Baked oven?
May 28, 2011 4:15 AM   Subscribe

I have an American Scientific TempCon oven I was hoping to use to bake enamel painted parts...the low temp (200F) it operates at is perfect for that. However, after a minute of use it trips my GFCI outlet. What should I look at?

I didn't pay a lot for this, and am generally not willing to spend a ton to fix it. The oven draws about 7 amps, and I think what might be happening is when the oven reaches 200 degrees (after roughly a minute), a thermostat tells the (typical-household-electric-stove-style-heating) element to quit, and that is tripping the GFCI.

I guess this could be a fault of the component (perhaps the coil when heated leaks too much juice, or the relay which acts on the thermostat's signal or whatever is bad), or the sudden change from drawing 7 amps to none alarms the GFCI...or?

So which of these (if any) are likely?

1) This unit was never designed to be plugged into a GFCI, and the way it draws current is always going to trip a GFCI. Find another outlet.

2) There is something wrong with component X, which I, as a fairly handy amateur, can diagnose with a multi-meter and replace for roughly $100 or less.

3) This thing is toast and/or is unlikely to be repairable for the amount I want to spend.
posted by maxwelton to Home & Garden (9 answers total)
Also likely is 4) the gfci receptacle is broken.

They work by measuring the inflow and the outflow of current in the hot and neutral wires. If they aren't the same, that means current is flowing out of the device and into the ground line, or through a person.

If it is the oven that is broken, I would bet that it is the thermostat. That it arcs when it breaks the contact and that's enough to trip the gfci. Either, replace the thermostat, or plug into a non gfci outlet.

if the case is metal, you might be able to hook a multimeter to the case and to ground and see if there is a voltage "pop" when the thermostat breaks.
posted by gjc at 4:35 AM on May 28, 2011

A few apartments back, I was having trouble with one of the outlets in my kitchen. Every time I'd run the microwave, it would trip the GFCI. Nothing else was plugged into it and it was just a microwave (a working microwave, I should add), so I couldn't figure out what was wrong. We had our super look into it, and it turned out that all of the kitchen appliances and a few other electronic things were all plugged into a series of outlets that were all somehow connected, and that the microwave was just one thing too many to run on that series.

He did some behind-the-scenes work to reroute the outlet that the fridge draws its power from to the dining room, and we never had the problem again.

I don't know what you'd have to do to test for something like that, but it's another option.
posted by phunniemee at 7:01 AM on May 28, 2011

As phunniemee points out in story form, GFCIs will trip from overcurrents as well as ground faults.

I'd guess that is what's happening, and that it takes a minute to occur because, in contrast to copper, nichrome wire drops in resistance as it heats up.
posted by jamjam at 11:40 AM on May 28, 2011

Response by poster: I have one of those devices you plug appliances into and then plug into the wall...I think they're sold so you can see just how much juice any particular appliance is using. Anyway, it shows a remarkably steady 6.5 amps until the GFCI trips, no sudden spikes or dips. Mind, the GFCI would hopefully be faster than anything I can read, but, still.

The GFCI is brand new and this device is the only thing plugged in on a 20-amp circuit.
posted by maxwelton at 11:54 AM on May 28, 2011

It's either a bad outlet or there really is a ground fault. GFCIs will not (reliably) trip on an arc fault. That's why they make AFCIs. Unless your 'GFCI' is really a combo arc-fault and ground-fault interrupter.

Something in the thermostat wiring is probably shorted to the chassis and there's a slight leakage current through whatever the oven is sitting on when the thermostat stops calling for heat. I would presume the thermostat activates a relay somewhere, given the 6.5 amp draw.

It could also be something as simple as a disconnected chassis ground, if the oven has a three prong plug. The chassis should be connected to the ground pin, if I'm remembering my safety ground theory correctly.
posted by wierdo at 2:34 PM on May 28, 2011

wierdo: yes, if it is a metal case, it must be grounded so that if a bare wire touches something on the inside, the power will go down the drain instead of through my arm.

Or it will be double insulated, but I think those things have no ground wire at all.

Back to the problem. If you run the thing with the door open, so that it heats up slower, will it take longer to pop the gfci? Or does it still happen in the same amount of time?

If wikipedia is to be believed, a gfci doesn't actually monitor the ground line. It measures the difference between hot and neutral for a difference in current. (6.5 amps in will be 6.5 amps out, if the thing is working correctly). If there is a difference, it pops.

Is it possible that oven uses some kind of induction heater? I couldn't find any schematics on teh webz.
posted by gjc at 2:53 PM on May 28, 2011

Well, that pretty much puts my answer out of its misery, maxwelton.

I too tried to find something online about this model oven, and I wonder if I might not have stumbled across the very oven you have:

S/N 0382-156. Unit is in good physical condition. Powers but immediately blows breaker.

If so, and you are not the seller behind the linked page, I'd say it's definitely defective.
posted by jamjam at 3:30 PM on May 28, 2011

Response by poster: jamjam, that's not my particular oven, but it is very similar. I'd guess you're right, it's defective.

(I took an ohm meter to the plug, and the results aren't encouraging. One side of the powered prongs (I think the neutral, not that it matters, really) is nearly open to ground. It should be completely isolated, unless I'm really behind on my "keep the smoke in the wires" knowledge. Though why the GFCI doesn't pop immediately given that is interesting.)

At this point I'll probably landfill it, though if I'm going to do that I'll probably take a peek at the guts just to see if there is something obvious wrong in there, like a broken or worn wire.

Thanks all for your help!
posted by maxwelton at 10:30 PM on May 28, 2011

Response by poster: I know many tens of thousands of you are watching this question hoping I found a solution...well, I took the oven apart, and the element is definitely bad--the internal insulation has failed. Why this thing doesn't instantly trip the outlet is, of course, the trick would be to see if parts are available.
posted by maxwelton at 8:15 PM on June 4, 2011

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