An investigation of earthing resistances

Abstract

The phenomena of polarization and endosmose which accompany the flow of direct current in soil were investigated in various laboratory experiments. Polarization was an effect of gas bubbles on the measured soil resistance—bubbles which formed owing to a small electrolytic decomposition of the soil moisture by the current and which accumulated at the electrodes. Endosmose was extensive and occurred even against an appreciable hydraulic back-pressure. The earthing resistances of electrodes of inextensive area depend directly on the resistivity of the surrounding soil, and so relations were obtained showing the effect of moisture, temperature and salt on soil resistivity. The resistivities of coal, coke, ashes, chalk and dried soil were also obtained.Various types of electrodes of inextensive area, viz. pipes, plates and strips, were installed in aplot of ground and tested over a period covering one year. The results are important as being typical of what may be obtained in practice, but not as criteria of comparative utility of the various electrode types on account of the soil resistivity being—there were reasons to presume— appreciably non-uniform. The earthing resistances of the various electrode types were, however, calculated theoretically, a soil of uniform resistivity being assumed, and the results obtained may be used as criteria. The seasonal variations in resistance were determined during the year, and the effect of moisture in this respect was greater than that of temperature.Some electrodes of extensive area, viz. water pipes, the lead sheath of a large size cable and the steel structures of buildings, that were in the vicinity of the other electrodes, were also tested. Their earthing resistances were much smaller than those of the other types, and the smallest resistance was that of a water pipe. The possible differences between alternating-current and direct-current measurements made on electrodes in the field were investigated. The greatest difference obtained was an increase in resistance of about 20 per cent with direct current and with the positive electrode one of inextensive area.The various types of electrodes of inextensive area are considered theoretically in an Appendix.