AFCI breakers are the latest advance in life safety and fire protection.  Basically - The arc fault circuit breaker continuously monitors the shape of the sine wave of the electricity passing through the AFCI breaker.  Anytime the sine wave is uneven or jagged, there may be a sparking condition somewhere down the line. In this case the AFCI breaker will click off.  Most of them also will light one or two tiny LEDs on the front of the breaker.  By reading the instructions you can determine if it was a hot-to-ground fault or a hot-to-neutral fault.

An AFCI breaker does not do the same thing as a GFI or Ground Fault Circuit Interrupter.
 
The AFCI uses a very simple computer chip that constantly checks for a sparking condition between the hot wires and the neutral wire.  It also checks for a high resistance or over heating condition in either the out going hot wire or the returning neutral wire.

Generally it checks for any sputtering, sparking or overheating condition that might indicate a loose or overheating connection.
 
So far these things are really hard to test on my workbench.  The engineers tell me that they work, that they save property and lives.  They have a portable demonstration setup in a case they say they will be glad to show me. Why can't they tell me how to make a test setup on my work bench?

Any way, it is smart to add them the next time you upgrade your main panel.  Be sure to provide plenty of space for these new electronic devices.  At this stage AFCI are still full-size breakers.  Tey are not available in half size or wafer type circuit breakers,  therefore they use up more spaces in the panel.  It can fill the panel up fast if you do not plan ahead.
 
Copyright 2010  John A. Peters, Your SF Electrician