The electrician was burnt when super-heated gases vented around the edge of the enclosure door of a 415V motor control cell after an arc fault occurred inside.
According to the inspectorate’s safety alert, the isolator switch handle mechanism had been replaced with a similar unit from a nearby spare cubicle. The original metal plate forming part of this mechanism had a cut-out to ensure clearances were met in relation to the adjacent fuses.
The replacement mechanism did not have this cut-out in the metal plate and clearance distances were severely compromised, the inspectorate said.
When the isolator handle was turned to the “on” position, a short circuit between one phase and the metal plate resulted in an arcing fault that spread to adjacent phases.
The inspectorate found a circuit breaker on the high-voltage primary side of the transformer feeding this 415V cubicle was the first protection/tripping device upstream of the cubicle.
Electrical personnel had previously identified this as an area for improvement and were in the process of installing quick acting circuit breakers in the 415V circuit upstream of these cubicles.
Before energising the motor control cell, the electrician secured both door latches, which appeared to have helped reduce the external exposure from the arcing fault.
The inspectorate found other motor control cubicles’ clearance distances were compromised by the lack of required barriers and inconsistent cable terminations.
“An arc flash study of electrical installations had been undertaken at this site, however the recommendations were not yet implemented. The standard AS/NZS 3000-2007 Wiring Rules states that arc fault protection must be provided for all switchboards with a nominal supply rated above 800 amps and should be considered for all other switchboards,” the inspectorate said.
The following recommendations were issued:
Ensure creepage and clearance distances are adequate in all electrical cubicles to ensure safe operation of energised apparatus;
Ensure arc flash energy levels are calculated and appropriate measures put in place to limit the extent of arc flash events. This may include reviewing protection system types and tripping times, as well as providing special protective clothing for working in the vicinity of high fault energy apparatus; and
Ensure that electrical cubicles have all fasteners in place and that securing the enclosure doors is part of normal maintenance practices.
