Next message: Randy Pickelmann: "T&T: Hot shore power connections"
There are two types of what are commonly referred to as short circuits: the
b bolted fault" and the b arcing fault." An example of a bolted fault would
be where a hot and neutral are mistakenly connected together; these are
usually wiring errors and are detected and "cleared" by circuit breakers when
the
circuit is energized. The destruction which can be caused by a bolted fault
in
the few microseconds it takes the breaker to clear is impressive in the case
of a typical residence service entrance. Do the same when mis-wiring a wall
outlet and the breaker merely trips.
Worse are arcing faults, which can occur for a prolonged period without
drawing enough current to "trip" the circuit breaker. There are 2 types of
arcing
faults. The current flowing in the b series arcing fault,b such as in a
conductor connected to a load which has frayed to the point where it breaks
and
pulls apart enough so an arc occurs, is current-limited by the load it is
connected to. Consequently, these do not generally create much heat.
Parallel arcing faults, which may occur between conductors (a b short
circuit
arcing faultb ) or between a current-carrying conductor and ground (a b
ground arcing faultb ) are limited only by the impedance of the system, that
of the
fault, and the rating of the circuit breaker protecting the circuit. But a
typical 15 amp branch breaker will likely not trip before a parallel arc
becomes hot enough to start a fire.
Most of us are familiar with GFCI breakers. Those interested in protecting
against arcing faults as well as shock protection offered by GFCIs may wish
to
investigate the relatively new AFCI breakers which protect against arcing
faults.
Disclaimer: I am not an electrical engineer; I have been fortunate to have
attended some seminars which addressed AC power distribution systems.
Regards,
John
b Seahorseb
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