Ignoramus29750 wrote:
On 2011-10-04, David wrote:
writes:

On 2011-10-03, David wrote:

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?


At startup, the usual reason is someone screwed up; and there
is a shorted cable, connection or...

In a case I know, it was a GE ElectoCenter; a housetrailer-size
structure filled with starters. They get shipped in& installed
onto a slab with a crane. Fed with 4160 through the roof; there
was a main breaker, and 3 busbars& gnd running along the wall
behind the main& starter cabinets. It also had a small 240/120
tranformer to power lights/tools/instrumentation.

A friend was working with a contractor on the installation.
With no starters racked in, he closed the main for the first
time so they'd have local power. Little did they know the
busbars had slid down inside the wall& were touching.

The breaker exploded, blowing the locked door open, knocking
Bill onto his side, with 2nd degree burns to his arms and hands.
Behind Bill, the contractor was also down.

The arcing continued, with the feeds down from the pole to the
building finally burning loose, and flapping in the wind -- the
arc would blow them apart [right hand rule] the arc would stop,
and they'd drift togther again .ZAAAP... ZAAAP...

Bill was trying to get up when the contractor ran OVER him, foot
right on his back, going for the utility substation to pull the
primary disconnect. Before he got there, the utility fuses went
"sounding like a 12 ga. going off in my ear BLAM BLAM".

He was in the hospital for 4-5 days.




Sounds like GE did a bad job on this Electrocenter.

Sounds dubious that the busbars "slid down". Busbars
have to be bolted solid in numerous locations to withstand
the forces generated by the magnetic fields created
from a short circuit. More likely would be a piece of
wire or other debris fell across them.