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What happened?
An arc flash occurred during de-isolation switching in a 690V switchboard for a hydraulic starter module.
The arc flash was inside the lower area of the tier displacing the vent panel.
The initial pre-trip current was approx. 500 amps and increased to approx. 17,000 amps during flash across the supply side of the fused isolator.
The nearest person was in immediate proximity to the module.
The arc did not make contact with any personnel.
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Why did it happen?
A phenomenon known as ‘tin whiskers’ contributed to the arc flash.
Thin strands or whiskers grow from bus bars over time, effectively reducing the air gap between bus bars, providing an arc pathway.
Tin whiskers are electrically conductive, crystalline structures of tin that sometimes grow from surfaces where tin (especially electroplated tin) is used as a final finish.
Tin whiskers have been observed to grow to lengths of several millimetres (mm) and in rare instances to lengths in excess of 10 mm (0.4 inches). However, tin whiskers are generally reported to be less than 50 μm (0.002 inches) long.
Module bus bar clearances were found to meet the minimum standard (>8mm or >0.31 inches). However, the standard does not account for the possibility of ‘tin whiskers’.
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What did they learn?
Prior to de-isolation, conduct 1000vDC insulation resistance testing for insulation resistance.
Prior to de-isolation, clean bus bars to remove any ‘tin whisker’ growth.
Change-out high-risk modules with fully insulated bus bars.
Update design specification to increase minimum air gap to account for ‘tin whisker’ risk.
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Ask yourself or your crew
What checks do you conduct during de-isolation of low voltage (LV) electrical equipment?
Have you identified and reported any ‘tin whisker’ growth?
Do you clean isolated bus bars to remove any ‘tin whisker’ growth?
Does the design of your LV electrical equipment adequately account for ‘tin whisker’ risk?
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What happened?
An arc flash occurred during de-isolation switching in a 690V switchboard for a hydraulic starter module.
The arc flash was inside the lower area of the tier displacing the vent panel.
The initial pre-trip current was approx. 500 amps and increased to approx. 17,000 amps during flash across the supply side of the fused isolator.
The nearest person was in immediate proximity to the module.
The arc did not make contact with any personnel.
Why did it happen?
A phenomenon known as ‘tin whiskers’ contributed to the arc flash.
Thin strands or whiskers grow from bus bars over time, effectively reducing the air gap between bus bars, providing an arc pathway.
Tin whiskers are electrically conductive, crystalline structures of tin that sometimes grow from surfaces where tin (especially electroplated tin) is used as a final finish.
Tin whiskers have been observed to grow to lengths of several millimetres (mm) and in rare instances to lengths in excess of 10 mm (0.4 inches). However, tin whiskers are generally reported to be less than 50 μm (0.002 inches) long.
Module bus bar clearances were found to meet the minimum standard (>8mm or >0.31 inches). However, the standard does not account for the possibility of ‘tin whiskers’.
What did they learn?
Prior to de-isolation, conduct 1000vDC insulation resistance testing for insulation resistance.
Prior to de-isolation, clean bus bars to remove any ‘tin whisker’ growth.
Change-out high-risk modules with fully insulated bus bars.
Update design specification to increase minimum air gap to account for ‘tin whisker’ risk.
Ask yourself or your crew
What checks do you conduct during de-isolation of low voltage (LV) electrical equipment?
Have you identified and reported any ‘tin whisker’ growth?
Do you clean isolated bus bars to remove any ‘tin whisker’ growth?
Does the design of your LV electrical equipment adequately account for ‘tin whisker’ risk?
An arc flash occurred during de-isolation switching in a 690V switchboard, causing a significant current surge but fortunately no injuries. The incident was attributed to 'tin whiskers,' which are conductive strands that grow from bus bars, reducing the air gap and creating a pathway for the arc.
Original material courtesy of Safer Together (Australia)
To access the PDF and PowerPoint versions, please visit https://www.safertogether.com.au/resources/sharing-library/working-with-electrical-equipment-contact-with-live-electricity-leb-toolbox
