Neutral-ground bonding for wall receptacles

On Wed, 9 Jan 2008 12:49:00 -0800 (PST), Gerald Newton


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
-
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
| Ungrounded 3 phase delta systems are often found in industrial | installatiions and in many cases permitted by the code.
These are under supervision of appropriately trained personnel who are supposed to be able to handle the exceptional conditions. They are "relatively" safe because most of the time they are just as safe as grounded systems. The times they are not would not get ignored since there would be alarms in the right places alerting the right people. You wouldn't have one of these in your home (even if you could get three phase power there at any voltage and configuration you wanted and had stuff that would be happy to use it).
--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 12 Jan 2008 17:09:20 GMT, snipped-for-privacy@ipal.net wrote:

Hello Phil,
I think an argument could even be made that 3 phase ungrounded systems are always at least as safe as grounded systems in the following sense.
Considering the line-to-ground capacitance, there is always some line to ground voltage, but it could be less lethal than the line-to-ground voltage of a grounded neutral system. Difficult to generalize, but even more difficult to imagine a grounded neutral system being less lethal at the same system voltage levels.
In the event of a line-to-ground short, the line-to-ground voltage of an ungrounded system would be no higher than with a grounded neutral system.
The ground fault indicators you mention are used to indicate a fault while allowing essential systems to continue functioning until it is economical or safe to undertake repairs. When a second ground fault occurs, it will clear both faults. While a ground fault indication suggests reduced safety levels for personnel coming into contact with an ungrounded line, this is a questionable function since voltage levels may have been lethal even prior to the ground fault. The trained supervisors have other objectives in their sights.
None of this is intended to question the wisdom of NEC grounding requirements for grounded systems.
Chuck
-
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
| On 12 Jan 2008 17:09:20 GMT, snipped-for-privacy@ipal.net wrote: |
|> |>| Ungrounded 3 phase delta systems are often found in industrial |>| installatiions and in many cases permitted by the code. |> |>These are under supervision of appropriately trained personnel who are |>supposed to be able to handle the exceptional conditions. They are |>"relatively" safe because most of the time they are just as safe as |>grounded systems. The times they are not would not get ignored since |>there would be alarms in the right places alerting the right people. |>You wouldn't have one of these in your home (even if you could get |>three phase power there at any voltage and configuration you wanted |>and had stuff that would be happy to use it). | | Hello Phil, | | I think an argument could even be made that 3 phase ungrounded systems | are always at least as safe as grounded systems in the following | sense. | | Considering the line-to-ground capacitance, there is always some line | to ground voltage, but it could be less lethal than the line-to-ground | voltage of a grounded neutral system. Difficult to generalize, but | even more difficult to imagine a grounded neutral system being less | lethal at the same system voltage levels.
In the absence of a line-to-ground short, I don't see a significant issue with an ungrounded system.
| In the event of a line-to-ground short, the line-to-ground voltage of | an ungrounded system would be no higher than with a grounded neutral | system.
How so? The neutral would be at the mid point. If a line were grounded, we're talking about (for example) 480 volts L-G, vs. 277 volts L-G. If there was a L-G fault in an ungrounded system, that becomes the same as a grounded corner delta.
| The ground fault indicators you mention are used to indicate a fault | while allowing essential systems to continue functioning until it is | economical or safe to undertake repairs. When a second ground fault | occurs, it will clear both faults. While a ground fault indication | suggests reduced safety levels for personnel coming into contact with | an ungrounded line, this is a questionable function since voltage | levels may have been lethal even prior to the ground fault. The | trained supervisors have other objectives in their sights.
I don't agree that a second fault is assured to clear both faults. The two faults may be in separate places. If at least one of them is not a fault relative to the EGC (for example a damaged line wire on the shop floor behind the equipment that just ran over it), you could see a low enough current level that would not clear the fault, but could still be a substantial fault current.
| None of this is intended to question the wisdom of NEC grounding | requirements for grounded systems.
There are valid industrial needs for various ungrounded, or unusually grounded, electrical systems. Those need to be handled in special ways and in some cases electricans always on site, or at least procedures for handling them (who to call, steps to isolate, etc).
I'm just saying, these things are not so maintenance free we could use them in our homes.
--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 12 Jan 2008 18:05:55 GMT, snipped-for-privacy@ipal.net wrote:

I should have said that differently. Ignoring capacitance and leakage, someone who is grounded could touch any of the hot lines without suffering injury. Not the case with grounded systems. The full L-G voltage at low impedance is always present.
Worst case for an ungrounded system is a L-G short which (keeping the voltage the same) is no worse than the normal case for a grounded system. I wanted to focus on the topology rather than specific voltage levels.

Here I was thinking of a L-G short mentioned earlier as the fault. Two concurrent L-G shorts becomes a L-L short and obviously will trip breakers. My mind is on naval vessels where "ground" is an extremely low resistance steel hull. You're correct that a high resistance ground fault might not be cleared. In a grounded system there might be a better chance of clearing the fault given the presence of the grounding conductor.

