HOT and NEUTRAL both reading 120V???

Discussion in 'Electrical and Wiring' started by jsbeckton, Aug 4, 2010.

  1. Aug 4, 2010 #1

    jsbeckton

    jsbeckton

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    Hello everyone, I have been remodeling an old house for about 4 months and this will be my first post. A contractor moved a wall and the wiring in it, while adding some new recessed lighting and an outlet. He said that the old power supply was in the ceiling which he ran over to the wall where the 3-way switches are.

    From the switch box he has wires going to the other switches on the 3-way lights (2 sets) and wires going to the refrigerator receptacle. Everything was working great for about a month and a half an then all of a sudden the refrigerator and lights stopped working. I pulled out the refrigerator and plugged my wiring tester in. The tester read “Hot/Ground reversed”. I then took readings with my mulitmeter and measured ~120V from the hot the ground, ~120V from the neutral to the ground, and obviously 0V between the hot and the neutral. My initial thinking is that there has to be a hot wire touching a neutral somewhere to make that 120V. Is this correct? I have tried changing out all of the switches and the receptacle with no luck.

    Curiously, I also found that on the red wires (travelers?) between the 3-way switches there is only an 85V reading between them and the ground while the hot to ground reads 120V.

    I have tried everything that I can think of here and still no luck. Anybody have any idea what is going on? Is there any other possibility than a hot/neutral contact somewhere and if so what is the best way to find this because I have checked everything on the circuit that I can think of.

    Any help would be GREATLY appreciated!

    Thanks in advance.
     
  2. Aug 5, 2010 #2

    GregC

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    Hi, sounds like someone wire both black and white leads to a breaker/fuse? ...creating 240 volt? did you trace back to the breaker box to see if white is attached to breaker/fuse instead or neutral/ground post? Its a thought, hard to understand without looking at it. Worth a try.;)
     
  3. Aug 5, 2010 #3

    john4153

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    I agree with GregC. There are a lot of possibilities. For example, black could be attached to ground, white and ground to the other terminals, etc. So, there are a lot of questions that might help sort it out.

    1) How widespread is the problem in your house? Or, is only one outlet affected?
    2) What prompted you to check that outlet?
    3) Have you located the circuit breaker or fuse for that outlet?
    4) Do you have a wire/circuit tracer. That is, a little tool that you can attach to a wire. It injects an audio signal that you can trace with the appropriate receiver.
    5) Have you looked inside the outlet box to see what color wires are attached to what?

    Finally, remember that a voltmeter puts almost no load on the circuit being tested. Sometimes, things change with a load. You might consider making something that will apply a load to the circuit being tested. A regular (incandescent) 60W bulb and a couple of alligator clips will work.

    John
     
    Last edited: Aug 5, 2010
  4. Aug 5, 2010 #4

    JoeD

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    Hot ground reverse is the indication of an open neutral. 120volts measured on the neutral confirms that indication.
    Look for a loose connection on the neutral wire. Sounds like the problem is back in the junction box where the feed comes from to the feed the switch box.
     
    Last edited: Aug 6, 2010
  5. Aug 5, 2010 #5

    jsbeckton

    jsbeckton

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    *** UPDATE ***

    Found the problem! It turns out that the contractor didn’t mount the lighting wires far enough away from the surface of the joists. When he installed the drywall one of the screws missed the joist and punctured through the wires insulation. After a month it finally tore the insulation enough to short the neutral and the hot causing both to be at 120V upstream of this point. Therefore the problem was the neutral being in contact with the hot, just as I suspected.

    Thanks for the replies.
     
  6. Aug 5, 2010 #6

    jsbeckton

    jsbeckton

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    The hot being attached to the ground woudln't make the neutral line 120V. Also, I couldn't have had a 120V potential between the hot and the ground if this were the case.

    Thanks anyways.
     
  7. Aug 5, 2010 #7

    jsbeckton

    jsbeckton

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  8. Aug 5, 2010 #8

    john4153

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    I am glad you solved your problem before it caused a much bigger problem.

    As for my comment, I was referring to wiring of a single receptacle. If Black is wired to ground and ground to where Black should be (i.e., the connections are reversed at the receptacle), from the receptacle side, you most definitely will see 120 V between what should be hot and ground and between neutral and ground. There will be zero volts (or close to it) between hot and neutral, as hot is actually ground in that situation. That was only one possibility, but it is why I asked whether you had pulled the receptacle from the wall.

