On Fri, 28 Jun 2013 07:28:22 -0400, "John Grabowski"
wrote:


Now the question is, did that GFCI preclude the circuit breaker's
ability
to do its job?
No. It was NOT a short. In most cases it is a loose, intermittenr, or
high resistance connection that causes a fire - not a short.

In order for this to happen there MUST be a load of some sort on the
circuit. A normal breaker will not respond to an "arc fault" of this
type, and nor will a CFCI - an "arc fault detector" breaker is
designed to trip under those conditions. Not sure how effective they
are in real life - or if they are more likely to false trigger.

I can add some comments about your last point from experience over the
last
12 years. I've had circuit-breaker AFCIs on several house circuits
during
that time and have had maybe 4-6 nuisance trips overall. A couple
happened
when we switched ceiling fan speeds using the fan's wall-box control.
The
others happened unexpectedly for no obvious reason.

Frankly, I expected more nuisance trips based upon what I had been
hearing
and reading when the installation was new since, at that time, I was
working
with one of the NEC code panels and there was a lot of discussion about
it.
As I understand it, AFCIs react to an electrical arc by sensing the radio
frequencies that the arc generates. But opening/closing a switch can
generate such an arc (usually very small) and the same thing can happen
when
an incandescent lamp filament fails or electric motors with brushes or
inertia switches power up.

Tomsic

Or the brushes "bounce" on ANY brush motor. I've heard of problems
running vacuum cleaners, older sewing machines, handheld mixers, and
food processors, among other devices. Anything that can produce
multiple sparks. Even unplugging a lamp while it is turned on - if
you don't pull the plug quickly - can trip an AFCI

All are certainly possibilities. So, there's some way in the AFCI sensing
circuit to adjust the level of RF that trips the AFCI. I wonder if there
are any AFCI products out there where that adjustment can be changed
within a range to maintain a level of protection while minimizing nuisance
tripping on noisy systems.


*About a year ago a customer called me about a circuit breaker that kept
tripping when they used their treadmill exercise machine. The breaker was
an arc fault type and the problem only occurred when the machine was shut
off . Long story short, I found that by using an extension cord to power
the treadmill from the same receptacle and circuit, the problem stopped.
A 10 foot extention cord coiled up beside the treadmill makes a
pretty good choke.