Effects of the metal sheathed cables upon the performances of the distribution substations grounding systems

A model for the analysis of the performances of the grounding systems of distribution substations in underground cable networks with uncoated metal sheathed cables is suggested. The model derived is utilized to analyze the effects of cable sheaths on the ground fault current distribution, on the potentials transferred to the consumers´ substations, and on the touch- and step-voltages that could appear in the cable network area fed from the source substation. Several typical cases are studied, and some general diagrams for assessing the effects of cables on the main grounding system performances are displayed. The proposed model takes into account mutual interaction among cable sheaths and substation ground electrode elements through the soil and the voltage drops along the cable sheaths. The analysis of the typical cases has shown that the metal sheathed cables might considerably affect the grounding system parameters and that, therefore, they must be properly taken into consideration