    A short between white and black should blow the breaker, if it is low resistance. If it is high resistance, you would have detected that by putting a load on the pin with the "wrong voltage," as that would create a voltage drop across the resistance.

    John
     
  9. Aug 5, 2010 #9

    jsbeckton

    jsbeckton

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    Ok, sorry I misunderstood what you were saying. I did mention that it was working for over a month so that couldn't have been the problem because it never would have worked in the first place.


    If somebody accidentially puts a hot on a receptical ground they should be banned from further work! I can see a hot/neutral reverse but not a hot or neutral on the ground!

    (But I am sure it happens...)
     
  10. Aug 6, 2010 #10

    JoeD

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    A neutral not connected somewhere can cause a reading of 120 volts on the neutral to ground. The voltage flow through any device that is plugged in. The device does not work because there is no current flowing.

    I also wonder why the breaker did not trip if the white and black were shorted. More likely the white was just cut by the screw.
     
  11. Aug 8, 2010 #11

    triple D

    triple D

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    That is exactly correct Joe, your trouble shooting skills are right on. The neutral slowly burned itself the rest of the way apart over time. The breaker would of never held, from the time the screw was installed. Was this person a licensed electrician by chance?
     
  12. Aug 9, 2010 #12

    jsbeckton

    jsbeckton

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    He is not a licenced electrician. I still am having trouble understanding how a open neutral can cause it to be at 120V?

    Also, should the breaker have tripped?
     
  13. Aug 9, 2010 #13

    john4153

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    Let me try to explain why one can see full voltage or almost full voltage from the com to ground, when com is open. Refer to the following diagram. I have represented each load as a resistance.

    [​IMG]

    It is important to note that the voltmeter draws almost no current. Consider first the switched load. If it is switched on, there is a direct path from hot to the com side. Since there is no current drawn by the voltmeter, there is no voltage drop across that load resistance. Thus, one sees the full voltage when measuring to ground. Now assume the switched load is off. Why can one still see a voltage in most cases? Well there is always some leakage which may be a very high resistance and might be measured. But more commonly in modern equipment, there is going to be some real, but very small load, such as an LED (whether seen or not), maybe a defrost timer, etc. In that case, the no-current voltmeter will see the voltage. That is why I suggested applying a load across the points being measured.

    Now, are there instances in which an open com may result in no voltage reading from com to ground? I am sure there are, but considering the complexity of the wiring in our houses and the number of potential non-switched loads on the com lines down-stream of the break, it might be uncommon. Also, given the close proximity of the com wire to the hot wire, coupling to the field around the hot line to com (think of a radio antenna) could give a voltage reading. Just look at how the value on an AC voltmeter will tend not to be zero when one lead is grounded and you touch the other or just leave it in free air.

    Regards, John

    OPEN COM.png
     
  14. Aug 10, 2010 #14

    jsbeckton

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    If the com was not open wouldn't you still have an open path between the hot and the com? Why does it read 120V if its open but ~0 if it is closed.

    So would there be any reason for the breaker to trip?
     
  15. Aug 10, 2010 #15

    john4153

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    I am not sure I understand the point of confusion, but let me try to exlain.

    First case: Assume the com is not open and the switched load off. Com-to-ground voltage will be real close to zero (I'll just call it zero from now on), because com is grounded.

    Second case: Assume com is not open and the switched load is closed (on). Com-to-ground voltage will still be zero. Why isn't it 120 measured through the load, as happens in the case of an open com? Because, there is a load (current) and the voltage drop across that load (i.e., between hot and com) will be 120 V (Ohm's law). Com is grounded, of course, and the only reason current flows to give that voltage drop is that the grounded conductor provides a return path.

    John
     
  16. Aug 11, 2010 #16

    jsbeckton

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    So basically you are saying the open neutral is no longer grounded ands thus retains the voltage when the switch is closed because there can be no current flow back to the ground?

    So there would be no reason for the breaker to trip if there is an open neutral?

    Thanks
     
  17. Aug 11, 2010 #17

    john4153

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    I think we are saying the same thing. John
     
  18. Aug 17, 2010 #18

    triple D

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    If the nail or screw had touched the hot to the neut as you earlier indicated causing some kind of 120volt anomoly on the neut, you would blow the breaker in less than a second. you would never have had the chance to even measure voltage.
     

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