Safely disconnecting a utility ground fault vs. NEC

I'm looking for a way to entirely and totally disconnect everything from utility power, while having a grounding wire that is still grounded.
Such disconnection would mean disconnecting the neutral wire, and do so not before disconnecting the live phase wires. As far as I can tell, a circuit breaker that disconnects all wires at the same time would comply with NEC code (so for a 240/120 volt single phase feed, using a three pole breaker normally intended for three phase would get the job done). The issue is with grounding. It's been pointed out to me that in NEC 250.24(A)(1), the grounded wire is to be connected to the grounding wire between the serice drop/lateral, and the terminals of the disconnecting means. Then 250.24(A)(5) prohibits connecting at any points on the load side of the disconnecting means.
The problem I have with this, is that by connecting the grounded conductor before the disconnection, it really isn't a 100% disconnection since the grounding system (e.g. the grounding wire, not the neutral wire) in the building is still attached to the utility neutral wire. Faults or breaks in the utility wiring, or even in neighbor wiring sharing the same transformer, could introduce voltage relative to ground on the neutral wire. While the grounding connection would make for a path for that voltage to go to ground, no ground is ever perfect, so if there is ever any other point in the grounding system (e.g. any equipment which has a chassis attached to the grounding wire) which might get grounded to earth, it also forms a path where current could flow. If that path includes a human being, then what we have is two paths in parallel, one near the service entrance (the normal grounding point), and one including a human. Since ground is a resistive component (relative to ground return paths) this will mean some part of the return current flows through the human. Since the grounding system has become part of the circuit under a supply fault, it is not doing its job in protecting people.
A ground fault detector at the main disconnect won't detect this since it is current flowing on the grounding wire itself once it has reached that wire from the connection with the neutral. Even if a disconnect breaker included disconnecting the neutral (allowed by NEC only if the neutral is disconnected with or after the other conductors), it won't stop this hazardous condition.
What I propose is that where a main service disconnect also disconnects the neutral conductors (simultaneously with all other conductors), that the neutral be connected to the grounding wire on the load side of the disconnect. This way, when the conductors are disconnected, there is no possible path for a supply/line side fault to energize the grounding wires in the building. This will also allow a ground fault current detection to work properly since it will be detecting all current coming in that does not have an equivalent return current. Such a detector would then be able to interrupt such faults via a shunt trip or such on the main disconnect.
I generally find when the NEC has a rule, there's a good reason for it. But I cannot figure out the reason for having the grounding point of the grounded conductor (neutral) being on the line side of the main breaker, as opposed to the load side. Obviously, in cases (and this is most of them) where the neutral is not switched at all, it's moot, electrically. But it is allowed to disconnect the neutral when the other conductors are as well, and in such a case, it now matters which side things are on. But I just do not see the reason why the NEC has it this way, and as far as I can see, it is safer as I proposed.
Reference from the NEC 2005 draft:
* 250.24 Grounding Service-Supplied Alternating-Current Systems. * * (A) System Grounding Connections. A premises wiring system supplied by a * grounded ac service shall have a grounding electrode conductor connected * to the grounded service conductor, at each service, in accordance with * 250.24(A)(1) through (A)(5). * * (1) General. The connection shall be made at any accessible point from * the load end of the service drop or service lateral to and including the * terminal or bus to which the grounded service conductor is connected at * the service disconnecting means.
[snip]
* (5) Load-Side Grounding Connections. A grounding connection shall not be * made to any grounded conductor on the load side of the service disconnecting * means except as otherwise permitted in this article. [ROP 5 65]
[snip]
--
-----------------------------------------------------------------------------
| 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
Phil, I'm not sure I understand the problem that concerns you. On a utility service with a grounded neutral, the neutral is grounded once at the utility transformer and again at the building service entrance. Are you concerned that, simultaneously, (a) the neutral-ground connections at the utility transformer and at the building service become disconnected, (b) the utility neutral is energized, and (c) a person contacts the utility neutral? If so, do you have personal knowledge of this happening?
Regards, Chris Johnston

