[UK-CONTEST] OT - Degraded mains filter capacitors

[Peter Hobbs]

The usual excuse - here resideth the fount of all knowledge!

Following the unreliability and eventual failure of a CH/HW controller, the fault turned out to be a 0.68mf 275VAC "suppression" capacitor, here used as a series mains voltage dropper to power the 48V switching relays.  When measured, it was down to 0.27mf after some 15 years service.
Now, there are several similar capacitors in the station mains filter boxes (built nearly 5 years ago now) and it never occurred to me that these might degrade progressively in this way.  Does anyone know if this is typical behaviour, which would lead me to check out my filters every so often (not a 5 minute job)?  These are not of course electrolytics, but metal film types, so I also wonder what the failure mechanism might be.

Any advice off line please.

Thanks
Peter G3LET