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Main protective bondingWhere does main bonding go, and what does it connect?

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ground (general mass of earth) Service head (supply earth) Consumer unit earth bar CPCs → final circuits MET main earthing terminal earthing conductor 16 mm² earthingconductor metal water pipe clamp nearstopcock / entry gas meter clamp ≤ 600 mm from meter outlet main protective bonding conductors (10 mm²), each with a BS 951 clamp and label
Main protective bonding connects every extraneous conductive part to the MET. If there's no separate external MET, the earth bar in the consumer unit acts as the MET, and all the bonding, the earthing conductor and the CPCs go there.
  • Make the bond on the consumer's side of the meter or stopcock, within 600 mm of the gas meter outlet union or the water point of entry, before any branch.
  • Plastic incoming service pipe? No main bonding needed, e.g. yellow plastic gas pipe or blue plastic water pipe. If even a short length of the incoming pipe is metal where it leaves the ground, you must bond it.
  • CPCs (the green/yellow in your final circuits) go to the earth bar in the consumer unit, not to an external MET.
  • Bonding clamps are BS 951. The conductor goes under the plate (lugged, crimped or wrapped round the screw). They carry a "Safety electrical connection – do not remove" label, and a label also goes at the MET. Labels are required by the regulations.
BS 951 clamp colour (common practice)Environment
Red / silverInternal, dry areas
BlueExternal or slightly damp areas
GreenCorrosive or hazardous areas

Electrician's Guide to the Building Regs · 3.3.2 Earthing conductor and main protective bonding conductors: list of services to bond · p.58

Electrician's Guide to the Building Regs · Plastic supply pipes: no bonding needed. Bonding within 600 mm of meter or entry · p.59 Electrician's Guide to the Building Regs · BS 951 earth clamp label image · p.63

On-Site Guide · Section 4 Bonding and earthing: earthing and main bonding figure · p.61

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