6.4 CPC, Main Protective & Supplementary Bonding

CPC Sizing, Main Protective and Supplementary Bonding

In BS 7671 (18th Edition Amendment 2), earthing and protective bonding represent the foundational line of defense against electric shock. Electrical candidates sitting the AM2 assessment must thoroughly master the distinct physical roles of earthing versus bonding, the mathematical sizing of Circuit Protective Conductors (CPCs) using the adiabatic equation, and the statutory requirements governing main equipotential bonding and supplementary protective bonding.

Distinguishing Between Earthing and Protective Bonding

While earthing and bonding both utilize green-and-yellow conductors, their electro-technical functions are fundamentally distinct:

• Earthing (Regulation 411.3.1.1): Earthing is the connection of exposed-conductive-parts (metallic enclosures of electrical equipment, such as consumer unit casings, motor frames, cooker frames) to the Main Earthing Terminal (MET). The primary purpose of earthing is to provide a low-impedance path to return fault current back to the supply source, ensuring that protective devices (MCBs, fuses, RCDs) disconnect the supply rapidly under an earth fault condition.
• Protective Bonding (Regulation 411.3.1.2): Bonding is the connection of extraneous-conductive-parts (metallic structural components that are NOT part of the electrical installation but can introduce an earth potential, such as incoming metallic water mains, gas installation pipes, structural steelwork) to the MET. Protective bonding creates an equipotential zone. If an earth fault occurs inside the premises, bonding ensures that all conductive pipework and metalwork rise to the same electrical potential simultaneously, eliminating dangerous touch voltages between accessible conductive parts.

Categorization of Conductive Parts

• Exposed-Conductive-Part: Touchable conductive metalwork of electrical equipment that can become live under fault conditions (e.g., metal light fitting casing, toaster chassis).
• Extraneous-Conductive-Part: Metallic pipework or structural steelwork not forming part of the electrical installation, capable of introducing an electrical potential (typically earth potential) from outside the building.

Sizing the Circuit Protective Conductor (CPC)

BS 7671 Regulation 543.1 provides two distinct methods for determining the minimum required cross-sectional area (s) of a Circuit Protective Conductor:

1. Selection Method (BS 7671 Table 54.7)

Table 54.7 provides a simplified selection rule based on the cross-sectional area of the associated Line conductor (S):

• If Line Conductor S ≤ 16mm²: Minimum CPC size Sp = S (must equal line conductor size, assuming same material).
• If Line Conductor 16mm² < S ≤ 35mm²: Minimum CPC size Sp = 16mm².
• If Line Conductor S > 35mm²: Minimum CPC size Sp = S / 2.

Note: Standard flat Twin and Earth cables (e.g., 2.5mm² line with 1.5mm² CPC) do not satisfy Table 54.7 directly because the CPC is reduced. Therefore, reduced CPC sizes in Twin and Earth must be mathematically verified using the adiabatic equation.

2. Calculation Method: The Adiabatic Equation (Regulation 543.1.3)

The adiabatic equation calculates the minimum thermal cross-sectional area required for a conductor to safely withstand the massive thermal energy generated by an earth fault current without melting its insulation:

s = √( I² × t ) / k

Where:

• s = Minimum calculated cross-sectional area of the CPC in square millimetres (mm²).
• I = Prospective Earth Fault Current (Ipf) in Amperes (A) flowing through the protective device.
• t = Operating time of the protective device in seconds (s) at fault current I (obtained from manufacturer time-current curves or BS 7671 Appendix 3 graphs).
• k = Thermodynamic factor (A·s½/mm²) reflecting the conductor material, insulation temperature limits, and initial/final temperatures (BS 7671 Tables 54.2 to 54.6).

Standard k Factors (Table 54.2 & 54.3):

• Copper conductor with 70°C thermoplastic PVC insulation: k = 115
• Copper conductor with 90°C thermosetting XLPE insulation: k = 143
• Steel wire armour of SWA cable: k = 51

If the calculated size s is less than or equal to the actual physical area of the CPC provided in the cable (e.g., s = 0.84mm² ≤ 1.5mm² actual), the CPC is thermally safe under fault conditions.

Main Protective Bonding Principles and Scope

Main protective bonding (Regulation 411.3.1.2) must be applied to all incoming extraneous-conductive-parts entering a building, including:

1. Incoming metallic water service pipes.
2. Incoming metallic gas installation pipework.
3. Other metallic service pipes (oil lines, compressed air).
4. Central heating and air conditioning metallic risers.
5. Exposed structural steel frame of the building.

Location of Main Protective Bonding Connections

Main protective bonding connections must be made:

• Within 600mm of the service meter outlet or point of entry to the building.
• Before any branch pipework distribution.
• On the consumer side of the main water stopcock or gas meter.
• On hard metallic pipework (downstream of flexible gas meter connectors), fitted with a BS 951 earth clamp and a permanent durable label stating: "Safety Electrical Connection - Do Not Remove".

Main Protective Bonding Sizing Rules by Supply Earthing System

The required cross-sectional area of main protective bonding conductors depends directly on the supply earthing arrangement:

TN-S and TT Supply Systems (BS 7671 Regulation 544.1.1)

Main protective bonding conductors in TN-S and TT installations must be sized to be not less than half the cross-sectional area of the main earthing conductor of the installation, subject to:

• Absolute minimum cross-sectional area = 6.0mm² copper.
• Maximum required cross-sectional area = 25.0mm² copper (no need to exceed 25mm²).

TN-C-S / PME Supply Systems (BS 7671 Table 54.8)

In Protective Multiple Earthing (PME) systems, a neutral supply conductor failure (open-PEN fault) forces circulating neutral current to return via the earth network and main protective bonding conductors. Consequently, BS 7671 Table 54.8 mandates larger bonding conductors based on the supply PEN conductor size:

For virtually all domestic and light commercial TN-C-S (PME) installations in the UK, the mandatory main protective bonding conductor size is strictly 10mm² copper.

Supplementary Protective Bonding Principles

Supplementary protective bonding (BS 7671 Regulation 415.2) provides localized equipotential bonding connecting exposed-conductive-parts and extraneous-conductive-parts within a specific room or high-risk zone where automatic disconnection times cannot be met or where heightened body conductivity risks exist (e.g., agricultural locations, locations with baths or showers).

The electrical resistance R between simultaneously accessible conductive parts must satisfy:

R ≤ 50V / Ia (for AC systems)

Where Ia is the operating current of the protective device (for an RCD, Ia = IΔn = 30mA = 0.03A, yielding R ≤ 1666 Ω).

Conditions for Omitting Supplementary Bonding in Bathrooms (Section 701)

Under BS 7671 Regulation 701.415.2, local supplementary bonding in bathrooms MAY BE OMITTED if ALL three of the following conditions are simultaneously met:

1. All final circuits serving or passing through the bathroom location are protected by a 30mA RCD.
2. Main protective bonding is correctly established to all incoming building services in accordance with Regulation 411.3.1.2.
3. All exposed-conductive-parts of equipment in the bathroom are effectively connected to the CPC of the installation, meeting ADS disconnection times.

If any of these conditions are unverified, local supplementary bonding (minimum 4.0mm² un-mechanically protected or 2.5mm² mechanically protected copper) must be installed between all pipework and equipment.