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 Post subject: Calculating cal/cm2Posted: Sun Mar 06, 2011 3:46 am

Joined: Sun Mar 06, 2011 3:12 am
Posts: 3
Upon turon OFF there is a one nano farad capacitor that stays charged with 8000 volts until it slowly self discharges.
my question is how to calculate and what should be the cal/cm2 value for such case as long as the capacitor is charged?

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 Post subject: Posted: Sun Mar 06, 2011 2:22 pm

Joined: Mon Jan 26, 2009 5:00 pm
Posts: 46
Location: Midwest
Upon turon OFF there is a one nano farad capacitor that stays charged with 8000 volts until it slowly self discharges.
my question is how to calculate and what should be the cal/cm2 value for such case as long as the capacitor is charged?

We are not talking about a lot of energy here. Unless I've missed a few zeros, it sounds like fractions of a calorie, even over 2 seconds.

The shock hazard may be the more serious concern here.

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 Post subject: Posted: Mon Mar 07, 2011 6:38 am

Joined: Sun Mar 06, 2011 3:12 am
Posts: 3
you are absulutly right. the energy stored in the capacitor is a fraction of a calorie.
yet, i am asked to calculate the cal/cm2 and I do not know how to approach it.

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 Post subject: Posted: Mon Mar 07, 2011 1:04 pm
 Arc Level

Joined: Tue Jan 13, 2009 5:00 pm
Posts: 579
In free air, the incident energy at a distance of r would be your stored energy over the surface area of a sphere of radius r.

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 Post subject: Posted: Tue Mar 08, 2011 6:17 am

Joined: Sun Mar 06, 2011 3:12 am
Posts: 3
thank you. this is clear.
just to make sure - for 1nF capacitor charged at 8000V the stored energy is 1/2CV^2 = 1/2 * 1nF * 8000^2 = 0.032 Joule = 0.0076 cal.
The only question left is what should I use as r in my case to get the arc flash value?

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 Post subject: Posted: Tue Mar 08, 2011 7:18 am
 Plasma Level

Joined: Wed May 07, 2008 5:00 pm
Posts: 863
Location: Rutland, VT
I believe the r is arbitary. How far do you want to be? 1 inch, 20 ft, 2 miles, etc. It is what you decide to be the distance of interest.

_________________
Barry Donovan, P.E.
www.workplacesafetysolutions.com

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 Post subject: Posted: Tue Mar 08, 2011 10:31 am
 Arc Level

Joined: Tue Jan 13, 2009 5:00 pm
Posts: 579
r would be your distance to the worker, usually determined by voltage level and working rules (rubber glove or hot stick).

Perhaps another approach would be to turn it around, and determine what r will result in 1.2 cal/cm^2.

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 Post subject: Posted: Tue Mar 08, 2011 11:02 am
 Arc Level

Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
thank you. this is clear.
just to make sure - for 1nF capacitor charged at 8000V the stored energy is 1/2CV^2 = 1/2 * 1nF * 8000^2 = 0.032 Joule = 0.0076 cal.
The only question left is what should I use as r in my case to get the arc flash value?

r is distance from arc to worker. Area is 4·pi·r², so your IE is negligible for any distance. For 1 cm, IE = 0.0076/(4·pi·1²) = 0.0006 cal/cm².

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