I don't know if this subject has been brought up before, but I recently had an inspector question me on whether or not the neutral was required to be switched on an automatic transfer switch. The application is: 480v, 100a, 3 phase automatic bus transfer switch from utility supply to emergency generator in an Oregon State Forestry office building. I'm not involved in this project at all, so I don't know what kind of load will be on the generator if that even makes any difference. Any code references would be appreciated.
Switching neutral in a transfer switch
- Thread starter magoo
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goldstar
Senior Member
- Location
- New Jersey
- Occupation
- Electrical Contractor
Re: Switching neutral in a transfer switch
I can't help you with a code reference but I recently had a situation where I installed a transfer switch in a gas station that planned to use a portable generator(120/240v-1 phase) so that they could pump gas in a power outage. The manufacturer of the generator said that the neutral on their generator should float and not be bonded to ground. So I used a 3 pole manual transfer switch (with the neutral on one of the poles) to power up a sub-panel for selected circuits. On the utility side the neutrals were bonded; on the generator side they floated.
I would check with the generator manufacturer. Every one is different. Hope this helps.
I can't help you with a code reference but I recently had a situation where I installed a transfer switch in a gas station that planned to use a portable generator(120/240v-1 phase) so that they could pump gas in a power outage. The manufacturer of the generator said that the neutral on their generator should float and not be bonded to ground. So I used a 3 pole manual transfer switch (with the neutral on one of the poles) to power up a sub-panel for selected circuits. On the utility side the neutrals were bonded; on the generator side they floated.
I would check with the generator manufacturer. Every one is different. Hope this helps.
- Location
- Massachusetts
Re: Switching neutral in a transfer switch
Goldstar
If you are switching the neutral at the transfer switch, the neutral must be bonded at the generator.
If the Neutral is left "floating" an ungrounded (hot) conductor shorting to ground will not trip any overcurrent device, in the subpanel.
Look at 250.20 250.20(B) and (D) also FPN No. 1 and 2
What you need to know from the maker is whether they bonded Neutral to Ground already in the generator, or in the language of the code is it a "Separately Derived System" or not, each has different requirements.
Some where this generator Neutral MUST be bonded to ground.
By they telling you to leave it floating they may be telling you that they bonded it, but you want to be sure.
[ March 29, 2003, 08:20 AM: Message edited by: iwire ]
Goldstar
If you are switching the neutral at the transfer switch, the neutral must be bonded at the generator.
If the Neutral is left "floating" an ungrounded (hot) conductor shorting to ground will not trip any overcurrent device, in the subpanel.
Look at 250.20 250.20(B) and (D) also FPN No. 1 and 2
What you need to know from the maker is whether they bonded Neutral to Ground already in the generator, or in the language of the code is it a "Separately Derived System" or not, each has different requirements.
Some where this generator Neutral MUST be bonded to ground.
By they telling you to leave it floating they may be telling you that they bonded it, but you want to be sure.
[ March 29, 2003, 08:20 AM: Message edited by: iwire ]
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: Switching neutral in a transfer switch
As mnetioned, if the gen has a bond from its neutral to ground, then the transfer switch must switch the neutral. If the transfer switch doesn't switch the neutral, then you must convert the gen to remove the n-g bond.
This assumes that the designer didn't specifically choose to have the neutral switched due to ground fault detection.
As mnetioned, if the gen has a bond from its neutral to ground, then the transfer switch must switch the neutral. If the transfer switch doesn't switch the neutral, then you must convert the gen to remove the n-g bond.
This assumes that the designer didn't specifically choose to have the neutral switched due to ground fault detection.
Ed MacLaren
Senior Member
Re: Switching neutral in a transfer switch
Magoo,
In addition to the good advice above, you may find these sketches to be of assistance.
They do not represent actual installations, and are offered to illustrate basic principles only.
Ed
Magoo,
In addition to the good advice above, you may find these sketches to be of assistance.
They do not represent actual installations, and are offered to illustrate basic principles only.
Ed
russ
Senior Member
- Location
- Burbank IL
Re: Switching neutral in a transfer switch
If the neutral is bonded at the service main and at the generator case, (Which is how they are usually manufactured), the equipment ground becomes parallel with the neutral.
