Neutral-ground bonding for wall receptacles

Have a bonding question,
For old houses that may be missing an available grounding wire (from the distribution panel) at the wall receptacle, the NEC forbids
bonding the ground terminal of a three plug receptacle to the neutral-- for purposes of providing a ground fault current path. What is the reason that this is forbidden and what is the specific hazard posed?
The main reason one would want to do this is to provide a three plug wall receptacle having an available ground terminal--for old houses that may not have an available ground fault path. I'm aware that you can use a three plug GFCI with the ground terminal floating but I'm curious as to what hazard is posed by the neutral-ground bonding.
Amos Kariuki
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for many appliances and devices the grounding pin is connected to the equipment chassis that could be touched by someone.
If the grounding pin in the outlet is connected directly to the neutral conductor an overload from another component plugged into the same circuit could cause a dangerous voltage to appear on that grounding pin due to the voltage drop from the current in the neutral.
Since a true grounding conductor does not carry anything but fault currents this condition is less likly to occur with correct wiring.
Dave 22

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Dave22 wrote:

If the two are bonded at the outlet and the neutral opens, the case can be at full line potential.
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wrote:

Agreed.
I was assuming that failure of the neutral inside the building wiring would be an unlikely event. In a similar case, it seems strange that neutral-ground bonding at the service distribution panel is allowed by the NEC, since the neutral conductor connection could fail at the (outside) transformer.
For this case, it seems like it would be safer to always require a separate ground wire, that is bonded only at the transformer.
Amos Kariuki
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wrote:
| wrote: |> If the two are bonded at the outlet and the neutral opens, the case |> can be at full line potential. | | Agreed. | | I was assuming that failure of the neutral inside the building wiring | would be an unlikely event. In a similar case, it seems strange that | neutral-ground bonding at the service distribution panel is allowed by | the NEC, since the neutral conductor connection could fail at the | (outside) transformer.
While it might be an unlikely event, when it might happen it can be a very serious event ... killing people.
And in the more common scenarios, problems can happen with the voltage drop of the neutral being applied to the appliance case.
| For this case, it seems like it would be safer to always require a | separate ground wire, that is bonded only at the transformer.
Or at least at a common point at the entrance where it is also earthed, which is exactly what the NEC requires.
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On Jan 8, 9:27 am, snipped-for-privacy@ipal.net wrote:

If the neutral fails at the transformer, you'd still have the same safety hazard since the earth bonding at the service entrance doesn't provide you any significant safeguards against electrocution (for this case). Furthermore, it's likely that you'd also have a brownout condition (equipment hazard) since the ground resistance would now be in series with your building load (from the utility transformer's perspective).
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Amos Kariuki wrote:

In the US, a seperate ground is used at the main disconnect.
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alt.engineering.electrical, snipped-for-privacy@earthlink.net says...

Define "main disconnect". My last house had the entrance panel located about 30' from the meter. There was a disconnect switch at the meter (and of course the main breaker in the panel). I thought the separate ground was connected back to the grounding point (usually the entrance panel).
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krw wrote:

"Service Equipment. The necessary equipment, usually consisting of a circuit breaker(s) or switch(es) and fuse(s) and their accessories, connected to the load end of service conductors to a building or other structure, or an otherwise designated area, and intended to constitute the main control and cutoff of the supply."
IOW it is the first place that the supply to the premise wiring system can be readily opened or disconnected. -- Tom Horne
"This alternating current stuff is just a fad. It is much too dangerous for general use." Thomas Alva Edison
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alt.engineering.electrical, snipped-for-privacy@veriqrmzon.net says...

