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I'm searching my electrical system for defects, fixing the last guy's work. He loved to cross neutrals (grab a hot from circuit 5 and neutral from circuit 19). I've found several instances, but it's really slow going.

It seems like I could speed this up a lot by turning on every load, putting dummy loads in every receptacle chain (night lights), and temporarily installing a GFCI, and see if it trips. I'm comfortable with that plan.

However, he also loved multi-wire branch circuits. I've given them special attention and I think they're OK, but want to test them nonetheless. This panel is obsolete, and a 2-pole GFCI breaker is prohibitively expensive. So I want to use the 1-pole GFCI to test half the MWBC at a time. The other hot would cause a false reading, so I just want to disconnect it temporarily.

Does this seem like a reasonable plan?

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I would suggest rather than leaving one hot disconnected you connect the two hot wires together. That way you can test the whole circuit at once and you don't have to deal with a loose and potentially hazardous wire.

This should only be done once you are reasonably confident that no hot-hot-neutral 240V devices are connected to the circuit.

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Outlet testers are inexpensive and with their 3 LEDs can tell you about a multitude of wiring errors. I wouldn't work on anything like this without one.

As for the neutrals, in an ideal world they should all be the same, (if they go back to the same box), but sometimes stuff happens and one may have more resistance than another.

Again, the outlet tester can usually tell you this.

The best way to test a GFCI is with the internal test and reset buttons.

And the best way to know whether it is protecting the downstream outlets is, (you guessed it), with an outlet tester.

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    Outlet testers will not detect borrowed/shared neutrals between circuits. – Peter Green Mar 6 '17 at 22:22
  • When you hit the test button on the GFCI you use the tester to see how many outlets branch off from it. Often it can be a whole room. – SDsolar Mar 7 '17 at 7:49

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