It seems a bit over the top adding RCD protection upfront of the circuit with circuit protection in place and SWA cables in the fixed installation.

You could add RCD protection at the sockets by replacing the existing fittings, not cheap though you should beat this price.

https://www.cef.co.uk/catalogue/products/1150765-16a-2p-e-110v-unswitched-socket-with-25a-rcd-ip44

What risks do you consider there are that require the additional 30 mA RCD protection?

You could also ask yourself what speed of RCD protection you needed in a system where the sort of live to CPC fault inside equipment that makes the case live, livens it to no more than about 28volts,  I suspect an infinitely slow RCD or none at all would do. You may like to still have an RCD for supplementary protection where one assumes that 2 layers of insulation are damaged but there is no short to CPC, so the full 55v RMS is available to touch, but it is hard to imagine a situation where a single fault to actual danger occurs.

However BS7671 does not see it that way,

Mike.

i was considering the potietial fire from mcb not tripping before the cable heats up. but looking at it the mcb for the cable size will trip in time. i think the regulations make it a little confusing. really the best soluotion would just be a rcbo in the socket as its the easy location for it. 

Andy "3-phase RLV system would be 63V L-PE and 110V L-L " why was I thinking 63.5V? Ok it`s only half a volt but did that change when we reclassed our 240V to be 230V and 415V to be 400V ?

well 110V  / sqroot( 3) is indeed 63.508 if we want to be fussy, so really we should round up to 64V not down to 63.
I suspect that a volt in 63 or an error of 1.8% need not worry us on a building site. With a real transformer unloaded  it could just as easily be be 66.6 which is a devil of a lot easier to remember.

Mike.

Andy "3-phase RLV system would be 63V L-PE and 110V L-L " why was I thinking 63.5V?

Yes, my number was just an approximation!  In my defence it's less inaccurate than the usual 230/400V notation (230V * √3 = nearer 398V than 400V).

   - Andy.