A typical earth fault circuit used on board ship’s electrical distribution system is shown below.
When “EARTH LAMP” switch (ELS) on a feeder panel is in OFF position, the neutral of the star connected primaries of three lamp transformers is disconnected from the earth, and the voltage across the primary is the same for each transformer regardless whether the ship’s feeder system contains an earth fault or not. Therefore, three indicator lamps (TLs) connected to the respective transformer secondaries are on at the same brightness.
To check the ship’s feeder system’s insulation resistance to earth, put “EARTH LAMP” switch (ELS) to TEST (this earths the neutral of the star connected primaries of lamp transformers). If there is an earth fault in phase R, for example, the lamp R would be less bright than other two; or if it is a complete earth fault, the lamp R would be off. Where there is no earth fault, switching of the ELS from OFF to TEST gives no change in lamps’ brightness.
See Also: Main Switch Board in Ships
See Also: Synchronizing Two Generators in Ships
See Also: Control Procedure for Marine Generators