...from a second separately derived system.

For example, the panel is fed from a building substation that is designated for emergency power (backed up by an onsite generator). Now you wish to temporarily shut down the power upstream from that panel in order to perform work on the system. This particular panel feeds equipment that is essential to the building and needs to be kept powered-up during the shut-down. In the same location there is a panel that has the capacity to feed the panel for the essential equipment but it is fed from a second substation that feeds non-essential equipment. Since this other panel is fed from a separately derived system do you have to switch the neutrals when transferring?

IMHO...Yes.

Not just 'yes,' but HELL YES.

Electricity wants / needs to go "home.' Home is where it is created. In most cases, this is the last transformer in the system, where the neutral was created.

Sending the neutral to another transformer is useless at best!

The ONLY place that neutral can terminate is at the same transformer that supplies the 'hots.'

Potseal:

As Ghost and Reno say...Yes! and Hell YES!!

Some bad things could happen by using the neutral of one SDS connected to another SDS neutral.

Thanks for the replies.

This question comes as a result of an actual project that I am involved in. I was asked by my supervisor to prepare to feed an Emergency power panel from a Normal power panel which will be part of a major electrical shutdown at the building where I work. When he stated that I only need to connect the hots I felt there was something wrong but decided I should do some research before discussing it with him. I looked in the CEC for a rule regarding this but found nothing. I then looked for articles online and read what I thought was related information. One article seemed to make it clear that a significant hazard would be compromising of any ground fault protection. As well, I contacted an electrical engineer whose projects I often work on. His main concern was backfeeding. Naturally I had to pose the question here. The results are unanimous and I can now approach him with confidence that we will need to switch the neutrals.