baseboard heater trips breaker - Bad stat?

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The baseboard heat in one of the bedrooms trips the breaker as soon as I turn the thermostat on. We didn't know this until now because it didn't get cold enough to need it until now.
But wait, there's more. The house (we just moved) had fuses until the panel was replaced with a modern looking circuit breaker panel. We tested that heater on inspection but that was while it was still on fuses. I never thought to try it with the new panel.
The electrician says most likely bad thermostat, he'll come back and look when he can schedule it, may be a while.
If so, I'd like to know what in a thermostat goes bad like that. I'm not sure how they're wired inside.
I put a meter on the thermostat. The stat has four terminals, two marked line and two marked load. With breaker off and stat off, I read 28 ohms across load. That sounds reasonable, it should pull 8.6 amps if my math is correct, and the 20 A breaker should be fine. Breaker off and stat on, 26.7 ohms appears across both load terminals and line terminals, that sounds okay to me too, .8 and .7 ohms from line to load across the contacts.
With the power on, I read 242 VAC across line, 0 across load, 120 from each line terminal to load terminal, 120 from each line terminal to ground. I didn't expect that. I thought a thermostat this old would be single pole, and so one load terminal would read zero. That's with the thermostat turned off, if I turn it on it trips the breaker immediately. Can this be a double pole stat? or am I just misunderstanding the readings?
Curious symptom: with the breaker off, I still read 1.12 VAC across line, 2.66 from line to one load terminal, 1.48 to another. It's a radio shack DMM, and I've always assumed low voltage readings like that were due to some kind of capacitative coupling, not real. Maybe not?
Anyway, heater sounds okay at 28 ohms, so it should be the thermostat or the breaker. But there would have to be a dead short in it to trip the breaker immediately, shouldn't that show up on the resistance readings?
Any more diagnostic suggestions? What else to try? I'm still suspicious of the breaker, that's the only thing known to have changed.
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TimR wrote:

Hi, Be logical. If it worked with fuse OK and now breaker trips wouldn't it mean the breaker is under rated? What is in there now?
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wrote:

Hi, Be logical. Do the math. What is in there now is the 20A breaker he said was in there. 240V / 28 ohms is less than 9 amps; how do you figure a 20A breaker is under-rated for a 9-amp load?
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Doug Miller wrote:

Hi, Yup, be logical. If nothing changed and fuse is replaced with a breaker which can feed 240V or 120V depending what type breaker is installed.
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Resistance of filaments changes, when the temp goes up. Thus, the big starting current. Can't calculate wattage like you can with smaller devices.
I'm thinking short to ground, or undersized breaker. Might also be wired wrong.
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Aparently a lower rating than the fuse with the penny behind it? What's really need is ammeter, to see what is the current draw. Also check the wire size at the breaker. And wire type. If it's aluminum wire, that's a problem. Different wire gage rated for different amperage of breakers. 10 gage, 30 amps. 12 gage, 20 amps. 14 gage, 15 amps.
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TimR wrote:

Quote: "With the power on, I read 242 VAC across line, 0 across load, 120 from each line terminal to load terminal"
120 from Line to Load may be the clue.
Difficult to be certain with a sensitive meter though. Repeat that test using a 120V lamp in place of the meter.
If the test lamp lights, the heater element is shorted to the sheath (or there is a wiring fault to Ground).
Jim
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yeah DONT use digital meters for this sort of testing......
light bulb or analog meter
digitals too sensitive give capacitive coupled readings
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Oh, fer cryin' out loud, haller, a digital meter is just fine for telling the difference between 120V and 240V. The 1- and 2-volt readings that he got have absolutely nothing to do with the breaker tripping.
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If I'm understanding this correctly, you suspect an actual heater problem? Not inside the thermostat?
Does that make sense with the resistance reading on the element?
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wrote:

Yup - makes sense, and would be first thing to check,
As for the digital meter, just load it and it will be accurate. Put a half watt 1 megohm resistor across the leads at the meter plugs.
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Might want to read from the element to ground. Isolate both ends of the element, and then read from element to ground. Should be OL (over limit) or infinity resistance.
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wrote:
The heater is grounded. (shorted to ground)
Pull the cover to the junction box on the heater (one end or the other) and disconnect both L1 and L2. Cap the wires with wirenuts. Chech resistance from eack L terminal to ground. You should read infinity. Then turn the stat on, with the heater disconnected. If the fuse still blows it could be the stat. If you have continuity to ground on the heater, it's the heater. Other possibility is a short to ground in the cable from the stat to the heater. With powewr off, check continuity from both L wires in turn to the neutral or bare ground.