Like I said, the above doesn't seem to be true then. The service in my previous house could be "disconnected" at the meter, some 30' (across the garage, on the front porch, with great-circle routing) from the entrance panel.
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krw wrote:

I consider the 'Main Disconnect' as the first breaker in any building, intended to shut off all electrical service.
A breaker, fuse or switch at the meter location, if separate from the building's wiring would be a 'Master Disconnect'.
By this logic I have one 'Master Disconnect' at the meter, and four buildings with a 'Main Disconnect' on my property. At one time there were two meters on the property. The electric company made the previous owner remove one pole that crossed the driveway. A line was run under the driveway, and an outdoor breaker box was installed on the stump, because that pole had supplied power to three of the buildings. What do you call that box? It has no main breaker, yet it feeds three additional breaker boxes?
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alt.engineering.electrical, snipped-for-privacy@earthlink.net says...

Ok, we still have the same problem. ;-)

As opposed to the "Main Disconnect"?

Sure, I would expect every building to have an "entrance panel" and "main disconnect". I don't think it's legal to have a separate building with only a sub-panel off the entrance, is it?

I'd remove the pole crossing my driveway without the power company telling me to. ;-)

Drop box? Power drop?

Oh, it has no outlets, just a junction box? I'd call it a "junction box". ;-)
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krw wrote:

Woth breakers?
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Michael A. Terrell wrote:

A breaker of switch located at the meter is just another form of Service Equipment. The conductors from a meter enclosure that is equipped with Service Disconnecting Means to the panel or panels they supply are feeders rather than Service Entry Conductors. If they serve a panel located in the same structure then the feeder has to have a separate Equipment Grounding Conductor (EGC) and all of the EGCs from those panels would be kept separate from the Grounded Current Carrying Conductors [neutrals]. All of the "neutrals" would be bonded to ground at the Service Equipment Enclosure older installations may have three wire feeders with the "neutrals" bonded at the Building Disconnecting Means (BDM). If a building has six or fewer circuits then the individual breakers mounted in a main lug only (MLO) panel may serve as the BDM.

What it is called would depend on whether or not it contains breakers, switches, or fuse pull outs. If it has no controls it is a meter enclosure or "meter pan". If it has controls that can shut off the current flowing to the four buildings then it is the Service Equipment and it contains the Service Disconnecting Means.
As a rather important aside it should also contain the only Over Current Protective Device for the water pump for that property regardless of whether that takes the form of a breaker, fused switch, or fused pullout. The reason that the farm bureau and other rural property interest prefer this is it allows cutting off the power to any building without shutting down the pump that provides the water for first aid fire fighting efforts. -- Tom Horne
"This alternating current stuff is just a fad. It is much too dangerous for general use." Thomas Alva Edison
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Amos Kariuki wrote:

It might be an 'unlikely event' to develop an open neutral, but I've seen more than a few in the last 40+ years.

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On Tue, 08 Jan 2008 20:13:55 -0500 Michael A. Terrell
| Amos Kariuki wrote: |>
|> wrote: |> > If the two are bonded at the outlet and the neutral opens, the case |> > can be at full line potential. |> |> Agreed. |> |> I was assuming that failure of the neutral inside the building wiring |> would be an unlikely event. In a similar case, it seems strange that |> neutral-ground bonding at the service distribution panel is allowed by |> the NEC, since the neutral conductor connection could fail at the |> (outside) transformer. | | | It might be an 'unlikely event' to develop an open neutral, but I've | seen more than a few in the last 40+ years.
In about as many years I have seen 3 of these without being an electrician one might be called to investigate. The first was in my own home when I was in junior high. My mother was quizzing me on why some of the lights would get dimmer while others would get brighter when she turned on the stove elements and adjusted their heat level. I immediately knew what the problem was, but didn't have the worse to explain it other than to say the wiring was bad. My parents at first were not willing to accept that since the lights were all on. I eventually convinced them the electrician needed to be called right now. Given there had been a couple other issues with the electrical wiring in the house before (one we rented), they finally agreed to do so. He found the loose neutral in the panel, as well as some exposed wiring for the stove itself. The 2nd incident with a neighbor down the street. Her stove was smoking, especially in and around the clock it contained (so it was using 120 volts and getting toasty from well above that). Some arcing happened behind it. The loose neutral turned out to be in the socket. The stove was replaced due to damage. The 3rd incident was a loose neutral in a large room of cubicles. And this was three phase. Maybe it was a burned out neutral due to harmonics from so many computers? There were an average of about 2 per person (I had 4). There were about 60 programmers in the room. Lights and computers were acting funny and things changed as people turned stuff off. Shortly, with someone else's help who knew where it was, we got to the breaker panel. After we did some pondering about where the cubicles were wired in, since it did not identify things specific enough, I just grabbed the main and flipped it off. We lost a half day of work instead of a room full of computers and other stuff to fire.
Does this mean loose/broken neutral are rare? I certainly don't go around hunting for them. But maybe they hunt for me?
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On Mon, 7 Jan 2008 16:11:09 -0800 (PST), Dave22

