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 Post subject: Arc Flash calculations
PostPosted: Wed Mar 23, 2016 6:29 am 
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Joined: Fri Nov 21, 2014 10:14 am
Posts: 15
Location: Fot McMurray, Alberta, Canada
For arc flash calculation purposes in a Main Contactor which value should I use for drop out time: the Opening Time or the Arcing time.

In this case I am working with a Toshiba HCV-6KAU and the Opening time is 25ms and the Arcing time is 10ms or less.

thx,


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 Post subject: Re: Arc Flash calculations
PostPosted: Wed Mar 23, 2016 12:46 pm 
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Joined: Tue Oct 26, 2010 9:08 am
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Location: North Carolina
Opening time.

In a vacuum interrupter the contacts open and an arc starts. Then at the next zero current crossing the arc quenches but won't restrike due to the insulation property of the hard vacuum. In air/SF6/oil breakers there are often mechanisms that physically stretch and/or cool the arc to the point where it is forcibly quenched via arc chutes, puffer mechanisms, melting/energy absorption of sand/boric acid (in fuses), or similar "quench" devices. Vacuum bottles rely on a hard vacuum to avoid restrike rather than to quench the arc.

Toshiba was really the first good quality vacuum bottle manufacturer on the market that began producing reliable interrupters and first made medium voltage become the interrupter of choice for medium voltage up to 40 kV, displacing air, oil, and SF6 for the most part due to low cost and reliable operation.


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 Post subject: Re: Arc Flash calculations
PostPosted: Thu Mar 24, 2016 7:53 am 
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Location: Fot McMurray, Alberta, Canada
Thanks so much for your answer Paul


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 Post subject: Re: Arc Flash calculations
PostPosted: Thu Mar 24, 2016 9:37 am 
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I usually use 50 ms when in doubt. Most vacuum breakers have an opening time of around 3 cycles and you can figure 1 cycle for the trip relay to function if it's a trip function that is not intentionally time delayed such as 50 or 87 relaying. In practice I've just used 50 ms based on recommendations by others.

Only thing that sounds odd about that Toshiba specification is that it takes 1/2 cycle maximum to reach a zero crossing and quench the arc or 8 ms. It sounds like Toshiba's specification is quoting a full cycle of arcing or 16.667 ms. Only reason I can see it taking longer is if there is something going on to clear the fault "softly" as opposed to the "hard" trip that VCB's are known for. Might not hurt to call Houston to find out why it takes 15 ms opening time. They are typically very helpful every time I've called them.


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