Following the events the NSW Department of Primary Industries recommended mines immediately inspect equipment in service to confirm integrity of earth connections.

In the first incident a power supply to an electrically powered hydraulic pump tripped on earth leakage. An operator found the handle of a jacking screw on a restrained coupler receptacle ‘tack welded’ to steel rib support mesh.

The DPI investigation found the scraping earth connection between the plug earth sleeve and receptacle body was not continuously effective.

In the second incident, power supply to a continuous miner tripped on earth leakage and a miner received a severe electric shock. The worker was resting his left forearm on the rear of the miner and grasping a sheet of rib support mesh with his (gloved) right hand.

The investigation found a trailing cable plug had an ineffective earth connection inside the plug body between the earth sleeve and the earth attachment ring mounted on the moulded interior. Earth leakage trip times had an intentional and unauthorised delay. The earth continuity relay was set for pilot control but the external circuit was not wired for pilot control.

The DPI made the following recommendations:

Mines should immediately inspect equipment in service to confirm integrity of earth connections;

Mines must ensure that tripping times for earth leakage relays and earth continuity relays are set to minimum values to achieve adequate prospective touch voltage/operating time characteristics;

Routine testing and recording of earth continuity must be carried out by competent people at a frequency sufficient to ensure that:

a. The earthing resistance is in accordance with the protective requirements and continuously effective, and

b. Earth fault currents and earth leakage currents will be carried without danger to persons from electric shock, effects of arcing or fire;

Manufacturers should immediately investigate the effectiveness of earthing arrangements used on plug and receptacle products and arrange for the provision of adequate information to recipients of the product, to ensure safe use. In particular, information on tests and inspections to assess ongoing compliance with AS1299; and

Cable Repair Workshops and Approved Workshops receiving receptacles and cables of these types for repair should apply information from the product manufacturers to assess the compliance of product and mechanical integrity of scraping and internal earth connections.