Seasonal grounding resistance variations on distribution systems

The authors present the results of a 16-month study focused on seasonal variations in distribution system grounding resistance and their effect on neutral-to-earth voltage. The earth resistivity, grounding resistance parameters, and neutral-to-earth voltages measured in this study exhibit seasonal fluctuations. Precipitation seemed to affect the measured values only nominally, and then only after large amounts. The freezing of the surface soil and temperature variations over the period of the study had a major effect on values for those soil types in which the upper layer of soil is more conductive than lower layers. Conversely, if the upper layer of soil has poor conductivity compared to lower layers, freezing of the surface layer results in smaller seasonal fluctuations of the resistance parameters. Neutral-to-earth voltages are highly dependent on primary and secondary distribution system grounding parameters. These parameters can fluctuate considerably with climatic seasons in northern latitudes. The magnitude of the seasonal fluctuations depends on the relative conductivities of the upper and lower layers of soil