The current-loading capacity of earth electrodes

The limits of current-loading capacity of earth electrodes are considered under the three types of conditions which may occur in practice, namely long-duration loading, which occurs where the earth current is insufficiently large to interrupt the supply; short-duration overloading, which occurs when the protective gear operates in the usual way; and long-duration overloading, which occurs on the failure of the protective gear or where the circuit is earthed through a tuned reactance coil and continuous operation with an earth on one phase is possible. The behaviour of various standard forms of earth electrode is compared under the three conditions of operation, and the effect is discussed of the size and shape of the electrode and of the physical characteristics of the soil on the change of resistance and temperature-rise in the ground. The report concludes with a résumé of published experimental work and a brief discussion of the effect of coke breeze and salt treatment on the loading capacity.