Aux 1 Unit 12 Q5

A preferential trip is a crucial safety mechanism in electrical power distribution systems, particularly on ships. It functions to prevent a complete system blackout by selectively disconnecting non-essential loads during an overload or partial power failure situation. Here’s how it operates:

1. Load Prioritization:

• Essential vs. Non-Essential Loads: Electrical loads are classified based on their criticality. Essential loads, like those required for navigation, safety, and communication, are given the highest priority. Non-essential loads, like air conditioning or galley equipment, are assigned lower priorities.

2. Overload Detection:

• Current Monitoring: The system continuously monitors the current flowing through the main busbar or a specific feeder.
• Overload Condition: If the current exceeds a predefined threshold for a specific duration, indicating an overload, the preferential trip system is triggered.

3. Selective Disconnection:

• Non-Essential Load Shedding: The system automatically trips circuit breakers connected to non-essential loads, starting with the lowest priority loads. This reduces the overall load on the system.
• Sequential Tripping: The tripping of non-essential loads usually occurs in a predetermined sequence, with delays between each trip. This allows time for the system to stabilize and potentially avoid further tripping.

4. Essential Load Protection:

• Main Breaker Prevention: By shedding non-essential loads, the system aims to prevent the main breaker from tripping, ensuring that essential services remain powered.
• Safety and Operational Continuity: This helps maintain critical functions like navigation, communication, and emergency systems, even during a power disruption.

5. Alarms and Indication:

• Audible and Visual Alerts: The system typically includes audible and visual alarms to notify the crew of an overload condition and the ongoing preferential tripping process.
• Load Indication: The alarms or indicators may also show which specific loads have been disconnected.

Key Components:

• Overcurrent Relays: These relays sense the overcurrent condition and initiate the tripping sequence.
• Time Delay Relays or Dashpots: These introduce a time delay between the tripping of different non-essential loads.
• Circuit Breakers: Special circuit breakers with preferential trip units are used for non-essential loads. These breakers have a time-delayed tripping mechanism that coordinates with the overall system.

Advantages:

• Prevents Blackouts: Helps avoid a complete loss of power by selectively disconnecting non-essential loads.
• Enhances Safety: Ensures critical systems remain operational during emergencies.
• Protects Equipment: Safeguards generators and other electrical equipment from damage due to overloads.
• Operational Flexibility: Allows for temporary load shedding to maintain essential functions.

Example Scenario:

• Overload: A sudden increase in load, perhaps due to the simultaneous startup of multiple heavy machinery, causes the current to exceed the preset limit.
• Preferential Tripping: The system might first disconnect the air conditioning, followed by galley equipment, and then non-essential lighting, based on their priority.
• Essential Services Maintained: This reduces the overall load, preventing the main breaker from tripping and ensuring that critical navigation and communication systems remain powered.

In summary:

Preferential tripping is an intelligent power management strategy that prioritizes essential loads during overloads or power disruptions. It’s a crucial safety and operational feature on marine vessels, ensuring that critical systems remain operational even in challenging situations.

In an electrical system with a preferential trip, the following types of circuits cannot be connected to it:

1. Essential Circuits:

• Propulsion Systems: The main engines and associated propulsion systems are critical for the vessel’s movement and safety. Their power supply must be maintained at all costs.
• Steering Systems: The steering gear is crucial for controlling the vessel’s direction, and any interruption in its power supply can lead to a dangerous loss of control.
• Navigation Equipment: Radar, GPS, AIS, and other navigation systems are vital for safe navigation and collision avoidance. They cannot be subject to preferential tripping.
• Communication Systems: Communication systems, both internal and external, are essential for safety and operational coordination. They must remain powered at all times.
• Emergency Systems: Emergency lighting, alarms, fire pumps, and other safety-critical systems must have a continuous power supply and cannot be connected to preferential trip circuits.

2. Critical Machinery and Equipment:

• Essential Pumps: Pumps that are vital for the vessel’s operation, such as bilge pumps, fire pumps, and certain machinery cooling pumps, should not be connected to preferential trip circuits.
• Other Critical Loads: Any other loads that are deemed essential for the safe operation of the vessel or its critical systems should also be excluded from preferential tripping.

Why these circuits cannot be connected:

• Safety: These circuits are vital for the safety of the vessel and its crew. Disconnecting them during an overload or power failure could lead to catastrophic consequences, such as loss of propulsion, steering, or navigation capabilities, or the inability to respond to emergencies.
• Operational Continuity: Essential systems need to be operational at all times to ensure the vessel’s continued operation and prevent accidents.

In summary:

The preferential trip system is designed to selectively disconnect non-essential loads during an overload or power failure to protect the main electrical system and ensure essential services remain powered.

It’s crucial to carefully identify and categorize the loads in the system, ensuring that essential circuits are never connected to the preferential trip and are always prioritized for power supply.

1. Preferential tripping in a marine electrical distribution system – DieselShip

The specific categorization of essential and non-essential loads might vary depending on the vessel’s type, its operations, and the applicable regulations. However, the general principle remains the same: essential services related to safety, navigation, and critical operations must be excluded from preferential tripping to ensure the vessel’s continued safety and functionality in any situation.Sources and related content