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
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.
design vs. use issue. Perhaps if you posted the specs from the switch plate, someone
could make a better estimate.
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.
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 pissant 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>
OK, I got your question in time to pull the remains of the fuses out of
the workshop trash can.
The fuse which opened this time, because the solder holding the fuse
link to the end cap next to the switch contacts melted, has GE's name on
it and is marked 60 AMP, One-Time Fuse, 250 VAC or less, 10,000 Amps
Internal Rating, Class H.
The other fuse in the disconnect, which fell apart when I pulled it out
because the fibre had toasted but the fuse link was still intact and
soldered to both caps, Was marked "Ace Hardware", Type NON, One-Time
Fuse, 250 Volts or less, 50,000 Amps internal rating, Class K5.
Gee, maybe now that Matt guy will come back in to tell me the problems
I've had with this disconnect were likely caused either by using an Ace
Hardware fuse in a GE fused disconnect or the fact that the pair of
fuses was "unbalanced" by 40,000 Amps. <G>
I am always suprised to see just how skinny fuse links actually are in
relation to the gage of the wires in the circuit they are protecting. I
just miked the link from one of those charred fuses and its only .008"
thick by .065" wide at the narrow spots. Obviously it has to be that
size to perform its function, but that small a cross section sure
doesn't "look" like it'd carry 60 amps continuously.
Sorry to waste so much bandwidth on this thread guys, it's my curious
mind always wanting to find the "real reason" why things happen.
in commercial disconnects of time gone past,
it wasn't odd to see 'fuse clip clamps' adding pressure to the clip the fuse
snaps into as they would get weak with age and heat up, usually just melting
clip clamps look like little springy packman jaws with a knob that tightens
them adding additional spring pressure. imagine there's still some out
there doing their job.
Yep, I remember those.
But, I'm darn cerain I've got this one diagnosed corectly, as the other
ends of the moving switch arms, the ones which mate with the contacts at
the input terminals, also oxidize and show heat discoloration. And
before somebody asks me again, the input wire terminal screws ARE tight,
the wires in them are copper, not aluminum, and there's no signs of
heating up there.
Also, the output ends of the fuses have never had any problems like
that, the clips on those ends are still in their original (tin plated?)
condition, with no signs of overheating there.
I also remember cartridge fuses with replaceable links, even in smaller
sizes like 20 amps. I don't know if they are still made now that we've
become used to living in a "throw away" economy.
Being a depression era baby, I really miss frugal stuff like that.
This is Turtle.
Yes the fuse link type fuses are still made but nobody buys them anymore. The
links cost $.80 and the replacement fuse cost $1.09. It's not really worth it.
Yes we are in the Throw away Economy Generation !
This is Turtle
Jeff , These fuse disconnects are stated as rated at 60 amps but if you read the
amp draw rating in the data plate with it. You will see it is suppose to be used
on amp draws of less than 40 amps.
It states that it will handle short time useage of near 60 amps but when put in
a contenous amp draw the 40 amps your drawing on it become the max it will
handle. I have the Connecuit Electric type I use on condenser outside and they
are rated at 60 amps but if you read closely you will see it says contenous use
40 amps is the MAX. it will handle. You just don't run anything at Max. By
running this disconnect at 40 amps is like it says on the Lexus LS-400 Maxium
operating speed is 161 M.P.H. and then you can run it at that speed and be safe.
So if you don't run your car at Max. Speed 160 M.P.H , Don't run your 60 Amp
disconnect Rated at Max. Amps 40 Amps.
I have a rule I use and that is I will rate the load at 1/2 the rated load on
the disconnect like it rated at 60 amps on the cover. I will not put more that
30 amps on it contenously.
And yes my Lexus LS-400 will go over 161 M.P.H. at 165 M.P.H. and is the model
before the Kill switches put on them. And yes I drive the speed limit most of
the time and also drive my disconnects below the amp limit.
Well now, I'll check the specs and if that's the case it sure makes
sense now! And too bad the original installer didn't see it that way
too, it would have saved me a lot of nuisance fixups.
Thanks, and I hope the 60 amp Aquare-D non-fused plug type disconnect
switch I just bough yesterday isnn's similarly rated. <G>
This is Turtle.
No that Square - D breaker looking disconnect is rated above the 40 amps but
usely in about 2 to 4 years the breaker will burn inside the breaker and go out
and then you can replace it with a real breaker square- D and keep going. A lot
of HVAC people don't like them for that call back coming in 2 to 4 years. Now if
you replace it now with a real breaker to apply to the application you will not
see it down the road.
Do they make a 100A disconnect that just has a pull-out block with 2
cartridge fuses in it? That's what I think I would use if they are
available that big. Put in whatever size fuses you actually need to
protect the heater, (45A?) or just put 100A fuses in it and let the
breaker back in the main panel provide the protection.
This is Turtle.
Yes they do but your looking at about $68.00 + Tax . It will come with a lever
on the out side and spaded type fuses and the fuses are about $8.00 each even
being 60 amps to 100 amps sizes. The 60 amp disconnect is the breaking point
between commercial and residentiual equipment and the price reflects it. Now
this type fuse box / fused type disconnect will run at 90 amps till the cows
come home but the 60 amp cheap o will run till the sun goes down. .
The standard 60 amp fuse / disconnect $15.00 verses the 100 amp fuse box / fused
disconnect at $78.44 is a far cry between them in quality and in price. At about
5 times the cost the public will not go for 5 times the price when Home Depot
sells them for $15.00 and the home owner can replace them pretty easy.
No, the one I bought is the "pull-out" kind. I looked at the one with
the "breaker looking" switch in it and decided not to take the risk of
its switch contacts fouling up by themselves. The plug-in contacts look
pretty robust and feel tighter than a virgin's cooze. I'll swap it in
and report back in a year or two. <G>
I'm still scratching my head in amazement that a closed switch with what
appears to be adequate ratings located in a benign environment will
slowly develop increased contact resistance, but I've seen it and
believe it, so it must be so.
Again, my apologies for wasting all the bandwith on this simple little
problem. It's my engineer's mind dammit, I have an overwhelming desire
to learn why things like that happen, I guess it's one of my my
retirement hobbies, thoroughly non-cost effective, but fun, like many
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