Earthing System Calculator — BS 7671

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. Check our Bonding Conductor Calculator for the correct bonding sizes for your earthing system.

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.

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.

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 BS 7430 rod formula R = ρ/(2πL) × [ln(8L/d) − 1] 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.

Yes — parallel earth rods reduce the effective resistance. Two rods at adequate spacing (at least 2× rod length apart) approximately halve the resistance. However, rods close together interact — their resistance zones overlap, so you never get a perfect halving. BS 7430 captures this with R = R₁(1 + λa)/n: the combined resistance is always higher than R₁/n. At wide spacing (≥2× rod length) two rods reach roughly 90% of the ideal benefit; at tight 1× spacing the combined resistance is only cut to about 60% of a single rod (around 40% reduction, not 50%). Space rods at least 2× their driven length apart for the best effect. Three or more rods in a line or triangle are effective for difficult sites.