Wima MP3-X2 failed in AC line filter

[japlytic]

Over a week ago, there was a strange smell in the house when the heater was running (the line filter was connected to the TV and other equipment, along with the heater - it was not overloaded). Yesterday, the 10A fuse in the line filter blew and upon opening it, a Wima MP3-X2 0.047uF capacitor was leaking, and tests shown it had electrical leakage.
This capacitor was apparently under stress, causing it to heat up and eventually started to short when it overheated.

[PeteS in CA]

Re: Wima MP3-X2 failed in AC line filter

The failure mechanism probably was something like this. Whether due to weak spots or voltage surges, arcing occurred at multiuple locations over time. While metallized caps "self-heal", there is residual carbon at the location of the arc, a high resistance current path. Each of these arc locations generates a certain amount of heat, and as the number of arc locations increases over time, the generated heat increases, eventually leading to overheating a dramatic failure. This has been a known failure mode with X caps for some 20 years that I'm aware of.

Was this a metallized film or metallized paper cap? BTW, Wima makes good film and paper caps, so this wasn't the film/paper equivalent of crap caps.

[starfury1]

Re: Wima MP3-X2 failed in AC line filter

Thought id really leave this to somebody who knows

I wouldn't think a current overload would take out one of these caps anyway
but I could be wrong

Given the weather conditions we have been having lately and your location Japlytic
well could be a lot more spikes etc on the mains...

Just out of curiosity what type of heater is it ?
one that cycles on and off or the old wire wound power resistor

wouldn't really have a clue here
But just wondering if maybe some switching action maybe generating spikes

Anyway here a link to a PDF on that cap type mp 3 x2 pdf

HTH Cheers

[gonzo0815]

Re: Wima MP3-X2 failed in AC line filter

Those Wima caps are usually really robust, so i think normal electrical stress should not cause any harm to them, at least not beyond the self healing capability.

I think PeteS has already described the failure mode for this kind of cap more than correct.
I have seen this problems in many TV Focus boards too, where the FKP1 6000v caps are used, two in series to get a healthy 12kv device.
But unfortunately, the cold soler joints, the resulting heat and smear will kill them too.

[japlytic]

Re: Wima MP3-X2 failed in AC line filter

Here is a picture of the failed cap.
Note the split on the top, which was caused by pressure building up when the capacitor started heating up, causing the impregnation material to melt and subsequently leak through this split.
This capacitor reminds me of the AEE Miniprint capacitors, which were once used in the speed control of Mistral Gyro-Aire fans, before "X" and "Y" classifications were established (http://qdg.sorbs.net/qdgnews.htm).