9.3 Main Protective Bonding Conductor Continuity

9.3 Main Protective Bonding Conductor Continuity

Core Equipotential Principle: Main protective bonding creates an equipotential zone within an electrical installation by connecting all incoming extraneous-conductive-parts directly to the Main Earthing Terminal (MET). Bonding limits potential differences between connected parts and works with automatic disconnection of supply. It does not guarantee zero voltage difference or a maximum 50 V touch voltage during every fault.

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The Role & Regulatory Scope of Main Protective Bonding

Under BS 7671 Regulation 411.3.1.2, in each installation, main protective bonding conductors must connect the Main Earthing Terminal (MET) to extraneous-conductive-parts entering the building.

An extraneous-conductive-part is defined as a conductive part liable to introduce a potential, generally an earth potential, and not forming part of the electrical installation. Key examples include:

• Incoming metallic water service pipes.
• Incoming metallic gas installation pipes.
• Other utility service pipes and ducting (e.g. oil, district heating, chilled water).
• Exposed metallic structural parts of the building fabric (structural steelwork columns).
• Lightning protection systems (BS EN 62305 interconnection).

Conductor Sizing Rules (Regulation 544.1.1)

Main protective bonding conductors must be sized in accordance with the earthing arrangement and supply characteristics:

• TN-S and TT Systems: The cross-sectional area must be not less than half the cross-sectional area required for the earthing conductor of the installation, with a minimum of 6 mm² and a maximum requirement of 25 mm² copper.
• TN-C-S (PME) Systems: Due to the risk of circulating neutral currents under open-PEN supply fault conditions, bonding conductors are sized based on the cross-sectional area of the supply neutral conductor:
  • Supply Neutral ≤ 35 mm²: Minimum bonding conductor = 10 mm² copper.
  • Supply Neutral > 35 mm² up to 50 mm²: Minimum bonding conductor = 16 mm² copper.
  • Supply Neutral > 50 mm² up to 95 mm²: Minimum bonding conductor = 25 mm² copper.
  • Supply Neutral > 95 mm² up to 150 mm²: Minimum bonding conductor = 35 mm² copper.
  • Supply Neutral > 150 mm²: Minimum bonding conductor = 50 mm² copper. Use the copper-equivalent neutral size and verify the distributor’s requirements for the actual PME supply.

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Testing the Individual Bonding Conductor

Testing the continuity of main protective bonding conductors is carried out using a suitable protective-conductor low-resistance tester complying with BS EN 61557-4 or providing equivalent performance and safety, using the familiar 4–24 V / 200 mA test parameters.

Why Parallel Paths Must Be Excluded

A measurement with both ends connected can include other metallic paths between the MET and pipework. A low reading then does not necessarily prove the individual bonding conductor continuous.

  PARALLEL PATHS CAN MASK AN OPEN INDIVIDUAL CONDUCTOR

          [ Low-Resistance Ohmmeter ]
            │                    │
    Lead 1  │                    │ Lead 2
            ▼                    ▼
         [ MET ]              [ BS 951 Clamp on Water Pipe ]
            │                         │
            │    SEVERED BONDING      │
            ├─── - - - ✗ - - - ───────┤ (Broken cable under floor)
            │                         │
            │  ═════════════════════  │
            └──► [ Buried Pipework ] ◄┘ ◄── Parallel Current Path
                 [ External Soil   ]
                 [ Shared Mains    ] ──► Meter displays: 0.02 Ω! (FALSE PASS)

If the conductor is tested while connected at both ends:

1. The test current does not flow solely through the copper bonding conductor.
2. Current flows through parallel earth paths: the buried underground metallic water or gas main, the conductive earth mass, central heating pipe loops, boiler heat exchanger connections, and even neighbouring properties via shared metallic utility grids.
3. If the bonding conductor is broken, a sufficiently low-resistance parallel pipe/metal path can still produce a low reading, such as 0.02 Ω. Its value depends on the paths actually present.
4. Recording that result as proof of the individual conductor would be a false pass. Establish the actual test path and investigate any uncertainty before accepting the bonding.

