Self repairing X2 Caps failing

[cosmos]

I have seen several examples lately where X2 caps are used as voltage reduction element  in mains connected low cost gear ( ventilation thermostat and so on)
These caps have failed with ~80% reduced value. This looks to me as self healing. Since these are 300VAC  parts it might be an interesting topic to look at how they can still break while being in a 240V circuit. I suspect the mechanism behind the self healing might not be so widely known since I have come across the same failure types also in industrial designs. just my 2 cents...

[Psi]

Self healing just means that any arc over inside will blow away the surrounding material thereby preventing the dead short failure mode.
Every time this happens the capacity is reduced because there is less surface area left. (think of a sheet of alu foil with many holes blasted through it)

So a 80% reduction in value just means the cap has had a lot of arc overs and self healed many times.
It's just a sign that the power in your area is dodgy and full of spikes


Of course there is always the likelihood of fake/counterfeit X2 rated caps.  Its not like the capacitor industry is fake free

[bktemp]

WIMA has some info about self healing:
http://www.wima.com/EN/selfhealing.htm

I have seen many X2 Capacitors with 10% of their rated capacity left. Almost all of them had quite small mechanical dimensions compared to other ones with the same ratings.
It looks like the manufactorer used thinner foils and the caps are not designed to tolerate overvoltage transients or have fake voltage rating.

[T3sl4co1l]

It's interesting that X2 caps are rated for quite high overload voltages.  They don't test them at that voltage for much time, however!  The smaller ones in particular are well known to, basically just disintegrate as the self-healing cuts off big hunks of otherwise-good capacitive area.

Tim

[Rufus]

I have seen several examples lately where X2 caps are used as voltage reduction element  in mains connected low cost gear ( ventilation thermostat and so on)
These caps have failed with ~80% reduced value. This looks to me as self healing.

So can you dismantle some of these examples and look at the film? 80% loss of plate area should be pretty obvious.

[SeanB]

The self healing is from spikes on the mains. The simplest way to reduce this is to place a MOV in the incoming mains, along with some inductance before the MOV, which will reduce the voltage spikes considerably, though there is a possibility that if you choose the values incorrectly you will make a series LC circuit, which the MOV will have to clamp.

Motor run caps I have taken apart ( much easier as they are oil filled and taped after winding, not heat fused into a solid block) you find massive areas of foil with only a tracery of aluminium left, with blackened foil as well from the self healing action. more modern ones often though have spray ends so the foils are heat fused making unwinding difficult unless you use a saw to cut the ends off.

[Chipguy]

I like that suggestion.
Maybe it is possible to open an X2 cap and show the fuse elements. I have only seen them as computer graphics, advertisement from Epcos or so.