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. |> | So if that is correct, you cannot (per NEC) disconnect the neutral from | the premises electrode grounding system under any circumstances. | Renders all the other words/issues moot.
Apparently.
But if the utility cuts the service, then it is disconnected. But if I cut my own service, then I cannot disconnect it.
I'm trying to find a way to disconnect it. Technically I know how. But making it legal is the issue. And it seems there is a hazard to having it disconnected, as well as having it connected.
Hell, there's even a hazard to having a grounding wire in the first place that I didn't realize before. This whole mess needs a lot more thought and perhaps a rather bizarre solution. I can see why people think this is all a black art.
I suppose I could have a motor generator pair connected by a big fiberglass rod.
Reply to
phil-news-nospam
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Grounding for me, means 2 things.
Bonding: By bonding the utility neutral to your ground, you are setting the phase wires at there desired potential. If you put the ground on the load side of the service and open the neutral. Whats the service voltage in the service panel relative to ground? You don't know.
Fault Clearing: By installing the ground on the service side and not switching the neutral, you are installing a clear path for fault clearing. If you installed the service ground on the neutral side and it opened, but a phase conductor didn't, because of a equipment malfunction. What would be the path back to the utility transformer.
YOU!!!! through ground
> 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] > > -- > --------------------------------------------------------------------------
Reply to
04escape
| Grounding for me, means 2 things. | | Bonding: By bonding the utility neutral to your ground, you are setting the | phase wires at there desired potential. If you put the ground on the load | side of the service and open the neutral. Whats the service voltage in the | service panel relative to ground? You don't know. | | Fault Clearing: By installing the ground on the service side and not | switching the neutral, you are installing a clear path for fault clearing. | If you installed the service ground on the neutral side and it opened, but a | phase conductor didn't, because of a equipment malfunction. What would be | the path back to the utility transformer. | | YOU!!!! through ground
So you're damned if you do, and you're damned if you don't.
It's looking more and more like what I need is a separately derived system involving two transformers.
Reply to
phil-news-nospam
I read in sci.engr.electrical.compliance that snipped-for-privacy@ipal.net wrote (in ) about 'Safely disconnecting a utility ground fault vs. NEC', on Sat, 17 Jul 2004:
This doesn't make sense to me. WHY are you disconnecting your system from the utility supply? To repair or modify it? To feed your system from a generator?
Reply to
John Woodgate
| This doesn't make sense to me. WHY are you disconnecting your system | from the utility supply? To repair or modify it? To feed your system | from a generator?
I need to be able to work on the RF filter safely. Every conductor needs to be able to be disconnected manually at a terminal block for RF filter maintenance. The filter needs to be in place for every wire (e.g. the path from the grounding wire to the service neutral also needs to be filtered), which means every wire needs to be maintainable, which means every wire needs to be disconnectable.
Further, I want to be able to isolate my grounding wire system from any problems outside that can introduce voltage on the neutral that would come into the system.
Reply to
phil-news-nospam
I read in sci.engr.electrical.compliance that snipped-for-privacy@ipal.net wrote (in ) about 'Safely disconnecting a utility ground fault vs. NEC', >
OK, that makes perfect sense. Since this is a maintenance activity, my previous suggestion (temporarily disconnecting your system grounding wire from the ground rod at the service entrance and grounding it instead, if you think it is necessary, to a ground rod at a distance from the one at the service entrance) appears to me to make the set-up as safe as you can reasonably expect. If it's not against Code, you could use a switch instead of disconnecting the wire, but I suspect that a switch is not allowed.
The 'reasonably unexpected' incident, of course, is a broken power line falling on your temporary ground rod!
Reply to
John Woodgate
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 building?s 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
Reply to
HorneTD
I guess I am missing the point 100%.
It seems to me that the "hazard" to which you refer also exists when the service is connected.
Why is it a safety issue when the service is disconnected, but not when it is connected?
> 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] > > -- > --------------------------------------------------------------------------
Reply to
BFoelsch
| 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.
Reply to
phil-news-nospam
| I guess I am missing the point 100%. | | It seems to me that the "hazard" to which you refer also exists when the | service is connected. | | Why is it a safety issue when the service is disconnected, but not when it | is connected?
Of course it's a hazard any time. I want to be able to disconnect if/when any such hazard does exist. That, plus I want to be able to disconnect to a level where even residual level voltages won't pose a hazard to someone who is disconnecting every conductor for maintenance purposes. If there is some current flowing from another secondary source back through the neutral to the grounding electrode, it may be safe enough to not cause any immediate problem, but would be a hazard when disconnecting the neutral when doing the RF filter maintenance (the potential across high impedance then rises to around 120 volts).
Reply to
phil-news-nospam
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
Reply to
HorneTD
| 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.
Reply to
phil-news-nospam

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