Spehro Pefhany fired this volley in
:

I suspect that what happens is that lighting and other transients
causes breakdown that causes a self-healing (because the short blows
away) short in the aluminized film. The value of the cap drops with
each surge until it's no longer high enough to support the motor
operation. The 5uF (?) one in my furnace draft inducer fan was down to
something like 1uF when it took out the fan motor (ouch$$$).

They've just pushed the thickness of the film down so far to save
money (and the volts/mil does not change) so the caps are more
susceptible to overvoltage spikes.


No, being an 'empirical chemist' one of my interests is in how polymers
act with time. Storage issues with pyrotechnic compositions containing
them is the matter at hand.

Most of the polymers used for the dielectric film undergo slow
degradation due to oxygen, chlorine, or sometimes even nitrogen in
contact with them.

They also can react (most slowly) to strong electrical fields across
them.

In any case, they slowly break down into compounds which are not
particularly good insulators, lowering the capacity, and eventually
allowing shorts to occur at the rated voltage.

It's not unexpected -- they make them to the BAREST minimums necessary to
manage the applied voltage, as it is.

Once, caps were 'over-rated'... no longer. Otherwise, the motor-start
capacitor guys would lose that 'replacement market'!

Lloyd