...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?
A malfunction at the junction