As a typical "Ancient Mariner", having gone to sea on merchant ships in the 1970s, I can confirm/correct the comments from mapj1.
The majority of ships (i.e. big things, as opposed to small boats) use a low voltage three phase, three wire IT system, i.e. without a neutral. The generators will often have a neutral terminal in their connection box but this is just for measurement and testing, not for connection to the system. The advantage of this is that when there is a fault the (required) earth fault detection system gives an alarm but the system continues to function without interruption - after all you don't want to go aground or collide with another ship just because there has been an earth fault on one circuit. The disadvantage is that when there is an earth fault the other two phases are raised to the line voltage wrt earth rather than the phase voltage, so the insulation needs to be over-rated by a factor of 1.73. Older earth monitoring systems (without an alarm) were of the form described by mapj1 with three lamps and a central earthing point.
The arrangements for HV on ships tend to be with a neutral earthed through an impedance, generally a resistor to limit the earth fault current to e.g. 5A. With the NER (Neutral Earthing Resistor) it is still an IT system but it does have a distributed neutral. As far as the idea of fusing the neutral goes, rather than agreeing it is not sensible to fuse it, I would say it is ****** DANGEROUS to fuse it. Certainly for marine systems it would be against every standard and regulatory requirement I can think of.
Ship systems were historically not referred to as IT, but rather were described as e.g. "three-phase, three-wire insulated" but with the latest issue of IEC 60092 the terms IT, TN-C, etc. have been included.
Late edit - of course I just realised my only "correction" to mapj1 is that he gave the impression that the earthing in the centre of the lamps was a main earthing point, which I hope I have clarified ......
As a typical "Ancient Mariner", having gone to sea on merchant ships in the 1970s, I can confirm/correct the comments from mapj1.
The majority of ships (i.e. big things, as opposed to small boats) use a low voltage three phase, three wire IT system, i.e. without a neutral. The generators will often have a neutral terminal in their connection box but this is just for measurement and testing, not for connection to the system. The advantage of this is that when there is a fault the (required) earth fault detection system gives an alarm but the system continues to function without interruption - after all you don't want to go aground or collide with another ship just because there has been an earth fault on one circuit. The disadvantage is that when there is an earth fault the other two phases are raised to the line voltage wrt earth rather than the phase voltage, so the insulation needs to be over-rated by a factor of 1.73. Older earth monitoring systems (without an alarm) were of the form described by mapj1 with three lamps and a central earthing point.
The arrangements for HV on ships tend to be with a neutral earthed through an impedance, generally a resistor to limit the earth fault current to e.g. 5A. With the NER (Neutral Earthing Resistor) it is still an IT system but it does have a distributed neutral. As far as the idea of fusing the neutral goes, rather than agreeing it is not sensible to fuse it, I would say it is ****** DANGEROUS to fuse it. Certainly for marine systems it would be against every standard and regulatory requirement I can think of.
Ship systems were historically not referred to as IT, but rather were described as e.g. "three-phase, three-wire insulated" but with the latest issue of IEC 60092 the terms IT, TN-C, etc. have been included.
Late edit - of course I just realised my only "correction" to mapj1 is that he gave the impression that the earthing in the centre of the lamps was a main earthing point, which I hope I have clarified ......
