What is the need for preferential tripping in marine electrical distribution system

 In marine electrical distribution system, a major consideration is that the supply of power to the switch board must be maintained, if possible. Otherwise a black out situation due to tripping of alternator breaker can endanger the safety of the vessel itself especially in restricted waters and maneouvring. The breaker therefore is to be arranged to trip instantly only in the event of fault such as short circuit. When over current is not so high, time delays with inverse definite Minimum Time relays are provided by way of preference trips which are designed to shed non-essential switchboard load. Preference trips are operated by relay set at about 110 percent of the normal full load, but with different time delays to open the breakers feeding non essential feeders such as galley load, ventilation fan, AC, etc, thereby reducing the load on alternator and saving the main breaker to trip