4.3 Earthing Arrangements & Protective Bonding Conductors

4.3 Earthing Arrangements & Protective Bonding Conductors

Core Principle: Earthing and equipotential bonding are the twin pillars of Automatic Disconnection of Supply (ADS) under BS 7671. While earthing provides a low-impedance path to return fault current to the supply source and trigger protective devices, main protective bonding reduces potential differences between exposed metalwork and extraneous services and works with the required automatic disconnection; it does not guarantee a safe touch voltage throughout every fault.

---

The Main Earthing Terminal (MET) & Disconnection Facility

The Main Earthing Terminal (MET) (BS 7671 Regulation 542.4) is the central common connection point connecting all protective conductors within an electrical installation.

Conductors Terminating at the MET

1. The Earthing Conductor: The heavy-duty conductor connecting the MET to the means of earthing (the DNO cable sheath in TN-S, the PME terminal in TN-C-S, or the consumer's earth electrode in TT).
2. Main Protective Bonding Conductors: Conductors connecting extraneous-conductive-parts (gas, water, oil, structural steel) to the MET.
3. Circuit Protective Conductors (CPCs): The protective conductors of distribution circuits supplying sub-distribution boards and final circuits.
4. Functional Earthing Conductors (where present): Dedicated earthing paths for telecommunications, data centers, or radio-frequency screening.
5. Lightning Protection Bonding Conductors (where present): Equipotential bonding connecting the building's BS EN 62305 lightning protection system to the MET.

Accessibility, Labelling & Disconnection Facility

• Accessibility & Identification: The MET must be located in an accessible position, protected against mechanical damage and corrosion, and fitted with a permanent label stating: "Safety Electrical Connection - Do Not Remove" (Regulation 514.13.1).
• The Disconnection Facility: BS7671 Regulation542.4.2 requires accessible means of disconnecting the earthing conductor for measurement, using a tool. This may be combined with the MET; a separate dedicated link is not essential.
• Testing Purpose: Where Ze is established by live measurement, excluding parallel installation paths may require disconnection of the earthing conductor. First consider available supply information and safer verification methods. Prospective fault current is measured with the earthing, bonding and CPC connections intact, representing the normal arrangement and its maximum fault current.

---

Circuit Protective Conductors (CPCs): Sizing & Fault Loop Integration

The Circuit Protective Conductor (CPC) forms the internal return path of the earth fault loop ($Z_s = Z_e + (R_1 + R_2)$). When a line-to-earth fault occurs, the CPC must carry high fault current without exceeding its maximum permissible fault temperature until the protective device disconnects.

Under BS 7671 Regulation 543.1, the cross-sectional area of a CPC must be determined by one of two methods:

Method 1: Sizing by Selection (BS 7671 Table 54.7)

Where the protective conductor is constructed of the same metal (e.g. copper) as the line conductor, Table 54.7 provides empirical sizing rules:

Note: In standard domestic flat twin-and-earth cables (BS 6004), the CPC is reduced (e.g., $1.5\text{ mm}^2$ twin cable has a $1.0\text{ mm}^2$ CPC; $2.5\text{ mm}^2$ twin cable has a $1.5\text{ mm}^2$ CPC). These reduced CPCs are verified using Method 2 (the adiabatic equation).

Method 2: Sizing by Calculation (The Adiabatic Equation)

Under Regulation 543.1.3, the minimum cross-sectional area $S$ can be calculated to determine whether a reduced CPC can safely withstand the thermal energy ($I^2 t$) let-through by the protective device during a fault:

$S = \frac{\sqrt{I^2 t}}{k} = \sqrt{\frac{I^2 t}{k^2}}$

Where:

• $S$ = Minimum cross-sectional area in $\text{mm}^2$.
• $I$ = Fault current in amperes (RMS) for a fault of negligible impedance.
• $t$ = Protective-device operating time at the calculated fault current, not simply the maximum permitted disconnection time. For operation below0.1s, use the manufacturer’s applicable let-through $I^2t$ data.
• $k$ = Factor taking into account the resistivity, temperature coefficient, and heat capacity of the conductor material and initial/final temperatures (from BS 7671 Tables 54.2 to 54.6).

Selecting the $k$-Factor:

• $k=115$: A copper CPC incorporated in or bunched with70°C PVC cables is a common example.
• $k=143$: A separate70°C PVC-insulated copper protective conductor not bunched with cables is another common example.
• Select the factor from the applicable table using the conductor material, insulation, installation and initial/final temperatures. Steel wire armour requires the appropriate steel factor; neither example is a universal armour or bare-copper value.

---

Main Protective Bonding: Principles & Equipotential Zones

An exposed-conductive-part is metallic electrical equipment casing (such as a metal consumer unit or toaster) that can be touched and may become live under fault conditions.

An extraneous-conductive-part is a conductive part not forming part of the electrical installation, but liable to introduce an electric potential (generally earth potential). Examples include underground metallic water service pipes, gas installation pipes, structural building steel, and central heating pipes.

