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Learned Something New Today-Electric Wall Oven

infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
So last night I'm watching TV and I hear the fan in my 27 year old electric oven running. I check the controls and everything is off. I open the top oven and it's very hot, the broiler is on. I flip off the breaker and today I begin to troubleshoot it. Internet says typically a burnt relay contact welded shut, makes sense. I open the control panel and check the relays, everything is working normally.

Since it's a double oven and I'm going to replace it in the fall I decide to just disconnect one of the leads from the relay going to the boiler element and I'll use the other oven. I disconnect the lead, turn on the breaker and it still gets hot. Now I'm baffled as to how is this working with one lead disconnected? So I take a closer look at the element and I see a bubble and a hole. Turns out that the element shorted to ground and was running off of one lead and the metal oven wall. Scariest thing is if I were away this thing could have been on for a week or at least until it burnt out.
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Dennis Alwon said:
The question is why didn't the breaker trip
Click to expand...
Unless it's GFCI protected, you wouldn't necessarily expect it to.

I gather the element is connected directly to L1 (say) on one end, and connected via a relay to L2 at the other end. At some point along it's length the element faulted to ground. So now we have a fraction of the element (so a lower resistance) connected to half the expected voltage (120V vs 240V).

If the element faulted to ground half-way along its length, the current draw will be the same as nominal (half the resistance, but half the voltage). Only if the fault is closer to the directly powered end connected to L1 would the current be higher. And even if the current is higher, you could have a situation where the element is 3600W nominal (15A @ 240V), but the double oven is on a 30A branch circuit. So the fault would have to be in the 25% of the element closest to the directly powered end in order for the fault current to have a chance of tripping the branch circuit.

Cheers, Wayne
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
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Journeyman Electrician
Dennis Alwon said:
Why isn't it 240V-- not that it would make a difference
Click to expand...
One side of the element was burnt clear so that leg of the 240 volts was not connected to anything. The other leg was connected to the side of the element that was grounded somewhere in the middle of the element so it was only seeing 120 volts from the one leg to the grounded metal wall of the oven.
 
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wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
tortuga said:
If you do use a Class B GFPE breaker (30ma) then you wont have any chance of a nuisance trip.
Click to expand...
Not following your point. With the failure mode in the OP, the element is faulted to the case, so the return current is on the EGC, and multiple amps will trip any GFCI. That would not be a nuisance trip.

Cheers, Wayne
 
tortuga

tortuga

Code Historian
Location
Oregon
Occupation
Electrical Design
wwhitney said:
Not following your point. With the failure mode in the OP, the element is faulted to the case, so the return current is on the EGC, and multiple amps will trip any GFCI. That would not be a nuisance trip.

Cheers, Wayne
Click to expand...
A oven might nuisance trip a 5ma class A GFCI in normal operation.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Occupation
EC
tortuga said:
A oven might nuisance trip a 5ma class A GFCI in normal operation.
Click to expand...
Yes, and recent editions of NEC think we need to use GFCI protection on cord connected ranges now.(n)

Well if you have it in the kitchen, basement, garage or on the porch anyway. Wouldn't need it if in the living room or a bedroom I guess.
 
Little Bill

Little Bill

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Location
Tennessee NEC:2017
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Semi-Retired Electrician
Not an oven, but years ago, I came home and noticed a noise and smell in the house. I went into the kitchen and the microwave was running and the glass tray had shattered, along with the glass on the door. That thing started all by itself. My wife had left before me and I know it wasn't running, and I sure didn't start or program it. Lucky the glass on the door stayed in and only cracked. I think it was double panes or at least very thick. Didn't trip my breaker either.
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
tortuga said:
A oven might nuisance trip a 5ma class A GFCI in normal operation.
Click to expand...
The April 18, 2022 revision of UL 858 "Household Electric Ranges," which appears to cover wall ovens as well per the scope, added section 55A which limits leakage current, so that should not be true for new products. Regardless, that's immaterial to the OP.

Cheers, Wayne
 
kwired

kwired

Electron manager
Location
NE Nebraska
Occupation
EC
wwhitney said:
The April 18, 2022 revision of UL 858 "Household Electric Ranges," which appears to cover wall ovens as well per the scope, added section 55A which limits leakage current, so that should not be true for new products. Regardless, that's immaterial to the OP.

Cheers, Wayne
Click to expand...
Probably because 2020 and 2023 NEC changes were going to mean more ranges will be on GFCI's in the future.

Still does nothing to solve headaches for cases when you have new code but older appliances built to lesser standards and basically isn't compatible with new codes.
 
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