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I'm in the United States. I have a handful of three-prong outlets that have been grounded to the underground water main just as it enters the crawlspace. All of the ground wires are home runs to the same clamp on the water main. This rules out ground-to-ground potential differences but maybe not ground-to-neutral differences, since I believe the panel is coupled to the water main only through dirt.

It's my understanding (correct me if I'm wrong) that the ground terminal on GFCI outlets can safely be left unconnected as long as the cover plate is marked "NO EQUIPMENT GROUND." Is this safer than the existing plumbing-ground situation I described?

I realize that neither choice is ideal, but replacing all of the 14-2 NM with modern 14-2/WG isn't an option at the moment.

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How is the panel grounded? Assuming there are ground rods in the soil, how close are they to the water main, and how is your soil? –  Jay Bazuzi Jul 24 '12 at 3:31
    
The panel is modern and has two separate ground rods. They are about 20 feet from the water main. The soil is clayey. –  ArgentoSapiens Jul 24 '12 at 15:32
    
My vote would be for GFCI with no ground. GFCI will eventually trip when there is a ground fault, and should prevent you from being killed. With no GFCI and receptacles bonded to water pipe, the ground fault might not kill you but it might kill the plumber holding the pipe while standing in a puddle. –  Tester101 Jul 24 '12 at 16:44
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3 Answers 3

up vote 2 down vote accepted

This is not even a contest. A non-GFCI outlet will happily send 1A through hot, into a faulty or misused appliance (think: submerged toaster), from there through you into some other ground (any ground will do, it doesn't have to be that same appliance's ground), without noticing anything's amiss. You, meanwhile, are dead several times over. The ungrounded GFCI-protected outlet, OTOH, trips as soon as the current imbalance reaches 30mA or so. If all the current leaving the GFCI eventually returns, there's none left to be going through your heart.

Yes, the GFCI could fail to open when it should (so test it regularly), but the counter-objection is that having a ground on the outlet doesn't guarantee that the circuit breaker will operate when a loose wire in an appliance touches the chassis. This is why my local electrical code doesn't like the idea of a single failure causing harm, so all socket outlets must be both GFCI-protected and grounded. For some reason I don't quite understand, lighting fixtures are exempt from this, as are stoves/ovens.

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It sounds like you're proposing to abandon the retrofit ground wire. Is that correct? It is of less than no value? –  ArgentoSapiens Jul 24 '12 at 19:03
    
Not sure I understand... I'm sticking to the brief of the question: choose the lesser of two evils. –  Bernd Jendrissek Jul 24 '12 at 19:24
    
I appreciate the directness of your answer. And I do like the idea of abandoning the janky retrofit that someone else did. –  ArgentoSapiens Jul 24 '12 at 20:13
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Is the pipe under ground galv steel or plastic? Assuming it is metallic pipe to a metallic box where the outlets are to be installed, you can run a green ground jumper from the box to the ground terminal on the GFIC. I would then be absolutely sure to run a #4 Copper bonding conductor from the panel ground to the water pipe in the house with an appropriate connector.

Regardless, always use GFIC outlets outdoors. They will work without the ground connected, but the extra work of bonding/grounding the boxes and outlets is worth the effort.

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The pipe is copper and the retrofit ground wires go all the way into the boxes and to the ground terminal on the receptacles. –  ArgentoSapiens Jul 24 '12 at 15:33
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Ground and neutral should be tied together at the main panel.

A GFCI will trip when it detects a current imbalance between hot and neutral. This means that current imbalance may go through your body for a short time. It's not likely to hurt you but there it is.

A grounded outlet should trip the associated breaker if hot ever touches ground. No current would go through your body if you were not touching the equipment.

That's if everything works as intended. There are failure modes that could increase the chances of you being hurt. For example, if the ground wire is broken somewhere, you might not know until it is too late. Likewise if the GFCI protection failed, you might not know either.

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Agreed, but in this case the ground wire does not go near the panel. –  ArgentoSapiens Jul 23 '12 at 22:45
    
In that case, what you have can not be considered grounded for safety reasons and your answer is use the GFCI. –  Philip Ngai Jul 23 '12 at 22:52
    
Better might be to run a ground wire from the panel to the same clamp. Nowadays the clamp should be within the first 5 feet of the water pipe's entering the building. The panel should also be grounded via an independent grounding electrode. –  Jeremy W. Sherman Jul 23 '12 at 23:27
    
@JeremyW.Sherman If you run a wire from the service panel to the clamp on the pipe, wouldn't that wire have to be large enough to carry the full amperage of all circuits connected to the pipe? This would also mean the pipe could act like your grounding electrode instead of your current grounding electrode, so the wire would have to be capable of handling the full service amperage? –  Tester101 Jul 24 '12 at 16:24
    
@Tester101 Not exactly. For example, 1 or 1/0 ungrounded service entrance conductor requires only an 8 AWG copper or 6 AWG aluminum/cu-clad aluminum grounding electrode conductor. Start at 250.24 Grounding Service-Supplied Alternating-Current Systems. (D) Grounding Electrode Conductor refers to 250.66 Size of Alternating-Current Grounding Electrode Conductor. You'll find a table there. 250.66(A) Connections to Rod, Pipe, or Plate Electrodes says the "portion of the conductor that is the sole connection to the grounding electrode [need not] be larger than 6 AWG copper or 4 AWG aluminum wire." –  Jeremy W. Sherman Jul 24 '12 at 16:43
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