Polarized appliance power cord giving unusual voltages?

Discussion in 'General Electronics Chat' started by GregSr, Mar 30, 2015.

  1. GregSr

    Thread Starter New Member

    Mar 30, 2015
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    I am an electronic tech with most of my experience in the PCB manufacturing industry. Usually I can diagnose and repair most household appliances - but this problem has me baffled. I'm not even sure it is a problem. We have one of those Showtime Rotisseries for cooking various meats. It uses a 1250 watt radiant heater element while rotating the meat to cook it. It has worked fine for years. The power cord is one of those two conductor polarized kinds. Well last night my wife tried to cook dinner on the rotisserie. The heater element stayed completely cold while the lights and motor continued to work normally. I assumed the element burned out. I opened the appliance up to discover that the spade connector on one end of the heater element had simply burned up and fell off. I used a crimp connector to reattach the broken connection to the heater element. I put everything back together, plugged it in and turned it on. Everything works as it should, including the heater element glowing cherry red hot. At this point I pat myself on the back and consider the repair a success.
    Now I'm getting to my question. With the appliance running, I measured for any voltage on the chassis relative to earth ground. Much to my surprise, my meter showed 115 VAC on the chassis of the rotisserie. When I switch the appliance off, the voltage goes to zero. If I run the appliance but not run the heater element, the chassis shows about 8 VAC. The power outlet I'm using is properly wired with a hot, neutral and ground. I confirmed that with one of those LED outlet testers. The outlet is protected by a GFCI device. I tested that also. It does not trip when running the appliance.
    Can anyone explain what is going on? How can the chassis be "live" and yet everything works normally? If the element was shorted to the chassis, I would expect to either trip a circuit breaker or the GFCI circuit. I think I liked appliances with the 3 conductor power cords better. Ground is ground...period.
    Thanks in advance for any comments or suggestions. This has me baffled. Am I missing something obvious?

    GregSr / Canyon Lake, CA
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Sounds like a death trap to me.

    Unplug it. Use your DC Ohmmeter. Measure each prong of the line cord to the case, with the switches in all positions. There should never be a path from either prong to the case. If there is, disassemble it and find out why.

    This makes the case why every kitchen appliance should have a three-wire cord. If it had, it would have tripped the GFI instantly.
     
  3. Lectraplayer

    Member

    Jan 2, 2015
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    Even that cord should trip a GFCI instantly. A GFCI looks for current "leakage," and it doesn't care where it's going. If it finds leakage (even into "the outside world...OW!) it yanks the circuit out.

    I had a fridge cord do that (though it had a ground) After putting a new cord on this fridge, it began working 100% properly.

    I will stress, USE A METER! I tried to diagnose this with a test light at first, and was finding absolutely nothing suspicious. When I grabbed my meter, I was like "Why do I only have 70 volts on the compressor?" "The fan has 70 volts!"
     
  4. profbuxton

    Member

    Feb 21, 2014
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    From your post I see your appliance has a two wire cord(active , neutral) correct?
    Your power outlet has active neutral and ground. If your appliance case is at 120V to earth then you have a dangerous situation. If someone touched the appliance case and an earthed object they would recieve a serious electrical shock. Your appliance needs to be fixed or discarded and should be connected to the power outlet via a thre conductor cord active neutral and earth for proper protection.
     
  5. DickCappels

    Moderator

    Aug 21, 2008
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    Leakage to chassis is normal. If its only a few milliamps, then it is probably safe. Underwriters Laboratories requires 100% leakage testing on most AC operated consumer appliances (maybe all of them), so if your rotisserie has a UL label and the safety was not compromised by your repair you are probably ok.

    Edit: (add) A GFIC probably wouldn't hurt; I use them on non-grounded appliances indoors and out.

    Using a three wire power plug and connecting the earth wire to the chassis will reduce the chance of shock or annoying tingle when the appliance is touched (I did this with my refrigerator and computer -neither came with a three conductor cord).

    You can measure the leakage current from the chassis to earth by running a wire between earth and the chassis and inserting a 1.5k resistor shunted with a .15 uf capacitor, then measuring the AC voltage across the resistor and calculating the current through it.

    leakage.png

    For safety, in your test, only connect your meter and shunt (R and C) to earth and not to Neutral.

