Failed or Absent Defences » LTA isolation hardware » LTA maintenance

28Transformer Primary Protection Fails To Isolate Fault

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Brief Description of Incident/Accident & Description of Consequences/Outcomes:

A plant operator received a severe electric shock as he was working to replace the lid on a cable pit. The cable pit housed consumers mains running from the supply transformer to the main switch-room, some 56m away. There was no protection installed on the transformer secondary with 25A fuses installed on the primary. After removing the top 150mm sand cover the operator continued to shovel down into the pit and at a depth of 300mm the sand was found to be very hot and a burning smell was noticed. The operator tried to exit the pit and as the steel shovel handle was placed onto the steel rim of the pit the shock was received.

Identified Root Causes:

The potential across the shock path was measured at 101volts. Cables installed in the pit had most likely been damaged during installation. Earth fault loop impedance was not low enough to ensure the protective devices operated. An earth fault had been repaired in the pit two weeks prior after employees reported seeing boiling water on the ground and sensing a burning smell. In both instances the fuses did not interrupt supply.

Energy Type(s) Involved:

Indirect contact with electricity

Equipment Type(s):

Transformer: 1MVA, 33kV/415V; Fuses: 25A; Cable: Single core, 185mm2, XLPE

Root & Contributing Cause(s):

LTA isolation

LTA maintenance

LTA risk assessment

Stated or Potential Consequence(s):

Potential Fatality

Preventative/Recommended/Accepted Steps of Risk Mitigation, Points of Interest:

1. Provide protection systems on electrical installations which reliably interrupt supply under all fault conditions. In the absence of earth fault loop impedance testing, calculation or other effective controls to protect personnel against indirect contact; install effective earth leakage protection with appropriate protection settings.

2. Develop, document, implement monitor and review a system of electrical installation and commissioning.

3. Provide an emergency/operational supply isolation point for an electrical installation in use at a mine.

4. Develop, document, implement monitor and review all procedures for emergency response to electric shocks or incidents involving electricity which recognises all the risks including monitoring of personnel who receive shocks.


Queensland Government Department of Employment, Economic Development and Innovation (DEEDI)


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Reference Type:

Safety Alert #109

Mine Type:

Any Mine Type







Australia, Queensland

ICAM Categories

Organisational Factors

• LTA design

• LTA isolation

• LTA risk assessment

• LTA work method

Task or Environmental Conditions


Individual or Team Actions

• LTA isolation

Failed or Absent Defences

• LTA isolation hardware

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