Clive Brittain:
TT earthing for an outbuilding is quite easy. You just do NOT export the earthing system from the TN-C-S of the house supply. TT is good for outdoor supplies such as caravans or farms etc. The supply is just Line and Neutral to the outbuilding, with say an earthed armouring of a supply cable from the house. At the outbuilding the earth is insulated back and does NOT continue to exposed conductive parts or socket earth terminals. The outbuilding is then earthed via a local earth electrode (rod) A 30mA double pole R.C.D. is normally used to provide shock protection. The earth fault loop impedance Zs can be up to 1666 Ohms. (Table 41.5 B.S. 7671) But anywhere around a stable 200 Ohms is good.
Agreed
TT earthing of the outbuilding is good as no metalwork can be raised to a dangerous Voltage by an external fault on the supply cable to the house.
As long as the installation earth electrode is separated by a suitable distance from buried metalwork connected to the PME earth. See Figure 16 of BS 7430
Exporting a TN-C-S earth to an outbuilding can introduce potential shock risks and should be avoided if possible, especially if there is metalwork that is in contact with the ground, true earth.
C.
Clive Brittain:
The Radio Society of Great Britain has a publication entitled "Earthing and the Radio Amateur".
http://rsgb.org/main/files/2012/11/UK-Earthing-Systems-And-RF-Earthing_Rev1.3a-.pdf
C.
It used to be the case that all extraneous-conductive–parts in a house with TN-C-S needed to be bonded, but with the recent changes to the IET Wiring Regs if you have a supply totally fed from RCD or RCBO protection devices, this requirement is no longer mandatory.
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