This question was asked of me at another web site and I don't have a good answer. Would you folks enlighten me so I can pass it on.

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 in your opinion do you have to have the 4 pole transfer switch and the separately derived system?

I would like your opinion on tapping service feeders also.





[This message has been edited by wolfdog (edited 03-25-2003).]