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I have two old circuits with no ground wires in which cables run parallel to each other for 20 feet or so. I had both circuits powered off today and was testing some things for unrelated reasons when I accidentally discovered that my multimeter thinks there is continuity between the neutral slots of an outlet on circuit A and the hot slots of an outlet on circuit B. I took both outlets apart, but couldn't see any wiring anomalies. I also traced the wires pretty thoroughly, but couldn't find anywhere that bare copper could be in contact (obviously I can't see inside the walls). The circuits (seem to) work fine, and it seems like if this were really happening, it would create a short that would trip a breaker. But I repeated the test several times, and the meter kept telling me there was continuity. And the meter has not lost its mind, because it gives sane continuity results in other places where I know for sure what's going on. Could this be some sort of inductive effect due to the lack of ground wires?

  • Are you sure there are no devices connected to either circuit, including night lights, lighted switches, motion detecting security lights, smart switches, etc? – Johnny Dec 27 '15 at 4:09
  • @johnny one of them does include a motion detecting outdoor light. Does that explain it somehow? – dlf Dec 27 '15 at 4:16
  • Yeah, the motion sensor will use a small amount of current for its operation (even if it uses a photocell to keep the light off during the day), your meter will show some small conductivity between hot and neutral. – Johnny Dec 27 '15 at 4:28
  • But it was between hot and neutral on two different circuits, and with both breakers off.. – dlf Dec 27 '15 at 4:31
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Depending on where you're measuring continuity and where the motion sending light is, the motion sending light on circuit could be causing the behavior you're seeing. For example, if the motion sensor is on circuit B (shown as a resistor in this diagram) that can cause your meter to see continuity between the hot on Circuit B and the neutral on circuit A since the neutrals are connected together:

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  • That makes sense, except my meter only signals continuity if the resistance is approximately zero. It doesn't sound across an ordinary lightbulb, for example. Could this still be the explanation? – dlf Dec 27 '15 at 5:23
  • Oops, I was mistaken. I did the test and it does sound across a lightbulb. I don't know what the resistance threshold is, but apparently it's much higher than I thought (which probably means I've drawn some bad conclusions from my tests). So your answer is probably right. I'll do some more tests tomorrow to confirm. – dlf Dec 27 '15 at 5:32
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    My meter shows continuity around 100 - 200 ohms, a 100W light bulb will have around 144 ohms of resistance when on, but it's actually much lower than that when it's "cold" -- cold filament resistance will be around 10 ohms. In any case, there are some active components in the motion sensing light that could present a very low resistance for the low voltage of your multimeter even if it only uses a small amount of current while in use. – Johnny Dec 27 '15 at 5:34
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All the neutrals are tied together. If there are devices on both circuits that complete the circuit (light bulbs, etc.), it's possible to measure continuity as you've described.

You'll measure from probe 1, along the neutral of circuit A, back to the panel. Then along the neutral of circuit B, across the load (light bulb, etc.) to probe 2 on the hot of circuit B.

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