The seat of the fire was on and around the main hydraulic pump, which had oil-soaked coal packed underneath.

A piece of engine-cover insulation foam beside the pump ignited, creating a small fire in a zone where methane levels of 0.5-2% might be encountered.

The loader was being used to provide hydraulic power to a pipe trailer, although for 45 minutes prior to the fire it had simply been manoeuvring the pipe trailer into a different position.

At the time of the fire the loader was idling and the operator was in the process of connecting the quick-detach power take-off (QDS/PTO) hydraulic hoses so the pipe trailer could again be operated.

Within five minutes of the fire being extinguished, temperatures as high as 200C were still being observed.

Investigations by the miens inspectorate 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.

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 was 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-centering hydraulic controls are safety devices and must not be overridden,” central safety and health manager Chris Skelding said.

“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. In this incident, it resulted in a fire in an explosion-risk zone in an underground coal mine, which may have had catastrophic consequences.”

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

“It is also recommended that the mine develop a training program for equipment operators on the hazards created when safety devices such as self-centring controls for hydraulics are overridden and incorporate training on hydraulic hazards in operator-training packages,” Skelding said.