Hi waross, I think the problem I faced with in the beginning is that there is no neutral bus in a fused 3-pole doublr throw switch. So when the load is switched over only the 3-phase is switched, but the neutral is not. Futher, they will be connecting to a portable genset in the future, which I am not sure how they will be connecting for sure. I am locating in Japan and grounding is a very foreign subject than what we accustomed to in the US, and it is no joke. Because there is no neutral bus in the DT switch, where else would they be connecting the ground. I fear they will be connecting to a separate grounding electrode (common in Japan) and in this case without any direction connection to the service neutral, it will completely violate NEC art. 250.20 & 250.30.
My intention is to assure that I give them a dedicated place to connect the neutral from the portable generator before I switch. The sequence of operation I am thinking is, first you disconnect the normal load with a 4-pole non-fused DTS, then you switched to a 3-pole + solid neutral fused safety switch (overcurrent protection is required in the design. The ground from portable genset will be connecting to the neutral at the safety switch. Let me know if I going about the wrong way.