Earthing Resistance

The earthing system provides the critical return path for fault currents in TT and TN-S installations and establishes the reference potential for safety. Earthing resistance depends on electrode type (rod, plate, strip, or mat), electrode dimensions, burial depth, and soil resistivity. Sandy or rocky soil may have resistivities exceeding 1,000 Ω·m, requiring extensive electrode systems, while clay soils may be below 50 Ω·m. The fall-of-potential method is the standard measurement technique: a test current is injected between the electrode under test and an auxiliary electrode, while a potential electrode measures the voltage gradient at progressive distances. For TT systems, BS 7671 requires RA × IΔn ≤ 50V, where RA is the total earthing resistance and IΔn is the RCD rated residual current. Typical targets are below 20Ω for domestic installations and below 1Ω for substations. Multiple electrodes in parallel reduce overall resistance, but spacing must be adequate to avoid overlap of their resistance areas. Seasonal variations in soil moisture significantly affect earthing resistance — measurements taken in summer during dry conditions represent worst-case values.