Flash is the result of a rapid release of energy
due to an arcing fault between a phase bus bar
and another phase bus bar, neutral or a ground.
During an arc fault the air is the conductor.
Arc faults are generally limited to systems
where the bus voltage is in excess of 120 volts.
Lower voltage levels normally will not sustain
an arc. An arc fault is similar to the arc
obtained during electric welding and the fault
has to be manually started by something creating
the path of conduction or a failure such as a
breakdown in insulation.
cause of the short normally burns away during
the initial flash and the arc fault is then
sustained by the establishment of a
highly-conductive plasma. The plasma will
conduct as much energy as is available and is
only limited by the impedance of the arc. This
massive energy discharge burns the bus bars,
vaporizing the copper and thus causing an
explosive volumetric increase, the arc blast,
conservatively estimated, as an expansion of
40,000 to 1. This fiery explosion devastates
everything in its path, creating deadly shrapnel
as it dissipates.
fault current is usually much less than the
available bolted fault current and below the
rating of circuit breakers. Unless these devices
have been selected to handle the arc fault
condition, they will not trip and the full force
of an arc flash will occur. The electrical
equation for energy is volts x current x time.
The transition from arc fault to arc flash takes
a finite time, increasing in intensity as the
pressure wave develops. The challenge is to
sense the arc fault current and shut off the
voltage in a timely manner before it develops
into a serious arc flash condition.