Short-circuit fault causes fire and equipment damage

The West Australian Department of Mines and Petroleum has reported a significant incident in which a short-circuit fault in a variable speed drive resulted in fire, arcing and equipment damage.

In an unidentified mine, four 1000 kW drive motors were being used to drive an overland conveyor. A 5.5 MVA transformer powered a 690 V motor control centre (MCC), which fed three variable speed drives (VSDs), each controlling a drive motor. A fourth motor was controlled separately.

A short-circuit fault in one variable speed drive developed into a line-to-ground fault. The resulting thermal stress caused a fire and arc flash within the variable speed drive. The electrical installation - including cables, the 690 V motor control centre and three variable speed drives - was severely damaged. Electromechanical forces generated by the short-circuit fault ripped the cables from the cable ladders. Fortunately, no-one was in the vicinity.

Simon Ridge, state mining engineer, identified a number of direct and indirect probable causes for the incident.

Direct:

• The variable speed drives were designed for a distribution system with a direct earth connection, at the neutral point of the supply transformer. The earth connection was separated from the neutral point and, therefore, the protection failed to operate.
• The designer failed to identify hazards associated with site earthing and protection on the engineering drawings and documentation.

Indirect:

• Testing of electrical equipment was only carried out at factory level. Site earthing and protection verification was not carried out.
• Site procedures and competencies were not adequate to ensure safe electrical installation.
• The cables were not secured to the cable ladder using clamps rated for potential short circuit.

Electrical installations at a mine site must comply with Part 5 of the Mines Safety and Inspection Regulations 1995. In particular, regulation 5.3 requires electrical installations and equipment at a mine to be designed, installed and tested in accordance with Australian Standard AS/NZS 3000 Electrical installations (known as the Australian/New Zealand Wiring Rules).

Ridge recommended a number of actions the mine take to achieve compliance:

• Design electrical installations to meet fundamental electrical safety principles, including protection against overcurrent (Wiring Rules clause 1.5.9) and earth fault current (clause 1.5.10).
• Provide engineering drawings and documentation with sufficient information to allow electrical equipment to be installed and maintained in a safe manner.
• Verify the installation, as far as practicable, prior to it being placed in service or use (clause 1.8).
• Develop, implement and maintain competency-based training systems so workers are aware of the critical tasks involved with verification of the electrical installation.

Under regulations 6.3, 6.4 and 6.5 of the Mines Safety and Inspection Regulations 1995, a designer is required to:

• Identify hazards associated with plant and assess the risks.
• Consider whether the risk of exposure can be reduced.
• Provide sufficient information for the plant to be manufactured in accordance with the design specifications.
• Provide sufficient information relating to installation, operation and maintenance of the plant.