Question

With reference to preferential tripping in a marine electrical distribution system.

(b) State why this facility is required.

(c) With the aid of a sketch, describe a typical arrangement to provide three stages of tripping as an instantaneous protection against short circuit.

If a generator develops an overload fault condition then the generator is liable to be disconnected by the overload trip and this could lead to a total power failure and shut down of machineries.

In such a case it is better to maintain the generators operating and connected by removing the overload fault by disconnecting some of the connected load.

This disconnection of loads must be carried out automatically

The load to be disconnected must not be essential to the safety of the ship.

These non essential may be a single, group or as several groups.

Preferential trip thus disconnect the non essential services in a definite sequence and so maintaining generator supplies available to essential services such as safety of person aboard and safe navigation and propulsion of ship.

A scheme for operating all of the overload-type trips from one load-current-carrying coil (Figure a) uses two instantaneous trip levers.

The top lever is arranged as an instantaneous short-circuit trip and opens the breaker directly through mechanical linkages.

The bottom lever closes instantly at the lower overload current setting and, by so doing, completes the circuit through two (or more) non-essential circuit trips and a main breaker trip all with dashpot time delay (similar to Figure b).

These relays will trip out non-essentials at 5 and 10 second intervals and finally, if the overload persists, the main breaker after 15 seconds.

Warning of overload is given by the alarm.

Overload protection is provided on both poles (see Figure c).

Figure a

Figure b

Figure c