Earth Electrode Resistance — TT System Reference

For a TT system protected by a 30mA RCD (the most common arrangement in the UK), the earth electrode resistance must not exceed 1667 ohms (calculated as 50V ÷ 0.03A). However, a maximum of 200 ohms is widely recommended as good practice to ensure reliable RCD operation and provide adequate fault current. BS 7671 Regulation 411.5.3 requires that Ra × Ia ≤ 50V, where Ra is the total earth fault loop impedance and Ia is the RCD operating current.

Earth electrode resistance is tested using a dedicated earth electrode tester (e.g., Megger DET4) with the fall-of-potential method. Two temporary test spikes are driven into the ground in a straight line from the electrode under test. The potential spike is placed at 61.8% of the distance to the current spike (the 61.8% rule). At least three readings should be taken at different spike positions to verify consistency. Testing should be done in dry conditions for worst-case results.

Yes, earth electrode resistance varies seasonally. It is lowest in winter when the ground is wet and highest in late summer when the soil is driest. Resistance can vary by a factor of 2-3 between wet and dry seasons. For this reason, testing in dry conditions gives the most conservative (worst-case) result. If tested in winter, apply a seasonal correction factor — typically 1.5 to 2.0 for UK conditions.

The most common earth electrode in the UK is a driven copper-clad steel rod, typically 1.2m sections that can be coupled together and driven to 2.4m or deeper. Earth rods should be at least 16mm diameter for copper-clad steel. Alternatives include earth plates (minimum 0.5m², 2mm thick copper), earth tapes (minimum 25mm × 2mm copper strip), and foundation earth electrodes. The choice depends on soil conditions and the target resistance.