While the mine was not disclosed, the inspectorate said the LHD was operating in an Explosion Risk Zone 1, where methane levels of 0.5-2% could be encountered.

The fire was quickly put out but temperatures were estimated to have reached as high as 200C.

The inspectorate said the fire was caused when the hydraulic pump and relief vale overheated because they were operating in a closed hydraulic circuit.

This heat set fire to a piece of engine cover insulation foam beside the pump.

The inspectorate said the pump also had oil-soaked coal packed beneath it.

For about 45 minutes before the fire the LHD was manoeuvring a pipe trailer.

The fire occurred when the LHD was idling and the operator was disconnecting the Quick Disconnect System and Power Take-Off hydraulic hoses so the pipe trailer could be operated.

“Investigations revealed that a chain fashioned from multiple safety clips was being used to hold the self centring QDS/PTO hydraulic lever in the engaged position,” the inspectorate said.

“This had the effect of supplying constant hydraulic pressure and flow to the QDS/PTO circuit.

“When the hoses were disconnected from the pipe trailer, valves in the circuit automatically closed.

“A closed circuit was created, leaving the pump operating against a closed valve, effectively ‘dead heading’ the pump. The only path for the oil back to the tank was over the 138 bar relief valve.

“The loader had been operated in this condition for the entire time it was manoeuvring the pipe trailer.

“Subsequent testing of the circuit in this configuration found a rapid and significant temperature rise at both the pump and relief valve. Self centring hydraulic controls are safety devices and must not be overridden.

“Most hydraulic circuits can overheat if the system is subjected to prolonged dead heading or stall conditions. Overheating of circuits and components such as hoses, pumps and valves can lead to early failure of components and injury to personnel.”

The inspectorate recommended that underground minesites consider hydraulic temperature monitoring for all machines where a risk assessment identifies the possibility to overheat a circuit and its components to above 150C.

“The monitoring system should activate the machine’s shut down system where the requirements of the Coal Mining Safety and Health Regulation 2001, Section 152 ‘Limit to external surface temperature of equipment used underground’ are not met.”

Mine operators were also recommended to develop a training program for operators on the hazards caused by overriding safety devices such as self centring controls for hydraulics.