4.4 Main Equipotential Bonding (within 600mm) & Direct Purging (IGEM/UP/1B)

4.4 Main Equipotential Bonding (within 600mm) & Direct Purging (IGEM/UP/1B)

Quick Answer: Main protective equipotential bonding requires a 10 mm² copper green/yellow conductor attached within 600 mm of the primary gas meter outlet, before any branch tee. Commissioning and decommissioning of domestic gas installations must follow IGEM/UP/1B Edition 4, calculating purging volume as PV = V_purge = 1.5 × IV (also written 1.5 × V_inst).

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1. Main Protective Equipotential Bonding Requirements

Equipotential bonding prevents dangerous electrical potential differences from arising between metallic gas pipework and accessible conductive parts of the electrical installation during an electrical fault.

Legal & Regulatory Framework

Main protective bonding is mandated by BS 7671 (IET Wiring Regulations) and Regulation 18(2) of the Gas Safety (Installation and Use) Regulations 1998 (GSIUR). Gas engineers must verify the presence and integrity of main bonding prior to commissioning gas work.

Bonding Conductor Specifications & Connection Rules

• Conductor Size: The main equipotential bonding conductor must be a single continuous copper conductor with a minimum cross-sectional area of 10 mm², insulated with green/yellow sheath.
• Connection Location: The bonding clamp (conforming to BS 951) must be connected to hard metallic installation pipework within 600 mm of the primary meter outlet (on the consumer side of the meter).
• Positioning relative to Tees: The connection must be positioned before any branch tee or appliance connection. This ensures that if any downstream pipe branch is disconnected, the remainder of the gas pipework network remains earthed.
• Surface Preparation: Bonding clamps must be fitted directly to clean, unpainted, bare metallic pipework. Clamps must never be attached to flexible corrugated meter connectors, plastic pipes, or painted surfaces.

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2. Gas Purging Fundamentals (IGEM/UP/1B Edition 4)

Purging is the safe displacement of air by gas (commissioning) or the displacement of gas by air or inert gas (decommissioning) without creating an explosive gas-air mixture.

Domestic gas purging procedures are governed by IGEM/UP/1B Edition 4 (Tightness testing and direct purging of small Natural Gas installations), published March 2026. Edition 3 may be used in parallel until 30 September 2026; Edition 4 is mandatory from 1 October 2026 (Gas Safe Register ISU 133 Edition 2). Edition 4 standardises purge volume as $PV = 1.5 \times IV$.

Direct Purging vs. Inert Gas Purging

• Direct Purging: Direct displacement of air with gas (or gas with air) without inserting an inert gas intermediate barrier (such as nitrogen).
• Scope Limits for Direct Purging: Direct purging is permitted under IGEM/UP/1B only when:
  1. MOP at the outlet of the ECV does not exceed 75 mbar (low-pressure scope).
  2. Maximum pipe diameter does not exceed 35 mm.
  3. Total installation volume ($IV$ / $V_{\text{inst}}$) does not exceed 0.035 m³ (35 litres).

If any limit is exceeded, direct purging is prohibited, and an inert gas purge must be executed under the appropriate larger UP/1 family procedure.

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3. Installation Volume Calculation ($IV$ / $V_{\text{inst}}$)

To calculate purge volume, the engineer must determine total internal installation volume, comprising meter volume plus pipework volume (plus fittings allowance).

IV = IVm + IVp (+ ~10% of IVp for fittings)

Standard Internal Meter Volumes ($IV_m$)

• U6 / G4 diaphragm meters: 0.008 m³ (8.0 litres).
• E6 ultrasonic smart meters: 0.0024 m³ (2.4 litres) — not 0.008 m³.
• U16 diaphragm meters: 0.025 m³ (25 litres).

Copper Pipe Volume Coefficients per Metre (NICEIC)

Allow about 10% of $IV_p$ for fittings unless a detailed schedule is used.

