I have recently seen several different proposals for the installation of some auxillary transfer switch installations. Most of these were 3ph. 4w. 277/480v. installations. Some of the engineers specked the xfer switch to be a four pole (switching neutral), others only had a 3 pole xfer switch specked. What would be the pros and cons of switching the neutral or letting it remain solid? Thank you.
Neutral switching for auxillary power
- Thread starter patc
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Senior Member
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- New York, 40.7514,-73.9925
Re: Neutral switching for auxillary power
If one source is a generator, then switching the neutral makes it easier to have GF protection, and for it to work properly. If you don't switch the neutral, then it is not a seperately derived source and there should be no n-g bond at the gen.
Hopefully the engineers you are working with you are on the ball, like most of them/us
If one source is a generator, then switching the neutral makes it easier to have GF protection, and for it to work properly. If you don't switch the neutral, then it is not a seperately derived source and there should be no n-g bond at the gen.
Hopefully the engineers you are working with you are on the ball, like most of them/us
Re: Neutral switching for auxillary power
4 pole changeover switch is must for two seperately derived sources like generator & transformer feeding same load alternatively (not parralleled). This is for following reasons
1. Proper functioning of GF protection - GF protection will not work in the event of fault or it may work when there is none.
2. Safety of persons working on transformer primary side when system is running on generator
4 pole changeover switch is must for two seperately derived sources like generator & transformer feeding same load alternatively (not parralleled). This is for following reasons
1. Proper functioning of GF protection - GF protection will not work in the event of fault or it may work when there is none.
2. Safety of persons working on transformer primary side when system is running on generator
Re: Neutral switching for auxillary power
Neelesh:
Could you explain point #2. I have not heard of this before, and I don't see any reason why the connection of a neutral would be a safety issue on the primary side of a transformer.
Steve
Neelesh:
Could you explain point #2. I have not heard of this before, and I don't see any reason why the connection of a neutral would be a safety issue on the primary side of a transformer.
Steve
Re: Neutral switching for auxillary power
If you have GFP you need to switch the neutral to prevent false tripping of the service while on generator.
Regardless of which method you use choose (SDS or not) the grounded circuit conductors (neutral) are always bonded together if code compliant. Think about it.
[ March 04, 2004, 10:08 AM: Message edited by: dereckbc ]
If you have GFP you need to switch the neutral to prevent false tripping of the service while on generator.
Regardless of which method you use choose (SDS or not) the grounded circuit conductors (neutral) are always bonded together if code compliant. Think about it.
[ March 04, 2004, 10:08 AM: Message edited by: dereckbc ]
Re: Neutral switching for auxillary power
Steve:
In case of SDS, generator is feeding load, neutral is common, transformer primary switch is off - current may circulate thru transformer neutral and bonding conductor, a connection (person, tool or vermin) between say 'R' pahse conductor of transformer secondary side with ground may circulate current (neutral point of transformer elevated wrt ground) This voltage will get transfered on primary side of transformer may cause hazard.
Steve:
In case of SDS, generator is feeding load, neutral is common, transformer primary switch is off - current may circulate thru transformer neutral and bonding conductor, a connection (person, tool or vermin) between say 'R' pahse conductor of transformer secondary side with ground may circulate current (neutral point of transformer elevated wrt ground) This voltage will get transfered on primary side of transformer may cause hazard.
- Location
- Massachusetts
Re: Neutral switching for auxillary power
The source of the current is the generator, only one line (neutral) is connected to the generator and the utility.
How do we have current flow without a complete circuit back to the source?
Neelesh, how would this happen?
The source of the current is the generator, only one line (neutral) is connected to the generator and the utility.
How do we have current flow without a complete circuit back to the source?
Re: Neutral switching for auxillary power
Generator neutral is solidly grounded, transformer neutral is solidly grounded as well there is seperate bonding conductor between them.
(I need to attach a diagram to explain. How do I do it here?)
[ March 05, 2004, 06:02 AM: Message edited by: Neelesh ]
Generator neutral is solidly grounded, transformer neutral is solidly grounded as well there is seperate bonding conductor between them.
(I need to attach a diagram to explain. How do I do it here?)
[ March 05, 2004, 06:02 AM: Message edited by: Neelesh ]
Ed MacLaren
Senior Member
Re: Neutral switching for auxillary power
Neelesh,
When the neutral is continuous (not switched) in the transfer switch, it should not be connected to a grounding electrode at the generator, nor should it be bonded to the generator frame.
Ed
[ March 05, 2004, 07:30 AM: Message edited by: Ed MacLaren ]
Neelesh,
When the neutral is continuous (not switched) in the transfer switch, it should not be connected to a grounding electrode at the generator, nor should it be bonded to the generator frame.
Ed
[ March 05, 2004, 07:30 AM: Message edited by: Ed MacLaren ]
Re: Neutral switching for auxillary power
Ed:
I agree with u for non SDS scheme, however in case of SDS scheme neutral of generator & transformer are grounded and ideally neutral of both the sources shall be seperate.
(both schemes follow code)
Group:
My point is in case of SDS scheme and the neutral of both sources is common (I know this is wrong but let us assume for time being it exist as a mistake) in this case with following conditions
1. System is fed by generator
2. Transfer switch is 3P and is on generator side
3. Transformer is switched off from primary side
A connection between say 'R' phase of transformer secondary with ground may lead to circulating current to flow.
Roger:
Thanks. Let me try one.
Ed:
I agree with u for non SDS scheme, however in case of SDS scheme neutral of generator & transformer are grounded and ideally neutral of both the sources shall be seperate.
(both schemes follow code)
Group:
My point is in case of SDS scheme and the neutral of both sources is common (I know this is wrong but let us assume for time being it exist as a mistake) in this case with following conditions
1. System is fed by generator
2. Transfer switch is 3P and is on generator side
3. Transformer is switched off from primary side
A connection between say 'R' phase of transformer secondary with ground may lead to circulating current to flow.
Roger:
Thanks. Let me try one.
- Location
- Massachusetts
Re: Neutral switching for auxillary power
Neelesh, I am not sure I followed your last post but code wise you are not allowed to have the neutral bonded twice.
Two code compliant ways to go.
1)Non-SDS generator (Eds drawing) must have a solidly bussed neutral at the transfer switch.
2)SDS-generator must have a switched neutral at the transfer switch.
A SDS-generator with a solidly bussed neutral, along with being a code violation would cause neutral current through the grounding system.
This may have been what you where getting at when you said
Bob
Neelesh, I am not sure I followed your last post but code wise you are not allowed to have the neutral bonded twice.
Two code compliant ways to go.
1)Non-SDS generator (Eds drawing) must have a solidly bussed neutral at the transfer switch.
2)SDS-generator must have a switched neutral at the transfer switch.
A SDS-generator with a solidly bussed neutral, along with being a code violation would cause neutral current through the grounding system.
This may have been what you where getting at when you said
I think we all know when electrical equipment is connected incorrectly it may present a hazard.
Bob
Ed MacLaren
Senior Member
Re: Neutral switching for auxillary power
As Bob has pointed out, SDS means that the neutral is switched.
Here is a sketch of the SDS configuration. Are you referring to unbalanced load (generator neutral) current?
Perhaps you could describe the path of the "circulating current".
Ed
[ March 06, 2004, 07:56 AM: Message edited by: Ed MacLaren ]
As Bob has pointed out, SDS means that the neutral is switched.
Here is a sketch of the SDS configuration. Are you referring to unbalanced load (generator neutral) current?
Perhaps you could describe the path of the "circulating current".
Ed
[ March 06, 2004, 07:56 AM: Message edited by: Ed MacLaren ]
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