Novel fail-safe earth-fault protection schemes for underground coalmines using restricted-neutral system of power supply

The paper reports the development of two novel fail-safe sensitive earth-fault protection schemes for coal mines using restricted-neutral system of power supply. Minimisation of earth leakage current for an underground coal mine´s power-supply system is of great advantage in reducing hazards like electrical shock from exposed metal works, fire and methane explosion. Restricted-neutral system of power supply can effectively limit earth-fault currents to a very low limit (100¿200 mA) as compared to that of 500¿1000 A in solidly earthed neutral system. With such drastic reduction of earth-fault current, the earth-fault relay design becomes quite complex and requires application of electronic circuits, because the weak output voltage of the core balance transformer can no longer operate a conventional relay directly during an earth fault. Most of the existing earth-fault protective devices used for this purpose are either nonfail-safe or partially fail-safe, and to the author´s knowledge no attempt has been made, so far, to study their fail-safe characteristics in a systematic manner. This leaves a scope of doubt about their degree of fail-safety feature. The new protection schemes developed by the authors use two novel circuits with fail-safe features, such that in case of failure of most of the circuit components the circuit breaker is tripped off thereby offering additional safety for underground application. The fail-safe feature of the protective devices developed have been critically ascertained by typical laboratory simulation of component faults. The protection schemes developed offer good reliability for earth-fault setting of 100mA and above.