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Based on response curves derived from field measurements, optimum number of vertical ground electrodes for grounding electrical installation is determined. On sites where the optimum number of the electrodes cannot reach a target ground resistance, a `critical resistance area concept' with conductive backfills is used. In this study, efficiencies of local conductive backfills for reducing ground resistance are investigated. Results show that application of tyre ashes as a conductive backfills lowers ground resistance to over 80%. It is also shown that the most favourable benefit in the ground resistance improvement is to limit the backfill to four electrodes.