Earthing & Bonding
Earthing systems, main bonding conductor sizes, supplementary bonding, and the adiabatic equation.
Earthing Systems
TN-S
Separate neutral and earth. Earth via cable sheath. Typical Ze: 0.8Ω
TN-C-S (PME)
Combined neutral/earth in supply, separated at origin. Typical Ze: 0.35Ω
TT
No earth from supply. Local earth electrode required. Typical Ze: 21Ω (with 200Ω electrode). RCD protection essential.
IT
Isolated earth. Used in special applications. First fault does not cause disconnection.
Main Bonding Conductor Sizes — Table 54.8
Main Bonding Conductor Sizes (PME) — Table 54.8
| PEN conductor, Cu equivalent (mm²) | Min main bonding, Cu equivalent (mm²) |
|---|---|
| 35 or less | 10 |
| over 35 up to 50 | 16 |
| over 50 up to 95 | 25 |
| over 95 up to 150 | 35 |
| over 150 | 50 |
Table 54.8 applies where PME conditions apply (TN-C-S), selected by the PEN conductor of the supply. Local distributor's network conditions may require a larger conductor.
Where PME conditions do not apply (TN-S, TT): not less than half the earthing conductor, not less than 6 mm², and need not exceed 25 mm² copper (Reg 544.1.1).
Supplementary Bonding
Minimum supplementary bonding conductor sizes between extraneous-conductive-parts: • 2.5 mm² Cu if sheathed or otherwise mechanically protected • 4 mm² Cu if not mechanically protected To exposed-conductive-parts, Regs 544.2.1 and 544.2.2 set a conductance rule instead (4 mm² minimum if not mechanically protected). Required where ADS cannot be confirmed by measurement, particularly in bathrooms (Section 701).
Adiabatic Equation
Minimum CPC Size
Variations
k values: Copper/PVC = 115, Copper/XLPE = 143, Copper/Rubber = 141, Aluminium/PVC = 76
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