Step-by-Step Testing Sequence

1. Identify the conductor and its endpoints. Apply safe isolation and the necessary precautions against diverted current before disturbing any protective connection. Do not proceed with an unsafe disconnection.
2. Null the complete test-lead arrangement or measure its resistance for subtraction. Identify which conductor end will be disconnected.
3. Pipe end disconnected: Leave the MET end connected. Measure between the MET/conductor terminal and the free conductor end at the pipe.
4. MET end disconnected: Leave the pipe end connected. Measure between the free conductor end at the MET and its conductor terminal at the pipe clamp. Do not use the disconnected MET terminal as though it were still part of the test conductor.
5. Exclude other connections to that conductor that could mask a break. Record and assess its resistance against the expected conductor value and instrument capability.
6. Restore the disconnected end to its correct terminal, following the specified preparation and torque. Confirm the conductor and connection are secure before energisation.
7. Inspect the clamp and its pipe contact. Where a separate joint-resistance check is needed, arrange the measurement to assess that joint; a MET-to-pipe result with other bonds connected can include parallel paths and does not by itself prove this clamp contact.

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Assessing Main Bonding Resistance

Compare measured resistance with the value expected from conductor material, size, length and temperature. Account for the instrument’s resolution/uncertainty and the test connections. An unexplained high value needs investigation; a low combined-path result is not proof of an individual conductor.

Calculating the Expected Value

For an approximate copper calculation with area in mm²:

$R = \frac{\rho \times L}{A}$

Use $\rho = 0.018\ \Omega\cdot\text{mm}^2/\text{m}$, $L$ in metres and $A$ in mm². Equivalently, use $18\times10^{-9}\ \Omega\cdot\text{m}$ with area in m². For comparison with standard cable values, use the applicable tabulated resistance at 20°C.

A 10 mm² copper conductor has a tabulated resistance of 1.83 mΩ/m:

$R_{\text{25m}} = 1.83\ \text{m}\Omega/\text{m} \times 25\text{ m} = 45.75\ \text{m}\Omega \approx 0.046\ \Omega$

A reading around 0.05 Ω is therefore plausible for that conductor. It is not a universal maximum: a correctly selected longer run has a higher expected resistance. A shorter conductor reading unexpectedly high still needs investigation even if below 0.05 Ω.

Lengths Giving Approximately0.05 Ω at 20°C

These are calculation examples, not maximum permitted lengths.

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Inspection of BS 951 Earth Bonding Clamps

Continuity testing must always be accompanied by a rigorous physical inspection of the bonding termination and clamp assembly. BS 951 specifies the design and performance requirements for metal clamps used to connect bonding conductors to pipes.

REQUIRED METAL SERVICE BOND
Internal meter -> consumer-side conductive pipe -> bond before branches
                  within 600 mm of meter outlet WHERE PRACTICABLE
External meter -> building entry -> bond near entry, before branches

Use an accessible connection, suitable clamp and sound conductive contact.
Provide the required durable safety label at the connection.

Key Inspection Verification Checklist

1. Location Criteria (Regulation 544.1.2):
   • Connect required service bonding as near as practicable to the building entry, on the consumer’s conductive pipework and before branch pipework.
   • For an internal meter, connect on its consumer side within 600 mm of the outlet union where practicable. With an external meter, connect near the point of building entry.
   • Assess insulating sections and whether the pipe is actually an extraneous-conductive-part; a metallic pipe downstream of an effective insulating entry is not automatically one.
2. Mandatory Safety Warning Notice (Regulation 514.13.1):
   • A durable, permanent label must be permanently fixed to or embossed upon every bonding clamp.
   • Inscription: "Safety Electrical Connection - Do Not Remove".
   • Missing, damaged, or paint-obscured labels must be flagged as a defect (Code C3 on an EICR).
3. Pipe Surface and Contact:
   • Use a clamp suitable for the actual pipe material and surface. Establish sound conductive contact in accordance with its instructions; remove obstructing contamination as necessary without damaging the pipe or its protective finish.
   • Do not attach a protective bonding clamp to plastic pipe or a plastic joint as though it provided electrical continuity.
4. Mechanical Security and Sizing:
   • Match the stated pipe-diameter range and terminal capacity to the pipe and bonding conductor. Product codes are manufacturer-specific; for example, one BS 951 range covers 12–32, 32–50 and 50–75 mm pipes with different band lengths.
   • Use the specified tightening torque and verify secure conductor and pipe connections. Do not infer suitability merely because a band can be tightened around the pipe.
5. Environmental Suitability:
   • Select a clamp rated for the actual dry, humid or corrosive conditions. Manufacturer ranges distinguish these applications and conductor capacities. Check material compatibility and corrosion condition rather than applying one universal material prescription.