2.2 Equipotential and Supplementary Bonding Requirements
Key Takeaways
- Main protective equipotential bonding eliminates touch voltage hazards by connecting extraneous conductive parts directly to the Main Earth Terminal (MET).
- Main bonding conductors must be sized to at least half the cross-sectional area of the main earthing conductor, subject to a minimum of 6 mm² copper.
- Supplementary bonding is required in high-risk zones (bathrooms, swimming pools, wet environments) where touch voltage could exceed safe thresholds.
- The maximum allowable resistance for supplementary bonding conductors between touchable conductive parts is governed by R <= 50V / Ia (or R <= 50 / IΔn for RCD-protected circuits).
- Continuity testing of protective bonding conductors must confirm low electrical resistance, typically requiring values under 0.05 Ω across connections.
2.2 Equipotential and Supplementary Bonding Requirements
While earthing provides a low-impedance path to return fault current to the source and trigger automatic disconnection, protective equipotential bonding is the vital safeguard that prevents dangerous voltage differences from appearing between touchable conductive surfaces during a fault. In DEWA and Dubai Municipality compliance inspections, bonding installation details are subjected to rigorous physical and electrical verification.
Fundamentals of Equipotential Bonding
Electric shock occurs when a person simultaneously makes contact with two conductive points at different electrical potentials, causing shock current to flow through the human body. During an earth fault inside a building, exposed conductive metalwork (such as a washing machine frame) rises to a high potential ($V_{fault} \approx U_0 / 2 = 115\text{V}$). If a person touches that live frame while simultaneously touching an earthed metallic water pipe or structural steel column at zero potential ($0\text{V}$), the resulting touch voltage ($V_t = 115\text{V}$) would prove fatal.
Equipotential Bonding eliminates this hazard by joining all accessible metallic structures together electrically. When bonding is correctly implemented, an earth fault causes all bonded metalwork to rise to the same potential simultaneously, reducing the touch voltage between accessible parts to near zero ($V_t \approx 0\text{V}$).
Key Terms & Definitions
- Exposed Conductive Part: Metalwork of electrical equipment that can be touched and is not normally live, but which may become live under fault conditions (e.g. electrical panel enclosures, motor frames, metallic conduit, cable tray systems).
- Extraneous Conductive Part: A conductive part not forming part of the electrical installation, but capable of introducing an electrical potential, usually earth potential (e.g. main incoming metallic water pipes, gas pipes, structural steel building frames, central heating/chilled water riser pipes, metallic window frames in wet areas).
Main Protective Equipotential Bonding
Main protective bonding connects all incoming extraneous conductive parts to the building's Main Earth Terminal (MET) at the service entry point.
Mandatory Connection Points
Under DEWA regulations, main bonding conductors must be connected to:
- Main Incoming Water Pipes: Connection must be made as close as possible to the point of entry into the building, within 600mm of the water meter or main stopcock, and on the consumer side before any branch pipework.
- Building Structural Steel: Connection to main exposed structural steel columns embedded in concrete foundations.
- Central HVAC Chilled Water Lines: Incoming main chilled water flow and return steel pipes serving central air conditioning units.
- Gas/LPG Piping Systems: Incoming metallic gas pipework (where present), connected before any regulator or meter.
- Lightning Protection System: Direct bonding between the building's down-conductor earth pit and the MET to avoid side-flashing during lightning discharges.
Sizing Main Protective Bonding Conductors (BS 7671 Table 54.8 & DEWA Rules)
Main protective bonding conductors carry substantial harmonic and earth fault currents. Their cross-sectional area (CSA) is determined based on the cross-sectional area of the main protective earthing conductor connected to the MET.
Sizing Rule for TN-S Systems
According to BS 7671 Regulation 544.1.1 and DEWA Guidelines:
- The main protective bonding conductor must have a copper cross-sectional area not less than half the required cross-sectional area of the main earthing conductor of the installation.
- Absolute Minimum Limit: $6.0\ \text{mm}^2$ copper (or equivalent in aluminum).
