What are the different UK earthing systems?

TN-S: The supply has a separate neutral and earth conductor — earth is via the cable sheath. Typical Ze: 0.35-0.8Ω. TN-C-S (PME): The neutral and earth are combined in the supply cable (PEN conductor) and separated at the origin. Typical Ze: 0.2-0.35Ω. TT: No earth is provided by the supply — you must install a local earth electrode (rod, plate, or mat). All circuits must have 30mA RCD protection. Found in rural areas and older installations.

How deep must an earth rod be driven?

Standard earth rods are 1.2m sections that couple together. The minimum practical depth is 1.2m, but achieving acceptable resistance often requires 2.4m (two sections) or 3.6m (three sections). In good soil (clay), a single 1.2m rod may achieve 30-50Ω. In poor soil (sand, gravel, rock), you may need multiple rods at 3m+ depth. The resistance decreases roughly proportionally with rod length — doubling the depth halves the resistance.

What earth electrode resistance is acceptable for a TT system?

For a TT system with 30mA RCD protection, the maximum earth electrode resistance (Ra) must satisfy: Ra × Idn ≤ 50V. With a 30mA RCD: Ra ≤ 50/0.03 = 1,667Ω. However, in practice, a resistance below 200Ω is recommended for reliable RCD operation. Very high resistance can cause nuisance tripping in damp weather and unreliable disconnection. Below 100Ω is considered good, below 20Ω is excellent.

How does soil type affect earth electrode resistance?

Soil resistivity varies enormously: wet clay = 40 Ω·m (excellent), dry clay = 100 Ω·m, loam = 150 Ω·m, wet sand = 200 Ω·m, chalk = 300 Ω·m, gravel = 500 Ω·m, dry sand = 1,000 Ω·m, rock = 3,000+ Ω·m. The formula R = ρ/(2πL) × ln(4L/d) shows that doubling soil resistivity doubles the electrode resistance. Seasonal variation is significant — resistance increases in dry summer months. Test during dry conditions for worst-case results.

Can I use multiple earth rods to reduce resistance?

Yes — parallel earth rods reduce the effective resistance. Two rods at adequate spacing (minimum 2× rod length apart) approximately halve the resistance. However, rods close together interact — their effective zones overlap, reducing the benefit. A coupling factor applies: at 1× rod length spacing, two rods give only about 60% benefit (not 50%). Space rods at least 2× their driven depth apart for maximum effect. Three or more rods arranged in a triangle or line are effective for difficult sites.

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Earthing System Calculator — BS 7671

Calculate earth electrode resistance for TN-S, TN-C-S (PME), and TT earthing systems. Determine rod depth and number needed based on soil resistivity.

Earthing SystemCheck your DNO supply type

Soil TypeDetermines soil resistivity

Rod Length (each)

Standard sections: 1.2m, 2.4m, 3.6m

Number of Rods

Parallel rods reduce resistance

How We Calculate This

This calculator estimates earth electrode resistance using the standard formula for driven rod electrodes.

Formula

R = (ρ ÷ 2πL) × ln(4L ÷ d)

Where ρ = soil resistivity (Ω·m), L = rod length (m), d = rod diameter (m)

Typical Ze Values

TN-S: 0.35–0.8Ω (cable sheath earth)
TN-C-S (PME): 0.2–0.35Ω (combined neutral-earth)
TT: Depends on electrode — typically 20–200Ω
Multiple rods: R_eff = R_single ÷ n × coupling factor

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.