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:
| Phil, I'm not sure I understand the problem that concerns you. On a | utility service with a grounded neutral, the neutral is grounded once | at the utility transformer and again at the building service entrance. | Are you concerned that, simultaneously, (a) the neutral-ground | connections at the utility transformer and at the building service | become disconnected, (b) the utility neutral is energized, and (c) a | person contacts the utility neutral? If so, do you have personal | knowledge of this happening?
Whan you say "at the building service entrance" you aren't specific enough to specify whether it is before, or after, the point where the main disconnect takes place. But I can understand the ambiguity since in most cases it is moot due to the neutral not being switched. But if the neutral is switched with the other conductors, then it matters. If the neutral is connected to the ground before the switch, then the switch will not protect against voltage coming across from the neutral to the ground wire.
Scenario 1: A broken neutral between the point where the transformer is grounded, and the point where the service splits between 2 service drops. Unbalance current in the neutral wire at another service drop will be divided among the various ground connections the service drop side of the neutral trying to find a way back to the transformer, since the proper path is broken. I don't remember which forum, but there was a case where someone actually experienced this. They had 5 amps of current across the bond between the neutral and ground, even with the main switched off (which did not switch the neutral). The power company eventually fixed "something" and it went away.
Scenario 2: (this might be what you described) The pole transformer becomes either ungrounded or has an internal fault. Primary voltage can be introduced on the secondary, either by capacitive coupling, or direct fault. I saw a video of a case where the fault actually happened and a lineman was hit. He survived (stunned by was able to walk away) because he was wearing gloves, but was injured because they were low voltage gloves (which otherwise make sense since he was attempting to cut the low voltage service drop).
My objective is to be able to disconnect entirely from the service drop, either by manual action, or by ground fault detection. That means all conductors, with a grounding wire system that is not connected around that switch ti the service drop. Connecting neutral to ground _after_ the main disconnect accomplishes that, but it does not comply with NEC.
--
-----------------------------------------------------------------------------
| 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
snipped-for-privacy@ipal.net wrote:

Your objective can be achieved by installing a service disconnecting means that opens the neutral. If the switch opens all conductors of the service simultaneously then that is permissible under the US NEC. The code permits the main disconnecting means to be adjacent to the service Over Current Protective Device (OCPD). They need not be the same device. So you could install a three pole switch ahead of a two pole main breaker or fuse pullout that would open all three conductors of the incoming three wire service. If the service is three phase then a four pole switch can be used ahead of a three pole breaker or fused pull out. The bonding screw or strap can be tied-in on the load side of the switch if it is located in the same enclosure. For protection from lightning induced surges you would install a secondary surge protector on the line side of the switch. -- Tom H
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
| Your objective can be achieved by installing a service disconnecting | means that opens the neutral. If the switch opens all conductors of the | service simultaneously then that is permissible under the US NEC. The | code permits the main disconnecting means to be adjacent to the service | Over Current Protective Device (OCPD). They need not be the same | device. So you could install a three pole switch ahead of a two pole | main breaker or fuse pullout that would open all three conductors of the | incoming three wire service. If the service is three phase then a four | pole switch can be used ahead of a three pole breaker or fused pull out.
This much was part of my plan.
| The bonding screw or strap can be tied-in on the load side of the | switch if it is located in the same enclosure. For protection from | lightning induced surges you would install a secondary surge protector | on the line side of the switch.
Can you cite NEC code that overrides 250.24(A)(1) and 250.24(A)(5) with respect to bonding on the load side of the switch?
What would the secondary surge protecter be connected to for ground?
Would that surge protecter deal with primary voltage?
--
-----------------------------------------------------------------------------
| 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

I'm sorry for typing too fast and not proof reading carefully enough. For this purpose the Service Disconnecting means continues to be the main breaker or fused pull out. The three or four pole switch is not the service disconnecting means it is an isolating switch. Your not overriding anything. The bonding point is in the service equipment enclosure. It is one of the points "from the load end of the service drop or service lateral to and including the terminal or bus to which the grounded service conductor is connected at the service disconnecting means." Isolating switches are used to isolate service equipment from its source of supply. This is often done with service rated transfer switches so that the transfer equipment can be serviced without utility company intervention. -- Tom H
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
|> | The bonding screw or strap can be tied-in on the load side of the |> | switch if it is located in the same enclosure. For protection from |> | lightning induced surges you would install a secondary surge protector |> | on the line side of the switch. |> |> Can you cite NEC code that overrides 250.24(A)(1) and 250.24(A)(5) with |> respect to bonding on the load side of the switch? |> |> What would the secondary surge protecter be connected to for ground? |> |> Would that surge protecter deal with primary voltage? | | I'm sorry for typing too fast and not proof reading carefully enough. | For this purpose the Service Disconnecting means continues to be the | main breaker or fused pull out. The three or four pole switch is not | the service disconnecting means it is an isolating switch. Your not | overriding anything. The bonding point is in the service equipment | enclosure. It is one of the points "from the load end of the service | drop or service lateral to and including the terminal or bus to which | the grounded service conductor is connected at the service disconnecting | means." Isolating switches are used to isolate service equipment from | its source of supply. This is often done with service rated transfer | switches so that the transfer equipment can be serviced without utility | company intervention.
If the isolating switch is not considered to be the service disconnecting means, then it should be able to be grounded _between_ the real service disconnecting means and that isolating switch, right? If that is true, then the isolating switch would only need to be 3 pole for 3 wire single phase (4 pole for 4 wire three phase for comparison), right? If right so far, then why not let the isolating switch actually be a circuit breaker itself (which adds automatic overload and fault interruption capability)? If that can be done, what we have is two main breakers in sequence, with the neutral grounded between them. Now the 2nd breaker is redundant, it would seem, and pointless (aside from a code issue) as all it does more is disconnect the neutral of the unenergized load side from ground.
--
-----------------------------------------------------------------------------
| 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
snipped-for-privacy@ipal.net wrote:

