Interesting Service Call

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
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.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
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.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
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.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
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.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
I have run into that on old houses. Someone told me that it was military electricians, that their hot wires were white. Don’t know if there is any truth to that....but anyway, probably the neutral got shorted to ground somewhere giving it an alternate path, would be the reason it worked. Ballast wire could have got pinched when or if it was changed. (I think someone else said something similar)
 
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