react
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Re: movs catching fire

Postby sqkybeaver » Fri Dec 21, 2012 12:54 pm

as a general rule i replace mine if there has been a surge or not every few years, I replace with ones that have a connected device warranty, and always UL and CE ratings.

like with any device manufactured in an uncontrolled setting, these are prone to penny pinching manufacturers using cheap parts.

I prefer the metal encased ones, they tend to be more robust(and more $$) but lessen the chance of fire spreading outside the surge protector.

these are one time protection devices, if you ever have to replace one for any reason cutting the cord(unplugged) as close to the body as possible will help prevent them from being used and possibly causing damage.

NEVER use one with the neutral pin removed, this is part of the protection circuit.

I have seen and repaired some pretty serious damage from faulty safety devices, a 100A mains breaker failed in a manner where it was putting 220V into a 110V branch whenever the hot water heater kicked on destroying several household apliances. fortunately only fire to happen was inside an outside breaker box.
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Re: movs catching fire

Postby Sleepwalker3 » Fri Dec 21, 2012 9:03 pm

Yeah, I've seen Mov's go up, but certainly the proper UL, CSA, etc. approved ones (i.e. name brand stuff like Siemens, etc.) will withstand more and should be self extinguishing material. Kapton is good here and I usually wrap a layer over the MOV's, if for no other reason than I've had fair size one go 'bang' on 415V supplies and copping the bits in the face certainly isn't something you want if you're working on it at the time!

Looking at that picture of the MOV (I'm assuming it is a MOV and not a Ceramic suppresion cap?), it looks like they are only protecting the downstream side. Looks to be a MOV on each line (A & N) to Earth, an X2 Cap, then a small fuse leading to a 3rd MOV on the downstream side. MOV's are designed for very short duration pulses (Spikes) and can handle huge fault currents for that short duration. If it's a longer duration Surge or a very intense Spike, the MOV will eventually fail and likely fail in a short-circuit condition. Under those circumstances a local fuse is needed to blow (or breaker trip) and stop that fault current.

From those pictures it appears like there is no local fuse (unless the switch is also a breaker), so the only thing that can blow (or trip) is the Fuse or Circuit breaker back at your switchboard and that might be a 20A or more breaker, perhaps with a C characteristic (relatively slow). Chances are the relatively light duty lead on the breaker would be light enough to make it hard for the breaker back at the board to trip because of the leads resistance. The lead will heat up, the MOV will still be trying to clamp (or may have gone short by now) and so it will keep heating and ... 1 x BBQ powerboard.

Sometimes the switch is also a breaker, but if it is it will be a cheap Thermal breaker and will have a very slow trip characteristic, typically 2 x Rated Current for 50 seconds or so, 4 x for about 8 seconds, 6 x for about 4 seconds and 10 x for nearly 2 seconds. If it was a breaker and was rated at say 10A, that could be passing 40A for 8 seconds or even 100A for 2 seconds which is plenty enough to turn the powerboard into an electric BBQ! Chances are the circuit board traces went up before the MOV and it may well have been the resulting fire that did most of the damage, not damage to the MOV may have been secondary to the PCB traces vaporising.

Actually I've collected a few fairly spectacular failures over the years, maybe we could start a new thread of spectacular failures. :)
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Re: movs catching fire

Postby sqkybeaver » Fri Dec 21, 2012 9:58 pm

Sleepwalker3 wrote:Actually I've collected a few fairly spectacular failures over the years, maybe we could start a new thread of spectacular failures. :)


sounds like a good idea, I'd have to see if the failed breaker is still around so i can take pics.
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Re: movs catching fire

Postby arhi » Fri Dec 21, 2012 10:07 pm

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Re: movs catching fire

Postby matseng » Fri Dec 21, 2012 11:26 pm

This reminds me a little bit of a power strip that I heard a rather loud "pop" from a while back.

Each outlet in the strip have a switch and a red LED connected to it. It seems like the resistors are severely underpowered to drop the led current down to a few mA in a 230 volt circuit. It seems like they don't even have reverse protection diode so the LED will have it's polarity reversed with a voltage far beyond the safe limits every cycle .

Terminator-fire.jpg
Destroyed by fire


Terminator-burn.jpg
Some of the resistors are only burnt - The soldering of the ground wire seems sucky as well.


Terminator-top.jpg
Yes - this power strip might Terminate your life, with its high quality engineering
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Re: movs catching fire

Postby sqkybeaver » Sat Dec 22, 2012 12:29 am

matseng wrote:Each outlet in the strip have a switch and a red LED connected to it. It seems like the resistors are severely underpowered to drop the led current down to a few mA in a 230 volt circuit. It seems like they don't even have reverse protection diode so the LED will have it's polarity reversed with a voltage far beyond the safe limits every cycle .


those look like they were 1/4 watt resistors, don't think i'd trust anything under 1 watt for a a led at that voltage.
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Re: movs catching fire

Postby Alex555 » Sat Dec 22, 2012 9:14 pm

Correct me if I'm wrong, but for a 230 volt circuit at say, 5 ma, would reguire 230*.005=1.15 watts? What were the resistor values?
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Re: movs catching fire

Postby sqkybeaver » Sun Dec 23, 2012 12:41 am

5mW is really high for an indicator, I would expect to see 200-500uA
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Re: movs catching fire

Postby matseng » Sun Dec 23, 2012 1:03 am

They are 150K resistors. 1.5mA = 350mW @ 230 volts.

They look smaller than regular 1/4w resistors, maybe 1/6w? But they are underdimensioned even if they are 1/4 w rated...
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Re: movs catching fire

Postby Sleepwalker3 » Sun Dec 23, 2012 9:27 am

[quote="arhi"]there is a small fuse inside with the mov, dunno if it broke or not but I don't see how a mov would catch on fire before fuse pops out :(

That's what I was saying Arhi, I think there are two MOVs on the *Upstream* side of the fuse and so the first two MOVs (which may potentially me Y1 Filter caps) are connected via the Switch (which may also be a thermal breaker) to the incoming Active wire.

The fuse looks like it comes after all that stuff and will only blow if the MOV on the left side (*Downstream* of the fuse) also clamps hard. Put another way, looks like the first two MOVs (or caps) which go to Earth are connected directly to Line in, only the MOV on the left is downstream of the fuse.

Pretty crappy design I'd suggest.
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Re: movs catching fire

Postby sqkybeaver » Sun Dec 23, 2012 9:35 am

perhaps some nasty letters/email to manufacturer are in order.
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