Arc Fault Protection - What its all About
By Mike Holt for EC&M magazine, back to basics.
Arcing is defined as a luminous discharge of electricity across an insulating medium. Electric arcs operate at temperatures of between 5,000 and 15,000°F and expel small particles of molten or burning materials from the center. Higher current arcs are more likely to cause a fire because of the higher energy in the arc. Greater current will melt more of the conductor metal and therefore expel more molten particles. The volume of hot, ionized gas emitted increases proportionally with energy.
Arcing faults can occur in one of two ways, series arcing faults or parallel arcing faults, but the most dangerous of these is the parallel arc. A series arc can occur when the conductor in series with the load is unintentionally broken. Examples might be a frayed conductor in a cord that has pulled apart or a loose connection to a receptacle or in a splice. A series arc is load limited, such that arc current cannot be greater than the load the conductor serves. Current with an arc in series has a lower rms value than current without the arc due to extinction and re-ignition. Typically, series arcs do not cause enough heat to create a fire.
Parallel arcing faults either occur in two ways, either a short circuit or a ground fault.
Short-Circuit Arc. A short circuit arc might occur if the wire insulation is cut by a staple or a cord is cut by a metal table placed on it. The current flow of a short-circuit arc is only limited by the system impedance and the impedance of the fault itself. A ground fault arc can occur only when a ground path is present, and this fault can be cleared by either GFCI or AFCI protection device. The rms current value for parallel arc faults, will be considerably less than that of a solid fault, and a typical 15 A might not clear this fault before a fire is ignited.
To protect against fires, the NEC requires AFCI protection of the branch circuit wiring in dwelling unit bedrooms, see 210.12. In addition, UL 1699 contains the requirement for listing AFCI devices. Each type of AFCI protection device is intended to protect different aspects of the branch circuit and extension wiring.
UL 1699 requires testing of the AFCI through a rigorous set of tests for arc detection ability, unwanted operation tests (to avoid nuisance operation), and operation inhibition tests. The operation inhibition tests assure that the AFCI will detect an arc even though it may be connected electrically in series or parallel with loads that might attenuate, mask or otherwise tend to hide the arc signal.
If you have any feedback, let me know, Mike@MikeHolt.com.
Why is AFCI protection only required for branch circuit conductors?
The NEC Code panel want the industry to gain experience with these devices in bedroom circuits so that in the future their usage might be expanded to other rooms and facilities that could benefit by the added protection to provide.
Studies have shown that over 60 percent of fires are from causes in the fixed wiring, switches, receptacle outlets and lighting fixtures that are part of the fixed electrical system of a residence. This data soundly supports the present NEC language that requires AFCI protection at the branch.
The following proposals were all rejected:
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