By Mike Holt for EC&M Magazine
Wiring and equipment installed in accordance with the National Electrical Code will be protected against arcing faults to a certain degree by a circuit breaker, fuse, or ground-fault circuit-interrupter. Circuit breakers and fuses are reasonably effective in preventing fire causes for conditions under which they are designed to operate. That is, when a bolted short circuit or ground fault occurs the circuits overcurrent protection device opens, fires are frequently prevented. (Graphic)
However, the current flow of a high-impedance arc is often too low to cause operation of the protection device, and a fire could occur. (Graphic)
To help reduce the hazard of electrical fires from a parallel arcing fault in the branch circuit wiring, the NEC requires a listed AFCI protection device be installed in branch circuit wiring in dwelling unit bedrooms.
Authors Comment: See the Back to Basics article for information on what causes an arc and how a AFCI protection device operates.
The 1999 NEC Rule:
The 1999 NEC Rule
Authors Comment: The 125 V limitation to the requirement means that AFCI protection would not be required for a 240 V baseboard heater or room air conditioner. For more information, visit http://www.mikeholt.com/Newsletters/Newsletters.htm, go to the Miscellaneous section and visit my AFCI links.
The intent of the change from the 1999 NEC is that arc fault protection be provided for all branch circuits that supply 125-volt, single-phase, 15- and 20-ampere outlets, not just receptacle outlets. In addition, the AFCI must be listed so that it will protect the entire branch circuit by de-energize the circuit when an arc fault is detected.
Controversy exists between circuit breaker manufactures and wiring device manufactures that make listed AFCI devices. The circuit breaker manufacture insists that the only acceptable device is the AFCI circuit breaker. Wiring device manufactures take the position that AFCI receptacles listed to protect the entire branch circuit should be suitable and Code compliant.
Both make excellent arguments, but the NEC is very specific. It requires the AFCI protection device to de-energize the circuit and protect the entire circuit from an arc fault. The only device that can de-energize the circuit when an arc fault is detected is the AFCI circuit breaker. AFCI receptacles of the type listed to detect upstream series arc faults will not de-energize the circuit from parallel type arcing faults that may occur upstream of the device. Therefor they cannot be used to meet the NEC requirement of 210.12. (graphic)
If you are in the practice of using one branch circuit for both lighting and receptacles, the 2002 Change will have no effect. But, the practice of separating the lighting from the receptacle circuits in dwelling unit bedrooms will now require two AFCI circuit breakers, or youll place them all on the same circuit.
A new subsection (B) was added to 550.25 to require all branch circuits that supply 15 and 20A, 125V outlets in bedrooms of mobile homes and manufactured homes to be protected by arc-fault circuit interrupter.
Why is AFCI protection only required for dwelling unit branch circuits? The NEC Code wanted 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.
The following proposals for the 2002 NEC were all rejected:
Omit AFCI protection for the lighting outlets, because light may be needed when the AFCI device operates.
Extend AFCI protection guest room branch circuits of motels and hotels.
Permit the AFCI receptacle outlet to provide the required protection.
Omit AFCI protection for the smoke detector circuit conductors.
Delete the AFCI requirement completely.
Common Questions about AFCIs
Q1. What happens when an appliance has a locked rotor condition
(bedroom window air-conditioner)? Will the AFCI breaker respond?
Q2. Will there be lots of nuisance tripping of these devices?
Q3. Will a AFCI prevent fires from loose connections at terminals
or splicing devices?
Q4. Are there any AFCI/GFCI combination breakers?
If you have any comments or suggestions, please let me know. Mike@MikeHolt.com.
Copyright © 2002 Mike Holt Enterprises,Inc.