According to zxcvbob :
You wouldn't need a transfer switch; pulling out the main fuses would be
a positive mechanical disconnect.

A positive mechanical disconnect, but not a foolproof _transfer_ mechanism.

The code insists on it being impossible to reverse energize the grid
from the generator and vice-versa.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.