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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.

BS 7671: Chapter 31

Main Bonding Conductor Sizes — Table 544.1

Supply Neutral (mm²)	Min Bonding Cu (mm²)
≤35	10
>35 ≤50 (PME)	16
>50 ≤95 (PME)	25
>95 ≤150 (PME)	35

Min 6mm² Cu for TN-S systems

Min 10mm² Cu for TN-C-S (PME) systems

BS 7671: Table 544.1

Supplementary Bonding

Minimum supplementary bonding conductor sizes: • 4mm² Cu (mechanically protected) • 2.5mm² Cu (not mechanically protected) Required where ADS cannot be confirmed by measurement, particularly in bathrooms (Section 701).

BS 7671: Reg 544.2

Adiabatic Equation

Minimum CPC Size

S = √(I²t) / k

S Minimum CPC cross-section (mm²)

I Fault current (A)

t Disconnection time (s)

k Material constant from Table 54.4

Variations

Find max disconnection time: t = (k × S / I)²

k values: Copper/PVC = 115, Copper/XLPE = 143, Copper/Rubber = 141, Aluminium/PVC = 76

BS 7671: Reg 543.1.3, Table 54.4 Open Calculator →

Conductor Resistance

Special Locations

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