Grounded Delta Transformers

[CuriousElectron]

Greetings,
I've got a distribution system which is getting upgraded to add a manual transfer switch with a back-up source. The caveat is that the MTS will be switching between Delta-Delta transformers with center tap on the secondary of both transformers grounded to provide 120/240, 3 phase, 4 wire service. The utility is feeding the MTS via a step-down transformer with circuit breaker on the load side for protection for the MTS. The alternate source is standby gen. feeding the MTS via a step-up xfmr.
Should I use 3-4 pole circuit breaker upstream of the MTS? There is no ground protection scheme.
I'm thinking solid neutral at the MTS and 3-pole breaker is fine, since I don't have any ground CTs. My only concerns is that you could treat the two power sources as separately derived sources, since both sources are feeding MTS via grounded delta transformers.

Thoughts?
EE

 

[RRaghunath]

I suppose you don't work two sources paralleled at any time. If true, I don't see any need to have 4-pole breaker.
Of course, you would like to have neutral isolation when there is work planned on generator when the grid source is supplying the load or vice versa. This is for safety reasons as the neutral may be above zero potential due to single phase loads.

 

[waross]

First I would ground the neutral point for safety.
If you make the ground connection downstream of the MTS, you may leave both neutrals solidly connected together and use a three pole switch.
It is cheaper and safer to ground the neutrals than to use a four pole transfer switch.
I have several times had to clean up after a neutral went open.
I will use a switched neutral only as a last resort.

Bill
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"Why not the best?"
Jimmy Carter