A mine worker received a minor shock when he touched a flameproof fluorescent light fitting mounted on a continuous miner, which was working in the mine’s hazardous zone.
The shock hazard, as well as a methane ignition risk, arose because of the combination of three factors.
The light had unearthed external metal despite the fitting case being plastic; three brass threaded inserts embedded in the boss assembly enable cable glands to be fitted. But these metal inserts are not connected to earth, and any metal gland screwed in may not be earthed.
The ingress protection failed as a result of a missing ‘O’ ring between the boss and nut assembly of the light fitting. As the light fitting was mounted vertically water was able to enter the fitting and accumulate in the light.
Investigators said the ‘O’ ring may become dislodged during re-assembly of the light fitting after maintenance. Also, the outer case collar of the light and the poly-carbonate case was cracked.
The water in the light fitting caused a fault current to flow to the earth screen of its supply cable, imposing a touch voltage onto the unearthed external cable gland.
Thirdly, the fault current was below the magnitude necessary to cause the electrical protection to operate. The circuit protection is a 10-ampere fuse and a toroidal earth-leakage relay set to trip at 500 milliamps, within 250 milliseconds. The light fitting continued to operate with the water present.
The DMR recommended that manufacturers of lights and continuous miners review the safety risks from their products. They were also told to stipulate earthing, protection, maintenance and inspection requirements.
Mines were advised to ensure maintenance workers were properly trained to prevent mistaken assembly; identify cracks in the flameproof assembly; and ensure adequate earthing.
“All mines should review the inspection, testing and maintenance of all electrical equipment to include the checking of the integrity of earthing of exposed metal parts and earth continuity testing,” the department said.
