Weird voltage

[hillbilly1]

Had a new brain twister today, a friend of mine has a camper set up for one of his workers, he had replaced the light fixture over the vanity with a flouresent. The light worked fine while turned on, but when it was turned off, it would flash once about every 30 seconds. Put my meter on it, and there would be no voltage on it at first, then it would steadily grow to 25 volts, flash, then start over at zero again. Turned the main off in the panel, the lights would blink then come right back on. Found another breaker in the panel that was back feeding. turned it off. The light still would flash at regular intervals. Checked voltage at the panel, 120 v neutral to hot, but nothing between hot and ground. Neutral to ground was 120 v! Aha! either the camper was not grounded and a circuit was shorted somewhere. But no, camper was grounded, found a plug in the luggage compartment that somebody had wired to the park power supply cord and had reversed the polarity on the receptacle they had installed in the compartment. Fixed that, the 25 volts went away, everything things good now, right, wrong. Turned on the breaker that had been backfeeding the panel, the range hood began to spark and smoke. My friend failed to mention that they had a problem with it and had cut the wire, but did not insulate it. Since the polarity was backwards before, the ground being common with the neutral, there was no boom boom when it was shorted, when I corrected the polarity it then properly tripped the breaker. Apparently the short was on the load side of the fan causing the phantom voltage.

 

[cadpoint]

Why don't you swap the Polority of the Fan circuit if it's that isloated...

See what you get, then meggar that puppy...

I don't want to qualify what it might be, becuase it might be a "stupid" statement...

 

[hillbilly1]

The panel's polarity was backwards, the circuit breakers were breaking the neutral, not the hot, and the hot was the neutral bar. Smaller campers have a 30 amp 120 volt feed to the panel, the factory jumpers together the hot buss's in the panel. The polarity was correct in the fan as long as the panel polarity was correct, that was the problem, since the hot was actually the neutral, it being shorted to ground would not trip the breaker because the neutral is common to ground at the service. And since the neutral bar was insulated as per code, the breaker at the pedestal did not trip. I would draw a diagram showing what happened, but the drawing program I have eats up too many bytes to post here.

 

[quogueelectric]

So the ground rod was conducting to earth spinning the meter like crazy??

 

[hillbilly1]

This was a camping trailer, the ground to the frame frame was intact and correct. The only current flowing on the ground was the parallel path of the neutral loads, when the main was turned off all of the neutral load current was flowing through the grounding conductor. Really weird series of events. Because the service to the camper is 120 volts only, the mis-wiring did not show up until the flouresent light and the hood fan had been messed with. There was no ground fault protection at the pedestal, as this problem would have immediately tripped it as soon as the fan shorted out to ground.