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On Nov 21, 9:04pm, snipped-for-privacy@snyder.on.ca wrote:

Heater shorted to ground.
I can't check that. Well, I can't check that tonight, I've had several beers while thinking about it, and I've got this rule about mixing beer and 240 V. Wish I'd stuck to wine, but I really don't like wine. <humor>
Anyway, thinking about it, your explanation makes perfect sense if the thermostat is a double pole. Then we'd have 120 V from each hot leg to ground, and heater is at ground.
I still have two logic problems with that. One is that I have trouble believing a thermostat this old is a double pole. If it's a single pole, it doesn't make as much sense. One hot leg ought to be connected to ground all the time. So it should trip before the stat makes, unless the short is on the other side of the heater. But in that case I shouldn't get 120 V to both load terminals, I should just get some kind of ratio. And if the short were in the middle of the heater, I should have enough resistance in the circuit to avoid tripping the breaker. Could be I just misunderstand how these things are wired though.
The other problem is the timing - it worked a month ago, we replaced the main panel, now it doesn't work........ oh, and I did check for nail holes, but nothing has been hung in the room yet.
gotta have another beer and think about this some more.
thanks for the help, by the way. I take it your diagnosis is most likely a heater problem, next likely short in the thermostat, least likely bad breaker.
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In article

From here, it seems perfectly obvious that something was wired incorrectly at the time the main panel was replaced -- like connecting a 240V circuit to a 120V device. CHECK THE RATING PLATE.
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Have you checked the rating plate on the heater to make sure it really is a 240V heater? Sounds more to me like it's a 120V unit connected to a 240V circuit, which of course would explain the instant breaker trip.
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On Nov 21, 9:22pm, snipped-for-privacy@milmac.com (Doug Miller) wrote:

That's a reasonable guess, but I didn't supply all the information. The house is from 1960 with no changes.
And I'm not sure that would have to trip the breaker immediately. That would double the amps, but 8.6 doubled is only 17.2, within the breaker rating and the 22 amp thermostat rating. I think it would overheat and trip something in short order, but not instantly.
No, something either goes to ground or shorts hot to hot the instant the stat makes.
Or something is wrong in the breaker panel. But it's not obvious to me what that could be. Or why a fuse did not trip but a breaker does.
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Well, obviously there was one change, at least: from fuses to a breaker. And it looks to me as though there's a decent chance that change was done incorrectly.

Exactly. Suppose you have, as I suggested, a 120V heater, not a 240V heater, and the neutral is bonded to the ground in the heater -- not a particularly far-fetched supposition for a heater original to a home built in 1960.
Then when a 240V supply is connected to that, one leg of the 240 goes to the hot side of the heater, and the other leg to the (grounded) cold side. Wham! Instant breaker trip.

HAVE YOU CHECKED THE RATING PLATE AS I SUGGESTED?
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TimR wrote:

Assuming that it is really a 240V heater, then it sounds like the load (heating element) is shorted to ground. What is the ohms reading from a load terminal to ground with t-stat off? If you don't have a nearby ground then use an extension cord to get a ground near the t-stat. The fault could be either in the t-stat or at the heater or the wiring in-between. Isolate the fault by removing the load wires then measure again.
Note that you can't calculate the amp draw or wattage by using the 'cold' ohm readings of heating elements or light bulbs. The resistance goes up as the element heats up.
Kevin

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Blindingly obvious - and yet I never thought of it. Good point.
Seems like an ampmeter on the main panel would be a useful tool. Wonder why there aren't any.
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