Isolating switches are often installed with a rating that would not be adequate as a service disconnecting means. Switches used as isolating switches are often not listed as suitable for use as service equipment. The reason that circuit breakers are seldom used as isolating switches is that since they open automatically they would have to be rated with a high enough withstand to open the circuit under load. That is a far more demanding function then just breaking an idle but energized circuit. -- Tom H
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
| Isolating switches are often installed with a rating that would not be | adequate as a service disconnecting means. Switches used as isolating | switches are often not listed as suitable for use as service equipment. | The reason that circuit breakers are seldom used as isolating switches | is that since they open automatically they would have to be rated with a | high enough withstand to open the circuit under load. That is a far | more demanding function then just breaking an idle but energized circuit.
I guess you meant that the breaker is rated to open the circuit under fault? The switch would have to be able to open under at least some portion of the load, and have the stay-closed rating at full load.
But if a circuit breaker were used anyway ... what then?
--
-----------------------------------------------------------------------------
| 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
snipped-for-privacy@ipal.net wrote:

Okay, you're correct the withstand rating has to do with matching the Over Current Protective Device (OCPD) that is used for overload protection of the service entrance conductors to the available fault current but an isolation switch does not have to be capable of interrupting any portion of the load. It need only be capable of carrying the load while closed.
Take the example of service equipment consisting of an unfused switch supplying a set of fuses that are located in an immediately adjacent assembly. That switch would have to be listed as suitable for use as service equipment and be capable of opening under the calculated load. The fuse holder and fuses would have to be suitable for the supplies available fault current.
An isolating switch on the other hand would not need to be rated for the calculated load because it is never opened with current flowing. Isolating switches are always labeled to indicate their purpose and that they must not be opened prior to opening the Service Disconnecting Means (SDM). One good example of a switch that might be suitable for use as an isolating switch but not as the SDM is a switch that is rated for the voltage and current but is not motor rated or is not suitable for use as service equipment. A common use for isolating switches is to de-energize a service rated transfer switch for servicing or to renew internal or external fuses. Such transfer switches are available with and without fuse holders or other OCPDs. The service OCPDs may be located on the line or load side of the switch. When they are located on the line side of the switch there is a need for an isolating switch on the line side of any fuses to permit safely renewing the fuses. The plant wireman or an electrical contractor opens the SDM, clears the fault, closes the SDM to the alternate source of power, opens the isolation switch, renews the fuses, re closes the isolation switch, and operates the transfer switch to the original position to reconnect the primary power source to the load. When you want to work on your RF filters, you open the SDM and then open the isolating switch.
If your ground fault detector is to protect you from an external power cross then I only have one more suggestion to make. You can install the service away from the building in question. If the service is overhead than you would have to install a yard pole. If you have another substantial structure on the property, you can install the Service Equipment in that building. You then have your SDM located away from the building that will be protected by RF filtering. The main power switch at the subject building then becomes the Building Disconnecting Means (BDM) that is required by NEC 225.31. The rule we have been struggling with does not apply to building disconnecting means but only to service equipment. The SDM would consist of a four-pole breaker equipped with Ground Fault Protection. The breaker would open all four wires of the feeder including the Equipment Grounding Conductor. The utility neutral at the service equipment is never disconnected from the service Grounding Electrode Conductor (GEC). At the building the BDM would be the main breaker of the panel or a separate breaker enclosure or switch. That building would also have a grounding electrode system which would be connected to the BDM enclosure and to its EGC buss bar but not to its neutral buss bar. Since this BDM is not service equipment, it can open all conductors of the feeder upstream from the point of termination of the buildings GEC if that is desirable.
The obvious draw back to this approach is the cost of; two sets of service equipment, the ground fault detection equipment, and the shunt trip on the service breaker. That combination of equipment is going to cost you a pretty penny. The biggest of those costs will be the four pole circuit breaker and it's enclosure that is functioning as the Service Equipment. I can see no way to avoid needing the four pole breaker if you want to provide automatic disconnection of the buildings GEC and its EGCs from the utility neutral. Since the building will need to have other utilities that are bonded to ground at neighboring premises you will need to locate the Network Interface Device (NID) for those utilities at the remote service equipment. If you use coax or shielded telephone cable between the NID and the building, you will have established a sneak current path for the stray high voltage you appear to be concerned about. I'm not even sure that the protectors on the telephone lines at the NID and at the building will not provide a current path about which you may be concerned. That would only happen in the event of a high voltage cross with the secondary of the utility transformer, the service entry conductors, or the telephone drop or lateral.
