An Introduction to Essential Electrics: Main Protective & Supplementary Bonding

Definitions

• Extraneous Conductive Part
  • Any metallic component not forming part of the electrical installation but liable to introduce a potential, e.g. gas, water, oil pipes, structural steelwork.
  • Must be connected to the earthing system to control voltage differences.

• Main Earthing Terminal (MET)
  • Central earthing point, normally in or near the consumer unit, where all protective conductors—including bonding conductors—are terminated.

• Equipotential Bonding
  • The practice of electrically inter-connecting all extraneous conductive parts so they remain at (approximately) the same potential under both normal and fault conditions.

Main Protective (Equipotential) Bonding

• Required by BS 7671 for every domestic installation.
• Purpose: minimise dangerous potential differences between simultaneously-reachable metalwork if a fault occurs inside or outside the installation.
• What gets bonded:
  • Gas service pipework.
  • Water service pipework.
  • Oil supply pipework.
  • Central-heating pipework if metallic throughout.
  • Structural steel or other building metalwork.
• Termination point: every main bonding conductor runs back to the MET located at or near the supply intake/consumer unit.

Supplementary Bonding

• Additional protective measure used only in specific areas—e.g.
  • Bathrooms, kitchens, or locations containing a bath/shower.
  • Places where metallic pipework has been interrupted by plastic fittings or lengths of plastic pipe.
• Links separate sections of pipework (hot, cold, central-heating, gas) to restore electrical continuity lost by non-metallic breaks.
• Achieves the same goal as main bonding but on a local scale—keeps hot & cold pipes, radiator pipes, etc., at the same potential so no harmful touch voltages appear between them.

Bonding Conductors & Installation Practice

• Bonding conductors must be sized in accordance with BS 7671 (note: sizing tables not provided in transcript).
• Connection method:
  • Use purpose-made earth clamps stamped with BS approval.
  • Clamps provide low-resistance, secure contact and are clearly labelled to warn occupants not to remove.
• All connections must be accessible for inspection, testing, and maintenance unless permitted otherwise by regulations.

Key Safety Purpose (Why Bond?)

• Limits conductive parts to the same potential, so if a live-to-metal fault occurs, touch voltage V_{touch} between two points a person can simultaneously contact remains acceptably low.
• Reduces risk of electric shock, especially where water and moisture lower body resistance.
• Complements automatic disconnection of supply (ADS); bonding restrains voltages while protective devices operate.

Quick Reference Summary

• Main bonding = whole dwelling, links to MET.
• Supplementary bonding = local areas, restores continuity where metallic paths are broken.
• Always bond gas, water, oil, structural steel.
• Use approved clamps; never remove a bonding conductor.
• Regulation source: BS 7671 Requirements for Electrical Installations.