I have 2 existing 4000Amp services in the same building sitting side by side in the same electrical service room.( Commercial building)I have an Emergency generator with several ATS ( Automatic transfer switch )in the building. The ATS are as follows 1-2pole, 3-3pole, 2-4pole. For a temporary shut down of one service on the street side by the utility, we want to do the following. On the load side of the services we want to parralel the services. This should be no problem. The question is when the generator is started and running can we disconnect and isolate the nuetral from the service, with out getting a back feed on the neutral? This needs to be done so we are completely isolated from the utility including the neutral. The generator needs to be running during the tie over for lighting and temporary power. Any help would be appreciated.
Service Neutral
- Thread starter kiss
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- Location
- Bremerton, Washington
Re: Service Neutral
Current will want to return to its source or the generator. The neutral should be able to be opened to the utility, however look and see where the grounding electrodes and main bonding jumpers are, take a look at section 701.9(B) and make sure you understand what happens if the utility neutral is opened, regarding grounding electrodes and main bonding jumpers.
[ March 10, 2004, 02:50 PM: Message edited by: tom baker ]
Current will want to return to its source or the generator. The neutral should be able to be opened to the utility, however look and see where the grounding electrodes and main bonding jumpers are, take a look at section 701.9(B) and make sure you understand what happens if the utility neutral is opened, regarding grounding electrodes and main bonding jumpers.
[ March 10, 2004, 02:50 PM: Message edited by: tom baker ]
Re: Service Neutral
I am slightly confused. If I remove the neutral to isolate it from the utility when the generator is running, you say that current will want to flow to its' source or the generator. I am assuming that if I remove the neutral I will have a load on it from the unbalance of my loads.I think this would be no different than removing the neutral from a panel under load. I read 701.9B
and I am not sure how it applies to my situation.I know that signs should be posted re:grounding locations. Thanks for your help.
I am slightly confused. If I remove the neutral to isolate it from the utility when the generator is running, you say that current will want to flow to its' source or the generator. I am assuming that if I remove the neutral I will have a load on it from the unbalance of my loads.I think this would be no different than removing the neutral from a panel under load. I read 701.9B
and I am not sure how it applies to my situation.I know that signs should be posted re:grounding locations. Thanks for your help.
Re: Service Neutral
If the neutral is transferred, after the service switch, the equipment ground conductor will maintain continuity between the generator neutral and the utility MGN.
The drawings in the orange book also show the equipment ground conductor being in parallel with the transfer switch neutral.
No switch will function as a switch, to open a circuit, when the equipment ground conductor is shunting the switch.
The code panel has ruled that the green wire does not count as a circuit wire, and will not carry current.
The utility neutral must be opened on the supply side to disconnect it from the premises wiring.
[ March 11, 2004, 09:44 AM: Message edited by: bennie ]
If the neutral is transferred, after the service switch, the equipment ground conductor will maintain continuity between the generator neutral and the utility MGN.
The drawings in the orange book also show the equipment ground conductor being in parallel with the transfer switch neutral.
No switch will function as a switch, to open a circuit, when the equipment ground conductor is shunting the switch.
The code panel has ruled that the green wire does not count as a circuit wire, and will not carry current.
The utility neutral must be opened on the supply side to disconnect it from the premises wiring.
[ March 11, 2004, 09:44 AM: Message edited by: bennie ]
Re: Service Neutral
When the transfer switch is on the load side of the service, there is more than one equipment ground connection. This procedure defeats the switch.
A generator should not have any electrical connection to the utility service, when on line.
This is my opinion, based on methods commonly used to connect generators to transmission grids for load peaking.
[ March 11, 2004, 10:06 AM: Message edited by: bennie ]
When the transfer switch is on the load side of the service, there is more than one equipment ground connection. This procedure defeats the switch.
A generator should not have any electrical connection to the utility service, when on line.
This is my opinion, based on methods commonly used to connect generators to transmission grids for load peaking.
[ March 11, 2004, 10:06 AM: Message edited by: bennie ]
Re: Service Neutral
Bennie: Thanks for your information and help. I agree with your opinion, but consider that I have a few ATS that are only 2 and 3 pole. These ATS do not break the neutral. Also I am not fimiliar with the orange book. Maybe you can send a picture as an attachment or other format.
Bennie: Thanks for your information and help. I agree with your opinion, but consider that I have a few ATS that are only 2 and 3 pole. These ATS do not break the neutral. Also I am not fimiliar with the orange book. Maybe you can send a picture as an attachment or other format.
