Enhanced selectivity of high resistance earth-fault clearing improves quality of electric energy supply in Slovenia

A normal earth-fault protection of a MV network is based on the assumption that fault currents that flow to earth will be determined by the earth-fault impedance of the line and associated plant. In some cases, however, the nature of the ground fault may be such that it makes non effective earth connection. In such a case a system fault to earth may result in a very small residual current and as a consequence voltage too small to overcome VT-s tolerances. This does not allow the earth fault protection (E/F) to determine direction and to operate correctly. If a line conductor should break and fall on to semi-insulating objects, such as trees or hedges, the circuit will, therefore, not be isolated and the conductor, which will be maintained at normal phase voltage will constitute a serious danger to life.