    UL.png
    The images above are from the publication at the URL below (Electrical Safety Testing Reference Guide). It contains a good tutorial on the subject.
    www.psma.com/ul_files/forums/safety/estguide2.pdf
     
    Last edited: Mar 31, 2015
  6. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    It seems that the heating element conductor has 'shorted' (or otherwise established a low resistance path) to its jacket and, hence, to the appliance chassis -- The GFIs balance is not disturbed because, owing to supply via an ungrounded power cord, the chassis is 'floating'.
    Moreover the impedance of your meter precludes 'leakage' of current sufficient to trigger the GFI during chassis to ground measurements.

    Best regards
    HP
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The leakage to chassis may be via a capacitor used to suppress sparking at switch contacts.
     
  8. GregSr

    Thread Starter New Member

    Mar 30, 2015
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    Thanks for this suggestion. I had already done that but forgot to mention it in my first post. There were no issues between each prong to the chassis or to each other.
     
  9. GregSr

    Thread Starter New Member

    Mar 30, 2015
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    I was tempted to force the chassis to ground with a temporary jumper wire to see what would trip - the breaker or the ground fault. I decided not to do that in case it becomes somehow destructive. Thanks for your feedback. This forum is an excellent source for brain power.
     
  10. #12

    Expert

    Nov 30, 2010
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    Forgive me if I misunderstand but, there ARE issues between the prongs of the plug and the case of the machine, even if only when you turn the machine on. I suggest that you DO attach a good ground to the case and see what blows before it is YOU that blows!

    I would much rather the defective part explode than myself.
     
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  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Rather than use a clip lead to ground to see what blows, use a 60W to 100W trouble light and extension cord. That way, if the lamp lights, you know you have a serious problem, but nothing blows up. If the lamp doesn't light, but the GFI trips, you still potentially have a serious problem.

    My next step would be to put my multimeter in ACamps mode starting on the highest scale, and then measure the AC current that flows between the cooker case and a non-GFIed Green wire ground (cooker plugged in, on, and turn over the line cord, if possible). Progressively switch to lower ACamps ranges till you get a reading. Anything over a couple of mA could kill you.
     
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  12. GregSr

    Thread Starter New Member

    Mar 30, 2015
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    Yes, it is a two conductor power cord with a polarized plug. The conditions to get a shock would be pretty unlikely to happen simultaneously. Although any chance is too much of a chance. The outside of the appliance is non-conductive. The internal metal is only electrified IF the heating element is energized. The metal surface inside would be VERY hot to the touch (thermally). So, the only combination of simultaneous conditions to get shocked would have to be as follows: Heating element is energized, door is open, someone touches the (thermally and electrically) hot internal metal while they are grounded. If you remove any of those conditions, there would be no way to get shocked. In any case, I will not be putting the appliance back into service until this issue is resolved.
    Thanks for your input. I am new to this forum but I can already tell it is a great resource.
     
  13. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    In my experience hazards find a way! --- Connection of a proper ground is cheap insurance! :)

    Best regards
    HP
     
  14. Lectraplayer

    Member

    Jan 2, 2015
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    If you can light an indicator LED, you are screwed.
     
  15. GregSr

    Thread Starter New Member

    Mar 30, 2015
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    Your lamp idea was excellent. I wired the threaded part of the 100w incandescent light bulb to ground. Then I switched on the rotisserie and the heating element. While holding the bulb, I touched the center electrode of the bulb to the appliance. The GFI outlet tripped immediately. I will open the appliance up again and look for shorts. But, as one poster suggested, I am now suspecting the heating element itself. I will order a new one and re-evaluate. Thanks for the great idea!
     
  16. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Try it again with the appliance plugged into a non-GFI outlet, where the lamp provides a ground path to a green-wire ground (third prong of another outlet?)
     
  17. GregSr

    Thread Starter New Member

    Mar 30, 2015
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    I think you hit the nail on the head! Everything points to the element being shorted to its jacket. As I mentioned in another post, I put a 100w light bulb between the chassis and ground. The GFI tripped immediately. I will order another heating element and then re-evaluate. Thanks again for your excellent observations.
     
  18. mattbullet

    Member

    Nov 29, 2008
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    The cost of a 3 conductor cord and a replacement heater is cheaper than a funeral.
     
    dl324 and Lectraplayer like this.
  19. Lectraplayer

    Member

    Jan 2, 2015
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    There's only one who should be allowed to pay that price, and he did.
     
  20. profbuxton

    Member

    Feb 21, 2014
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    Are you saying this is a "double insulated " appliance? Should have a little double square on the label indicating such. If so it does not need to (and usually cannot be earthed) since there is no external metal
    Although if you can reach the internal metal element this should only be when power off and isolated.
     
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