Main protective bonding: the clamp, the conductor and the METWhere must main bonding be clamped?

To stop this happening, we bond the extraneous conductive parts to the MET with main protective bonding conductors (MPBC).

• Connect them with an earth clamp to BS 951. It's a strap that wraps around the pipe and screws tight, with a separate terminal for the conductor.
• Don't strip a bonding conductor and wrap it around the pipe held on with tape. It does get done, but it's not the right way.
• Every time a metal pipe comes up out of the ground inside the premises, it's an extraneous conductive part and needs its own bond. In our drawing the water pipe splits, goes back under the ground and comes up elsewhere, so the installation needs three MPBCs: gas, water, and the re-emerging branch.

• Gas meter outside in a box on the wall (common in houses)? Bond where the gas pipe comes into the building, within 600 mm.
• Never on the supplier's side. That pipework belongs to the supplier, who could alter it and disconnect your bond.
• Before any branch. Every cut, fitting and soldered joint adds resistance, and the whole point is the lowest possible resistance between the pipe and the MET.

Bonding makes the extraneous conductive parts the same potential as the MET. The MET is connected to the exposed conductive parts and to the star point, so everything is at the same potential. Before bonding, the potential difference between the faulty exposed part and the pipe was 230 V. After bonding, it's (near enough) gone. Put the person back in the drawing and they feel nothing.

BS 951 · earth clamp standard BS 7671 · Reg 544.1.2 location of main bonding connections (600 mm, consumer's side, before branches) · check your copy for the page Reg number added for reference

Electrician's Guide to the Building Regs · 3.3.2 list of services to bond Electrician's Guide to the Building Regs · plastic pipes, bonding within 600 mm · p.59 On-Site Guide · Section 4 earthing and main bonding figure