On 7/3/2013 3:55 AM, Robert Green wrote:
wrote in message
b.com...
On 7/1/2013 7:59 AM, Robert Green wrote:

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?

RF from the arc is not picked up by circuit conductors acting as antennas.

AFCIs look at the current on the conductor. The arc "signal" is created
by current variations through the arc.

From what I've been reading at least
some part of the detection process is based on detecting the RF noise from
an arc.

That is the current variations created by the 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.

A "series" arc (loose connection) is limited by the normal load that is
on the circuit. The AFCIs that are used now may detect a 5A or larger arc.

Parallel arcs (H-N and H-G) can be a lot larger. The current can be up
to the available short circuit current at the point of the arc. If I
remember right, an investigation found that is very likely over 60A out
to 6 ft of line cord in a house.


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.

The signal from the camera isn't very large so a small noise can be
quite visible.

AFCIs need a 5A or larger "noise" on power wiring that runs at much
higher voltage and current than the camera.