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As described here (for example), in a correctly-wired receptacle, there will be a small potential difference between the neutral and ground wires that increases slightly with increasing load. I'm thinking that with a bootleg ground, this voltage will always be 0 no matter how much current is running through the circuit. Is my reasoning correct? If so, is measuring the neutral-to-ground voltage a good way to test for the presence of a bootleg ground?

Backstory: I'm in the middle of buying a house in the US. The home inspector discovered that some of the three-pronged outlets had no ground wire. The listing claimed recent electrical upgrades, so I asked the seller to rewire these. I meant for this to be the starting point of a negotiation, but somewhat to my surprise, he agreed to do it. In the spirit of finding a cloud for every silver lining, I'm concerned this means he's planning to do something like a bootleg ground to fool me and the inspector into thinking the problem is solved, and I'm trying to come up with a way to test for this without buying a $300 circuit analyzer.

Backstory update: It turns out the guy just replaced all the ungrounded outlets with GFCI boxes. Which is not exactly what I asked for, but good enough.

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    Do you still have room in your agreement to specify "NEC compliant repairs"? That would open the door to legal action if you discovered bootleg grounds. (Though it might also mean you end up with 2 prong plugs...) – Aloysius Defenestrate Sep 23 '15 at 13:38
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    @Aloysius It's too late to specify that for this round, but if I'm dissatisfied with the work as I see it, I could include it next time. Time is not on my side, though. At this point, unless problems are major, I'll probably just need to fix them myself (or demand a discount). – dlf Sep 23 '15 at 15:11
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    Out of curiosity, can you check resistance for bootleg instead of voltage difference. If there is a bootleg, I'm guessing resistance will be about .001 ohms. – diceless Sep 23 '15 at 17:02
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    @dlf , in order to get accurate readings yes. But in this case you are only looking to see if it is more than near zero. – diceless Sep 23 '15 at 20:10
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    @diceless I thought it was to protect the meter from getting fried. Is that not the case? – dlf Sep 23 '15 at 20:12
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The best way to check for the bootleg ground, is to open up one of the boxes and look at how the receptacle is wired.

Since the grounded (neutral) and grounding conductors are bonded at the service equipment, they should always be at about the same voltage potential. If you had a long circuit, and an accurate meter, you might be able to measure a slight difference.

Since wire has a resistance, it's certainly possible to measure a voltage difference. However, a reading of 0 volts, does not necessarily mean there's a bootleg ground. The only way to know for sure, is to actually inspect the wiring.

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    Understood, but since I don't own the house yet, I'm not sure I'm technically allowed to do this. I'd like to anyway, and could insist on it as a condition for waiving my inspection contingency, but I'd rather avoid the whole discussion if possible. Also, for what it's worth, I tested my multimeter on a correctly-wired outlet at work, and it was able to detect a 0.5V difference. – dlf Sep 23 '15 at 13:32
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    A home inspector won't unscrew face plates, but assuming you're willing to accept liability on your own walk through, most everyone looks the other way to the buyer doing a non-destructive check like this. – BMitch Sep 23 '15 at 13:39
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    @BMitch My home inspector removed a few cover plates, when I was buying an older home. I guess it just depends on the inspector, and the area. – Tester101 Sep 23 '15 at 13:41
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    That seems to make sense. What about the converse--if I do see 1 volt or so, can I be pretty confident there is not a bootleg ground? (I guess the guy could have wired them together with a weak resistor in between, but I don't think I'm dealing with a criminal mastermind). And if I'm going to pick just one receptacle to take apart, I should prefer one that shows zero voltage over one that shows more? – dlf Sep 23 '15 at 14:28
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    Check any one of the receptacles the homeowner "fixed". If they're installing bootleg grounds, they're likely going to do the same on each receptacle. Or, remove the cover plate on all the receptacles that were "fixed". Or if you really want to be sure, ask the homeowner to show you the repair, or give you a copy of the bill for the repair (if an Electrician did the work). – Tester101 Sep 23 '15 at 14:56
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  1. Get a big load, like a space heater or an old-school theatrical spotlight.
  2. Plug it into each outlet in question and measure the voltage between neutral and ground.

    • If there's no difference, it's bootlegged.
    • If there is a significant difference, then it's probably done right.
0

The likelihood of coming across 0.0 volts AC on a properly wired system is not great. So if you check them and do find 0.0 volts, it would warrant some investigation.

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    Disagree. If there is zero load on the circuit, the current through the neutral wire will be zero, and thus the voltage at the outlet's neutral will be zero. – Jonathon Reinhart Sep 23 '15 at 22:00
  • Disagree. Even with no load on the circuit, you are not likely to actually get 0.0 volts. I just tried it on a completely empty, but powered up circuit and I show 24.8 mV. Even if you put your meter leads together, you still won't get 0.0. And I'm using a newly calibrated Fluke 789 Processmeter. – Edwardt Sep 24 '15 at 0:19
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    @edwardt If you have a $900 digital multimeter, you'll probably never measure 0.0000 volts neutral to ground. For most diyers with a $20 meter, 0.0 volts is definitely a possibility. – Tester101 Sep 24 '15 at 1:12
  • @Tester101 Fair enough. My job requires me to have the processmeter, so I haven't used a diyer meter in many many years. I can agree with you on that point. – Edwardt Sep 24 '15 at 1:15

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