On 21/05/2012 22:48, MAT wrote:

For what it's worth this was signed off by a firm specialising in
outdoor, multiple source, non-grid installations who get a lot of MOD
and FC contracts.

What documentation has been provided (if any)? There really should be
some, but it might be stashed in the filing cabinet of somebody who
doesn't understand a word of it. Have you enquired?

Questions in my mind a

The big genny (how big?) is earthed locally. Is the line to the cabin
2-core (Line + CNE) or 3-core (L, N, E). The armour of a 2-core SWA
cable is valid as a protective conductor, provided it's properly glanded
at both ends. If there is a separate earth core, what happens to it at
the cabin end?

Does the VFX thing provide any isolation, or are its input and output
neutrals solidly bolted together? Are the manufacturer's instructions
available?

Points:

Nothing here needs to rely on a TT or IT system. All sources are local
and under your control, so TN earthing can be employed throughout.

The VFX (source) and cabin mains distribution appear to be wired as a
PNB system - a variation on TNC-S. The source neutral is earthed in the
first (2-way) isolator box, by dint of the strapped busbars and
connection of the main earth electrode. Thereafter it should be
conventional TN-S (SNE) to the two consumer units (so no N-E strapping
in those!).

Despite TN earthing, RCDs (as RCCBs and/or RCBOs) should be used. The
inverter in particular may have limited output s/c current, to protect
the semiconductor devices, so you can't assume that the normal
disconnection times will be met, based on its output impedance under
normal load. And then there's all the usual 17th ed. reasons for RCDs
too, as well as the general nature of the place and its users.

--
Andy