"bud--" wrote in message
b.com...
On 7/1/2013 7:59 AM, Robert Green wrote:
wrote in message ...
As I understand it, AFCIs react to an electrical arc by sensing the
radio
frequencies that the arc generates.
...
Who would have thunk of monitoring the RF
emissions of an arc ...? As for the AFCI, it
makes sense because the wire leading to the arc must act as a fairly
efficient antenna. I always wondered how an AFCI could differentiate
between normal current draw and the creation of a dangerous arc. Does
that
mean you can't use an arc welder on an AFCI protected circuit?
AFCI look for current components that are higher than power line
frequencies. Wires are not antennas.
I'm not sure I understand the last sentence. Do you mean that the AFCI's
don't use the powerline to transmit the "noise" created by an arc to the
detector/signal processor in the AFCI? From what I've been reading at least
some part of the detection process is based on detecting the RF noise from
an arc. I also read that detecting an arc is difficult because it may not
be drawing detectibly larger amounts of current than a normal device might.
One of my CCTV cameras is badly shielded an acts as an arc detector.
Whenever I run a motor with brushes near it, the image starts to show random
blips across the screen. It's really noticeable when an old power drill is
operated anywhere near the camera. I've been assuming that noise is one
part of what an AFCI "looks" for in trying to determine if arcing is
present.
I'm asking because I've got three different items that appear to use the
home powerline as "antennas" although perhaps that's not the right term.
Each devices uses the powerlines in a slightly different way. X10 home
automation controllers inject a 120kHz RF signal onto the powerlines to
communicate commands. My Netgear Ethernet adapter sends networking signals
over the home powerlines with plug-in adapters (sometimes - it's not very
reliable):
http://www.netgear.com/home/products...line-and-coax/
I've seen numerous discussion refer to the HomePlug technology as
"transmitting" ethernet signals over the powerline. Is the powerline acting
as an antenna in those cases or is their a better terminology for what's
happening?
And it is not just the higher
frequencies. AFCIs look for arc "signatures" to separate normal arcs
from "bad" arcs. My guess is they use digital signal processing but I
don't know.
I would also guess that the AFCI's are looking for several parameters to
determine if there's an arc fault. I would suspect that sort of detection
has improved greatly since the first units hit the market.
This site (from 1999!) claims:
http://ecmweb.com/content/using-arc-...idential-fires
Enormous progress has been made recently by manufacturers of AFCIs and
engineers/scientists developing the standard in understanding the variety of
conditions an AFCI must respond to in order to be effective. A reflection of
that understanding is in the variety of tests and conditions under the three
categories of the draft standard. These address the majority of arcing
conditions known to lead to fire.
The inductance of a welder transformer might hide a welding arc.
Haven't been able to find information about how AFCIs and arc welders
interact but it seems most run off 240VAC and would have their own circuit,
probably without an AFCI.
As for AFCI's, my understanding (from way back when) is that they were
recommended for bedrooms and bathrooms where a high current device like
a
space heater or a hair dryer could create an arc big enough to start a
fire
very quickly. I can attest that a space heater plugged in only
partially
can create enough heat to melt plastic. DAMHIKT. (-:
The NEC originally required AFCIs only for bedrooms. Those AFCIs could
only detect "parallel" arcs - H-N & H-G. Arcs might have to be 60A to be
detected (series arcs would be 15 or 20A max). The idea, I believe, was
largely to detect arcs between wires in extension cords which were
walked on or abused in other ways
This site talks about the original bedroom requirements and says it's
because that's where many home electrical fires start:
http://www.peterspirito.com/afci_faq.htm#12
Why do the 1999, 2002, and 2005 versions of the NECŪ require AFCI
protection for only bedroom circuits?
NFPA fire statistics show that a high percentage of electrical fires occur
in bedrooms. There are many appliance cords in bedrooms, for example,
radios, clocks, blankets, air conditioners, heaters, TVs, vacuums, as well
as, lamp cords. All of these cords can be trapped/abused leading to arcing
faults. Further, there are long runs of installed wiring (M-B, "Romex")
between the loadcenter and the bedroom outlets. The wiring can be abused
during installation (e.g. stapling) and after installation (driving nails
into the wall etc.) Therefore, the most logical room to start with would be
the bedroom.
The same site also implies that arc welders do not generate nuisance trips
with the newer AFCIs.
AFCIs now detect arcs at a 5A level. That can detect a series arc (loose
connection).
And AFCIs include ground fault detection at about 30mA. If there is a
ground present in a cord an arc is likely to include leakage to ground
soon. Arcing at a receptacle can also wind up with leakage to ground.
The same site has a good roundup of arc fault types and possible causes:
What is an arcing fault?
According to UL 1699, Standard for Arc Fault Circuit Interrupters, an arcing
fault is an unintentional arcing condition in a circuit. Arcing is defined
as a luminous discharge of electricity across an insulating medium, usually
accompanied by the partial volatilization of electrodes.
There are 3 basic types of arcing faults: line-to-neutral, line-to-ground,
and series arcing.
What causes an arc fault?
Arc faults may occur anywhere in the electrical system and may be a result
of the following:
a.. worn electrical insulation or damaged wire
b.. misapplied or damaged plug in appliance cords and equipment
c.. loose electrical connections
d.. drill bits, nails, or screws driven into the wire
e.. wire staples driven too deep
f.. furniture pressing against electrical cords
g.. broken wires
h.. frayed wires
More reading is in order before I bite the bullet and begin replacing the
rather new sigh breakers I installed in the panel just recently. Does
anyone reading this know if AFCI's come in "dual skinny" formats? One of
the site above talks about how AFCI's run warmer than normal breakers
because of the built-in power supply for the electronics. I wonder if they
can squeeze all the required electronics into the dual space-saving
breakers?
Thanks for your input, Bud!
--
Bobby G.