3

Long story. I'll try to make it short:

Goal: Replace cooktop exhaust fan/light

I've done this before. Should be no big deal... Removed old fan/light. Started to install new fan/light. Connected cable - armored (BX?). To make sure I have the grounding right, I checked continuity from metal case of fan/light to armor and to neutral. All good. But then checked to hot and that also showed continuity, not good.

I figured the problem was the old (~65 years old) cable with insulation falling off the wires. Went to a big orange store to get something to cut the armor, new bushings, etc. Cut off some of the armor (fortunately this cable had some length to spare) to get some good wire - same problem! Figured it must be down the line. Next fixture is overhead light. Removed fixture to get to the wires. Disconnected the cable going to the fan/light (that's all good) and problem is coming from cable from the switch box.

Opened up the switch box and found a big mess. Two switches, one cable coming in, one cable (3 wires black hot/red switched hot/white neutral) to overhead light and fan/light, one cable for second switch light fixture and one cable going on to at least two rooms of the house. Hot wire looped around a screw and then one to another screw instead of using a pigtail. At least one wire around a screw the wrong direction. Etc. Cleaned it up, marking wires as I went with colored tape (I'm learning) since all the old wires are shades of gray.

Isolated the problem to the ongoing cable. Capped the two wires of that cable.

Put overhead light and fan/light back together - all worked. Having a bit of trouble getting the switch box (but with the bad cable wires capped) back together, but I'm sure I can get that done.

The question: Since everything was working when I turned off power to this circuit to start this "little" project, and I found the problem (hot shorted to neutral in a cable far from where I was working), any idea this would happen? And any idea how to find the source of the problem, or is it just the usual "keep taking things apart until you find the problem"?

Resolution: Ed Beal and Harper's comments were 100% correct. Unplugged everything and the phantom short went away. The interesting thing though is that some of the devices (the kitchen LED light fixture and the new kitchen fan/LED light fixture that started this journey) did not cause a phantom short. In fact, leaving those lights on let me have an easy way to check (yell upstairs after restoring power "did the kitchen lights go on?"). So this is not an absolute "must turn off/unplug everything". But oh what a mess...turns out downstairs refrigerator & freezer on the same circuit and I hadn't realized it. Freezer definitely OK, refrigerator almost certainly lost some stuff. Ugh. Live 'n learn.

8
  • 2
    Wait, is your only indication of a problem, continuity between hot and shell/ground? Because that’s normal enough; any load that is switched-on will create continuity (at medium impedance) from hot to neutral to the N-G bond to ground. – Harper - Reinstate Monica Jul 20 '20 at 5:16
  • I'm suspecting that might be the case. I'll check in the morning. Half the house (except the 240V loads and the kitchen circuits added 20 years ago (which didn't touch the problem circuit)) is on this circuit. Neutral to ground of course I expected. But what bothers me about this is why ohms law doesn't come into play - if 3 ohms hot to neutral shouldn't that mean 40 amps at 120 volts? Or is this just another quirk of digital multimeters? I didn't want to put everything back thinking it had a short. – manassehkatz-Moving 2 Codidact Jul 20 '20 at 5:38
  • Yeah, the problem is some loads are not linear/ohmic; their resistance at 1 volt and microamps is quite different than at 120V warmed up. Incandescent bulbs being a poster child for this. – Harper - Reinstate Monica Jul 20 '20 at 13:10
  • And as @EdBeal pointed out, lighted switches can be an issue - I don't have those, transformers are an issue - pretty sure I 've got those (just told the kids to unplug everything) and while I don't have any on this circuit, any smart switches, timers, etc. would be problems too. Once I get this straightened out, I'll update the FAQ-that-never-worked-but-really-would-work-if-SE-had-a-good-place-for-it. – manassehkatz-Moving 2 Codidact Jul 20 '20 at 13:49
  • 1
    @isherwood Actually, it wasn't in the range hood wiring, and I determined that much before I wrote the question. But you're right, a better title is needed. – manassehkatz-Moving 2 Codidact Jul 20 '20 at 17:07
2

Not only the possible continuity from a lamp being the issue but if any of the switches are lighted can also be the source. Last and most common is a plugged in transformers on the same circuit Will look like a dead short. This messes with new apprentices every time. A transformer Looks like a short to a continuity or DC ohm meter test but AC has a reactance so it is not actually a short.

1
  • 2
    Similar to Harper's comment. Transformer actually makes sense and among other things I'm pretty sure there are some plugged in. That actually makes sense - a coil of wire is just wire to DC. Duh. I'll check it in the morning. If that's all it is then my kids will be happy, and I can get back to everything else I was supposed to do Sunday. Good part is that I ended up cleaning up some of the old mess. – manassehkatz-Moving 2 Codidact Jul 20 '20 at 5:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.