Purpose

Explain main protective bonding as control of potential difference between the earthing system and extraneous-conductive-parts.

Learning objectives

• Distinguish earthing from bonding
• Identify candidate extraneous parts
• Explain bonding location
• Avoid bonding by material alone

Main protective bonding connects qualifying extraneous-conductive-parts to the main earthing terminal so dangerous potential differences are limited during faults.

Core theory

An extraneous-conductive-part can introduce earth potential into the installation; metallic appearance alone does not prove that classification.

Main bonding is normally connected near the point where the service enters, subject to the actual insulating sections, accessibility, current rules, and competent assessment.

Bonding conductor size and PME-related requirements depend on the earthing arrangement and supply information; service pipes must never be used as the means of earthing.

Assumptions: The plastic section reliably prevents introduction of earth potential.

1. Classify: Assess whether the part can introduce earth potential; do not classify by material alone.
2. Evidence: Inspect the service and consider testing under the applicable safe method.
3. Decision: A competent designer records whether it is extraneous and applies current requirements.

Not solely because it is metal; classification depends on whether it can introduce a potential.

Reasonableness check: Bonding controls imported potential, not the existence of metal as such.

Knowledge check

Answer: It is liable to introduce a potential, generally Earth potential. Material alone is insufficient.

Practical exercise

Classify six fictional building parts as exposed, extraneous, neither, or insufficient evidence, and justify each classification.

Summary

• Earthing and bonding have different functions
• Bond qualifying imported potentials
• Supply and service evidence govern design