Generators as Separately Derived Systems

[mshields]

I've encountered an installation at a hospital consisting of several 4 pole ATS's and 1 3 pole ATS. The three pole switch, having a solid neutral, it seems to me, provides a permanent connection of the system to the generator and therefore, even though all three of the other switches, switch the neutral, this generator IS NOT a separately derived system. As such the neutral should not be bonded to ground at the genset.

Do you agree?

 

[mshields]

Ground Fault

Ground Fault

By the way, the building service is 800A at 480V. There is no ground fault. One of the concerns with bonding the neutral to ground under these circumstance is that you'd get ground fault trips. True?

Since we do not have ground fault, what are the other implications if one made the mistake of bonding at this generator?

Thanks,

Mike

 

[Smart $]

I agree, not an SDS.

Not quite sure how to interpret the first question of post #2. Are you asking about trips if the system(s) were ungrounded? As such, I believe the issue is where in the system the detector is located. When grounded and no detector, you want ground fault trips. :happyyes:

Post #2, question #2, objectionable current on the grounding system because you'd have parallel neutral current paths.

 

[qcroanoke]

I've encountered an installation at a hospital consisting of several 4 pole ATS's and 1 3 pole ATS. The three pole switch, having a solid neutral, it seems to me, provides a permanent connection of the system to the generator and therefore, even though all three of the other switches, switch the neutral, this generator IS NOT a separately derived system. As such the neutral should not be bonded to ground at the genset.

Do you agree?

Not an SDS.
However I am curious. Is there only one generator?
It seems if there is only one generator there is something wrong here.

 

[Maders]

By the way, the building service is 800A at 480V. There is no ground fault. One of the concerns with bonding the neutral to ground under these circumstance is that you'd get ground fault trips. True?

Since we do not have ground fault, what are the other implications if one made the mistake of bonding at this generator?

Thanks,

Mike

You should check out Chapter 7.9 of IEEE Standard 466 (a.k.a. The Orange Book) as it goes into great detail on generator grounding (both intentional and unintentional). Figure 7-17 (b) illustrates the effect that improper neutral grounding will have on ground fault protection.

 

[Pharon]

I've encountered an installation at a hospital consisting of several 4 pole ATS's and 1 3 pole ATS. The three pole switch, having a solid neutral, it seems to me, provides a permanent connection of the system to the generator and therefore, even though all three of the other switches, switch the neutral, this generator IS NOT a separately derived system. As such the neutral should not be bonded to ground at the genset.

Do you agree?

Are you certain that the 3 pole ATS has a neutral in it? If it feeds a 3-wire board, this could still be a SDS.

 

[texie]

I've encountered an installation at a hospital consisting of several 4 pole ATS's and 1 3 pole ATS. The three pole switch, having a solid neutral, it seems to me, provides a permanent connection of the system to the generator and therefore, even though all three of the other switches, switch the neutral, this generator IS NOT a separately derived system. As such the neutral should not be bonded to ground at the genset.

Do you agree?

Yes, but if the neutral is not bonded at the genset in this setup then the fault current path for the switched neutral part of the system would be back through the solid neutral ATS. There is no way to make this mix and match of ATSs compliant.

 

[brian john]

Yes, but if the neutral is not bonded at the genset in this setup then the fault current path for the switched neutral part of the system would be back through the solid neutral ATS. There is no way to make this mix and match of ATSs compliant.

There is always a way, it just depends on how much money you have. Utilizing the 3 Pole ATS setting a transformer on the load of the ATS would resolve the issue. Not practical and expensive but .......

First thing I would check is what they did with the neutral at the generator.

As for nuisance tripping (if you had GFPE) that would depend on the setting of the GFPE relay, but you will have ground current and this can result in power quality issues.