On Mon, 03 Oct 2011 13:40:23 -0500, Ignoramus19762
wrote:

On 2011-10-03, David Lesher wrote:
" writes:


My friend had a 440 motor or line short right in front of him in an
elevator motor room. He was temporarily blinded for a couple days. He
said it felt like someone poured sand in his eyes.

OSHA has gotten religion on the arc flash issue. The #1 chance
for such that will kill folks is when you close a large
main/sub-main breaker. They can and do explode outward with
enough force to blow apart cabinets, taking out the chump
standing there closing the breaker.

So, why exactly do they explode? Can you explain?

I should think shorts and overloads. F'rinstance, turning off a
breaker while all the machinery is still running would cause a large
arc, but when the contacts were closed again, the load would be
gigantic--with all of those motors starting again. Inductive startup
current is much greater than running current, as we all know, right?

Part of the danger is probably from all companies cutting corners to
provide exactly, no more, the required material to handle the rated
load. "Cheap" can be a bit more dangerous. Maybe Square D parts are
worth their extreme prices after all.

--
Worry is a misuse of imagination.
-- Dan Zadra