English Maritime Terminology on Naval Engineering

Essential and non-essential loads 

All ship power distribution system is divided into two major parts: the essential one, which feeds the equipment necessary for safe navigation, e.g. navigation lights, navigation and radio equipment, steering gear and its auxiliaries, lifesaving and firefighting equipment, etc. The non-essential part of the main switchboard provides power for all the remaining consumers, such as galley equipment, sewage system, air conditioning, other distribution boards, that are not crucial for safe navigation. 

This division of the loads is necessary to ensure power for the essential loads in case of blackout, as the essential load part of the main switchboard is directly connected to the emergency switchboard and in case of main generator failure, the emergency generator takes over the essential loads.