zxcvbob wrote:
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.
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 cap 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.
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.
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.
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.
Is it just that GE fused disconnects are likely to be ****e, or am I
possibly overlooking something?
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?
Comments?
Jeff
I think a loose or corroded connection to the wires is getting hot, and
the brass contacts conduct the heat to the fuse cap. The fuse probably
has nothing to do with it other than be the weakest link.
Is this aluminum wire? If so, you need to make sure the disconnect is
listed for AL, and use some emory cloth on the ends of the wire, and
apply de-oxide goop.
Bob
Thanks Bob. And no,it isn't the wire connections, they are all copper
and tighter than a tick and don't show any signs of overheating. All the
discoloration and oxidation is on the moving switch arms and the
stationary contacts they rest against when the switch is closed.
My best guess now is that the leaf springs which create the contact
force may take a set with time and not press the contacts together hard
enough to prevent heating. That would be analogous to your reference to
the wire connections not being tight.
What I was really fishing for was someone with wider experience to
confirm my suspicion that those GE switched disconnects have a
proclivity for failing that way after long periods of use without
having been switched off and back on. Switching them occasionally might
even help prevent the problem by mechanically wiping over the contact
areas and scrubbing off the oxidation. I was also looking for
confirmation that a 60 amp non-fused disconnect would be an applicable
replacement since the circuit is already protected by a 60 amp breaker.
I will let the group judge Matt and his responsess for themselves, as
he's convinced me of his inability to follow and understand a technical
description of something as simple as overheated switch contacts caused
by contact resistance. I made plain that was what was happening in the
second paragraph of my OP. Further responses to Matt from me would be
useless.
As for Matt's suggestion that the fuses be replaced by copper pipe
shunts, that wouldn't do squat to eliminate the contact heating, it
would just let that heating continue and get worse until the nonmetalic
switch parts charred enough so that the switch contacts fell apart and
opened the circuit.
Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
"As long as there are final exams, there will be prayer in public
schools"
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