Analytical and numerical simulation techniques have been developed for the calculation of earth resistance/ impedance and to estimate the potential distribution in the vicinity of earth electrodes. However, very little literature is available on experimental validation of these calculation techniques. To address this, a programme of experimental tests on various earth electrodes has been carried out at the lower water reservoir of a hydro pumped-storage power station in North Wales. In this paper, the earthing test facility at Dinorwig power station is described including the details of the experimental set up. The results from experimental tests on a 5 m× 5 m earth grid, immersed in water and energized under ac, dc and impulse, are presented. The values of measured earth resistance/impedance and water surface potential distributions are compared with those obtained from analytical calculations and detailed numerical computer simulations.