Australian mine workers injured by arc flash
27 July 2015
Two workers were injured in an arc flash incident in a West Australian underground mine. The workers, an electrician and mechanical fitter, were investigating water overflow issues at the unnamed site when the incident occurred, according to a WA Department of Mines and Petroleum (DMP) report.
The workers came across across a 90kW submersible pump, supplied from a 415 V motor control centre, that had faulted. The electrician found that a control relay contact had fused in the closed position and a control circuit breaker had tripped, causing the fault.
He then replaced the control relay for the main contactor and reset the control circuit breaker, but without properly securing the pump control cubicle door properly after closing it.
When he switched the main circuit breaker on it caused an instant arc flash and blast, blowing the cubicle door open.The electrician received first degree burns to his neck, face, and ears, while the fitter also received a minor face injury.
The subsequent DMP investigation revealed that the main circuit breaker has suffered a phase-to-phase arcing fault, which started within the main circuit breaker terminal on the line side of the blue phase (which was shorted to earth), and then transferred to a three-phase fault.
Direct causes were:
*The pump supply cable blue phase was shorted to earth (as confirmed by an insulation
resistance test after the incident).
*The electrical circuit was not designed to automatically disconnect the main circuit breaker
under an earth leakage or earth fault.
*The protection settings were not coordinated correctly, therefore the upstream air circuit
breaker connected to the 415 V MCC busbars failed to operate. The high voltage circuit
breaker connected on the primary side of the 750 kVA transformer cleared the arc fault current
of about 9.5 kA in more than 1 second.
Contributory causes included the trip circuit of the motor overload protection being bypassed allowing the pump to operate under overload conditions, inadequate inspection and maintenance processes for testing the functionality of protective devices, and the workers not wearing adequate facial protection.