Depends on the transfer switch timing. That's the buildings stuff. They want to make sure before a transfer that all IR has collapsed on the supply side and the current is 0 before the transfer from NP to EP or vice versa.

It Don't mean a thing if it aint got that swing.

Cool, Free, Johnny Smith courtesy of NPR..HERE
http://www.npr.org/ramfiles/asc/asc25.smith.asx