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Electrical Power Flow Diagram

Electrical Power Flow Diagram

Every ship has an electrical power flow diagram. This is usually known as the single line diagram. This is posted in the engine control room, listed in the electrical manual and used during blackouts, faults and isolation.

Understanding this diagram can give you all the information needed to answer questions like if this generator trips what will still be on? What will be lost if I open this breaker? Where does the emergency board get its supply? This post will explain how to read and understand the power flow diagram.

Want to understand how electrical power is generated, distributed, and supplied across different systems on a ship? Explore the Electrical Circuit Diagram Course by Merchant Navy Decoded to improve your understanding of shipboard electrical circuits, connections, and power distribution systems.

Electrical Circuit Diagram

Why is it called a single line diagram?

As electrical power on a ship is 3 phase there are 3 conductors. To show all 3 phases on each cable would make the diagram very complicated. Instead we draw this as one line, therefore it is referred to as a single line diagram. This is a simplified version showing how the components are connected.

Common symbols

You don’t need to know all these symbols but some will be used on most ships:-

  • Generator – Circle with a G inside. Sometimes with a rating (kW/kVA, Volts and Frequency) written next to it.
  • Motor – Circle with M inside.
  • Transformer – Two circles joined together or two coils. Voltages will be written next to them (e.g 440/220V).
  • Busbar – Thick horizontal line. This is the main supply for that section.
  • Circuit Breaker – Small square or square with cross in. Often labelled (e.g. ACB / MCCB).
  • Fuse – Small square on line.
  • Isolator / Disconnector – Small diagonal line showing an isolator switch.
  • Earth – Set of horizontal lines getting smaller.
  • Battery – Long and short lines in pairs.
  • Meter / Relay – Circle or square with letter (e.g Voltmeter – V, Ammeter – A, Protection Relay – ).

Usually all components will have a tag or label. This will match the tags on the actual equipment and the breaker list

Table of Contents

Diagram Layout

  • Generators at top (No.1 DG, No.2 DG, No.3 DG, Shaft gen and Shore supply).
  • Main busbars in the middle (thick line with sometimes a bus-tie breaker in the middle to split into 2).
  • Feeders going down to loads.
  • Transformers feeding lower voltage boards.
  • Emergency switchboard will usually be separate with its own generator and link to the main board.
  • Power flows from top to bottom.

Actual diagrams will be more complex with more feeders and protection. The general layout will be the same with supply coming from the generators to the busbars and then everything else.

Normal Operation

  • At sea – 1 or 2 diesel generators running or the shaft generator supplying power. Breakers will be closed and power is supplied to the group starter panels via the feeders. This powers the pumps, fans and compressors. Another feeder goes to the 220V boards after going through the transformer. The emergency switchboard is also supplied from the main board via the interconnector so everything remains live and ready.
  • Manoeuvring – More generators are started and put in parallel on the busbars due to high power requirements for the thrusters, steering gear and extra pumps. All the generator breakers can be seen on the diagram.
  • In port – Different generators may be required to be running due to extra equipment such as cargo pumps and cranes. Or the ship may be connected to shore supply. Shore supply will be shown as another supply with its own breaker.

Shore Supply

When the ship is connected to shore supply the breaker will be closed and generators turned off. This can be seen on the diagram with another supply. There is an interlock between the shore breaker and the generators to ensure you cannot accidentally put both on. The voltage, frequency and phase sequence must all match before connecting to avoid the motors running in reverse.

Blackout

If the main supply is lost:-

  1. Main busbars will lose power and the generator breakers will open.
  2. Emergency generator will start automatically.
  3. Interconnector breaker between main and emergency boards will open to prevent the emergency generator supplying back to the main board.
  4. The emergency generator breaker will close onto the emergency switchboard. Emergency loads will return within the required time.
  5. Batteries will supply the 24V DC system to keep emergency lighting and alarms on.
  6. Standby generator will start and close onto the dead main board. Essential services will start up in sequence.

You can follow this on the diagram by seeing which breakers will open and close. It is a good idea to practice this on paper.

Conclusion

The power flow diagram acts as a map. The generators are at the top, busbars in the middle and loads at the bottom. Breakers, transformers and emergency supplies are also shown. This will tell you where the power is coming from, going to, what is protecting it and what will happen if there is a fault. Once you are confident reading the diagram, you will be able to follow the power flow during normal operation, shore supply and blackouts. This will allow you to safely isolate equipment and troubleshoot much quicker.

Frequently Asked Questions (FAQs)

          A simplified diagram showing a 3 phase electrical system. Each circuit is represented by one line to show how the supply, busbars, feeders and loads are connected.

     Normally from top to bottom. From the supply (generators, shaft generator and shore supply) through the busbars and feeders to the loads.

           This connects the two sections of the main busbar. This allows them to be run together or separated. If there is a fault on one section the other will still work.

To prevent them both being accidentally connected to the busbars. This could cause damage to equipment.

           The interconnector breaker will open. The emergency generator will start and the     breaker will close onto the emergency board. Emergency loads will return.

Disclaimer :- The opinions expressed in this article belong solely to the author and may not necessarily reflect those of Merchant Navy Decoded. We cannot guarantee the accuracy of the information provided and disclaim any responsibility for it. Data and visuals used are sourced from publicly available information and may not be authenticated by any regulatory body. Reviews and comments appearing on our blogs represent the opinions of individuals and do not necessarily reflect the views of Merchant Navy Decoded. We are not responsible for any loss or damage resulting from reliance on these reviews or comments.

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