ARC FLASH FROM "TIN WHISKERS"

T

TVH

Senior Member
Interesting Arc Flash info on a phenomenon known as "Tin Whiskers" on bus bars:


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Arc flash incident in 690V switchboard due to tin whiskers

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.
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Hoo-boy... I've heard of zinc whiskers causing problems with circuit boards when lead was removed from solder, but this is the first I've heard of them growing on bus bars. Something for me to shoot over to our reliability guy!!
 
Side note while working at a large candy plant years ago the room they made air puffed candy ( air bons ) from sugar went thru 120' long vacuum tunnels were they grew to over 5 /8". Machines were old and produced sugar dust during production. Starters were in conventional enclosures instead of dust tight. So much sugar would build up on starters the 480 volt would flash over luckily only taking out one or two fuses. Thus occurred at least once a year. Was on our list to install all of the starters in a large Hoffman panel but they scrapped it.
 
I have heard of silver whiskers, which is why silver plating on bus bars is never (supposed to be) used in places with H2S gas present, like WWTPs and refineries. I personally witnessed this at a geothermal power generation plant in the early 80s, when whiskers formed on silver contacts in circuit breakers that sat unused in an open condition and eventually flashed. They wanted to have all of the breakers replaced with those that had gold plating (talk about expensive!) but settled on installing H2S scrubbers for the electrical rooms. But I’ve never heard of “tin whiskers” before, in fact tin plating is the industry common alternative to silver for this very reason.

Reading through that article, and another from a NASA materials science investigation of an incident at a paper mill in Sweden (also would have had H2S present), it appears that these tin whiskers can grow to as much as 10mm, which is less than 1/2”. Bus spacing in 690V IEC gear can be as little as 20mm, which means two whiskers from each side can reach each other. Bus spacing here for ANSI 600V class gear is a minimum of a 1” (25.4mm) air gap, which to my mind may explain why we don’t hear much about it on ANSI spec equipment with tin plated bus. Good to know though so that we are aware of it.
 
Jraef said:
....when whiskers formed on silver contacts in circuit breakers that sat unused in an open condition and eventually flashed.
Click to expand...
All this makes me interested in prophylactically flipping breakers, every few years, as a routine service action. Just to keep
things lubed up and disrupt any weird growths....
 
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