Neutral-ground bonding for wall receptacles

On Tue, 08 Jan 2008 20:13:55 -0500 Michael A. Terrell wrote: | 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. | | | 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

Does this mean loose/broken neutral are rare? I certainly don't go around hunting for them. But maybe they hunt for me?

Amos Kariuki wrote:

question,

Use your common sense and compare the nature of the connections at the neutral buss, meter enclosure, demarcation point (the splices that connect your service entry conductors to the utility's supply conductors), with those that are made in an outlet box. The ones at and before your Service Disconnecting Means (SDM) are tightened by the most experienced wireman on the job, using torque measuring tools, with anti corrosion paste applied. They are then closely inspected by the electrical inspector and checked by the utility company's outside wiremen prior to power being applied. Which of the two do you think is more likely to suffer a failure? That doesn't mean that they cannot fail because they can and do but that is a far less frequent fault then an open neutral on a back stab receptacle in one of dozens of outlet boxes most of which were made up by the least experienced helpers who are being pushed for production. Just as importantly the bonding that is done during the construction that keeps all of the conductive surfaces at the same voltage reduces the likelihood of a fatal shock and the malfunctioning of the served loads in the entire home provides the impetus for the occupants to report the trouble and have it cleared. If just one circuit is affected the occupants are more likely to try to muddle through and the other conductive surfaces in the building are not at the same elevated potential as the bonding pins of the faulted circuit. The 120 volt difference between various surfaces in the home makes a fatal shock far more likely.

-- Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use." Thomas Alva Edison

bonding question,

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

Gerald Once again, as I opined in another answer on this question, the difference is in the nature of the connections themselves. The circuit must originate in the Service Equipment enclosure, the conductors are heavier, invariably multi stranded, and the connections are made using single conductor terminals designed for the purpose. The rate of failures was perceived to be low enough to be acceptable. It was actually the advent of better data collection and analysis that was the three wire 120/240 volt circuits undoing. The NEC code making panel became aware that failures were far more common then was believed previously.

-- Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use." Thomas Alva Edison

On Tue, 08 Jan 2008 11:54:32 -0500 snipped-for-privacy@aol.com wrote: | On Tue, 08 Jan 2008 08:51:15 -0500, Chuck | wrote: | |>>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? |>

|>How do you know the green wire is really grounded? |>

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

And no one, trying combinations to see what works, would ever hook up a receptacle between black and green?

| 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.

You mean there was a separate grounding wire before WWII?

| 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.

They should be quite safe with a neutral in place of ground when:

1. All the loads in the appliance are wired L-L, e.g. 240 volts
2. The appliance is on its own dedicated circuit
3. The circuit is sourced from the main panel that has the N-G bond

Most dryers in the US have 120V motors, and lamps. Many ranges also have 120 V. oven lights, and clocks, and sometimes blowers and, as I remember from my childhood, often there was a two prong convenience outlets mounted near the clock.

.... This means that there will be at least some neutral current. So maybe it isn't so safe if the neutral is connected to the frame?

Phil had a good question. Assuming that they had 220V appliances before WWII, did they actually run a separate conductor to ground these things? (and a neutral?)

A related interesting side note from Wikepedia:

Philip F. Labre invents the grounded outlet in Milwaukee (1928)

For the code I just looked at the requirement was

- insulated ground wire or

- service entrance cable originating in the service panel.

That eliminates Romex unless it has an insulated ground wire but some methods could use a subpanel.

When they do use Romex you will typically see the bare ground wire going to the box or backstrap of a surface mount receptacle. The insulated white wire goes to the center pin of a 3 terminal receptacle. That is why I always have people look before they assume they can't convert to 4 pin receptacles.

As I recall from reading the original Soares book on grounding where he devoted a whole chapter on the history of grounding, at one time in the early 1900's there was an international debate on whether grounded systems were better than ungrounded systems. As I recall at one time all the grounding was removed from the services in New York City because they thought ungrounded systems were the way to go. Eventually they decided that grounded systems provided superior protection for humans while ungrounded systems were better for not acting as a source of ignition for fires. But I still maintain that if the ranges and dryers could be grounded using a neutral for 60 years that this is a viable alternative that could work throughout the electrical system. Just because the code says it can't be done does not necessarily mean it is not a safe option. I for one am convinced there is a great deal of lobbying by the copper industry to use more copper in our electrical systems and that not all options are given equal opportunity when decisions are made.

A bit of a red herring. Appliance cases grounded through the neutral were definitely safer than those not grounded at all. That's why it was done. The question you need to address is whether grounding through the grounding wire is more or less safe than through the neutral.

The US Navy uses no grounded conductors at all on its ships, which is really not intended as a safety measure, but rather to prevent disruption of critical functions in the event of a (first) line to ground fault. Also to avoid currents flowing in the hull, esp. through welded joints.

Even with no grounded conductors, lethal voltages can and do arise between ungrounded conductors and the hull due to the capacitance effects of miles of wiring.

Ungrounded 3 phase delta systems are often found in industrial installatiions and in many cases permitted by the code.

Chuck

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).

Not nearly as likely, I suspect. The "green" terminal on most fixtures is, well, green. Many people don't know there is a difference between the other two or that outlets are polarized. At one time there wasn't and they weren't.

"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

It is at least less likely because the ground screw is usually painted with green metallic paint. It is somehow beyond some people to match the black to the brass colored screw and the white to the silver colored screw. Then you add in the conductors that the painters spray gun made the same color as the room's finish, the ones that are so old that the original color coding is gone...

-- Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use." Thomas Alva Edison

Well you're convinced so there's an end to all discussion with you on that subject.

Now for the benefit of those who are willing to read on I'll recap. Individual conductor terminations made to listed terminal lugs using anti corrosion paste as necessary have a far lower rate of failure then production receptacle installations. This is especially true were push in spring loaded receptacle terminals are used.

If I had my way we would have a separate bonding conductor all the way back to the utility's neutral connection at the transformer so that the hazards of an open neutral would be reduced even further. Modern services in Europe are equipped with service ground fault protection so that if the neutral goes open so does the main breaker.

-- Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use." Thomas Alva Edison

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?

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

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". ;-)