This possibility of a high voltage cross with the utilities wires does not seem likely enough to take such expensive precautions. -- Tom H
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
| An isolating switch on the other hand would not need to be rated for the | calculated load because it is never opened with current flowing.
But there can be current flowing between the service neutral and ground. It might not be substantial, but the isolation switch would need to be able to interrupt it safely.
| Isolating switches are always labeled to indicate their purpose and that | they must not be opened prior to opening the Service Disconnecting Means | (SDM). One good example of a switch that might be suitable for use as | an isolating switch but not as the SDM is a switch that is rated for the | voltage and current but is not motor rated or is not suitable for use as | service equipment. A common use for isolating switches is to | de-energize a service rated transfer switch for servicing or to renew | internal or external fuses. Such transfer switches are available with | and without fuse holders or other OCPDs. The service OCPDs may be | located on the line or load side of the switch. When they are located | on the line side of the switch there is a need for an isolating switch | on the line side of any fuses to permit safely renewing the fuses. The | plant wireman or an electrical contractor opens the SDM, clears the | fault, closes the SDM to the alternate source of power, opens the | isolation switch, renews the fuses, re closes the isolation switch, and | operates the transfer switch to the original position to reconnect the | primary power source to the load. When you want to work on your RF | filters, you open the SDM and then open the isolating switch.
I understand what you mean by an isolating switch. So why would such a switch be used instead of something else? Lower cost? Would this be the "unfused safety switch" like I see in a Square-D catalog?
| If your ground fault detector is to protect you from an external power | cross then I only have one more suggestion to make. You can install the | service away from the building in question. If the service is overhead | than you would have to install a yard pole. If you have another | substantial structure on the property, you can install the Service | Equipment in that building. You then have your SDM located away from | the building that will be protected by RF filtering. The main power | switch at the subject building then becomes the Building Disconnecting | Means (BDM) that is required by NEC 225.31. The rule we have been | struggling with does not apply to building disconnecting means but only | to service equipment. The SDM would consist of a four-pole breaker | equipped with Ground Fault Protection. The breaker would open all four | wires of the feeder including the Equipment Grounding Conductor. The | utility neutral at the service equipment is never disconnected from the | service Grounding Electrode Conductor (GEC). At the building the BDM | would be the main breaker of the panel or a separate breaker enclosure | or switch. That building would also have a grounding electrode system | which would be connected to the BDM enclosure and to its EGC buss bar | but not to its neutral buss bar. Since this BDM is not service | equipment, it can open all conductors of the feeder upstream from the | point of termination of the building?s GEC if that is desirable.
So by having the SDM at some distance from the BDM, the ground is not a viable conductor to worry about. The building would then be a subfeed of the remote SDM for purposes of separate neutral and protection ground (EGC), unless I decide to also do a separately derived system.
| The obvious draw back to this approach is the cost of; two sets of | service equipment, the ground fault detection equipment, and the shunt | trip on the service breaker. That combination of equipment is going to | cost you a pretty penny. The biggest of those costs will be the four | pole circuit breaker and it's enclosure that is functioning as the | Service Equipment. I can see no way to avoid needing the four pole | breaker if you want to provide automatic disconnection of the buildings | GEC and its EGCs from the utility neutral. Since the building will need | to have other utilities that are bonded to ground at neighboring | premises you will need to locate the Network Interface Device (NID) for | those utilities at the remote service equipment. If you use coax or | shielded telephone cable between the NID and the building, you will have | established a sneak current path for the stray high voltage you appear | to be concerned about. I'm not even sure that the protectors on the | telephone lines at the NID and at the building will not provide a | current path about which you may be concerned. That would only happen | in the event of a high voltage cross with the secondary of the utility | transformer, the service entry conductors, or the telephone drop or | lateral.
Suppose I feed all power through a transformer at the building, where the BDM is just before the transformer primary. Would I even need to feed the neutral conductor from the remote SDM to the building, given that the transformer (240 to 240/120) won't connect to it at all? If I can leave the neutral out of the feeder, then can I also leave it out of the breaker? If so, then that would get me to a 3-pole breaker, which is more readily available.
| This possibility of a high voltage cross with the utilities? wires does | not seem likely enough to take such expensive precautions.
I've seen a case where it happened. And I fully understand why the fire department just stood there watching the house burn down.
Still, I'm more concerned with the voltage across the terminal block in the RF filter enclosure. One thing I thought of doing there is to use not one, but two ground conductors in the enclosure. Extra filters would need to be used, but I could work on them one at a time, and never have both ground conductors disconnected at the same time. Or I could do two whole RF filter systems in parallel with 4 isolating switches to take one or the other out of service for maintenance.
--
-----------------------------------------------------------------------------
| 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
Replies are in line.
snipped-for-privacy@ipal.net wrote:

