Underground workers burned by arc flash

Two workers have been injured in an arc flash and blast 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 WA Department of Mines and Petroleum reports.

During their shift they came 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.

The worker then replaced the control relay for the main contactor and reset the control circuit breaker.

However this worker then failed to properly secure 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 mechanical fitter also received a minor face injury.

According to a subsequent DMP investigation 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.

The DMP also found that the circuit was not designed to automatically disconnect the main breaker under an earth leakage or earth fault, and that the protection setting were not co-ordinated correctly, which caused the upstream breaker connected to the 415 V motor control centre to fail to operate.

“The high voltage circuit breaker connected on the primary side of the 750 kVA transformer cleared the arc fault current of about 9.5kA in more than one second,” the DMP stated.

It also found that the drawings did not match the actual electrical installation, “specifically the trip circuit of the motor overload protection had been bypassed, allowing the pump to operate under overload conditions”.

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