Hazard Assessment of Low-voltage Insulated Neutral Networks with Unstable Single-phase Fault

The paper considers the influence of transient processes on the safety of single-phase faults in low-voltage electric networks with insulated neutral, taking into account the resistance at the point of phase-to-ground fault. Practically a significant part of the short circuits has a nature of an unstable contact between the phase and the grounded housing, therefore, it is necessary to take into account the effect of transient processes on the indicators characterizing the hazard of an electrical installation. Experimental studies carried out on the physical model of a low-voltage electric network have shown that the surge fault current can be several times higher than the amplitude of the steady-state fault current. In addition, surge currents lead to an increase in the energy released at the damage point. In this case, the magnitude of the surge current and the released energy increase with a decrease in the short circuit resistance. During the studies more than a twofold increase in the released energy was recorded when the nature of the circuit changed from stable to unstable. A similar trend was recorded for the surge fault current. Calculation of the correlation coefficient between these values showed their close relationship, which may indicate the same factors that form these processes. Accordingly, the means aimed at reducing the short-term surge currents in the transient process can be considered as a way to improve the safety of the electrical installation.