If the concern there is a high voltage circuit crossed with the neutral it is unlikely that the switch would safely open at the primary voltage. The fault would either burn clear or continue until the utility circuit breakers opened for the third time. Most utility circuit breakers are set up to re close three times in order to restore service without outside wireman intervention.

That would be one type of switch that could be used as long as it was suitable for the voltage.

That is correct.

Yes you could leave the neutral out of the feeder to the transformer and thus use a three pole breaker as the service disconnecting means. But if that breaker was also the interuptor for your ground fault protection of equipment then be aware that it is unlikely to withstand the fault current caused by a high voltage power cross. Even a 45000 symmetrical ampere withstand rating may be inadequate at the fault currents possible in a high voltage fault situation.

The parallel conductors or arrays would be permitted if the conductors involved were 1/0 or larger. That is true if the service is one hundred and fifty amperes or larger. -- Tom H
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
| If the concern there is a high voltage circuit crossed with the neutral | it is unlikely that the switch would safely open at the primary voltage. | The fault would either burn clear or continue until the utility | circuit breakers opened for the third time. Most utility circuit | breakers are set up to re close three times in order to restore service | without outside wireman intervention.
Depending on the cause, it might not be a high voltage. It could just be 120 volts. It could also be high voltage with sufficient impedance as to not appear to be a fault, such as capacitively coupled.
|> Suppose I feed all power through a transformer at the building, where |> the BDM is just before the transformer primary. Would I even need to |> feed the neutral conductor from the remote SDM to the building, given |> that the transformer (240 to 240/120) won't connect to it at all? If |> I can leave the neutral out of the feeder, then can I also leave it out |> of the breaker? If so, then that would get me to a 3-pole breaker, |> which is more readily available. |> | Yes you could leave the neutral out of the feeder to the transformer and | thus use a three pole breaker as the service disconnecting means. But | if that breaker was also the interuptor for your ground fault protection | of equipment then be aware that it is unlikely to withstand the fault | current caused by a high voltage power cross. Even a 45000 symmetrical | ampere withstand rating may be inadequate at the fault currents possible | in a high voltage fault situation.
So what do you do? Just depend on the fact that a primary to secondary cross is very unlikely to happen?
At least with a 3 pole breaker, there are products to be found. And with a transformer at the building entrance, a ground fault system might not be needed with this.
|> Still, I'm more concerned with the voltage across the terminal block in |> the RF filter enclosure. One thing I thought of doing there is to use |> not one, but two ground conductors in the enclosure. Extra filters would |> need to be used, but I could work on them one at a time, and never have |> both ground conductors disconnected at the same time. Or I could do two |> whole RF filter systems in parallel with 4 isolating switches to take one |> or the other out of service for maintenance. |> | The parallel conductors or arrays would be permitted if the conductors | involved were 1/0 or larger. That is true if the service is one hundred | and fifty amperes or larger.
Probably 200 or 225 amps.
--
-----------------------------------------------------------------------------
| 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
A solution would be to use a 4-pole main circuit breaker at the service, with the 4th pole breaking the neutral. The service neutral would be grounded on the line side of the main breaker, thus complying with the NEC. When the breaker opens,the load side neutral is disconnected from the line side neutral and grounding connection.
The part that puzzles me is how the breaker is to be automatically tripped under the two scenarios you mention. In scenario 1, no one with an interest in continuous service is going to set a GF relay at 5 amps. In scenario 2 you could use an overvoltage relay to trip the breaker.
A personal comment, please. I sometimes wonder whether the folks who write the NEC ever consider situations such as you describe. I know that they continue to give the utility companies a pass on using multi-grounded neutrals on ballfield and parking lot lighting. Every so often you read of someone being electrocuted by this situation. I find it disturbing that the utility companies maintain a prominent presence on some of the NEC committees, yet maintain that, in most circumstances, they are not subject to the NEC.