- Location
- Illinois
- Occupation
- retired electrician
Re: Service Neutral
Bennie,
Don
Bennie,
That is not what the definition of SDS says. It is just saying in other words that current can only return to its source. The conductors on the secondary side of the transfomer are a new system and no current from the secondary side will try to return to the primay system via a tie in the grounding conductors of each system. The connection of the two systems with the EGCs is not a connection of a normal circuit conductor of the two systems. As far as current flow goes the systems are isolated from each other.
Don
Ed MacLaren
Senior Member
Re: Service Neutral
As stated above by Don, the two sources are completely isolated, with regard to neutral (unbalanced load) current flow.
There is no "parallel neutral" condition, or objectionable current flow over any grounding conductors.
Ed
kiss, here is a basic sketch of a typical stand-by generator installation, configured as a separately derived system (SDS).
As stated above by Don, the two sources are completely isolated, with regard to neutral (unbalanced load) current flow.
There is no "parallel neutral" condition, or objectionable current flow over any grounding conductors.
Ed
- Location
- Massachusetts
Re: Service Neutral
[ March 11, 2004, 04:31 PM: Message edited by: iwire ]
Bennie I do not see any blue wires.
[ March 11, 2004, 04:31 PM: Message edited by: iwire ]
Ed MacLaren
Senior Member
Re: Service Neutral
Bennie and I have been through this before, and we've agreed to disagree, but I think it should be addressed again in case the new folks are interested.
A double throw switch (transfer switch) is actually two switches in one, and in this SDS installation it?s purpose is to isolate the circuit conductors of the two systems.
There are actually two separate systems, with regard to neutral (unbalanced load) current flow, and it helps to visualize them one at a time
Note- The sketches below are for a single-phase installation, but the principle illustrated also applies to three phase systems.
When the load is on utility power, it is as if the generator does not exist. Diagram 4 The unbalanced load (neutral) current path is from X to Z, and there is no parallel path.
Likewise, when the load is on generator power, it is as if the service (utility) does not exist. Diagram 5 The unbalanced load (neutral) current path is from X to Y, and again there is no parallel path.
Each system is grounded and each system?s EGC lands at the point where it?s GEC is connected. (Single Point Grounding)
Ed
Bennie and I have been through this before, and we've agreed to disagree, but I think it should be addressed again in case the new folks are interested.
A double throw switch (transfer switch) is actually two switches in one, and in this SDS installation it?s purpose is to isolate the circuit conductors of the two systems.
There are actually two separate systems, with regard to neutral (unbalanced load) current flow, and it helps to visualize them one at a time
Note- The sketches below are for a single-phase installation, but the principle illustrated also applies to three phase systems.
When the load is on utility power, it is as if the generator does not exist. Diagram 4 The unbalanced load (neutral) current path is from X to Z, and there is no parallel path.
Likewise, when the load is on generator power, it is as if the service (utility) does not exist. Diagram 5 The unbalanced load (neutral) current path is from X to Y, and again there is no parallel path.
Each system is grounded and each system?s EGC lands at the point where it?s GEC is connected. (Single Point Grounding)
Ed
electricman2
Senior Member
- Location
- North Carolina
- Occupation
- Retired Electrical Contractor
Re: Service Neutral
Quote by Bennie
Quote by Bennie
I dont see what the switch is doing either. It is effectively bypassed by the the green EGG and the blue GEC, at least on the diagram I'm looking at.
- Location
- Massachusetts
Re: Service Neutral
The switch is preventing a parallel path for neutral current.
If the all things in the graphic stayed the same, except change the switched neutral to a solidly connected neutral. you would have neutral current flowing on the grounding electrode conductor (the blue wire) when either system was in use.
[ March 11, 2004, 05:25 PM: Message edited by: iwire ]
The switch is preventing a parallel path for neutral current.
If the all things in the graphic stayed the same, except change the switched neutral to a solidly connected neutral. you would have neutral current flowing on the grounding electrode conductor (the blue wire) when either system was in use.
[ March 11, 2004, 05:25 PM: Message edited by: iwire ]
electricman2
Senior Member
- Location
- North Carolina
- Occupation
- Retired Electrical Contractor
Re: Service Neutral
I think I am beginning to see this now. Pardon me for being a little slow. With the neutral switched there wil be no current flow over the EGC or GEC even though they effectively parallel the switch. Is this right?
I think I am beginning to see this now. Pardon me for being a little slow. With the neutral switched there wil be no current flow over the EGC or GEC even though they effectively parallel the switch. Is this right?
