Personal Safety for Persons in Vicinity of MV-Cables During Earth Faults

The potential differences in the immediate vicinity of an earth fault location of 20 kV and 30 kV cables in earth fault current-limited grids (ohmic or resonant) are analyzed with regard to personal safety for general public, which might be barefoot. Not only occurring step voltages are assessed but also accessible touch voltages on the surface for persons in push-up-position. Two main fault scenarios are in the focus. The first scenario is a cable earth fault, where some part of the fault current flows through the damaged insulation into earth and causes a rise of surface potential in the vicinity, which can lead to step or touch voltages. In the second scenario the transferred potential via accompanying earth electrode in the case of earth faults in the nearby ring main unit (RMU) or cable termination tower is analyzed.Based on the investigations carried out, in the case of internal faults cables in medium voltage grids (resonant or current-limited) in practical cases, no danger to persons is to be expected with regard to impermissible body currents or potential differences (touch voltages, step voltages). Likewise, it can be assumed in the typical networks and situations, that there is no danger to persons due to transferred potential with the accompanying earth electrodes, especially if the earth potential rise (EPR) of the connected earthing system is less than four times the permissible touch voltage.