OT -- switching heating elements

to

ey?

So, the thermostat just opens one side of the 230V line?

Is this something you surmised by looking at the wiring, or just a result of a meter measurement? In the latter case, it could be that the alleged qualified electrician who installed the appliance has connected it the wrong way wroung.

Sylvia.

I should have explained that I went to remove the element without opening the breaker -- on the assumption that no engineer in his right mind would leave one side of the heating element powered.

As I pulled out the element, I got a big (though not fat) spark. I was more surprised than frightened.

You're correct. It appears the thermostat switches only one side of the heating element.

By the way, the replacement was ordered Thursday morning and arrived Saturday morning at 9:30. Not too shabby. (Or should I say shabbes?) It took only a few minutes to replace the element, and the oven works fine.

Are you stupid? NOTHING gets worked on while connected to power.

Jeff

--
"Everything from Crackers to Coffins"

"Foolish" might be more apt than "stupid". And people commonly work on powered TVs and other electronic equipment -- with proper precautions, of course.

We still need an answer to the original question... Why?

Um, stupid is the correct term William.

"Bench testing live equipment" is NOT the same as working on an oven connected to a 240 circuit.

As to why it only opens one side of the circuit, that's simple, it only requires one switch to open a series circuit. It takes two to remove power. Hence the coupled dual pole breakers on the source (breaker) panel.

Jeff

--
"Everything from Crackers to Coffins"

I think it's perfectly normal for appliances to switch only one pole of the power supply, that pole being the live one. Same with power points and light switches. I don't think I've seen an appliance with a double power power switch.

What's not normal, and dangerous, is for the neutral wire to be the one switched. Either the oven is miswired internally, is miswired to the mains supply, or the mains supply is miswired. Whichever it is needs to be fixed.

Sylvia.

Jeffrey, that's a tautological answer. I'm looking for an engineering and/or safety justification for not providing a dual-pole switch within the oven. (The economic "justification" is obvious.)

I have have. I've owned two of the classic GE/B&D toaster ovens, and they switched both sides of the line when you opened the door. This is a good safety precaution. My current B&D toaster oven does not remove the power when the door is opened.

With respect to this particular oven, something "live" was NOT being switched. How do you explain the spark, otherwise? The 208V or 240V are (presumably) taken from across two phases (or whatever you want to call them -- we needn't rehash that argument), one of which is not switched. The oven's "metalwork" is grounded/"neutraled" so that a short from the heating-element circuit to the metal will trip the breaker -- which it did.

If I understand multi-phase wiring, the 240V is taken from across two phases. Connecting or disconnecting the neutral would have no effect on that voltage.

In single-phase systems, it would, of course, be foolish to switch just the neutral. This would leave the hot "hot", and open the possibility of electrocution from hot to ground, such as a plumbing fixture.

Yet again, an example of how a simple question becomes a tsimmes.

Residential wiring is, AIUI, typically "anti-phase" rather than multiple (usually three) phase. One phase of the 3-phase distribution from the substation is dropped with a center-tapped service transformer. The center is earthed (at one point) and that becomes the neutral for the

120 volt services. 240 volt service doesn't have a neutral; both sides are "hot" with respect to earth ground, so in the situation described you'd expect to see 120 V to ground on the heater element.

Presumably, what you're seeing meets UL requirements. Perhaps (just speculating here) the intent is that the oven should have an on-off switch that does isolate both sources, and a thermostatic switch that cycles on only one?

Also: tsimmes; I learned a new word! ;-)

--
Rich Webb     Norfolk, VA

Did AP sell you the part?

Have you RTFM?

Would you prefer that the part not be available?

Will you be working on other appliances without removing the power?

Is what normal?

-- Cheers, WB .............

of

We are talking at cross purposes here. In the UK and Australia, the

240V has one side neutral and the other 'hot' while in North America the 240V is both sides hot and centre neutral. In my stove the oven thermostat is the typical single pole switch in one side of the element circuit, but both sides pass through the 'oven function' selector switch that offers 'OFF', 'Bake', 'Broil' and 'Clean'. When that switch is OFF then the element is dsconnected from both hot feeds. In Canada, a heating thermostat for controlling 240V space heaters [permanently connected] may be a single pole type, but if it has an 'OFF' position marked, it MUST open both circuits.

Neil S.

From: "Rich Webb"

That was my general assumption.

I'm not sure whether UL requirements have any direct connection with /consumer/ safety. * The UL evaluation is more "generic" -- is this device likely to catch on fire, or start a fire? Is it inherently unsafe to use? etc, etc, etc

The user manual says the owner should not perform any service on the oven -- but if s/he does, the power should be first disconnected at the breaker.

The Calrod(tm) heating elements are nichrome wires sealed in a metal tube full of magnesium oxide. The probability of the wire shorting to the metal tube is extremely small. But if the user lifted the hinged bottom element to clean under it (which is not "service" in the manual's sense -- you are not told to shut off the power), s/he would be grabbing a live element, with possibly fatal results.

For those unfamiliar with it, it means "stew", in both the literal and figurative senses.

• To clarify... On a segment of "I've Got a Secret" that involved a giant boa constrictor, Steve Allen said "Would you believe it? This snake is perfectly safe. You're in terrible danger, but the snake is safe!"

I hadn't thought of the selector switch. Good point.

Needless to day, the oven switch was in the Off position.

So what other justification do you think you are looking for ? It works. It's perfectly safe until some fool works on it 'live' in the expectation that he's going to be safe, and it saves money on the build cost of the appliance. When the equipment is used 'closed up' in its normal operational configuration, the manufacturer has no legitimate safety concerns beyond that. Also, given the way American society sues at the drop of a hat, I'm pretty sure that the manufacture will have his arse covered in that doing it this way meets any and all safety regulations that might apply.

Sorry, but I can only echo Mr Angus's sentiments that working on such equipment without fully isolating it first, is highly irresponsible, especially for someone of your usually pedantic persuasion, who I'm sure would normally be shouting "never assume anything !!" at anyone else who posted a similar story here ...

Arfa

I guess I'm looking for a groveling "Gee-ee, we're stupid" apology from GE.

expectation

As another poster pointed out, there are ovens with both sides switched at the "bake" selector.

Criticism accepted... But in this case I might very well have taken "anyone else's" side.

Let's let this drop.

I got an extremely useful response -- one I didn't expect -- about some ovens "properly" disconnecting the AC at the oven-mode selector. This gives me something useful to discuss with GE.

Thank you all for your interest and help.

E.

the

t e o

ne

Interestingly I also own a GE stove and coincidentally my bake element developed a hot spot one day and "exploded" open and also needed to be replaced. I never even thought of the possibility that only one side of the 240 was being switched off from the control unit when I replaced it, Like you William I just would have expected such a design to be both impractical as well as unsafe. However I did unplug the stove first, but just purely from habit, ( and because I really don't like working on live circuits, unless there is no other alternative). So in all honesty I made the same assumption that you did with the exception that my stove was unplugged. The model number of my stove is JBP64 and it is almost ten years old. GE was nice enough to provide a service page with their appliances, (at least they were still doing it at the time we purchased this model), and it included a schematic. I just looked at it and incredible as it may seem, sure enough L2 is directly connected to one side of both the bake and broil elements! I've never forgotten the words of one of my teachers many years ago when I was in electronics school. He never seemed to tire of reminding us to "never assume that the power is off". And I try very hard not to. Good advice when you run into something unexpected like this. Lenny

Quite by chance, I saw it burning. I thought it was a bit of food. Near the "socket"?

Mine's from a (probably) related series, JBP26, and a year or two older.

quoted.