Need some assistance troubleshooting this circuit

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mark32

mark32

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Currently in NJ
Hey guys!

I maintain the lights for a small pre-k school, but the owner asked if I could stop by and see why some of her receptacles are not working. The story goes, one of her breakers was tripping, she'd reset it, and it would trip again (Not sure if immediately or not). This happened a few times, then, the breaker stayed on, but as noted, a few receptacles never powered back on. The breaker in question powers a window a/c, so I put my clamp on it, and it was fine, only pulling 6-7 amps. The receptacles in question are across the hallway in another room, hmm, seems odd the circuit would be routed like that. Anyway, I pulled out the receptacles and the connections were fine. I checked for continuity from ground (This circuit is in emt) to the grounded conductor, it's good. For the hell of it, I went across the prongs of the receptacle and I was getting 300+ ohms across the grounded and ungrounded conductors. "Something must be plugged in" I said, but what? After farting around a bit, I unplugged the a/c, and the ohms dropped to zero??!! So here is my question, how can I be reading that (Functioning) a/c on this circuit, when this circuit is dead? The a/c circuit is run from the panel in emt (Along with 4 other circuits) into a junction box above a drop ceiling. There, the circuits go out in mc cable. As far as I can tell, the a/c circuit then runs above one of those old acoustic tiled ceilings, into another junction box, and transitions to wiremold. At this point, I'm unsure where the circuit that feeds the receptacles originates. I don't mind putting a little more time into troubleshooting this, but after spending over two hours here, just refeeding the receptacles is starting to sound like a good idea :)
 
Are you asking why you are reading ohms when the a/c is plugged in? Anything plugged in will complete the circuit and show as a short. The power does not need to be on to show that. Screw in a light bulb and you will get an ohm reading that is not zero. So if you unplug the a/c unit and you have zero ohms then there isn't a short unless it is something showing up occasionally.

Does the breaker stay on when the a/c is unplugged?
 
Was the circuit off when you checked continuity between the grounded and ungrounded conductors?

If you check for continuity on an energized circuit, you will get an unreliable reading. Meters inject a small current/voltage into the circuit to measure the continuity, so when the circuit is energized it will 'trick' the meter. You should de-energize the circuit before you check for continuity.
 
Thank you Dennis and Meterman for stopping by. My post might not have been as clear as it could've. The circuit I was troubleshooting had no voltage present. The only reason I was checking continuity between the grounded and ungrounded conductors, is because of the following: In an attempt to figure out how the circuit is configured, I wanted to take power from an extension cord and energize the conductors at one of the receptacle locations. As a precaution, I first used a pigtail with an incandescent bulb from the cord to the ungrounded conductor. And much to my surprise, it lit up brightly! Surprised because, the ungrounded conductor should be open somewhere, yet, it somehow is reading that (Running) a/c, which is on a circuit that appears to have nothing to do with the circuit in question.
 
1) It's possible if not already checked that the breaker has an intermittent buss to breaker connection issue.

2) try to determine the line side wired origin of the first receptacle that is out amongst the others involved.
 
Based on your description, it appears that the ungrounded wire initially shorted to the ground, and after multiple tripping of the breaker, the ungrounded wire melted and split. Based on your ohm measurement and your last test, it seems that the side connected to the dead receptacles is still connected to ground, while the other side of the ungrounded wire has split and is not connected anywhere. It is possible that the short circuit occurred due to a scratch on the ungrounded wire when it was being pulled through the metal conduit between dead and live outlets. If my guess is right the ohm between ungrounded wire and ground should be zero in dead receptacles. I would disconnect the ungrounded wire in dead outlets and see which one is still shorted to ground. The outlet that its open ungrounded outlet has zero ohm to ground is first outlet in dead outlets circuit.
 
Voltage. I want voltage checks. Did the problem start from day one?
Wiremold means to me, someone picked up a circuit wherever they could. Not necessarily properly. Wouldn't be the first time of two breakers feeding the same branch circuit. Possibly different phases. Wiremold also means sharp corners and cuts that weren't properly bushed or even reamed.
 
A voltage sensor (stick) is invaluable for troubleshooting. ………..
Also try to make a list of everything that’s on each circuit if we’re talking about two separate circuits.
Then you can look for junction boxes where those two circuits might cross paths.
 
Great stuff guys!! I really appreciate the input. If time was not an issue, I would've done some more troubleshooting, but as it stood, I had already spent 2.5 hours on site, and I don't feel comfortable charging someone when nothing had been fixed. I think of myself bringing a vehicle to a mechanic, and after three hours I'm told the issue was never resolved, and then being charged for it. In the end, I just pulled a new line from the panel to the first receptacle in question, and just added one hour of troubleshooting time. To add to the story, a number of years ago something similar happened at this school, when a few outlets stopped working, although in that case, no breaker(s) were tripping. A number of the receptacle circuits at this school are routed underground in emt within concrete, using solid #12 with that thick old insulation, although in good condition. I can only surmise some of the conductors had been compromised when being pulled in, but taking 60 years to rear it's ugly head?

Thanks again!
 
mark32 said:
Hey guys!

I maintain the lights for a small pre-k school, but the owner asked if I could stop by and see why some of her receptacles are not working. The story goes, one of her breakers was tripping, she'd reset it, and it would trip again (Not sure if immediately or not). This happened a few times, then, the breaker stayed on, but as noted, a few receptacles never powered back on. The breaker in question powers a window a/c, so I put my clamp on it, and it was fine, only pulling 6-7 amps. The receptacles in question are across the hallway in another room, hmm, seems odd the circuit would be routed like that. Anyway, I pulled out the receptacles and the connections were fine. I checked for continuity from ground (This circuit is in emt) to the grounded conductor, it's good. For the hell of it, I went across the prongs of the receptacle and I was getting 300+ ohms across the grounded and ungrounded conductors. "Something must be plugged in" I said, but what? After farting around a bit, I unplugged the a/c, and the ohms dropped to zero??!! So here is my question, how can I be reading that (Functioning) a/c on this circuit, when this circuit is dead? The a/c circuit is run from the panel in emt (Along with 4 other circuits) into a junction box above a drop ceiling. There, the circuits go out in mc cable. As far as I can tell, the a/c circuit then runs above one of those old acoustic tiled ceilings, into another junction box, and transitions to wiremold. At this point, I'm unsure where the circuit that feeds the receptacles originates. I don't mind putting a little more time into troubleshooting this, but after spending over two hours here, just refeeding the receptacles is starting to sound like a good idea :)
Click to expand...
It seems that there is a connection issue between the air conditioning circuit and the circuit powering the receptacles. When the air conditioning unit is plugged in, it creates a resistance across the grounded and ungrounded conductors of the receptacle circuit, causing a loss of power. Unplugging the air conditioning unit resolves the power Vehicle issue. To troubleshoot further, it is necessary to investigate the wiring path, including the junction boxes and wiremold, to trace the origin of the receptacle circuit. Considering the time spent already, it might be worth considering refeeding the receptacles as a potential solution.
 
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