Consider a commercial building with a three phase service and 100% generator back up.

If you use a 3 pole transfer switch, you don't break the neutral.

If you use a 4 pole switch, you break the neutral, add some extra grounding, and the system is now "Separately Derived".

When do you have to have the 4 pole transfer switch and the separately derived system?

This same question is asked in the General Discussion Area.
Look for a post by wolfdog (backup generators)

Russ
I added this link to it. Just click it
https://www.electrical-contractor.net/ubb/Forum1/HTML/002728.html

[This message has been edited by russ m (edited 03-26-2003).]

I just went into the search section of this forum (ECN) and typed in transfer switch. There are alot of threads on this subject.

Russ

[This message has been edited by russ m (edited 03-26-2003).]