muskrat
Member
- Location
- St. Louis, MO
Can't/don't find in art.700-Have seen it both ways. Should the grounded current carrying conductor be switched in a transfer switch for optional stand-by generator?
muskrat / Generator Bonding?
- Thread starter muskrat
- Start date
- Status
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
You haven't seen it because it is not there. It is not a code requirement, but rather is a design choice. It can go either way, and both ways have their advantages and disadvantages. For my part, I will switch the neutral unless someone gives me a good reason not to (i.e., switching is my "defalt" setting). Welcome to the forum.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Can't/don't find in art.700-Have seen it both ways. Should the grounded current carrying conductor be switched in a transfer switch for optional stand-by generator?
By no means am I an authority on this but I will attempt to answer. I see one thing in art. 700.6(A) that could be construed as requiring the grounded conductor to be switched but I am not sure that is the intent.
Does this mean the grounded conductor? I don't know.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Charlie is that always the case-- a design issue or are the scenarios where you must switch the neutral? Would you mind sharing an advantage to a switched neutral?
- Location
- Massachusetts
If you have a GFP main breaker there are some conditions that can trip the GFP main of you do not switch the neutral at the transfer switch.
OTH a 3 pole transfer switch is less money than a 4 pole transfer switch.
OTH a 3 pole transfer switch is less money than a 4 pole transfer switch.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
The path and connections for the equipment grounding conductor are simpler and more nearly sane, when you switch the neutral. If you have the 2008 handbook, take a look at exhibits 250.6 and 250.7. It's not a major thing, but I just don't like the layout of 250.6.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I have heard this, in fact, I have heard of problems with AFCI and transfer switches that didn't switch the neutral. I only heard of a few cases so I assume it is not a regular problem.
If that required a switched grounded conductor, I would think the ATS companies would stop making and selling 3 pole ATS's.
A three pole ATS requires the removal of any bonding between the neutral and the ground at the generator. A 4 pole ATS requires the bond between the neutral and ground at the generator.
Are there any generators that you can't remove or bond, or where it would violate the generator's manufacturer's instructions to remove the bond?
Another place where a 4 pole ATS may be needed is for multiple generators, or for generators serving multiple services or multiple transformers, or multiple buildings.
For example, imagine a single generator that serves two 3 pole ATS's where the normal is fed with two separate utility transformers. The neutral and ground have to be bonded at each utility transformer. So with a 3 pole ATS, the solid neutral connection will place the ground wires in parallel with the neutral wires. Then the neutral current will be flowing all over the grounding system.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Well the section I quoted only applies to art. 700- emergency systems so I wasn't sure. It would not apply everywhere unless that wording is used everywhere.
Yes, I didn't think about that.
But after looking at the quote again, I noticed it says the "inadvertent interconnection". I guess having the neutrals tied together would be considered an intentional interconnection.
- Location
- Massachusetts
I agree.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
That could be the case, I read it and a flag went up. I never worked on an emergency system generator so I only had the code words and was confused. As far as I knew the switched neutral setup is never required or am I wrong on that?
Charlie, Do you specify closed or open neutral transition?
I'm not sure there is anything in the code that requires the neutral to ever be switched in a transfer switch.
But on the other hand, most people don't want their GFP tripping due to the conditions Bob mentioned. And having neutral currents flowing over ground paths isn't good either. But if there is a specific code article that disallows either, I couldn't find it.
- Location
- Massachusetts
Here is a drawing by Ed MacLaren a forum member that used to be a regular but has not been around in a long time.
ceb58
Senior Member
- Location
- Raeford, NC
I agree that if there is GF protection then switch the neutral. But IMHO switching the neutral in normal situations is a waste of time and money. If the system is installed per code you will never break the the connection between the utility neutral and the gen. neutral even though you switch the neutral.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Most of the facilities I have dealt with recently were labs with a backup generator. The function of the generator was to supply egress lights and a minimum of lab equipment, should the utility fail. That would not require a closed transition, so I have only specified open transition.
muskrat
Member
- Location
- St. Louis, MO
Thanks all. And its for the Comms building at an ANG base,stand-by power for all the equipment, manual x-fer. I'm switching the neutral.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
And for me, I don't switch the neutral. Its simpler, and then no ground rods are required at the generator. There are only a few times where the neutral must be switched, one is for services with ground fault protection.
hillbilly1
Senior Member
- Location
- North Georgia mountains
- Occupation
- Owner/electrical contractor
Of all the ones I've installed, the services that were ground fault protected, used four pole switches, while the ones that did not have GFP, were three poles. I do believe it must be an issue with false tripping of the GFP using the three pole, even though the neutral is not grounded at the generator when the three pole is used. If there is a fault to ground while the generator is supplying the load, it could trip the GF main because the fault current would flow thru the GF neutral CT. This would be a problem if only part of the building is on generator.
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