SDS and common neutral

[fsck]

So from what I've read, when you have a generator or other alternate power source, and transfer switch, each is a SDS, and that switch should switch the neutral as well. Neutral is grounded at the source.

But the case I'm looking at will have a utility xfmr w/wye secondary at the transfer switch, and an adjacent genset, and the inverters, all within a few feet. I would think all would share the same ground point -- the rod+Ufer etc. at that point. If so, is there still a requirement to switch the neutral?

 

[lume7006]

Hello,

In this case, I would recommend you:

1)Take a look at NEC, there is a whole chapter about grounding and as far as I remember there are some sections explicitly related to what you mentioned.

2)You should do an analysis about:
When you have as a source the xfmr, there will be a ground reference, because it is the basic scheme to feed your loads
When you have the genset, will the ground also be there?
Remember that ground is important to give a reference to your voltages and also the ground path has to be kept in order to allow ground protections to "do their job"

 

[waross]

Although there are more complex systems where separate neutrals must be used to avoid ground loops, current sharing, and to facilitate ground fault detection, my preference is to ground the neutral if at all possible. In cases where a compromise must be made, I try to consider switching the neutral to be the last compromise.
The neutral may be common for the grid feed and for the generator. Solid neutral through the ATS.
I have seen expensive damage done several times when a switched neutral opened early or closed late. None of the systems was of a size or complexity to warrant neutral switching. I have never seen an issue with a solid neutral on a simple system.
How do you know if your system is simple or complex? Do you have a protection engineer determining the various settings on a sophisticated protection relay? No, then it may be a simple system. If a complex system may be configured to allow a solid neutral, that is my choice also.
There are other opinions on this topic.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

If all of the sources are close to each other, it will make more sense to ground the neutral only at the utility source and leave the generator neutrals ungrounded. The generators will not be separately derived sources and the transfer switch would not switch the neutral.

 

[waross]

Further to jghrist's post; the generator neutrals will be grounded by the main panel ground by virtue of the solid neutrals. Leaving the generator neutrals ungrounded would refer to not grounding the neutrals at the generator, but using the panel ground.
Often, the ATS has a neutral bus on which all incoming neutrals are connected. The main panel or location designated by the utility has the one ground on the system and that serves all sources.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[fsck]

Leaving the generator neutrals ungrounded would refer to not grounding the neutrals at the generator, but using the panel ground.

I sorta figured that out.