On Thursday, December 20, 2012 2:50:30 AM UTC-8, N_Cook wrote:
2x 250V , 2.2uF yellow epoxy sealed polyester? caps used as intermediary high DC storage in an electric fence unit. Failed as one of these caps is fairly consistently ohmic at 370 ohm. Now repaired 195V dc over them ,peak. Used in the wettest part of the UK , Cumbria, so assumed due to condensation getting inside the cap, no direct rain ingress traces seen. But removing the cap casing and pulling back the leads , no trace of green/corrossion. Heating , with low heat of a hot air gun , ohms drop to 350 and then back up again on cooling. Failure due to damp or HV punch through? . This is PbF solder but is RoHS a consideration inside such caps , ie dendrite formation? If punch through then does the oscillator need knocking back a bit, 2 presets on the board. Replaced both of these caps and another one used in the pulse shaping drive to the "pulse Tx" , the usual backwards mains Tx. Thinking perhaps if unit is in direct sunlight perhaps the pump circuit gives out 300V say. Schematic not seen but HV zeners in that area 270V + 16V plus another , voltage not seen but probably 16V. I was inclined to drop the value of the 270V one but I assume these are fudge values, "selected on test" for nA leakage at less than combined knee voltage of about 300V and has been working for 5 years , then presumably ok to leave as is , as these zeners DVM diode test at least as normal.

As you say, there are several factors involved here. Indeed it could be moisture ingress or it could be punch through. The punch through could be from a spike or just a weak point in the dielectric that failed under long term stress somewhat below rating. Since this is in a fence charger I would also consider lightning. I have repaired a couple of fence chargers that suffered from induced transients from nearby lightning, and in both cases the caps had failed to high leakage. One unit was an electromechanical type where the cap across the points had gone leaky, the other was similar to what you have and the storage cap was blown.

Neil S.