On Mon, 03 Oct 2011 15:30:44 -0500, "Lloyd E. Sponenburgh"
lloydspinsidemindspring.com wrote:

Ignoramus19762 fired this volley in
m:

Right, but why does the arc flash in the first place?

All electricity needs is a low-enough resistance path to start flowing,
and as it raises the temperature of its surrounds (say, by fusing the
metal grinding dust that started the connection), the air itself becomes
a conductor.

A "spark" is nothing more than the voltage overcoming the insulation
resistance of the air; when it begins to flow through the air, it heats
the gasses to plasma temps, and that's what you see as the "spark". If
something like a filament of lathe turning can assist in starting that
current flowing, all the easier for a big, fat arc to form.

Such an arc can form from having foreign material between the leads (like
your 3ph box), or it can form from having two high-voltage leads too
close together. Excess humidity shortens the required gap.

The "classic" man-killer short happens when an inattentive technician
creates a dead-short with a tool, then the tool AND the air around it
vaporize into plasma.

LLoyd

Plus a whole lot of copper vapor. Blown switchgear at my old company
has vaporized pounds of copper. This was more often 2300V than 480V,
but either one will blow the doors way off. We had to watch movies of
phase to phase faults in our training to work in energized enclosures.
I was approved solely due to the large number of VFD's I had and
needed to program in 480V enclosures. Contrary to normal plant
practice, I used low voltage control circuits. I didn't want live 120
V. stuff behind me on the door to the enclosure when I was inside
programming stuff.

Highest voltage I ever personally switched was 15000 V. Hated that
****.

And I never had to deal with grinding dust in my stuff. Water once, a
lot of it, but no conductive dust. Yuk. By the way, I was a chemist,
not an electrician. That's R&D for you.

Pete Keillor