You may be correct. The systems are certainly different. Nonetheless, I don't think a case has been made in this thread that for residential installations grounded systems are inherently safer than ungrounded systems.
Chuck
-
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Gerald Newton wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
| 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.
Where would you bond the neutral to ground with a "bonding conductor" (I don't know if you mean "grounding conductor" by that term)? I see three or four possible scenarios:
1. What we have now, which is that the neutral is earthed at the entrance as well as at the transformer by the utility.
2. The neutral is earthed at the entrance only.
3. The neutral is earthed at the transformer only.
4. The neutral is not earthed.
I'll rule out #4 immediately with no discussion.
I would not accept #3 at all. That puts the inspection and maintenance of a critical element of the electrical system outside my scope of control. If it came to this, I'd get a big dry-type transformer and derive my own 120/240 system out of the 240 L-L coming in and earth my own neutral.
FYI, I have seen several cases of broken ground wires on utility poles. Since those were not getting maintained, and what I could see was only a subset of possible places they could fail, I would not want to ever depend on the utility to get things right.
So #2 is essentially equivalent to what exists in many places already.
I'll stick with having my own grounding electrodes, whatever the utility does.
| "This alternating current stuff is just a fad. It is much too dangerous | for general use." Thomas Alva Edison
Mr. Edison was a patent monger who didn't like that Mr. Westinghouse had a bunch of them for AC. His electrical system sucked. His light bulb filament sucked. His phonograph sucked. I can't think of anything he invented that didn't need some major improvements before they become practically usable. He did just enough to get enough of a patent to be sure he got money even if someone else fixed his ideas. I'd bet that if he had never been around, most of the things he "invented" would have been invented, anyway, by the time they were actually used on a wide scale, according to our history.
--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 9 Jan 2008 12:49:00 -0800 (PST) Gerald Newton
| 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.
As long as the ciruit originates from the main panel, and assuming things are installed right at that panel, the electrical potential on the appliance frame should not be contributed to by other circuits any more so that would be the case with a separate EGC. There would be more potential in the frame due to the imbalance in the appliance itself. As long as the heavy loads, e.g. the heating elements, are connected L-L at 240 volts (not all are), then the imbalance is limited to little things like the light and clock and that convenience outlet.
I personally still believe that with the EGC it is safe_R_ enough that I wouldn't even think of doing it differently even of the NEC changed and went back to allowing imbalanced loads and "EGC" to share the same wire. That's not where I would want to "save copper". Although it is one of those "impossible battles", I'd much prefer eliminating any load connected to the neutral and not have the neutral (at least for the 240 volt loads). We have 3-wire (with or without separate EGC) utilization devices, and thus circuits, IMHO, strictly for legacy reasons. Today we don't need the 120 volt outlet on the stove (I've not seen one on a new one in decades). Today we can run all the components of a stove or dryer directly on 240 volts via a L-L connection and have the third wire strictly for EGC. Motors can be made with dual windings that can be strapped for 120 or 240 volt operation so they can be the same motor in a 240 volt electric dryer and a 120 volt fossil fuel dryer. Electronics control can be operated from a switching power supply that easily operates on the 100 to 240 volt range. Lights can be operated from that 12 volt DC output of that electronic PSU and be more reliable that way (thicker filaments). There is no longer any big _economic_ need for a neutral. I would contend that it would also, in a tiny way, contribute to saving a small amount of energy. The remaining issue is whether such appliances would need a NEMA 14-XX plug to fit the outlets of older homes or a NEMA 6-XX plug to fit the outlets of homes that are wired without the neutral to these dedicated receptacles. It would have to be a choice made at purchase for self-installed or by the installer if installed by the tech.
--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 8 Jan 2008 02:04:21 -0800 (PST) Gerald Newton
| 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
--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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)
http://en.wikipedia.org/wiki/Domestic_AC_power_plugs_and_sockets#Earth.2FGround
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
| |> |>| 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?
Aside from the convenience outlet (which decades ago was treated as more of a necessity ... my grandmother frequently used it due to not enough outlets in the kitchen), all the rest could be redesigned for 240 volts. The light would have to have a different kind of socket to prevent contact with either conductor as well avoiding any wrong voltage bulb. Today, we don't need that convenience outlet, thanks to the NEC requiring plenty of outlets in the kitchen. I'd rather see the big appliances run on NEMA 6-30, 6-50, etc. Eliminate a wire; copper is getting expensive.
BTW, have you ever compared a NEMA 6-15P to a NEMA 14-15R :-)
--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
news:3ba2d237-85c6-4e19-a169-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.
That is a single appliance with no other connections on the branch circuit, fed directly from the service panel. In the case of driers, they are often within a few feet of the panel. That is why we have gotten away with it. Don't extrapolate from that to the conclusion that it is a safe practice universally
--
Benjamin D Miller, PE
www.bmillerengineering.com. As many others have pointed out, it is not.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:
| That is a single appliance with no other connections on the branch circuit, | fed directly from the service panel. In the case of driers, they are often | within a few feet of the panel. That is why we have gotten away with it. | Don't extrapolate from that to the conclusion that it is a safe practice | universally
Specifically the main service entrance panel and not a subpanel. And don't forget that the bulk of the load is L-L leaving a relatively small neutral current to worry about. You can't say that for other kinds of things that someone might try to avoid a ground wire with.
--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.