How often do you replace your RFI filter caps..

Some time ago, I made a circuit that was powered off the mains with a proper mains rated series capacitor.
A 275vac yellow block type film capacitor, with all the safety approval stamps on it. The ones used in RFI filters.
After 6 months of service, the circuit started to fail. Got me stumped.
It took me some time to realise, the 100n capacitor had slowly gone down to just 30n. There is a 220ohm 1/8 watt resistor in series with the cap that looks just fine. No big power spikes in my house.
Am I missing something?
I found some similar info on the net. Apparently these self-healing caps can destroy themselves within a year.
Leo..

Hi,
curious about this one indeed... I have no real solution to offer but I wonder about a few things....
But beforehand, I assume you know your way arouds in electronic.
How did you measure the value of those caps?  (Which instrument and were the caps still in circuit or removed or at least isolated from the circuit)
Very often these caps are somehow in series or parallel with a coil and some high value resitors.

Also you mention in your post that there is no large spikes on the house supply. How do you know for sure?
I am thinking here lightning strike on the power grid nearby your home, a neighbour with a 500Mwatts arc welder for hobby purpose!!  ;D  etc..  I found also that anything that use a compressor ( A/C unit, fridge, freezer etc...)  produce quite a high level of spikes on the house power system.
A compressor is not like other motors, say like the dishwasher or the washing machine. A compressor at start-up is under a rather heavy load and appears like a jammed motor for the first few cycles...

Please understand that my questioning is really not to quibble about your knowledge or personal IQ

In my home, I have a completely separated sub panel for the studio.
I start from the main house panel with a 240Vac 60 amps fused cutoff switch, then go to a dedicated sub panel in the studio. This sub panel then holds 4X 15amps breaker that ends in 2 duplex outlet each.
The main computer and the audio rack are on the same UPS. I use a UPS not much for the 10minutes of power they can supply during a power failure, but they provide the best protection during brown-outs and also a very efficient line filtering.
The other lines are fed trough a 'corcom' EMI/EFI filter box and to a duplex wall outlet.
Since I have wired the studio this way.... never had ANY problems related to the AC power feed!!

Oh, I almost forgot, I have a rather thick ground wire ( I think AWG 6) welded to a copper bar in the studio and to the cold water copper pipe that feeds the house water. If that is not a good ground point....

Luc

Hi Luc.
Thanks for your reply, and yes, I should know what I'm doing after being a consumer-electronics service tech for more than 40 years.
There is no lightning here in this part of New Zealand.  
And we live in a quiet street, no welders or big machines.
Radio and TV are quiet. No crackling.
Strange thing is that there was a 220ohm resistor in series with the cap, so no big currents possible.
Resistor (1/8watt) looked like new.
Investigating further, all arrows are pointing at (cheap) X2 capacitors.

http://www.vintage-radio.net/forum/showthread.php?t=59610

I removed a few caps from old supplies (RFI filters), and some caps are still perfect, while others are completely gone.

Leo..

There has been an issue with some brands of "X" rated capacitors slowly losing their capacitance!
The EMC Compliance Club covered the problem in Issue #90 "Banana Skins" section.

http://www.compliance-club.com/

A brief quote from the article:

Next I took a new ballast from our shelf
and it was found compliant. After
searching I found that the X2 capacitors
(a commonly-used type from a well known
European manufacturer) had lost
more than 90% of their value. Instead of
100nF they were down at less than 10nF!
That was the reason for being out of
compliance.
I contacted the manufacturer
immediately. Their answer was: “You are
describing here a well-known problem on
X2 capacitors (across the line) X2
capacitors are safety capacitors designed
to fulfil the IEC60384-14, UL specs, CSA
specs.... Means the cap is allowed to do
everything, but not to fail in an unsafe
way! This has to be guaranteed and will
be checked according to the requirements
of the IEC.”