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 -- To terminate puerile preservation prattling among pals and the uncoffee-ed, or to change your settings, go to: <http://maelstrom.stjohns.edu/archives/bullamanka-pinheads.html>