Each bus-tie between a main switchboard and an emergency switchboard must:

1. (a) Disconnect automatically upon loss of potential at the emergency switchboard;
2. (b) Be arranged to prevent parallel operation of an emergency power source with any other source of electric power, except for interlock systems for momentary transfer of loads; and
3. (c) If arranged for feedback operation, open automatically upon overload of the emergency power source before the emergency power source is tripped off the line from the overload.