Russ
PS By switching the neutral you can break the bond and elminate the parallel between the neutral and equipment ground.
[ March 29, 2003, 10:45 AM: Message edited by: russ ]
If the neutral is bonded at the service main and at the generator case, (Which is how they are usually manufactured), the equipment ground becomes parallel with the neutral.
Russ
PS By switching the neutral you can break the bond and elminate the parallel between the neutral and equipment ground.
[ March 29, 2003, 10:45 AM: Message edited by: russ ]
- Location
- Massachusetts
Re: Switching neutral in a transfer switch
What are you getting at Bennie?
What are you getting at Bennie?
Re: Switching neutral in a transfer switch
Switch the neutral or not switch the neutral? It is a design issue and not a code issue. You can do it either way. If the neutral is switched, or not ran from generator it is considered an SDS. If the neutral is not switched and ran from the generator it is not an SDS.
The main determination factor is whether or not GFP is used at the service, generator, or both. If GFP is used you would configure the generator as SDS. Otherwise it doesn?t make much difference and comes down to economics. The code only tells you what has to be done with either configuration, not which configuration has to be used.
Dereck
Switch the neutral or not switch the neutral? It is a design issue and not a code issue. You can do it either way. If the neutral is switched, or not ran from generator it is considered an SDS. If the neutral is not switched and ran from the generator it is not an SDS.
The main determination factor is whether or not GFP is used at the service, generator, or both. If GFP is used you would configure the generator as SDS. Otherwise it doesn?t make much difference and comes down to economics. The code only tells you what has to be done with either configuration, not which configuration has to be used.
Dereck
Re: Switching neutral in a transfer switch
Please read 250.34 (C). Please make note..."Where the generator is a component of a separately derived system"
A generator is not a separately derived system, it is only a source of power for one.
A premises wiring system, with no external electrical connections, is a separately derived system.
All conductors capable of carrying current are electrical connections. Equipment ground wires are capable of carrying current.
A transfer switch must be on the supply side of the service switch, to effectively break any electrical connection to the utility supply.
Please read 250.34 (C). Please make note..."Where the generator is a component of a separately derived system"
A generator is not a separately derived system, it is only a source of power for one.
A premises wiring system, with no external electrical connections, is a separately derived system.
All conductors capable of carrying current are electrical connections. Equipment ground wires are capable of carrying current.
A transfer switch must be on the supply side of the service switch, to effectively break any electrical connection to the utility supply.
Re: Switching neutral in a transfer switch
The reason for disconnecting the utility neutral/ground is to make the premises wiring a separately derived system.
During a storm, or other events, when an active line conductor breaks, the first contact is often the multi-ground neutral conductor.
This contact will impress the high voltage on the metal surfaces of the premises.
This is also one reason for the quick response of generator units in coming on line.
This is the only reason for disconnecting the utility neutral.
[ March 29, 2003, 11:51 AM: Message edited by: bennie ]
The reason for disconnecting the utility neutral/ground is to make the premises wiring a separately derived system.
During a storm, or other events, when an active line conductor breaks, the first contact is often the multi-ground neutral conductor.
This contact will impress the high voltage on the metal surfaces of the premises.
This is also one reason for the quick response of generator units in coming on line.
This is the only reason for disconnecting the utility neutral.
[ March 29, 2003, 11:51 AM: Message edited by: bennie ]
- Location
- Massachusetts
Re: Switching neutral in a transfer switch
[ March 29, 2003, 12:28 PM: Message edited by: iwire ]
Yes, but all these surfaces will be at the same potential, where's the danger.
I am missing the connection between the quick response of the generator and the switching of the grounded conductor.
If you look at Ed's drawing "B" and you changed it to a solidly connected grounded conductor don't we now have a grounding conductor in parallel with a grounded conductor, and if there is nothing wrong with that can I start bonding the neutral at sub panels?
[ March 29, 2003, 12:28 PM: Message edited by: iwire ]
Ed MacLaren
Senior Member
Re: Switching neutral in a transfer switch
Magoo,
The two reasons for switching the neutral in the transfer switch are as stated above -
by Russ - "If the neutral is bonded at the service main and at the generator case, the equipment ground becomes parallel with the neutral."
and by Dereck - "The main determination factor is whether or not GFP is used at the service, generator, or both. If GFP is used you would configure the generator as SDS."
