Personally I would not run any out building from the house RCD, I would want it on it’s own RCD even if it is in the house in the form of a RCBO, as don’t want to lose house power because of fault with outbuilding, if this can’t be done, then some double pole switch so both live supplies are switched both line and neutral.
ericmark wrote:Personally I would not run any out building from the house RCD, I would want it on it’s own RCD even if it is in the house in the form of a RCBO, as don’t want to lose house power because of fault with outbuilding, if this can’t be done, then some double pole switch so both live supplies are switched both line and neutral.
I don't think you understand how a RCD works, it does not actually detect current flowing to earth, it detects an imbalance line to neutral and it assumes if not the same, then some current must be going to earth, so you can't have a RCD which is solely for the SWA cable.
Personally I would not fit an earth rod to a PME supply, I have seen the result when the supply authority lost their neutral/earth, with a rod at 60 ohm not really a problem, but with a good earth, and with the case in question there were 4 earth rods connected with bare earth tape designed as the earth for a radio transmitter, when the supply earth/neutral was lost the whole street tried to use his earth, so likely swinging up to 300 amp, and the 4 mm earth cable melted.
It is OK when every house has water and gas earthed as the load is shared between all the houses, but if only one house is earthed that house takes it all, so let the DNO do any multiple earthing, either use PME (TN-C-S) or TT don't mix them.
The problem with TT is you need to test, there are two methods, one you use a loop impedance tester and compare your earth with the DNO earth, and second is you fit temporary test probes at 15 meters +/- 2 meters and 30 meters and test the earth using a proper earth test meter, both require expensive equipment to test, and the second needs access to the ground to fit probes. So for the DIY man it is an expensive exercise to use TT because of the testing involved.
Of course all installations should be tested, including the RCD, we all know you can't measure 40 mS with a stop watch so again should use a special tester, but in the main if the RCD trips at 30 mA then we just cross our fingers that it does it fast enough.
I've ditched the earth rod idea on your advice. DNO have informed that PME is sufficient for cable run length.
I have 40A MCB connected to RCD on main, if I have the issue of tripping the whole house, I will break mcb and reevaluate.
The reason I'd like to use Armour as earth is because I am running 2 Neutral, 2 Live lines as 6mm is cutting it fine with ampage required for hot tub. So my thoughts were to use sheathing to make up the loss while still using 1 earth core as well.
So to recap,
100A 30ma RCD on MCU Feeding 40A MCB
sheathed 5 core, earthed from armour and 1x 6mm core, 2 Neutral 6mm, 2 Live 6mm
18m run (DNO approve distance on PME)
Cable run to CU. Armour earthed along with 1x core 2 Neutrals to N busbar 2 lives to 40A RCD
6A for lights 16A sockets 40A run through earthed armour to Rotary isolator 40A
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