Bruce:

With your electrical background I thought you might be able to help me
figure out arch fault circuit interupters (AFCIs) and how they can be used
in old houses.

Our "research question" is:
Can AFCIs be used in retrofit applications in older dwellings that use
two-wire circuits without a grounding conductor? More specifically:
wouldn't it be neat if these devices could be used to protect old knob &
tube branch circuits to reduce the risk of building fires?

Background:

These devices work similar in theory to the ground fault circuit
interrupter (GFCI); however, their purpose is a little different. The arch
fault circuit interrupter (AFCI) is used to sense an arch fault. An arch
fault occurs when electric current jumps through an air gap between two
current carrying or a current and non-current carrying piece of metal. It
takes 80 millivolts to jump across an air gap of one 1000th of an inch.
This means in theory that an arch occurring at a 125 volt receptacle could
span a gap of one and one half inches.

An arch is a high resistance short circuit. For this reason, it is
difficult to extinguish the arch once it begins. The temperature within an
arch is extremely hot. It has been theorized that arches occurring at
bedroom receptacles have been linked to the eventual fires in residential
homes. In 1999, the NEC code panel added a requirement to the code which
requires these devices to be used on all 125 volt receptacle circuits in
the bedroom. The code panel further revised this code in the 2002 edition
to include all 125 volt outlets within the bedroom, which includes smoke
detectors, lighting, etc. The AFCI device works as a current limiting
device disconnecting the source voltage quickly and thereby extinguishing
the arch once it occurs.

Our understanding is that these devices are in their infancy stages. Based
on their current design, they will not extinguish archs in every situation.
Their design is based on extinguishing parallel archs not series archs. A
parallel arch is one which is in parallel with the load. Likewise, a series
arch is one which might occur when the load is disconnected from the supply
such as during switching operations or when a cord cap is pulled from the
receptacle while still under load. Niether the series nor the parallel arch
are desirable, but the series arch occurs under normal operation. We expect
future technology will resolve the problems surrounding these devices, and
we plan to stay abreast of the future changes as they occur.

What do you think?

John (kneeling to Electra, praying she will bless us all) Leeke

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