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.

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? Ok it`s only half a volt but did that change when we reclassed our 240V to be 230V and 415V to be 400V ?

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. 

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.

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?

the circuits from the transformer are installed in SWA cable. fly lead from banjo to the transformer earth. 

My experience of fixed 110 volt installations has been in pub cellars with everything run in three core cable.

So, what is the earth path from the socket back to the transformer with two core cable, is it in conduit?

The “star” point in a 55-0-55 transformer delivering 110 volts is the centre tap in the transformer, I’m not picturing how the sockets earth terminal are connected to the centre tap. This earth is referenced through the incoming supply earth, is that what is being relied on? 

I presume you are testing loop impedance and fault currents live using an electricians multi-function tester, rather than dead testing resistances with a multi-meter.

In real life I have only ever seen tool transformer fuses blow due to prolonged over load, not due to a damaged cable or the like.

When I started work a firm I worked for was using the old scratch earth 110 volt leads onsite, some of these had fuses which were actually the pin of the plug, you had to unscrew the plug pin and replace it if you blew the fuse, others had solid fixed pins so you had to ensure you laid them out in the correct order and watch what you plugged in to multi-way adapter boxes.

Why 4.0 mm two core cable? the transformers and sockets where updated some years ago and the guy here just reused the cable. 

What is the output rating of the transformer? not to sure. probably a 5kva 

Does the transformer already have fuses in it on the output? the transfomer has two outgoing ways 16a c double pole. 

fixed installation

i got .25ohms on the z at tx and .85zs at the furthest socket. the short circuit on the multimeter had a result of 142a and around 70a down to earth. the results line up with the calculation. so line to line calculation need 110v and line to earth 55v. makes scene. i just saw a calculation of phase to phase short circuit and wanted to check if that was used 

thanks guys