Building regulations questions.

[pc1966]

So theoretically speaking, hypothetically: if there was an incoming water pipe which was INSULATED and no gas pipe, and a PME/TN-C-S failed the metal accessories could become live and will not trip any protective device as there will be no active earthing system in place for the house?

The accessories will go to the PME potential in any case. If bonded to significant lengths of metal gas/water pipes that is probably fairly close to 0V (in the tens of volts range) but if insulated pipes then it goes to whatever the remaining N/E load might be, so conceivably to 230V if the fault is on the last TN-C leg to the property.

The lack of any trip is a problem, but in any case the external risk is independent of the RCD tripping as that only isolates the L & N and is not isolating the E.

However, the EV protection system do isolate all 3 so if a fault is detected the car is not left at a potentially dangerous voltage that someone might be washing, etc.

Now if you did TT the shed, then the sheds stuff (any tubs metal light fittings etc) will be earthed, BUT because the regs have said NOT to earth the armouring to the OUTBUILDING you are then lacking earthing in the MAIN building (house)... So shouldn't a TT earth ground rod be installed for the house too then or shouldn't they advise to connect the armouring/cpc to the outbuilding too as well as have a TT ground rod for the outbuilding?

If they are connected (and the house is not TT) then the outbuilding is not TT either, it is TN. The 2nd 'T' is the direct connection to the local Earth!

Some other countries require an earth rod for the house as well, in effect adding more "multiple earths" to the PME arrangement. The ROI is fairly close to the UK in many ways and does this, as does the USA and Australia. The 2nd amendment regs put up for discussion before Xmas look as if that will be added to the UK as well (at least new builds to provide an earth in the foundations).

Now the problem in reality is it is hard to get any sort of low impedance rod. Typically you might get a few tens of ohms for a 2.4m rod in many areas, so with a typical neutral current in the tens of amps range you are well over 200V. Where the rods would help is for a PME fault on a large section of the DNO network, then having 50+ rods and some balance between the 3 phases would keep the fault voltage down to safe level.