Cheers, Chris Johnston
- snipped-for-privacy@ipal.net wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:
| A solution would be to use a 4-pole main circuit breaker at the | service, with the 4th pole breaking the neutral. The service neutral | would be grounded on the line side of the main breaker, thus complying | with the NEC. When the breaker opens,the load side neutral is | disconnected from the line side neutral and grounding connection.
Now to find a 4-pole breaker.
| The part that puzzles me is how the breaker is to be automatically | tripped under the two scenarios you mention. In scenario 1, no one | with an interest in continuous service is going to set a GF relay at 5 | amps. In scenario 2 you could use an overvoltage relay to trip the | breaker.
If there is current coming in on the supply neutral seeking ground, that tells me there is something wrong. I'd want my GF detection set way down there for these things, though I'm not sure where. I don't want to have to switch neutrals on all the branch circuits, yet I do want to open the circuit on 5 milliamps. The traditional ground fault which can be interrupted by opening hot wires should catch that in the branch circuit where GFCI is installed. But coming in on the neutral remains the issue. And if, for now, that means opening the main breaker to clear it, so be it. If that happens too much I'll address that when it happens.
I have more of an interest in safety over continuous operation for my home. In a data center I might install, it could be different; it might have a high impedance ground on multiple separately derived systems. But I'm the networking person on those; I just work with electricians sometimes (like providing them with load needs for the cabinets).
| A personal comment, please. I sometimes wonder whether the folks who | write the NEC ever consider situations such as you describe. I know | that they continue to give the utility companies a pass on using | multi-grounded neutrals on ballfield and parking lot lighting. Every | so often you read of someone being electrocuted by this situation. I | find it disturbing that the utility companies maintain a prominent | presence on some of the NEC committees, yet maintain that, in most | circumstances, they are not subject to the NEC.
I would wonder why they want multiple grounded neutrals.
--
-----------------------------------------------------------------------------
| 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
I read in sci.engr.electrical.compliance that snipped-for-privacy@ipal.net
utility ground fault vs. NEC', on Fri, 16 Jul 2004:

Draw a schematic including the cables as resistors. With long cable runs, there may be too much loop resistance for a line to ground short to cause enough fault current to clear the protective device (not a GFCI but an ordinary fuse or thermal-magnetic circuit breaker) in a short enough time. Grounding the neutral at several places reduces the loop resistance.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
You want a 3 conductor transfer switch. That will allow you to connect another independant power source to the other leg or just disconnect completely. That is the simplest, code compliant, way to cut the cord.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
| You want a 3 conductor transfer switch. That will allow you to connect another | independant power source to the other leg or just disconnect completely. | That is the simplest, code compliant, way to cut the cord.
It may be a transfer switch, or it may be just a plain switch or breaker. But definitely a 3 pole unit would be needed to disconnect the neutral with the other conductors.
But where do you attach the neutral to ground?
If before the switch, then the ground is still connected to the utilty even though the switch opens or the breaker trips.
If after the switch, then it runs into NEC 250.24(A)(1) and (5).
--
-----------------------------------------------------------------------------
| 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
| You want a 3 conductor transfer switch. That will allow you to connect another | independant power source to the other leg or just disconnect completely. | That is the simplest, code compliant, way to cut the cord.
But what about the grounding? Where will you attach the ground in a code compliant way while not providing a path between the ground inside and the neutral outside?
--
-----------------------------------------------------------------------------
| 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
wrote:

another
You can't really do it. If you drive a separate ground rod, it will still provide a path to the ground rod on the other side of the switch. The earth is a decent condutor.
I think you are worrying to much about a non-issue.
Charles Perry P.E.
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.