Pop wrote:

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?

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.

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.
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.

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

Pop

All good stuff thanks!

The only offsetting thing I can say is that the auxillary heaters
typically only get switched on when it gets too cold outside for the
heat pump to take care of the heating requirements, and that's the time
of year when the attic is cold, not hot. The rest of the year the only
load is just the fan motor, a ****ant 2.5 amps. So, I doubt that high
ambient temperature is the troublemaker.

Rather than turn this "one off" situation into a major research project,
I think I'll go with replacing the GE fused disconnect with an unfused
disconnect and see what happens in a year. G

Thanks again.

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
schools"