What is ship’s electrical system?
An overview of a ship’s electrical system is presented and describes various types of circuit diagram used in electrical work. Electrical calculatons, safety precautions, circuit diagrams and testing methods are outlined together with a description of general electrical maintenance and fault techniques. The electrical power system on board ship is designed to provide a secure supply to all loads with adequate build-in protecton for the equipment and operating personal. The general scheme of a ship’s electrical power system is common to nearly all ship.
The main a.c.generators (sometimes called alternators) produce the electrical power. It is supplied to the main switchboard an then distributed to have various auxiliary services comprising the electrical load. An emergency generator and emergency switchboard maintain supplies in the event of a main power failure.
Power generation On board
Shipboard power is generated using a prime mover and an alternator working together. For this an alternating current generator is used on board. The generator works on the principle that when a magnetic field around a conductor varies, a current is induced in the conductor.
The generator consists of a stationary set of conductors wound in coils on an iron core. This is known as the stator. A rotating magnet called the rotor turns inside this stator producing magnetic field. This field cuts across the conductor, generating an induced EMF or electro-magnetic force as the mechanical input causes the rotor to turn.
The magnetic field is generated by induction (in a brushless alternator) and by a rotor winding energized by DC current through slip rings and brushes. Few points to be noted about power on board are :
- AC, 3 phase power is preferred over DC as it gives more power for the same size.
3 phases is preferred over single phase as it draws more power and in the event of failure of one phase, other 2 can still work.
Power Distribution on board
The Power Distributed on board a ship needs to be supplied efficiently throughout the ship. For this the power distribution system of the ship is used.
A shipboard distribution system consists of different component for distribution and safe operation of the system. They are :
- Ship Generator consisting of prime mover and alternator
Main switch board which is a metal enclosure taking power from the diesel generator and supplying it to different machinery.
- Bus Bars which acts as a carrier and allow transfer of load from one point to another. Circuit breakers which act as a switch and in unsafe condition can be tripped to avoid breakdown and accidents. Fuses as safety device for machinery.
- Transformers to step up or step down the voltage. When supply is to be given to the lighting system a step down transformer is used in the distribution system.
- In a power distribution system, the voltage at which the system works is usually 440v.
- There are some large installations where the voltage is as high as 6600v.
- Power is supplied through circuit breakers to large auxiliary machinery at high voltage.
- For smaller supply fuse and miniature circuit breakers are used.
- The distribution system is three wires and can be neutrally insulated or earthed.
- Insulated system is more preferred as compare to earthed system because during an earth fault essential machinery such as steering gear can be lost.
In case of the failure of the main power generation system on the ship, an emergency power system or a standby system is also present. The emergency power supply ensures that the essential machinery and system continues to operate the ship.
Emergency power can be supplied by batteries or an emergency generator or even both systems can be used.
Rating of the emergency power supply should be made in such a way that it provides supply to the essential systems of the ship such as :
a) Steering gear system.
b) Emergency bilge and fire p/p.
c) Watertight doors.
d) Fire fighting system.
e) Ships navigation lights and emergency lights.
f) Communication and alarm system.
Emergency generator is normally located outside the machinery space of the ship. This is done mainly to avoid those emergency situations wherein access to the engine room is not possible. A switch board in the emergency generator room supplies power to different essential machinery.