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).]

Try these links.
http://www.mikeholt.com/cgi-bin/codeforum/ultimatebb.cgi?ubb=get_topic;f=18;t=005123
http://www.mikeholt.com/codeforum/ultimatebb.php?ubb=get_topic;f=18;t=005233#000000

Short answer: When ground fault protection is required on either the service entrance or generator or both.

Thanks for the responses and links.

Obviously, I have not been keeping up very well with the Mike Holt forum.