The Touch Voltage Hazard

During a line-to-earth fault, exposed conductive metalwork rises to a potential determined by the fault current and the impedance of the CPC. If a person simultaneously touches the faulted appliance and an unbonded metal water pipe connected to true earth, the person bridges the two potentials, experiencing a dangerous touch voltage ($U_t$) that can cause ventricular fibrillation.

Main protective bonding reduces the potential difference but does not eliminate every shock hazard. Connecting extraneous-conductive-parts to the MET helps them rise in potential together. Voltage drops in protective conductors can still produce dangerous touch voltages, so the selected protective device must provide the required automatic disconnection. Bonding alone does not guarantee $U_t\le50\text{ V}$ during a fault.

Determining Whether a Part Is Extraneous

A metal part needs main bonding when it meets the definition of an extraneous-conductive-part. An internal pipe fed by plastic service pipe is not automatically extraneous; check its actual connections and ability to introduce earth potential.

Inspect first. Where resistance measurement is needed, account for parallel paths through connected protective conductors and select a suitable safe test method. IET GN8-based guidance relates the resistance criterion to $R_{CP}>U_0/I_B-Z_T$, using chosen body current and body impedance assumptions. For example,230V,10mA and1000Ω give22kΩ. This is an illustrative assessment, not a universal GN3 pass limit; consider the environment and whether the separation will remain effective.

---

Extraneous Parts Requiring Bonding & Connection Geometry

Under BS7671 Regulation411.3.1.2, the following parts require main protective bonding when they are extraneous-conductive-parts:

1. Metallic water installation pipes entering the building.
2. Metallic gas installation pipes entering the building.
3. Other service pipes and ducting (e.g. oil fuel supply lines, district heating pipes).
4. Metallic central heating and air-conditioning systems (where entering from outside).
5. Exposed metallic structural parts of the building (structural steel columns).

Connection Location Rules (Regulation 544.1.2)

The connection of a main protective bonding conductor to an extraneous service must comply with strict physical geometry rules:

Internal meter/stopcock → consumer-side metal pipe → branches
                         ↑ bond here, within600mm where practicable
External meter → point of entry → internal metal pipe
                 ↑ bond as near the entry as practicable

1. Internal Meter/Stopcock: Connect to the consumer’s hard metal pipework before branch pipework, within600mm of the meter outlet union or main stopcock where practicable.
2. External Meter: Make the connection as near as practicable to the service entry into the premises.
3. Insulating Sections: Connect on the consumer side of any insulating insert and of the meter/stopcock; assess whether the pipe is extraneous before bonding it.
4. Connection and Label: Use a suitable secure connection, with a BS951 clamp where applicable and a permanent "Safety Electrical Connection - Do Not Remove" label.

---

Sizing of Main Protective Bonding Conductors (BS 7671 Table 54.8)

Main bonding conductors are sized according to the type of earthing system and the size of the supply conductors:

1. TN-S and TT Systems

Under Regulation 544.1.1:

• The main protective bonding conductor must have a cross-sectional area of not less than half the required cross-sectional area of the earthing conductor of the installation.
• It must have a minimum cross-sectional area of $6\text{ mm}^2$ copper.
• It does not need to exceed $25\text{ mm}^2$ copper (or equivalent conductivity in other metals).

2. TN-C-S (PME) Systems

Because PME installations are subject to diverted neutral currents under normal load balance or broken neutral conditions, main protective bonding conductors must carry sustained currents without overheating.

Under BS 7671 Table 54.8, bonding conductors are sized strictly against the copper-equivalent cross-sectional area of the supply neutral / PEN conductor:

(Use the copper-equivalent supply neutral size and confirm any additional distributor requirements.)

---

Supplementary Bonding & Bathroom Exemption Conditions

Supplementary equipotential bonding (Regulation 415.2) is localized bonding applied in high-risk locations (such as swimming pools, agricultural buildings, and medical treatment facilities) to interconnect all simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts.

Disconnection Criterion for Supplementary Bonding

To verify the effectiveness of supplementary protective bonding, apply the resistance criterion:

$R \le \frac{50\text{ V}}{I_a}$

Where:

• $R$ = Resistance between simultaneously accessible exposed and extraneous parts ($\Omega$).
• $I_a$ = Operating current of the protective device in amperes (for an overcurrent device, $I_a$ is the current causing disconnection within 5 seconds; for a 30 mA RCD, $I_a = 0.03\text{ A}$, giving $R \le \frac{50}{0.03} = 1667\ \Omega$).

Exemption in Bathrooms & Shower Rooms (Regulation 701.415.2)

Historically, bathrooms required visible supplementary bonding wires linking all copper pipes, radiator valves, and metal bath frames. Under BS 7671 Regulation 701.415.2, supplementary equipotential bonding may be omitted in a room containing a bath or shower if ALL THREE of the following conditions are simultaneously met:

If any one of these three conditions is not satisfied (for instance, an electric shower circuit protected only by an MCB without 30 mA RCD protection), supplementary equipotential bonding is required by BS7671 unless the missing condition is remedied so that all three omission conditions are met.