Main and supplementary bonding continuity
Inspection Testing · Lesson 8 · Advanced
Purpose
Test main and supplementary protective bonding while distinguishing bonding purpose, conductor identity and parallel paths.
Learning objectives
- Plan a defensible sequence
- Verify instrument and test boundary
- Interpret topology from patterns
- Record evidence without invented limits
Main protective bonding connects qualifying extraneous-conductive-parts to the main earthing terminal; supplementary bonding locally reduces touch voltage where required. Neither should be confused with functional bonding or an incidental pipe path.
Core theory
Inspect conductor presence, route, size, identification, terminations and connection near the service entry as applicable. Determine whether the metalwork is actually extraneous using the installation assessment.
For a meaningful continuity measurement, isolate safely and manage parallel paths only where necessary and safe. Never disconnect main bonding while energized or leave it removed.
Record low-resistance continuity and reconcile it with conductor length/area and connections; a near-zero reading through parallel services may conceal a disconnected intended conductor.
| Term | Meaning | Symbol | Unit |
|---|---|---|---|
| Main protective bonding | Connection of extraneous-conductive-parts to the main earthing terminal | Not applicable | Not applicable |
| Supplementary bonding | Local equipotential connection used where required | Not applicable | Not applicable |
| Parallel path | Alternative conductive route influencing measurement | Not applicable | Not applicable |
Assumptions: Values are supplied test data; The installation is safely isolated for dead testing.
- In situ: 0.01 Ω may be dominated by parallel metalwork.
- Identity: Open result shows the labelled conductor path itself is discontinuous or misidentified.
- Correct: Trace, repair/identify, reconnect and retest before restoration.
The initial low value did not prove the intended bonding conductor; repair and re-verify it.
Reasonableness check: The conclusion uses expected topology and comparative patterns; no single convenient reading overrides inspection or missing evidence.
- Starting live tests before dead-test defects are resolved
- Calling a calibration label a pre-use functional check
- Declaring a universal maximum continuity value without design context
Where this appears in practice
Initial verification combines inspection and ordered tests to prove protective measures before energisation; periodic work records the extent, limitations and observed condition.
Knowledge check
Can a low in-situ bonding reading prove the labelled conductor is continuous?
Not always. Parallel metallic paths can mask an open or misidentified conductor.
Answer: Not always. Parallel metallic paths can mask an open or misidentified conductor.
Practical exercise
Assess service metalwork, decide bonding applicability and design safe continuity checks for a supplied building.
Summary
- Inspect and dead-test before avoidable live testing
- Instrument validity has several independent checks
- Continuity patterns prove topology and support later calculations
Sources and review
- BS 7671 and Guidance Note 3: Inspection & Testing: IET; BS 7671:2018+A4:2026 / GN3 10th edition; transition rules apply; United Kingdom
- Model forms for BS 7671: IET; A4:2026 and earlier applicable editions; United Kingdom
- GS38: Electrical test equipment for low-voltage systems: HSE; Fourth edition; Great Britain
- Ring-final continuity Step 3 resistance readings: IET Wiring Matters; 2022; United Kingdom
Editorial review date: 2026-08-22. Professional electrical review is pending.