Ed MacLaren
Senior Member
Re: Service Neutral
It is easier to visualize if you concentrate on one supply at a time. (My post of 4:51 PM above)
The neutral (unbalanced load) current will only flow over conductors that will carry it back to the source from which it originated, and that can only be the utility service or the generator, one or the other but not both.
Ed
That is right, John.
It is easier to visualize if you concentrate on one supply at a time. (My post of 4:51 PM above)
The neutral (unbalanced load) current will only flow over conductors that will carry it back to the source from which it originated, and that can only be the utility service or the generator, one or the other but not both.
Ed
Re: Service Neutral
The purpose of transferring the neutral is to prevent the generator from feeding into the utility line. And the utility neutral from feeding into the generator line.
Parallel current has nothing to do with the reason for switching the neutral.
Now everyone is going to say "how can one feed into the other" Think of a LN fault, the neutral will be energized until the fault clears.
A lineman may get killed when a generator faults.
The neutral was not switched originally for GFP reasons as many claim. GFP's did not exist in the beginning.
Dumping a GFP on a dead feeder is very difficult.
Dumping a GFP on a live circuit is preferable to zapping a mean lineman.
I will not address this topic again, but I do need to document my concern with the present procedure for connecting a generator.
I ask anyone interested to study the schematics and prove the wiring method is correct.
This method works fine for a backup generator, but it does not qualify as a separately derived system. Read the definition and show me the generator is not electrically connected to the utility MGN.
The original post indicated the desire to disconnect the service neutral for safety purposes. This would make a separately derived system, only when there is complete isolation of neutrals.
[ March 11, 2004, 07:48 PM: Message edited by: bennie ]
The purpose of transferring the neutral is to prevent the generator from feeding into the utility line. And the utility neutral from feeding into the generator line.
Parallel current has nothing to do with the reason for switching the neutral.
Now everyone is going to say "how can one feed into the other" Think of a LN fault, the neutral will be energized until the fault clears.
A lineman may get killed when a generator faults.
The neutral was not switched originally for GFP reasons as many claim. GFP's did not exist in the beginning.
Dumping a GFP on a dead feeder is very difficult.
Dumping a GFP on a live circuit is preferable to zapping a mean lineman.
I will not address this topic again, but I do need to document my concern with the present procedure for connecting a generator.
I ask anyone interested to study the schematics and prove the wiring method is correct.
This method works fine for a backup generator, but it does not qualify as a separately derived system. Read the definition and show me the generator is not electrically connected to the utility MGN.
The original post indicated the desire to disconnect the service neutral for safety purposes. This would make a separately derived system, only when there is complete isolation of neutrals.
[ March 11, 2004, 07:48 PM: Message edited by: bennie ]
- Location
- Massachusetts
Re: Service Neutral
Bennie you always seem to look at this like the generators always pick up entire buildings, I have never seen that, always generators that pick up bits and pieces of buildings.
The generators we install do not pick up the entire building.
We have many buildings that have multiple generators, one will be for the Article 700 loads, the others and they may be quite a few are owned by individual tenants for there use to keep them in business.
You do not see the parallel path for neutral current on the GEC in Eds drawing?
Or you feel that this circulating current that could be 100s of amps is OK?
This seems to fly in the face of your opposition to the low levels of current flowing between grounding electrodes in the earth.
Please explain with more than one sentence, where is the "other" conductor for this circuit to feed or return current on?
A line to case fault would only circulate current between the generator and the bonding point, well away from the utility workers.
Bennie you always seem to look at this like the generators always pick up entire buildings, I have never seen that, always generators that pick up bits and pieces of buildings.
The generators we install do not pick up the entire building.
We have many buildings that have multiple generators, one will be for the Article 700 loads, the others and they may be quite a few are owned by individual tenants for there use to keep them in business.
Once a week the generators run for exercise about 30 to 60 minutes, multiply that by the number of generators that the building has and you find a lot of time when both systems are active and could dump the GFP mains.
Bennie what the heck do you mean?
You do not see the parallel path for neutral current on the GEC in Eds drawing?
Or you feel that this circulating current that could be 100s of amps is OK?
This seems to fly in the face of your opposition to the low levels of current flowing between grounding electrodes in the earth.
Call me one of the everyone, I do not understand this, how would the generator feed into the utility line.
Please explain with more than one sentence, where is the "other" conductor for this circuit to feed or return current on?
A line to case fault would only circulate current between the generator and the bonding point, well away from the utility workers.
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