On 10/03/2011 02:31 PM, Ignoramus19762 wrote:
On 2011-10-03, Lloyd E. Sponenburghlloydspinsidemindspring.com wrote:
fired this volley in
:

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

Rapid heating (to plasma temperatures) of the air. It gets heated to
several thousands of degrees in a few tens of microseconds. That releases
a physical shock wave (due to the expanding ball of super-heated gas -- if
you don't know them read the Gas Laws) equivalent to a small charge of
explosives.

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

i
OK, in fault conditions, there is a lot of current flowing. There is
inductance in the transformer, wiring, and possibly the load. At 480
and above, it doesn't take a huge inductance to make a substantial arc.
As the breaker is attempting to disconnect the load from the source, it
has to break the arc. This is done by separating the contacts while
the arc is flowing between them. As you separate them, you get a higher
voltage dropped across the arc. Amps times Voltage is power. The
breakers and transformers have some inductance built in to limit
fault currents, but they can develop toward 100 K Amps (that's what
those big 480 breakers are designed to handle one time when interrupting
a major fault.) Well, 100 K amps time 480 V is 48 Million Watts!
That is a HUGE amount of energy, and even if it only is produced for
microseconds, it will go bang. If something is wrong with the breaker,
the installation (to properly limit fault current) etc. then it can be
a major explosion if current flows for milliseconds.

Jon