Just read thru all the posts so far and -think- I understand your =
situation. It's pretty hard to tell, but one thing no one mentioned is =
the "area" of the switch contacts. The smaller the contact area, the =
faster it can heat up, so if there's any hot weather relationship, it's =
possible the "set" you mentioned in one post was enough to actually =
"offset" it a tad, and thus drop the sq. in/sq. cm area of the =
connection. 40A running current is enough to cause some heat buildup =
during normal operation; so, on a hot day with a start-surge, in a =
super-heated attic, the buildup could take a run-away attitude. The =
hotter the metal gets, the more impedance it presents via its own =
properties, plus that faster oxidation can form, and off it goes. The =
heating process is often a logarithmic as opposed to linear pattern, so =
what takes ten seconds at first, may only take 0.1 seconds in a minute, =
and so on, meaning the heat increases exponentially also. Is there a =
heat-rise spec on the plate? =20

Something tells me that 40A ac might be at or beyond the "running" =
(constant current) limit of the contacts and that the 60A spec is a =
timed spec: eg, 60 A for x minutes or hours vs. constant current. No, =
I'm not talking slo-blo type of stuff. If so, then 40A would be too =
much for it. This would be worth taking up with the manufacturer or =
better yet, their eng dept if you can find an "in" to them. Some =
places, Square D for instance, make it easy to talk to an engineer, =
others won't let you in a million years, but it's worth asking for =
someone with design experience, or the actual technical design specs of =
the switch. =20

Since this has happened so often, relatively speaking, it just about has =
to be a design vs. use issue. Perhaps if you posted the specs from the =
switch plate, someone could make a better estimate. =20
Any chance you can parallel a fan to cool the contacts when they =
close, and see if that extends the time it takes, or even stops the =
burning? Probably not; sounds like an awful long time between problems. =
Nah, guess it wouldn't work; too close to the fuses themselves to =
separate the air flow. =20

Anyway, them's my inejukatid thoughts. A little far out, but still =
within the realm of possibility. =20

Pop


Jeff Wisnia wrote:
We have a 60 amp 240 volt two pole GE fused disconnect switch in our
attic supplying the air handler/auxillary electric heat for one of our
heat pump HVAC systems. It is fed from a dedicated 60 amp breaker in
the home's load center.
=20
That switch has been a nuisance since the house was built about 19
years ago. Every year or so the switch contacts start heating up and
will eventually heat the end carp on one of the fuses enough to melt
the solder joining the fuse's link to the cap, shutting down things.
The overheated fuse usually falls apart when I take it out, as its
fiber tube is crisped.
=20
I take the switch apart, clean up all the discolored switch parts
with a fine file, paint some Kopper-Shield on them and put it all
back together with a new fuse. My cleaning fixups last for another
year or so and the same contact heating thing repeats.
=20
About six years ago I gave up and figured maybe I just had a "bad"
disconnect switch, so I bought an same model GE disconnect and just
swapped in the guts to avoid having to mess around changing the
housing and cable entries. The same switch contact heating problem
happened again a year later.
=20
The disconnect is in a dry area, and the switch is never thrown except
when I have to fix it, so why does this happen? The current draw with
the auxiliary heaters on is less than 40 amps, and as I'm using
regular quick blow cartridge fuses, I doubt if there's much surge
even when those heaters are cold, or the fuses would blow. After a
cleanup I've let the auxiliary heaters run for ten minutes and then
felt the disconnect switch parts (with the breaker off of course).
They feel like they're only a few degrees above ambient then.
=20
Is it just that GE fused disconnects are likely to be ****e, or am I
possibly overlooking something?
=20
Methinks I'll just pick up a non-fused disconnect and next weekend
deep six that darned fused GE disconnect I spent an hour cleaning up
this morning, when we woke up with no heat. I can't really understand
why the installers used a fused disconnect there anyway. I appreciate
the need for a disconnect in close proximity to the equipment, but
having fuses in it when it's fed from a dedicated breaker of the same
rating seems redundant. Am I right about that?
=20
Comments?
=20
Jeff

--=20
--=20
One should not be so p-h-i-l-o-p-o-L-e-m-i-c
lest they be seen as disputatious.