OperationReferring to Figure 22.11, when the double pole switch is closed the navigation light is illuminated. Current in the relay circuit causes the relay coil to energise, which pulls the NC (normally closed) contact open so that the audio alarm (buzzer) circuit is now open. Only a low voltage lamp is needed for the indicating lamp. This ensures a small voltage drop across that part of the circuit. Keeping regulatory requirements in mind, if the indicating lamp fails, the circuit is completed through the back-up resistor, so the navigation light does not fail. If the navigation light fails, or if a fuse blows, the current in the circuit ceases and the relay is de-energised. The NC contact springs back to activate the buzzer circuit. In case of failure of the ship's mains, the double pole switch may be changed over to emergency supply. A similar circuit is depicted in Figure 22.12. The recommended caged filament lamp is also shown. Caged filaments last longer as they can withstand shock and vibration.
Now various signal lights with colours of red, green, white, and blue are arranged on the signal mast. These lights are switched-on to provide a particular combination in order to signal states relating to various international and national regulations. Pilotage requirements, health, dangerous cargo condition, etc., are signalled with these lights. White Morse code flashing lights may also be fitted on the signal mast. The NUC (Not Under Command) state is signalled using up to two all-round red lights (depending on the length of the vessel), vertically mounted and at least 2 metres apart. Such important lights are fed from the 24V d.c. emergency supply system but some ships may also have an additional NUC light-pair fed from the 220V a.c. emergency power supply. Lights and signals are to be exhibited appropriately to other ships so as to indicate whether the vessel is under way, at anchor, not under command or even aground. The required lights are to be shown from sunset to sunrise, and also during bad visibility. Note: A ship is 'Under Way' when she has no ropes running ashore or to the buoy, nor is she at anchor or aground. A ship is 'Not Under Command' generally when either the propeller or the rudder is not functioning satisfactorily.
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