In a marine electrical distribution system, “preferential tripping” refers to a protective scheme that selectively trips or disconnects specific electrical circuits or equipment in a predetermined order during fault conditions or abnormal situations. The goal is to prioritize the protection of critical or essential equipment and systems while maintaining the overall stability and safety of the vessel’s electrical system.
Here’s how preferential tripping works in a marine electrical distribution system:
- Electrical Fault Detection: The electrical distribution system is equipped with protective devices such as circuit breakers, fuses, relays, or other protective devices. These devices constantly monitor the current, voltage, and other parameters in the electrical circuits.
- Predefined Priority: The system’s design includes a predetermined priority list of electrical circuits or equipment based on their criticality to the ship’s operation. For example, essential services like propulsion, steering, communication, and safety systems might be assigned a higher priority than non-critical systems.
- Fault Occurrence: When an electrical fault occurs, such as a short circuit or overload in a specific circuit or equipment, the protective devices detect the abnormal condition.
- Tripping Sequence: Instead of indiscriminately tripping all the protective devices simultaneously, the preferential tripping system follows the predefined priority list. It starts by tripping the least critical circuits or non-essential equipment first, gradually moving towards more critical systems.
- System Stability: The preferential tripping scheme aims to maintain the overall stability of the electrical distribution system during fault conditions. By ensuring that critical systems remain operational for as long as possible, the vessel has a better chance of avoiding a complete blackout or loss of essential services.
- Fault Isolation: Once the fault is cleared or isolated in a particular circuit or equipment, the system may automatically or manually restore the power to that circuit, if possible, to resume normal operations.
Preferential tripping is particularly crucial in marine electrical systems, as a complete loss of power or critical systems can pose significant risks to the vessel’s safety and operation. By selectively disconnecting less critical loads first, the ship’s crew can have more time to address the fault and avoid catastrophic consequences.
Working of Preferential Trip
As shown in the diagram, when overload current increases and reaches the lower loads of the system,the bottom lever pulls up and completes the circuit. The completion of the system gives an alarm signal and supplies current to the overload trips which are set for 5, 10 and 15 seconds delay.
If the overload current is extremely high,the upper levers of the system get trip, which instantaneously cuts off the the generator thus preventing it from any kind of damage.
5 second Air – conditioning and ventalation
10 second Refrigerated cargo plant
15 second Deck equipment
It’s important to note that the specific implementation of preferential tripping can vary based on the vessel’s design, the complexity of the electrical system, and the specific requirements of the ship’s classification society and regulations. Proper planning, design, and testing of the preferential tripping scheme are essential to ensure the safety and reliability of the marine electrical distribution system.
