Clive is correct in his comments but ships are treated very differently as often continuity of supply is of prime importance - with an insulated system an earth fault on one phase will not trip the protection, and if it is your steering as you come into harbour you definitely don't want that to suddenly fail. However the insulation to earth is monitored and an earth fault has to be dealt with promptly as if you get a second earth fault on a different phase before you have the first one fixed, you could be in big trouble! The alarm is normally (or at least should be) set at 1MΩ or thereabouts.

On ships with insulated systems ELCBs are of limited use as an earth fault doesn't necessarily trip the ELCB, since there is no earth leakage current.

Land based systems are different - if you have an earth fault in an office, it is not going to run aground. On a ship the personnel on board are trained to deal with the expected problems, but in a land based situation they call in a specialist who may or may not turn up promptly, even assuming they have called promptly. Therefore a land based system uses an earthed distribution so an earth fault immediately trips the circuit protection, and on these systems ELCBs work as intended.

Alasdair

[Edit]

I should have also added that ships do not comply with BS7671, since they have a different set of standards that they comply with, viz. IEC 60092, and for British ships there is additionally BS8450, which is the old IEE Code of Practice for Electrical Installations in Ships taken under the BSI wing in much the same way as BS7671.

It is my understanding that many, if not most, solar inverters fitted to the larger systems use unearthed three phase between the inverter output and step up transformer. These can be at 2.5MVA. I think these systems are ignored as they are in the middle of a field, and are not offered for inspection. I also believe that they have no circuit protection or isolation and would be interested if anyone can confirm or correct me.