The power rating of an emergency generator is determined by the size and role of the ship.
On some small vessels a few kW will suffice for emergency lighting only. Larger and more complicated vessels, e.g. LPG carriers, passenger liners, etc., may require hundreds of kW for emergency lighting, re-starting of the main engine auxiliaries and to supply fire-fighting pumps.
The construction and operation of an emergency generator on ship is similar to that of a main generator. Excitation supplies, either static or rotary, will usually be governed by an automatic voltage regulator.
In some cases where a static compounded exciter provides a reasonably constant generator voltage, the A VR may be omitted.
Emergency generator will usually operate at 440 V and the emergency switchboard will be interconnected with the Engine Room 440 V switchboard in normal operation.
An emergency generator is connected to its own emergency switchboard and they are located together in a compartment above the water line, e.g. on the boat deck.
In normal operation the emergency board is supplied from the main board by u cable called the interconnector.
It is not normally possible to synchronise the emergency and main generators.
Special interlocks in the control circuits of the circuit-breakers, at each end of the interconnector prevent parallel running.
Starting of the emergency generator prime mover is generally automatic. The run-up is initiated by an electrical relay which monitors the normal voltage supply.
Falling mains frequency or voltage causes the start-up relay to operate the engine starting equipment. The prime mover may be electrically cranked from its own 24 V battery and starter motor or air started from its own air reservoir fitted local to the generator engine.
A manual start-up may be initiated by push buttons in the main control room and in the emergency generator room.
Small generator prime movers can usually be manually cranked with a starting handle.
Correct functioning of the auto-start equipment is obviously vital to the production of emergency power.
Weekly testing of the emergency generator should include simulation of the loss of normal power.
The start-up equipment may provide a push button to interrupt the normal voltage supply to the control panel which then triggers the start-up sequence. Loss of main power supply can easily be simulated by pulling a fuse in the auto start panel which supplies the under-voltage or under-frequency relay.
Emergency generators on ship should be regularly checked and run up to speed for short test runs to comply with safety regulations. These no-load running checks should, when practicable, be supplemented occasionally by an actual load test.
This requires the disconnection of normal mains power from the emergency board while the emergency generator is loaded up to near its rated value.
Only a proper load test will prove the performance of the emergency generator, its prime-mover, and the circuit-breaker operation.