Another way to look at the problem is to see that without the grounding conductor, a "ground fault" would allow return current to flow through the ground, pipes, etc., all of which would be in parallel with the neutral.
With the grounding conductor, fault current sees a low impedance path back to the service ground.
An important consideration is that ground fault currents may be tens of amperes but be insufficient to trip protective devices and clear the fault, even for long periods of time. Meanwhile, a portion of that current would be travelling through the ground (earth, pipes, etc.) in parallel with the neutral.
Chuck
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Doesn't the same argument apply whether the neutral-ground bonding is done at the wall receptacle or at the distribution panel, i.e. a voltage drop in the neutral will be seen at the ground terminal for both cases?
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wrote:
question, |> |> If the grounding pin in the outlet is connected directly to the |> neutral conductor an overload from another component plugged into the |> same circuit could cause a dangerous voltage to appear on that |> grounding pin due to the voltage drop from the current in the neutral. |> | | Doesn't the same argument apply whether the neutral-ground bonding is | done at the wall receptacle or at the distribution panel, i.e. a | voltage drop in the neutral will be seen at the ground terminal for | both cases?
If the bonding is at the entrance panel, and earthed there, then any neutral voltages will be held near ground potential by the low impedance path to ground at that panel. No current will flow over any of the grounding wires with the exception of one where a ground fault might happen. In the case of bonding at the receptacle with no grounding wire, and no earthing, you get the neutral voltage on the case of the appliance through its grounded case.
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snipped-for-privacy@ipal.net wrote:

Phil In most cases the impedance of the Grounding Electrode System GES is too high to actually keep the Equipment Grounding Conductors at or close to earth ground potential. But since all of the conductive surfaces of the building will rise to the voltage that is being dropped across the building's GES and the transformers Grounding Electrode in series the voltage of the conductive surfaces in the building will be the same. Persons coming in contact with the voltage will get a very unpleasant shock but it is less likely to be a fatal one. IN homes on public water systems no such voltage rise occurs because the neutral current is distributed across all the GESs of the buildings that are served by that transformer and across the remaining intact neutral connections if any. Even in the unlikely event that the utility neutral is open at the transformer their combined parallel resistance is usually low enough to prevent any substantial rise.
One of the reasons that service equipment installed to serve manufactured buildings post production cannot be mounted in or on the building is that too much of such a building is likely to be both conductive and poorly grounded making the exterior of such a structure uniquely dangerous if an open neutral occurs in its supply conductors if that neutral was bonded to the frame of the manufactured building. Such services must be mounted off of the structure and a separate Equipment Grounding Conductor must be run with the supply feeder to the structure. This assures that there is a low likelihood of an open neutral energizing the entire building because the utility neutral is usually run underground to those services reducing the likelihood of physical damage. The arrangements spelled out in the NEC for recreational vehicles and mobile equipment are similar. -- Tom Horne
"This alternating current stuff is just a fad. It is much too dangerous for general use." Thomas Alva Edison
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