Emerg Gen Grounding with 3 and 4 Pole ATS's

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doubleclawhammer

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Is there a grounding sceheme to follow when an emergency system contains both 3 Pole and 4 Pole auto xsfr switches? Or should an effort be made to replace ATS's so that the system has only one switch configuration?

Thanks much.
 
If the reasons to use 4 pole switches are not present, such as ground fault protection, then either 3 pole or 4 pole are fine.
If 4 pole are really needed, then the 3 pole xfer sw's should be replaced.
 
Ron,

Just need further clarification. Are you saying that if an existing emergency system has both 3 pole and 4 pole switches, then "no harm done", everthing is fine, Code legal, etc?

Thanks again!
 
doubleclawhammer said:
Ron,

Are you saying that if an existing emergency system has both 3 pole and 4 pole switches, then "no harm done", everthing is fine, Code legal, etc?

Thanks again!
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Providing the system bonding jumper is made at only the source and not the gen set also, IMO everything is fine and to code. In a 3-pole switch, the main bonding jumper is used to return the fault current back through the windings of the source to complete the circuit. With a 3-pole and a 4-pole the same jumper is used for the same purpose. The only difference is the grounded conductor of the 3-pole feeder is used to carry the fault current from the 4-pole feeder through the gen set because of it being switched.That would be the debate as to whether it is allowed. Nothing says either way.

Rick
 
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It is fine to leave both types if 4 pole type are not required to make the system work properly. Often if there is ground fault detection upstream, 4 pole switches are needed to ensure proper fault current flow, and then you would need to replace 3 pole switches.
 
The 2008 in section 250.20 D tells us if supplied with transfer equipment that switches the grounded conductor then it shall be grounded in accordance with 250.30A, it is by definition a separately derived system. The fine print note tells us that if a solidly interconnected grounded conductor, a 3 pole transfer switch, then the generator is not a separately derived system. I do not believe that you can have it both ways, a separately derived and not a separatelt derived system. From my understanding you cannot mix transfer switches, meaning thay all have to switch the grounded conductor or not switch it, the transfer switches cannot do both.

Fred
 
I may be missing something but how can you have both 3 pole and 4 pole ATSs in the same building?

If you bond at the generator then the 3 pole ATS will have parallel grounds, the 4 pole will function properly.

If you don't bond at the generator than the 4 pole ATS will lose the ground-neutral bond when it switches to generator.
 
MIEngineer said:
If you don't bond at the generator than the 4 pole ATS will lose the ground-neutral bond when it switches to generator.
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The effective fault path is still intact via the main bonding jumper at the main service disconnect. The path continues via the grounded conductor of the non switched feeder. I'm not saying this is a good way of wiring, but is there a code restriction that prevents using the grounded conductor of one feeder to carry the fault current of another feeder? Attached is a sketch showing the 3 and 4 pole switches.

View attachment 2322
 
The code referrence is 250.20 which requires the generator to be bonded and grounded in accordance with 250.30 when it is provided with transfer equipment where the grounded conductor is not solidly interconnected to the service grounded conductor. This requirement says when a 4 pole then the generator must be bonded at the source or the first disconnect. Even though there would be a effective fault current return path via the 3 pole in your diagram that is not allowed. The only time a 3 pole and a 4 pole can be on the same generator is if the three pole does not have a neutral load.

Fred
 
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