When I was first learning the trade, the boss was most insistant that there be BOTH a mechanical and electrical interlock to prevent back-feeding - similar to what you see on a reversing motor starter.
So, we tied the breakers together for the 'mechanical' interlock. We also set up a pair of contactors so that the coil would energize (connecting generator power to the panel) only if both the generator was running and the house power was off. This was the 'electrical' interlock.
