Earth fault current limiter parameters

Mine Type

Underground coal mines

Incident

Recently at an underground coal mine a shuttle car ran over and damaged its own power supply cable, causing a short circuit between two live phase conductors and the earth conductor. The fault caused a power surge in the local distribution system, and at the same time two people observed a spark between the continuous miner machine working nearby and the rib mesh. Earth leakage protection tripped to both outlets at the distribution control board, therefore no electric shocks or other injuries were sustained.

Investigation

During the mine's incident investigation, the power factor of the Earth Fault Current Limiting Impedance was questioned. Such neutral-connected impedance systems, in which an impedance is placed between the neutral point of a transformer and earth, are used to limit earth fault currents. It was found that several precautions should be observed as some switching operations and some fault types can compromise the operation of neutral-connected impedance systems as earth fault current limiters.

In particular, the use of a neutral earthing reactor (as opposed to a resistor) may contribute to high inrush currents in some system configurations, particularly associated with clearing earth faults. Neutral earthing impedances are described in Australian Standard AS/NZS 2081.5:2002 Electrical equipment for coal and shale mines—Electrical protection devices Part 5: Earth-fault current limiters.

This standard requires that the peak value of inrush current in the event of a phase-to-earth fault under the worst conditions shall not exceed 12 times the current value under steady-state phase-to-earth fault conditions. The power factor of the earth-fault current limiting impedance, assuming zero cable capacitance, shall be not less than 0.65 lagging. This implies that the device requires a significant resistive component as part of its total impedance.

However, further testing by the mine and information from the manufacturer showed that the units do not meet this standard. What the investigation highlighted was the lack of compliance to the relevant standard AS/NZS 2081.5:2002, as well as some confusion in the industry as to the properties of this equipment.

Hazard

Aside from the potential electric shock hazard, this situation must also be recognised as a potential flammable gas ignition source when such equipment is in an explosion risk zone.

Recommendation

The mining community, including manufacturers and suppliers, must be aware of this issue, and the requirements of the Australian Standard. Possible hazards from low power factor neutral connected current limitation must be considered in the development of a coal mine's safety and health management system. Independent engineering advice may be required to confirm that risk is at an acceptable level.

All such neutral connected impedances in service should be checked for compliance with the relevant Australian Standard AS/NZS 2081.5:2002, which is referenced in the Department of Mines and Energy Recognised Standard 01, 'Underground electrical equipment and electrical installations'.

Authorised by Brian Lyne - Chief Inspector of Mines

Contact: minesafetyandhealth@dnrm.qld.gov.au

Issued by Queensland Department of Mines and Energy