Dave Platt wrote on 8/24/2017 2:32 PM:
In article , rickman wrote:

I thought the X and Y capacitors were used when a short would be a safety
problem as in a shock hazard. Why would there be a shock hazard if a cap
across the power line shorts? If you use a fuse any concern about a fire is
eliminated. Then why would you need the X cap?

You can't count on a fuse to eliminate all of the risk of a fire.

Some types of cap can fail with a "near short circuit" - they get
leaky enough to start drawing a good fraction of an ampere, but aren't
a dead-short. Imagine what happens if such a cap is "protected" by a
1-ampere fuse, but is drawing 100 mA at 120 volts... that's more than
10 watts, heating up the capacitor. If the cap doesn't either short
itself well enough to blow the fuse, or go "open", it can definitely
heat up enough to smoke and burn.

I've seen this happen... a non-X/Y-rated film cap was used "across the
line", and it overheated and nearly started a fire.

"X" and "Y" caps are intended to be at least somewhat
self-healing... if they develop a pinhole and start to short, the
localized heating burns away the metallized film in the area of the
short, and it opens. If I recall correctly they're also required to
use an insulating resin which is at least somewhat flame-resistant.

If you're going to the trouble of replacing an across-the-line cap
in some equipment you're refurbishing, I'd suggest going right to a
suitable "Y" cap. The additional cost is modest and the labor to
install is the same.

Ok, so there is a failure mode where the cap won't draw enough current to
blow the fuse, but can locally heat up enough to smoke and burn. Isn't that
true for other parts in the device?

What is the purpose of this cap anyway? I assume noise filtering. Wouldn't
that be just as effective on the secondary of the transformer?

--

Rick C

Viewed the eclipse at Wintercrest Farms,
on the centerline of totality since 1998