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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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dlf
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3 Answers3

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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.

Tester101
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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.
AaronD
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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.

Edwardt
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