- Absolute Maximum Limit: $25.0\ \text{mm}^2$ copper (for TN-S systems).
| Main Earthing Conductor Size (Copper) | Minimum Main Protective Bonding Conductor Size (Copper) |
|---|---|
| $6\ \text{mm}^2$ | $6\ \text{mm}^2$ |
| $10\ \text{mm}^2$ | $6\ \text{mm}^2$ |
| $16\ \text{mm}^2$ | $10\ \text{mm}^2$ |
| $25\ \text{mm}^2$ | $10\ \text{mm}^2$ |
| $35\ \text{mm}^2$ | $16\ \text{mm}^2$ |
| $50\ \text{mm}^2$ or greater | $25\ \text{mm}^2$ |
Note: Bonding clamps must comply with BS 951, featuring a permanent metal tag clearly embossed with the legend: "Safety Electrical Connection — Do Not Remove".
Supplementary Equipotential Bonding in Special Locations
In environments where human body resistance is drastically reduced due to moisture, wet skin, or lack of footwear, main protective bonding alone is insufficient. In these special locations, DEWA and BS 7671 mandate Supplementary Equipotential Bonding.
Mandatory Locations
- Bathrooms and Shower Rooms (BS 7671 Section 701): Bonding together exposed conductive parts of electrical equipment (e.g. water heater frames, towel rails, light fittings) and extraneous conductive parts (metallic hot/cold water pipes, waste pipes, metallic bath tubs, shower trays).
- Swimming Pools and Fountains (Section 702): All metallic handrails, ladders, pool drainage grates, and submerged lighting niches within Zones 0, 1, and 2.
- Commercial Kitchens & Food Prep Areas: Stainless steel sinks, metallic counters, and exposed water supply taps.
- Medical Locations (Section 710): Patient treatment rooms and operating theaters (equipotential busbars).
Minimum Conductor Sizing for Supplementary Bonding
Under BS 7671 Regulation 544.2, the minimum size of a supplementary bonding conductor depends on whether it is mechanically protected:
- Mechanically Protected (e.g. in conduit or trunking): Minimum $2.5\ \text{mm}^2$ copper.
- Not Mechanically Protected (e.g. surface clipped exposed): Minimum $4.0\ \text{mm}^2$ copper.
- Connecting Two Exposed Parts: Size must equal the smaller CPC connected to the exposed parts.
- Connecting Exposed Part to Extraneous Part: Size must equal half the CSA of the CPC attached to the exposed part.
Mathematical Derivation of Supplementary Bonding Resistance ($R \le 50 / I_a$)
Supplementary bonding is deemed effective when the electrical resistance ($R$) between any two simultaneously touchable exposed conductive parts and/or extraneous conductive parts satisfies the equation:
Where:
- $U_L$ (Safe Touch Voltage Limit): $50\text{V AC}$ under normal dry/wet conditions ($25\text{V AC}$ in agricultural/swimming pool environments).
- $I_a$: Operating current of the protective device within the required disconnection time (0.4s or 5s).
Application with RCD Protection
Where supplementary bonding is installed on a circuit protected by a 30mA RCD ($I_{\Delta n} = 30\text{mA} = 0.03\text{A}$), $I_a$ is taken as $I_{\Delta n}$:
Because $1666\ \Omega$ is a very high resistance value, the presence of a functioning 30mA RCD makes satisfying the supplementary bonding criterion virtually guaranteed, provided the bonding conductor is intact.
Inspection, Testing, and Verification of Bonding Conductors
During DEWA installation verification, contractors must perform Continuity of Protective Conductors Testing using a low-resistance ohmmeter (calibrated test current $\ge 200\text{mA}$):
- Null or zero the test leads prior to measurement.
- Measure resistance from the MET to the furthest point of each main bonding conductor.
- Measure resistance between accessible metalwork in bathrooms and kitchens.
- Acceptance Benchmark: A sound metallic bonding connection across clamps and conductors should yield a measured resistance of $\le 0.05\ \Omega$. Any reading exceeding $0.05\ \Omega$ indicates loose bonding clamps, surface corrosion, or undersized conductors requiring immediate rectification.
What is the primary objective of installing main protective equipotential bonding in an electrical installation?
In a TN-S installation with a main protective earthing conductor of 25 mm² copper, what is the minimum required cross-sectional area for the main protective bonding conductor under BS 7671 Table 54.8?
For a supplementary bonding conductor installed in a bathroom without mechanical protection, what is the minimum permitted copper conductor cross-sectional area?