Impact of arc impedance on earth fault currents in medium voltage cable networks

This paper demonstrates the influence of the fault arc on the magnitude of the fault current for single phase to ground faults in underground power cables. A method is suggested which allows incorporation of this impact in practical fault current calculations. Earth faults in underground cable networks depend on many parameters and are therefore complex to model and calculate exactly. Field measurements have shown the magnitude of actual occurring fault currents to sometimes be significantly lower than calculated, occasionally leading to non-selective fault clearing. A model of the arc impedance was developed and subsequently applied and verified in several case studies of actual measured faults. This showed that the arc impedance may cause a substantially lower fault current, but that its relative impact also varies significantly with the location of the fault. The proposed approach can be readily integrated into fault calculations to include this variable impact and allow for more precise determination of actual earth fault currents.