WorkSafe Tasmania

WorkSafe Tasmania

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Notify WorkSafe Call 1300 366 322

Moulded case circuit breaker: Arc flash incident

An electrician and safety observer at a mineral processing plant were subjected to a large arc flash and blast when conducting a ‘test for dead’ verification before replacing a damaged contactor and overload fuse in a 415V three-phase pump circuit.

Fortunately, the workers were not seriously injured. The electrician sustained minor burns to his hand.

This image shows damage from the arc flash:

damage thru arc flash

Contributing factors

An investigation by WorkSafe Tasmania mines inspectors revealed that the following chain of events had occurred:

  • the circuit that was being repaired was feeding a 415V three-phase pump
  • the pump bearing at the non-drive end had failed, resulting in mechanical damage to the insulation, which caused the motor to suddenly ‘pole’
  • this caused a high fault current to flow, which severely damaged the circuit breaker, contactor and overload fuse
  • the arc flash and blast occurred while the electrician was testing the outlet of the damaged circuit breaker, with the circuit breaker switched off. It was most probably caused by pressure from the tester causing a short circuit between the phases inside the damaged circuit breaker
  • the use of appropriate arc-flash protective clothing, including a face shield and gloves, was not being enforced when the incident occurred.

This image shows the failed bearing in the motor:

failed bearing motor

Recommendations

  • Where electrical components have been subjected to high fault currents, de-energise the electrical installation (where possible) and consider the risk of a short circuit developing inside the damaged components.
  • Enforce the use of appropriate arc flash protective clothing when there is a risk of arc flash.
  • Consider using a thermal imaging camera scan of all motor control centre and distribution board components before starting fault finding and testing.
  • Consider that an arc flash incident may result in electrical power to the area being cut off. Ensure that electricians are aware of exit routes and have emergency equipment ready before starting the task.
Updated: 4th April 2022
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