I'm pretty good when it comes to electrics but I confess I don't know it all. Regulations have changed recently, and what used to be accepted practice has changed in recent years. Good then, to do a little R&D before attempting a job.
The van bodies are going to plug in using Commando connectors and as such have a similar hookup to a touring caravan on a campsite. Most pitches are provided with a weatherproof 13A socket or a Commando hookup and are protected by a RCD device. This is all well and good with a short run of interlink cable, but in our situation the extension from the Stone Barn is significantly longer.
A consequence of this is that the loop impedance (resistance) of a L-E or N-E fault is too high for a RCD to trip in the required time. The whole house protection of RCD's in the split-load consumer unit will protect the sockets in the barn, and will give an element of protection to the cable itself because we have separate MCB's in the barn, but at the other end of the cable, the situation is not so good.
The earth conductor in the 90m of extension simply isn't sufficient to protect against earth faults in the bodies.
Ah well. I'll need to implement what is known as a TT earthing system on the bodies themselves, protect the entire system with a separate RCD in each body and have separate protective earths at the body end of the link. The earth from the house, while sufficient to protect the cable itself from faults, won't handle the earthing correctly. Each body will need it's metalwork grounded using an earth spike as well as main bonding on the entire electrical system. Not an insurmountable issue, but to do it 'by the book' is going to cost another £50 or so per installation. On the upside, it does mean that when we go for a fully installed permanent supply, the systems will be compatible. Phew.