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I noticed with a two wire cable (with a ground), the ground and neutral wires in the circuit breaker box all connect together. If I have a two wire only cable to a new three wire receptacle can I install a jumper from the neutral to the ground terminal in the receptacle box with the same results?

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Although the neutral wire will generally be near ground potential, there are some situations where it might not be. For example, when switching on a large motor, the voltage on the neutral wire may jump briefly but significantly. Further, if the neutral wire breaks between a device and the panel, it will very likely end up with full line voltage on it. If the neutral wire were bonded to the ground terminal on a piece of equipment, any voltage on it could shock anyone touching the equipment. Because the current would be flowing on the ground wire which the ground-fault interrupter does not switch, the GFCI would provide no protection against it.

The accepted way to handle the situation where no ground is available is to use a GFCI with its ground lead attached to nothing but with a label affixed reading "NO EQUIPMENT GROUND". A GFCI used in such fashion will not detect some failure modes that would be detected if the grounding wire were bonded to neutral but the danger of putting neutral voltage on the ground wire exceeds the safety gain from catching those particular failure modes.

It would be helpful if someone were to sell a GFI which was designed to connect ground to neutral well enough to trip the detector immediately if a device has a leak between its hot and ground leads, but not well enough to pose a dangerous condition. I am unaware of any such devices being for sale, however.

  • Interesting. What are these additional failure modes? I'd always assumed that a GFI was basically the same piece of equipment that we have in the UK called an RCD, but those do not have a connection to ground (for example, the ones in my house look very much like the one shown here: electricalonline4u.com/2015/11/…). They detect an earth fault by ensuring the current flowing through the live circuit is equal within 30mA of the current on the neutral circuit. What else do US GFCIs do with an additional connection? – Jules Jun 13 '16 at 21:08
  • @Jules: If a device that's plugged into an RCD/GFCI has a case which is connected to the earthing/grounding lead, and a fault develops that causes the hot lead to be connected to the case but the earthing lead isn't connected to anything, no fault currents will flow until something or someone touches the case. While the RCD/GFCI would likely ensure that fault currents couldn't flow long enough to kill someone, they would not prevent the ground fault from delivering a nasty shock; if the ground fault occurred as a result of connecting pieces of electronics, they would likely be fried. – supercat Jun 13 '16 at 21:25
  • @Jules: An outlet-mounted GFCI with a "cheater" between the grounding lead and the line side neutral would trip immediately if a short developed between hot and the case, rather than leaving the case electrified until someone touches it. – supercat Jun 13 '16 at 21:27
  • Ah, I think I see the difference -- in UK wiring, the earth lead is always connected somewhere, but just not to the RCD. It joins with neutral at the point where power enters the distribution board, bypassing the RCDs. And as conductive cases are required to be always connected to the earth circuit, the situation you describe should never occur. – Jules Jun 13 '16 at 21:30
  • @Jules: Many homes in the US have outlets at which hot and neutral leads are available, but no separate earthing/grounding lead. The "problem" is figuring out how to make those outlets safe without having to rip out walls to install new wiring. – supercat Jun 13 '16 at 21:32
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Short answer: NO

Bonding the neutral and the ground anywhere but the service is prohibited.

This puts neutral current on the ground wire that people expect to be safe under normal conditions.

I know they go the same place, but they serve two completely different functions.

Good luck!

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    But there's no ground wire, so the described connection doesn't bond anything to the ground. – Random832 Jun 10 '16 at 16:40
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    @Random832 The metal parts of the receptacle itself are bonded to the grounding terminal. Connecting neutral to ground terminal, puts neutral currents on the metal parts of the receptacle. – Tester101 Jun 10 '16 at 17:46
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    @Tester101 Yes but none of that goes back to the ground in the house wiring, because there's no ground wire for the receptacle to be tied to either. – Random832 Jun 10 '16 at 19:42
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    @Random832 Unless some poor sap comes along to plug something in, and provides a current path. Then ZAP! – Tester101 Jun 10 '16 at 20:50
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    @Random832 Right, you won't energize any grounds except in the GFCI outlet itself, but you will energize those, including the cover plate/screws of the outlet. – Harper Jun 11 '16 at 0:42
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Long story short, if you have no proper source of ground, then the proper choice is to use a GFCI with no ground bonded at all.

Why this is okay is interesting though:

Suppose you have a metal tool with a ground fault. If you are totally isolated from any grounding conductor while using the tool, you will never know it has a fault. Once you become grounded, then the GFCI will sense a fault and trip.

The only real difference you would see between the two scenarios is if the fault is really low impedance. If there were a proper ground, then the breaker would trip immediately with an arc at the receptacle when the tool was plugged in GFCI or no. Now you will not get that immediate shut off. Instead either you will become part of the path and the GFCI will trip, or you will place the tool against something grounded, at which point there will be an arc at the tool and the breaker will trip.

So it's not as good having a ground conductor, but it's acceptable (and it meets code requirements) to install that way. But ONLY if there's no ground available at all. That is to say that you can't skip wiring the ground just because.

[EDIT] As Hot Licks says in the comments, make sure to label the receptacle that it is not grounded. :)

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    It should be noted that an ungrounded GFCI outlet is supposed to be labeled as such. – Hot Licks Jun 10 '16 at 22:39

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