The mine in question was evacuated after a fault in the 33-kilovolt overhead line on the mine surface led to a “cross country fault” in the 33kV reticulation system.

Over the next six minutes a failure occurred on a 33kV pole-mounted circuit breaker and a 33kV cable splice underground, causing a fire to start.

Monitoring systems logged significant disturbances on the high voltage network throughout this period, leading to the HV protection system shutting down power to the site.

Mine Safety Operations director Rob Regan said the initial fault might have been caused by a fallen branch or wildlife coming into contact with the 33kV overhead line.

A series of smaller shorts occurred across two different feeders but failed to trip the relevant earth leakage protection relays.

In the alert, Regan said investigations following the power trip identified a flashover at a separate 33kV recloser on the surface and the insulation failure and subsequent fire on the underground 33kV feeder cable splice.

One of the major contributing factors was that the 33kV splice fire was not discovered until 36 minutes after the initial fault.

The splice is believed to have burnt for about two hours, periodically dropping burning insulation material onto the roadway.

The situation was compounded when another electrician, who was resetting power to the mine, failed to identify that the 33kV underground feeder involved had tripped on an earth fault, and reset power to the effected cable.

This occurred about 55 minutes after the initial failure and subsequent fire.

Regan made a number of recommendations to prevent such incidents from occurring again, including ensuring that underground HV networks are isolated from surface HV overhead lines to avoid a risk of interference from wildlife and fallen branches.

He said effective surge diverters and earthing should be provided to help prevent transients transferring to underground cable networks.

Additionally, electrical protection on IT network HV supplies should be set with the fastest clearance time possible, risk assessments needed to be carried out and mine emergency response plans should be in place.

Partial discharge testing should also be conducted when HV cable networks are commissioned to assist in assessing the quality of the installation and to establish a baseline figure for future analysis.