Question

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

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

(b) State why this protection is required.

Answer 1

When the generator load reaches 110%, preferential Trip comes into operation as follows

First Stage Preferential Tripping (PT1):

Initiated when the current on a running generator exceeds 100% of the generator rating for a period of 10 seconds.
Protects against overcurrent by releasing the 1st stage preferential tripping.
Shut down non-essential loads (air-conditioning, entertainment, accommodation fans, cargo hold fans, amplifiers, etc.) to reduce the generator load

Second Stage Preferential Tripping (PT2):

Initiated if the current on a running generator continues to exceed 100% of the generator rating for an additional 5 seconds.
Shut down additional loads such as cargo hold vent fans and packaged air conditioning units. (service required for running the ship properly, leaving loads of top priority services to maintain propulsion and navigation) if the generator load is still high

Third Stage Preferential Tripping (PT3):

Initiated if the current on a running generator persists in exceeding 100% of the generator rating for 15 seconds.
Shut down the main generator as the last action, if the load is still too high, it may be due to a short circuit or insulation breaking.

Short Circuit Protection (Instantaneous Tripping):

Current transformers (CTs) monitor the current in each phase. In the event of a short circuit, the CT's secondary coil energizes the short circuit trip coil.
This generates a strong magnetic pull that trips the main breaker immediately, isolating the fault.

Main Breaker Trip

If the overload condition continues after non-essential loads have been shed, the final time-delay relay (e.g., 60 seconds) trips the main breaker to protect the alternator from damage.

Overload Protection and Alarms

Overload protection relays monitor all three phases and provide audio-visual alarms as warnings before tripping occurs.

Part (b)

In marine electrical systems, continuous power supply to the switchboard is essential to maintain vessel safety. A blackout resulting from the tripping of the alternator breaker can compromise the vessel’s operation and safety.
The preferential tripping system ensures that the alternator breaker only trips instantly in the event of a severe fault like a short circuit.
For less severe overcurrent conditions (e.g., 110% of full load), time-delayed relays perform preferential tripping by shedding non-essential loads such as galley equipment, air conditioning, and ventilation fans.
By reducing the alternator's load incrementally, the system prevents the main breaker from tripping unnecessarily and avoids a complete power blackout, ensuring essential systems remain powered.

✓ Verified Model Answer (Text Solution)