I've never seen a fixture with a large electrolytic across the power
line. Are you from the UK since you use the word "mains" for the
power line? Any capacitor is probably just to filter out
high-frequency noise from the lamp arc, 50 or 60 Hz and many, many
harmonics. The capacitor is probably breaking down when exposed to
the full line voltage, but not under the low voltage from the ohmmeter
section of the multimeter. Almost any capacitor rated at twice the
line voltage will reduce the "noise going back onto the power line.
It is most evident when listening to AM radio. If you can use the
radios in your house with the light on, without the capacitor, then it
probably is ok to continue to use it without interfering with your
neighbors radio reception. But if you are in an apartment house, you
may cause interference to your neighbors radio due to the ac power
lines radiating the noise.


The capacitor has nothing to do with filtering noise, it's to raise the
power factor. Your diagnosis that it's breaking down under the high voltage
is likely correct though.