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    Interesting Service Call

    I had an interesting service call today. It was for a light that wouldn't go off with the switch. The customer had already put in a new switch and it didn't help. This was in an office building with just one office effected. I pulled the switch out to check the wiring and immediately saw two white wires on the switch. I thought either someone just used the wrong color or they were switching the neutral. There were two NM cables in the box. The blacks were tied together and the whites were on the switch.

    I turned the power off and disconnected the wires. Turned the power back on and touched the two blacks together and the lights came on. Now while doing this, the whites were still separated. So that was a clue right there. I turned the power off, put the blacks on the switch, and tied the whites (neutral) together. The switch now turns the light on/off. I questioned the customer about changing the switch and she assured me that they only took the wires that were on the screws off and put them back the same way. Normally I would doubt someone telling me this but after seeing the wires I didn't see where the blacks had ever had a "hook" for the screws and the whites only had the hooks with no sign of being twisted from a wire nut. She also assured me that no work had been done in the office. As far as I know, I'm the only person in the last 8 years that has done anything electrically in the building.

    I have a theory of what is going on with this light & switch but wanted to give others a chance to give their opinion.
    [COLOR=navy]If you aim at nothing, you will hit it every time![/COLOR]

    #2
    Parallel paths on the neutral.

    Comment


      #3
      Originally posted by Little Bill View Post

      I have a theory of what is going on with this light & switch but wanted to give others a chance to give their opinion.
      i'm thinking you didn't get the whole story

      ~RJ~

      Comment


        #4
        Originally posted by romex jockey View Post

        i'm thinking you didn't get the whole story

        ~RJ~
        Normally I would think that too but I know the customer. I also checked the old switch and it was still good. Also, I know some of the history of the building and there is a lot of "questionable" wiring in it.
        [COLOR=navy]If you aim at nothing, you will hit it every time![/COLOR]

        Comment


          #5
          Originally posted by 480sparky View Post
          Parallel paths on the neutral.
          A similar cause could be a ground fault that appeared on the neutral on the way to (or even within) the light fixture. If so then switching the neutral would no longer turn off the light, and it would require the hot to be switched to make it turn off.

          Comment


            #6
            Originally posted by Little Bill View Post
            ..there is a lot of "questionable" wiring in it.
            Light fixture screw shells could also be tied to Hot.

            Compared to remodel disasters most existing work is golden, since laborers are doing the wiring for remodels.
            Roger Ramjet NoFixNoPay

            Comment


              #7
              Originally posted by ramsy View Post
              Light fixture screw shells could also be tied to Hot.

              Compared to remodel disasters most existing work is golden, since laborers are doing the wiring for remodels.
              This was a fluorescent light.
              [COLOR=navy]If you aim at nothing, you will hit it every time![/COLOR]

              Comment


                #8
                Originally posted by Little Bill View Post

                This was a fluorescent light.
                Listed luminary requires black & white conductors, unless you installed it with unlisted zip cord.
                Roger Ramjet NoFixNoPay

                Comment


                  #9
                  Something doesn't make sense or I am misunderstanding.......you said the old switch was good? Then the lights should have worked. The electrons and the switch were not getting along??
                  Ethan Brush - East West Electric. NY, WA. MA

                  "You can't generalize"

                  Comment


                    #10
                    Originally posted by electrofelon View Post
                    Something doesn't make sense or I am misunderstanding.......you said the old switch was good? Then the lights should have worked. The electrons and the switch were not getting along??
                    I think post #2 tells us why, the trick is to find out where (or just ignore it I guess).

                    Comment


                      #11
                      Ok, to be clear........
                      The neutrals being switched has been working until recently. The customer had been turning off the breaker to kill power to the light. With the light working even with the neutrals in the box separated/not tied together, tells me what 480sparky said in post #2, there was another path for the neutral. After moving the blacks/ungrounded conductors to the switch and that worked the light, I didn't take the light down to investigate since this was a busy office. My theory is the light on/off was working by switching the neutrals until something cause the parallel path. Since they told me no work had been done, I'm guessing that another load on the same circuit using the same neutral was turned on and the neutral current was backfeeding to the light. Probably another light that maybe had a bad bulb and not used for a while. Another possibility is a neutral to ground fault or two neutrals were faulted/shorted from a pinched wire somewhere.
                      [COLOR=navy]If you aim at nothing, you will hit it every time![/COLOR]

                      Comment


                        #12
                        So when you got there blacks were nutted, whites on good switch, no lighty worky? Then you take switch out, whites separate, touch blacks together, light worky?
                        Ethan Brush - East West Electric. NY, WA. MA

                        "You can't generalize"

                        Comment


                          #13
                          Originally posted by electrofelon View Post
                          So when you got there blacks were nutted, whites on good switch, no lighty worky? Then you take switch out, whites separate, touch blacks together, light worky?
                          The light was on when I got there, blacks were nutted, whites on the switch. Took switch out and separated all wires. Turned power back on, touched the blacks together, with whites still separated, light comes on.
                          [COLOR=navy]If you aim at nothing, you will hit it every time![/COLOR]

                          Comment


                            #14
                            Like you’ve said, Assume a neutral to ground fault at the fixture. Remove the EG, and any source to it. We’ve found fluorescent fixtures that energize an entire ceiling grid.
                            Tom
                            TBLO

                            Comment


                              #15
                              It's also possible there is a parallel neutral.
                              They say I shot a man named Gray and took his wife to Italy
                              She inherited a million bucks and when she died it came to me
                              I can't help it if I'm lucky

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