Neutral-ground bonding for wall receptacles

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

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

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

question,

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?

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

If wired correctly, the neutral, which is a current carrying conductor, cannot come in contact with a human under normal circumstances. The ground, which is _not_ a current carrying conductor, is regularly in contact with humans at many metal case appliances and tools.

It is obvoius that you do *not* want a human to come into contact with a current carrying conductor. You want to keep the current away from the human. Here's a "picture" of what can happen.

------- Hot------------------|Recept |---Appliance---+ | | | Neutral--- Defect ---|Nuetral|---+-----------+ | | | |

How do you know the white wire is really grounded? Somebody night have swapped them on an upstream receptacle. Then what do you think you would have?

We grounded ranges, cooktops, ovens, and laundry dryers for about 60 years using the neutral. They claim this practice was adopted during WWII to save on copper. Today in many older homes this practice remains in place. Although the NEC forbids this practice for 120 volt 15 and 20 ampere receptacles, I think it might be an acceptacle practice as it has proven to be safe for the ranges and dryers for many years.

How do you know the green wire is really grounded?

Chuck

On Mon, 7 Jan 2008 17:12:31 -0800 (PST) Amos Kariuki wrote: | On Jan 7, 6:32 pm, "Michael A. Terrell" | 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.

It is very common to find white and black swapped in receptacles.

Range and drier circuits had to originate in the service panel and IIRC had to be connected with service entrance cable.

If you are talking about 15/20A circuits, there can be many splices back to the panel where the neutral can open or black and white be swapped. If wired correctly now, the black and white can be swapped when a receptacle is replaced.

I beg your pardon; "originate at the service" for dryer and range circuits was not a NEC requirement for many years. I wired many four plexes where the circuits originated at the subpanel and that was to code. I also read that in some European countries they use the neutral for grounding. To me it makes more sense to use the neutral for grounding since if you lose it the circuit stops working. There is no way of knowing if you lose the grounding conductor or not unless you test for it.

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).

In the US, a seperate ground is used at the main disconnect.

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

I've run into several places where the wire is spliced to another color, and pulled through conduit. One was pink at on end, and yellow at the other end. :( It was between floor outlets in a school office, and the janitor had removed the screw in covers before using a floor scrubber. Then he put them back, without the rubber seals. The conduit was full of dirty water and floor wax. The power for the school's intercom system ran through the same conduit, and it had opened, due to electrolysis. I though I was going to need a blasting cap and new conduit, but after a couple hours of work, I managed to open the conduit enough to run new wire. Needless to say, it was an expensive repair, and they fired that janitor, before they found other stupid things that he'd done.

A lot of code rulings are partially based on probability. The probability of a ground/neutral connection opening up on a range or dryer (assuming it was installed correctly in the first place) is very, very low. These conductors are typically larger than other wiring and make a home run to the disconnect panel with few or no splices. This is why the neutral was also allowed to function as a ground for so many years.

Even though a very small (probably insignificant) number of "engergized dryer or range frame" accidents have been reported over the years, it is safer still to isolate the frame with a separate ground and keep the neutral separate, as well.

There is an added cost for doing this (running an extra wire to the junction box - using a four conductor instead of a three conductor hookup cord and plug, but apparently the authorities in charge think that it is worth it.

The new 2008 code is going to require arc-fault protectors in many new home spaces (instead of just bedrooms). Once again... this will be at added cost to electricians, new homebuilders, and new home purchasers. Once again, the code authorities believe that this will prevent a few fires from happening that might otherwise burn down your house.

Beachcomber

On Mon, 7 Jan 2008 17:05:38 -0800 (PST) Amos Kariuki wrote: | On Jan 7, 6:11 pm, Dave22 wrote: |> On Jan 7, 3:49 pm, Amos Kariuki wrote:> Have a bonding 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.