All I remember is that the Neutral needed to be twice the size of the phase conductors - including the neutral conductor to the generator which cause all sorts of procurement difficulties.
The real nonsense was that the designers were so paranoid about neutral currents that they also had three phase three wire distribution to a delta star transformer in each Sub distribution Board so the neutral current was very small as only the office lighting had a neutral connection back to the Main Board.
The point of course is that the neutral is not really anything of the kind, as it is quite likley to be at a voltage no-where near earth, as the whole point of this sort of system is that you can ground one of the phases, and it carries on worrking. Variations on the IT theme used to be popular with DC on small ships and I believe for some electric train systems, so that first fault can be detected, without the supply being disconnected.
Indeed the earthing would somtimes be the centre of some warning lamps so that they all lit dimly, when all was well, and earth was in the centre of the live conductors, and when one side was faulted to earth, then one lamp or group of them would go out, and the remaing come on brighter, so show a fault state on one side. The problem with having a 'neutral' is that it is not sensible to fuse it, when it is at or near earth potential, but the whole ADS needs rethinking when either side could be live or not. Wiring so all cores are treated as live is a far safer way to operate - some odd applications like undersea power works this way for example, and often at odd voltages too, like 690 single phase for example. I understand that on ship AC 3 phase is often with "insulated neutral" , but we'd need someone like Ancient Mariner to confirm if that is true IT, though I fear we may have lost him in the forum move.
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 ......
Thanks for that clarification - in my mind the lamps are acting as a sort of NER. There is, or at least once was at least one building on theArdnamurchan peninsula that had that arrangement for single phase 220 ish volts, but that was an ancient lister genset that had probably been wired in by chaps more used to fixing trawlers then houses. As last seen in the 1970s, I suspect it has been modernised in the intervening decades.
As a side note Ancient Mariner was the handle of one of the regular contributors to the wiring forum in its previous place and something of the go-to man for electricity on all things water based, though he does not seem to have survived the transition, please understand that I'm not trying to be disparaging about ex navy folk or anything.
I did realise that Ancient Mariner was the handle of a user from the way you refenced it, but decided to use it in the way I did to introduce my credentials.
If you use the central earthing point of three lamps as your main earth you tend to have a bit of a problem when one of the bulbs fails, so I suspect you may have misunderstood the arrangements.
With regard to being disparaging about (ex-)mariners and the tale of the house on the Ardnamurchan peninsula, I would also be very wary if it was wired by individuals more used to wiring trawlers. There is a saying "worse things happen at sea", and I have a wealth of horror stories about what people have done to prove it!
I have come across an unearthed neutral system in Algeria, wired under 1960's French regulations (which may well have changed by now).
This was an otherwise normal 3 phase and neutral system but the design called for a neutral earthing system which had a high resistance and an alarm so that the neutral was normally around earth potential but in the event of a Live- Earth fault, the alarm would go off, the neutral rise to 230V and the circuit remain in operation.
What had happened was that the Neutral Earth alarm unit was stuck in Customs and the system had been put into operation without it. The first fault had earthed one phase and subsequent faults had wither blown fuses and been found or, if they were in the same phase, had simply reinforced the earthing of that phase.
The idea that for months the system was operating with 400V Live -Earth on the other two phases is distinctly worrying.