The sketch below (Single Phase System) illustrates the code violation of a neutral/EGC parallel path.
Ed
Magoo,
The two reasons for switching the neutral in the transfer switch are as stated above -
by Russ - "If the neutral is bonded at the service main and at the generator case, the equipment ground becomes parallel with the neutral."
and by Dereck - "The main determination factor is whether or not GFP is used at the service, generator, or both. If GFP is used you would configure the generator as SDS."
The sketch below (Single Phase System) illustrates the code violation of a neutral/EGC parallel path.
Ed
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: Switching neutral in a transfer switch
I personally will avoid switching the neutral as it puts one more link in the chain to break. and if the chain breaks I don't want it to be the neutral that I loose. so with the experience of a failure of a transfer switch contact. I think switching the neutral would be avoided when ever possible.
As we all know what would happen if that happens.
So no SDS's for me.
With that said I do under stand that some manufactures don't provide a easy way to separate the neutral to ground bond. with out voiding the listing of the generator or even the warranty on it, so we sometimes have to use a SDS type hook up to make it code compliant.
I personally will avoid switching the neutral as it puts one more link in the chain to break. and if the chain breaks I don't want it to be the neutral that I loose. so with the experience of a failure of a transfer switch contact. I think switching the neutral would be avoided when ever possible.
As we all know what would happen if that happens.
So no SDS's for me.
With that said I do under stand that some manufactures don't provide a easy way to separate the neutral to ground bond. with out voiding the listing of the generator or even the warranty on it, so we sometimes have to use a SDS type hook up to make it code compliant.
Re: Switching neutral in a transfer switch
Ed: With the neutral/ground interconnected in the first switch, why have the green wire from the transfer switch to the generator frame? Remove the wire, prevents a parallel problem.
A generator is a power producing device not a power consuming device.
Ed: With the neutral/ground interconnected in the first switch, why have the green wire from the transfer switch to the generator frame? Remove the wire, prevents a parallel problem.
A generator is a power producing device not a power consuming device.
Re: Switching neutral in a transfer switch
A generator on the supply side of the service will act the same as the utility supply. The GFP system will not change. The ground electrode system will be the same.
The fast switching of a generator coming on line will remove any voltage impressed on the neutral, somewhat a slow GFI.
A primary line, when broken, falling across the MGN, will raise the voltage of the MGN to the line potential. The recloser will be set at withstand rating of the conductor. The MGN can and will burn open back towards the upstream transformer, this will guarantee the full high voltage is on the premises neutral and ground system.
A generator on the supply side of the service will act the same as the utility supply. The GFP system will not change. The ground electrode system will be the same.
The fast switching of a generator coming on line will remove any voltage impressed on the neutral, somewhat a slow GFI.
A primary line, when broken, falling across the MGN, will raise the voltage of the MGN to the line potential. The recloser will be set at withstand rating of the conductor. The MGN can and will burn open back towards the upstream transformer, this will guarantee the full high voltage is on the premises neutral and ground system.
- Location
- Illinois
- Occupation
- retired electrician
Re: Switching neutral in a transfer switch
Bob,
Don
Bob,
Bennie has a good point here. You are correct that the surfaces that are bonded will be at the same potential, but there will be high voltage between these surfaces and the earth for the time that the fault exists. In order to "isolate" the building system completely from the utility system the 4 pole transfer switch would have to be on the line side of the main bonding jumper. As others have stated, this is a design and not a code issue. It appears that the use of a four pole transfer switch on the line side of the main bonding jumper would be a good design practice.
Don
Re: Switching neutral in a transfer switch
Don I agree with you 100%. An SDS configuration is the best design approach for a generator. Like you said, it is a design issue not code.
Wayne or Hurk27, I understand your eggs in the basket theory, but if GFP is involved, SDS is the only practical design solution. Otherwise false trips will occur on either the generator or service
Don I agree with you 100%. An SDS configuration is the best design approach for a generator. Like you said, it is a design issue not code.
Wayne or Hurk27, I understand your eggs in the basket theory, but if GFP is involved, SDS is the only practical design solution. Otherwise false trips will occur on either the generator or service
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