According to the alert issued by the NSW Department of Primary Industries, both diesel engines suffered major internal damage at the unnamed longwall mine, but no one was injured in the incident.

The two turbo-charge Cat 3126 engine-LHDs were both operating together towing a longwall tailgate drive assembly out of a panel in a return heading.

The length of the train made it necessary to open two consecutive sets of ventilation double doors at the same time, allowing return air from the longwall to short-circuit over the explosion protected diesel LHDs.

“It is most likely that this ventilation disruption caused excess methane to be drawn from the goaf into the return air,” NSW DPI mine safety operations’ branch director Rob Regan stated in the alert.

He said both diesel engines then started revving at a high speed and out of control.

“One of the LHD operators activated the emergency shutdown system (strangler valve) on his vehicle after about 30 seconds, which shut the engine down as required.

“The second diesel engine shut down of its own accord after approximately 50 seconds of high revving.”

At this point in time, Regan said the train had travelled far enough to allow the inbye set of doors to be closed which returned the ventilation circuit back to normal.

Regan said monitoring data from nearby showed methane levels hitting 3% for four minutes.

He also noted the monitoring equipment was calibrated to read a maximum of 3%.

In the investigation of the incident, Regan said the air velocity across the longwall face probably increased which consequently drew a body of methane across the goaf.

The out of control behaviour of the LHDs was deemed to be the result of methane-charged air entering their fuel systems.

While both diesel engines were in an explosive protected condition, Regan said the poor state of the roadway added to the need for using two LHDs to tow the tail gate assembly.

He recommended the design and location of ventilation structures to allow safe passage of vehicles and equipment without damage or disruption to the ventilation circuit.

“In particular, adequate roadway dimensions and clearances around equipment and vehicles during passage must be maintained when installing such structures,” he said.

Maintaining high-standard roadway surfaces and ensuring all vehicles have gas detection on board was also recommended.

Vehicle operators were advised to understand the hazards of altering ventilation circuits and be trained to carry and use a methane detector, and to perform the emergency shutdown of a diesel engine.

Regan said the competencies of mechanical staff, to inspect, service and repair explosion-protected diesel equipment, should be reviewed.