Earthing and bonding in outbuildings

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Question: 

Both enquiries relate to the electrical installation of a house supplying a (15 metres detached) outbuilding which currently is without any electrics. The outbuilding needs a twin power socket and a spur lighting circuit.

1. If the house earthing system is TT, and the supply to the outbuilding is an SWA radial circuit from the house consumer unit, can the outbuilding rely upon the earth rod of the house for it's earthing too? The outbuilding supply circuit Zs is estimated to be 100 Ohms max.

2. If the house earthing system is TN-S or TN-C-S or TT, and there's a water supply to the outbuilding from the house, does the accessible water service pipe in the outbuilding require a main bonding connection? The water pipe is routed underground from the house and is made from copper. The water pipework in the house is all metallic and is main bonded to the house main earthing terminal. All pipe connections and house main bonding connections are sound and the house stopcock also turns off the water to the outbuilding. The electrics supply to the outbuilding is planned to be a 3 core 6mm SWA radial circuit from the house consumer unit.

Answer:

NICEIC Tech Team

There are no specific regulations stating that an additional earth electrode is required for the outbuilding. Regulation 411.5.1 requires exposed-conductive-parts to be connected to a common earth electrode. Earth loop values should be in accordance with regulation 411.5.3 and Table 41.5. For stability, an Ra value not exceeding 200Ω is recommended in accordance with Note 2 of table 41.5.

Regulation 41.3.1.2 requires that protective equipotential bonding is required in each building and protective equipotential bonding conductor sizes should be in accordance with regulation 544.1.1.