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OT -- switching heating elements

Discussion in 'Electronic Repair' started by William Sommerwerck, Jun 19, 2011.

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  1. I just had to replace the bottom element in my GE oven, and discovered -- to
    my great surprise -- that one side of the element is always "hot" -- that
    is, it has voltage on it. I will be calling Appliance Park next week and
    verbally tearing someone a new oven cavity.

    Is this normal? And if it is, is it for some reason other than saving money?
     
  2. Sylvia Else

    Sylvia Else Guest


    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.
     
  3. 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.
     
  4. Are you stupid? NOTHING gets worked on while connected to power.

    Jeff
     
  5. I should have explained that I went to remove the element without
    "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?
     
  6. 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
     
  7. Sylvia Else

    Sylvia Else Guest

    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.
     
  8. 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.)
     
  9. 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.
     
  10. Rich Webb

    Rich Webb Guest

    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! ;-)
     
  11. 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. <beat> You're in terrible danger, but the snake is safe!"
     
  12. I hadn't thought of the selector switch. Good point.

    Needless to day, the oven switch was in the Off position.
     
  13. I guess I'm looking for a groveling "Gee-ee, we're stupid" apology from GE.

    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.
     
  14. 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.
     
  15. 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.
     
  16. I've never forgotten the words of one of my teachers many years ago
    Also applies to guns... Always assume the weapon is loaded.
     
  17. Guest

    That is common. The resistance element is encapsulated in a
    protective sheath. You cannot remove the heating element without
    removing the back of the appliance. I would wager the appliance was
    shipped with a warning label indicating it should be removed only by
    qualified servicers.

    Also, recall if either a mechanical or solid state double pole relay
    were used, one side could fail (contacts welded or SCR shorted) and
    you would never know the difference.

    There are titles for those who work on appliances without first
    ensuring it is disconnected. "The Dear Departed" is one. "Candidate
    for a Darwin Award" is another. I prefer 'Fool".

    PlainBill
     
  18. You don't have to take off the back of the oven. You can remove the element
    in situ. Just undo two 1/4" hex-head screws, pull out the base of the
    element a bit, and pull off the friction-fit connectors.
     
  19. If both sides were connected, the element would be on! Wouldn't it?

    Well, I was clothed. And the door was off.
     
  20. For those who enjoy calling me an idiot... I used to service klystrons with
    20kV anode supplies. One was vehwy, vehwy kehful around these. You not only
    made sure the power was off, but you used a conductive pole to short the
    supply.
     
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