Detail

Batteries
batteries and emergency systems on ships enable a number of essential services to be maintained under emergency conditions. The requirements vary with type of ship and length of voyage. Self-contained emergency sources of electrical power must be installed in positions such that they are unlikely to be damaged or affected by any incident that has caused the loss of main power. Batteries are fitted to provide temporary or transitional power supply, emergency lights, and navigation lights, watertight door circuits including alarms and indicators, and internal communication systems.

Switchboards and Distribution Systems
Dead front switchboards are a safety requirement for A.C. voltages in excess of 55 V. The air-break circuit breakers used for marine installations are frame-mounted and arranged for isolation from bus bar and alternator input cable contacts by being moved horizontally forward to a fixed position. The isolating plugs are not designed for making or breaking contact on load so the breaker must be open before the assembly is withdrawn. Interruption of current flow results in the production of an arc between contact faces. D.C. switchboards and distribution system switchgear unlike that of enclosed A.C. equipment, is mounted on what is literally a board or screen of panels. On the front, the open main circuit breakers, distribution breakers, knife switches, and instruments are easily accessible for inspection and maintenance. Safeguards against contact are only an insulated handrail and a rubber mat. A closed gate normally prevents entry to the passage behind the board where the copper bus-bars and rheostats are mounted. Enclosed or dead-front boards are only required for D.C. installations if the voltage is greater than 250 volts.

Generators
Operation of a generator relies on the principle that when magnetic lines of force are cut by a conductor, a voltage is induced in the conductor. Size of induced voltage and resulting current are dependent on magnetic field strength, length of conductor, and speed of cutting. The direction of current flow is dictated by the relationship between magnetic field and direction of movement of the conductor. A practical direct current generator has a large number of conductors that are caused to rotate in the magnetic field.

Motors
Motor is constructed with inward projecting field poles in the yoke and a rotating armature with conductors in the slots. Input current is applied to the shunt and or series windings on the field poles and also, through brushes and commutator, to the lap or wave wound armature conductors that are momentarily passing under the poles. Current in the pole windings produces fixed magnetic fields. Current in the armature conductors sets up a small magnetic field in each of those under the poles. Reaction of the fields associated with the conductors and the magnetic fields of the poles produces a turning effect in accordance with the motor rule.

Miscellaneous Electrical Items
These include light switches, torches, sockets and plugs, pushbutton contacts, relays, electric bells, starters, circuit breakers, and open fuses. Fuses may be used as the only protection in a steady-load circuit, such as for lighting. An A.C. motor with its high starting current and varying load has fuses in each of the supply conductors, but fitted as a back-up for the other forms of protection and to break the circuit in the event of a short-circuit current greater than that which the ordinary contact breakers are designed to interrupt without damage. Very accurate time/current characteristics are needed for fuses used in conjunction with other safety devices to ensure that the overload trip is allowed time to operate for moderate over-current but that the fuse blows first if there is very high short-circuit current.