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4. Purging Volume Determination ($PV$ / $V_{\text{purge}}$)

To ensure complete displacement without leaving stratified pockets of combustible gas-air mixtures, IGEM/UP/1B Edition 4 requires:

PV = V_purge = 1.5 × IV

Worked Purging Calculation Example:

A domestic installation consists of a standard U6 gas meter (0.008 m³), 10 metres of 22 mm copper pipework, and 6 metres of 15 mm copper pipework (ignore fittings for the exam arithmetic, then note the 10% uplift in practice).

1. Meter Volume ($IV_m$): 0.008 m³
2. 22 mm Pipe Volume: 10 m × 0.00032 m³/m = 0.0032 m³
3. 15 mm Pipe Volume: 6 m × 0.00014 m³/m = 0.00084 m³
4. Total Installation Volume ($IV$): 0.008 + 0.0032 + 0.00084 = 0.01204 m³ (12.04 litres)
5. Required Purging Volume ($PV$): 1.5 × 0.01204 m³ = 0.01806 m³ (≈ 18.1 litres)

E6 variant: With an E6 meter ($IV_m = 0.0024$ m³) on the same pipe, $IV = 0.00644$ m³ and $PV = 1.5 × 0.00644 = 0.00966$ m³ — substantially less purge volume than a U6 on identical pipework.

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5. Direct Purging Operational Safety Procedures

When executing a direct purge on site:

• Venting Hose Attachment: Attach a flexible anti-static purging hose to the purge point (typically the furthest appliance outlet).
• External Venting Location: Route the purge hose outdoors to a well-ventilated open-air location, keeping the hose outlet at least 1.0 metre away from potential ignition sources, doors, or openable windows.
• Ignition Control: Eliminate all naked flames, hot surfaces, smoking, and electrical switching within the purge zone.
• Gas Concentration Monitoring: Use an intrinsically safe gas detector to verify gas concentration. Purging is complete when 100% Natural Gas (or 0% gas when purging to air) is confirmed at the vent outlet.
• Flammability Range: Natural Gas is flammable in air between 5% (Lower Explosive Limit - LEL) and 15% (Upper Explosive Limit - UEL).

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Domestic Purging Calculation Reference Matrix

Volumes use NICEIC copper coefficients and correct meter $IV_m$ (no fittings uplift shown).

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6. Re-establish Gas Supply and Relight Appliances (CCN1 Competency 15)

After pipework alterations, meter changes, temporary isolation, or any work that has left the installation purged of gas (or filled with air), CCN1 competency 15 requires a controlled return to service — not an informal “open the ECV and light everything.”

Recommended Sequence

1. Confirm tightness under IGEM/UP/1B Edition 4 (let-by 7–10 mbar / 1 min; tightness 20–21 mbar; 1 min stabilisation; 2 min test). New or disturbed pipework must show no perceptible movement. Existing systems follow the Edition 4 perceptible-movement / IV Table 3 rules with no smell of gas.
2. Re-open the ECV only after a satisfactory tightness result and with appliance isolation valves controlled so that purge and light-up remain orderly.
3. Purge air from the installation per UP/1B Edition 4 using $PV = 1.5 \times IV$, venting outdoors away from ignition sources, and confirming purge completion with a suitable gas detector.
4. Relight each appliance to manufacturer instructions (MIs): restore electrical supplies where required, follow the appliance lighting sequence, and establish correct pilot/main flame as applicable.
5. Check flame picture, operation, and safety devices: visually assess flame quality; confirm controls modulate as designed; prove flame supervision (thermocouple FSD or electronic ionisation), ODS/ASD where fitted, and relevant limit/overheat devices shut gas off when tested to MIs.
6. Confirm safe to use: verify working pressures / gas rates where required by the work scope, check for smells or LDF bubbles at disturbed joints, leave the installation labelled and documented, and only then advise the customer that appliances are safe to operate.

Skipping purge, relighting into an air-locked burner, or returning an appliance with an unproven FSD/ODS is a competency failure and a live safety risk.