Many mining power systems utilize ineffectivelygrounded sources to restrict the residual current of single-phaseearth fault in order to reduce outage and shock hazard. Inpractice, with the system expansion, topology changing, andinsulation aging, the potential residual current and zero-sequencevoltage for earth fault vary dynamically and some arcing earthfaults can easily cause over-voltage and induce multiple faults. Inorder to improve the system safety, on-site condition monitoring,safety evaluation, and preventive maintenance for earth fault areproposed in this paper. Some techniques for online parametersmeasurement based on resonance-measurement principle arepresented. Power system operation states are classified intonormal secure state, alert state, incipient fault state, and faultstate. Some security enhancement methods (preventive action andremedial action) for each state are implemented. The prototypefor safety evaluation has been developed and installed in industrypower systems for several years.