Excellent article on Arc Flash

Thanks..its an excellent article indeed, on issues I had grossly underestimated...along with various other factors, paralleling utility transformers and interrupt ratings...very valuable. In todays litigious society a must read if for no other reason than to include the proper cya provisions in ones contracts.

Phil Scott

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Good read. I never thought about the fact that a slower interruption due to a lower fault current (because of there not be an idealized impedance) can actually result in a greater total energy dissipation at the point of arc. But this makes sense. The performance curves of breakers are not nice straight lines.

I have heard of power companies just adding more transformers to network service areas in many cities to handle expanded loads. But that sure would be raising the available fault current for everyone.

yes it is. I sent it to a number of engineers I work with.

There is a statement about a 10,000 Amp arc in a 480 volt circuit being 8 megawatts and the equivalent of 8 sticks of dynamite.

1) I don't "get the math." 2) I think I would prefer being at certain distance from an arc than "8 sticks of dynamite."

The piece should have been edited a few more times.

Why, for example, does the "energy/time" chart use a non-linear axis for time?

My guess would be since protective device clearing times are often dealt with on logarithmic scales?

Charles Perry P.E.

Maybe.

But something that was LINEAR was made to appear non-linear by the choice of scale.

Some years ago (maybe 20) someone wrote a book about good and bad charts.

The BEST chart the writer reported was one showing the shrinkage of Napoleon's Army on the advance to and retreat from Moscow. A lot of understandable information on a 1/2 page.

The WORSE came from someone in the EDUCATION journal. The information content of a near incomprehensible graph was a SINGLE NUMBER!

circuit being 8

Thats 5 million watts approx. or 48,000 100 watt light bulbs... the depending of the duration of the arc achieve the same mount of force in the 8 sticks of persuasion. But I see your point.

there are distribution issues as well... the arc is more concentrated in its effects... the 8 sticks more diffuse. Id take the arc as well in preferance to the 8 sticks.

one time when I was even dumber than I am today, I opted to change a 600 amp breaker in a 3,000 amp 480vac panel live... I was real careful...I taped up the extension on my ratchet wrench...stood on a rubber mat, and put my left hand in my ass pocket.

it was about a 16" reach to the back buss bar bolting, very little space...but I had a steady hand. I had the damn thing out and the new one going back in, when for some reason that I still cant fathom the socket and half of my 1/2" snap on extension was vaporized by the flash.

Being utterly insane in those days, i sauntered out to the truck and got another extension..was real careful this time to wrap it in TWO layers of chinese electrical tape... and finished the bolt up.

I was in a closed electrical room, and that room was in a closed supply room... and people came running from the offices beyond saying they saw a huge flash coming from under the doors... these were tight fitting doors... it might have been a fire ball going to ground spreading across the floor? , I didnt notice one though. The farhest door was 40' away or more.

an arc than "8

non-linear axis for

Because the arc surge is non linear based on conductive plasma formation? or capacitance in the feeders and transformers, blowing off at the initial surge? or the gound energizing reducing the voltage potential to ground or whatever... probably a combination of these factors.

The article did address the fastly increased hazards of a slow acting fault protection (say a second or so) vs a fast acting fault protective device at say a tenth of a second or less.

Phil Scott

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non-linear axis for

are often dealt

by the choice of

That assumes an arc is linear... its probably not, when you get into the micro second phase of the arc...it only appears linear in the larger context of the complete event... for instance an arc through clean air will have an initial resistance of say 'x ohms' but then with the metal plasma that instantly develops the resistance approaches zero ...increasing the current flow exponentially as the plasma field developed and became larger and more dense.

and bad charts.

shrinkage of

A lot of

We are going though that now... the invader is over confident in the final analysis.

information

NUMBER!

PBS had a grade school level science show, explaining electricity.. it showed how to splice a wire.... two insulated wires were tied in a knot, and it was explained that the electricity flowed outside of the wire and in the insulation... thus the splice done in that fashion. Rush Limbaugh was explaining one time how you got as much radiation from being out in the sun as by breathing uranium dust. he got a fax shortly there after... he was referencing the US dept of Energies bogus spin trying to obfuscate its internal contamination problems (radiation on the affected cells 24/7 forever) by equating those to transient solar load external source radiation.

Phil Scott