A vessel has an extensive electrical system designed to carry out more functions than can be found in a small community. Lighting, instrument panels, steering systems, stoves, and so forth are spread throughout the vessel. Every electrical item must be serviced regularly. Locating the electrical wires can be frustrating. In an emergency situation, restoring electrical power for the dewatering and fire pumps can mean the difference between life and death.
To understand the electrical distribution system, the different system components need to be examined in closer detail.
Distribution Center on ship
The distribution center controls the ship service generated power and shore power. Automatic overcurrent protective devices connected to the bus monitor and protect the feeders and branch circuits.
The feeder consists of the cables that extend from the main switchboard to the distribution panels. In some cases, the feeder may provide power directly to a large motor.
Bus Tie on ship’s system
The bus tie is the electrical connection between the main and emergency switchboard bus bars. It is not considered a feeder.
Emergency Switchboard on ship
The emergency switchboard is a smaller version of the main distribution switchboard. Power is received either from the emergency generator or the main switchboard through a bus tie. This switchboard provides power to vital services, including the fire main, communications, emergency lighting, steering, and so forth. When the ship’s main power is lost, emergency power is automatically provided by the emergency generators through the emergency switchboard by an automatic bus transfer (ABT) switch.
Motor Control Center (MCC) on ship
The motor control center consolidates all the motor controlling equipment for all the major motors on board the vessel. Overcurrent and overload protection is provided to the motor and immediate circuitry.
Distribution Panel on ship
The distribution panel is an enclosed metal panel that supplies power to components for a localized section of the vessel. Circuit breakers or fuses are installed to protect the branch circuits. The distribution panel has three bus bars (there is a fourth when the neutral is used). When the distribution panel is used to provide three-phase power to loads, then each bus bar is connected to one terminal of a three-pole circuit breaker.
The other side of the circuit breaker is connected to the loads.
If the distribution panel is used to supply single-phase power to loads, then only two of the bus bars are used, and a two-pole circuit breaker is employed.
To keep the three-phase generator balanced with single-phase circuits, each phase (A-B, B-C, or C-A) is designed to carry a specific percentage of the load. The single-phase loads are equally divided between the three phases. Each distribution panel receives three-phase power. When single-phase loads are connected, the distribution panel divides up the three phases so that each single-phase circuit (A-B, B-C, or C-A) has one third of the total power available from that panel.
This electrical balance is continued by equally dividing the load at each panel throughout the vessel. All the other panels are divided in the same manner because in the event of a distribution panel casualty, the overall generator load will be decreased evenly across the phases. The generator will still be electrically balanced.
Ship’s Power Distribution Panel
This panel is generally dedicated as a source of power for operating other components, such as the power supplied to the emergency batteries or to other distribution panels. More often, it can be distinguished as a panel used to distribute three-phase power to three-phase components.
Branch Circuits on ship
These are the conductors that exist between the final overcurrent protective device in the distribution panel and the load; for example, motors, lights, and receptacle outlets.
Lighting Branch Circuits on ship
These conductors supply power to lighting systems, bracket fans, small heating appliances, circuits, and motors 1/4 HP or less.
Appliance Branch Circuits on ship
These conductors supply power to the outlets for the use of portable or nonfixed electrical apparatus.
Communication Branch Circuits on ship
These conductors supply power to communication, audio, and visual signaling devices.
These are heavy-duty conductors used to carry current between a source and a load. Alternating current cables should contain all three current-carrying conductors in a single cable to cancel out heating caused by inductive effects.
Distribution Cables on ship
These cables are used for the power distribution up to the rated voltage and ampacity of the cable. Low-voltage (600-volt). They are used for most electrical connections.
Ship’s Control Cables
These are multiple parallel conductor cables used for:
- Control circuits where an electrical signal energizes a magnetic control device to physically open or close the main contacts of a motor.
- The control cable does not carry the main motor operating current, but only the current used in energizing the coil of the magnetic control device.
- Indicating circuits in meters and other audio and visual indicating apparatus.
- Communication, electronic, and other similar circuits.
- Signal Cables Signal cables of twisted pairs of conductor cables are used for signal transmission. Each twisted pair of conductors will have a shield to prevent interference.
- Portable Cords Portable cords are used for the temporary connection of portable appliances. They are not to be used for fixed wiring.