How combination arc-fault circuit interrupters enhance electrical safety in the home.
NFPA Journal®, September/October 2008
By Alan Manche and Andy Haun
Although there has been plenty of discussion in the electrical industry recently about arc-fault circuit interrupters (AFCIs), they are not a new residential circuit protection technology. NFPA 70®, National Electrical Code® (NEC®), first required them in the 1999 edition, and by 2005, the NEC required the installation of combination AFCIs, which respond to both parallel and series arcs, for all electrical circuits that supply electrical outlets in the bedrooms of new homes by January 1, 2008. The 2008 edition of the NEC, published in September 2007, further expands the use of combination AFCIs beyond bedroom circuits to other areas of a home, such as the family room, the dining room, the living room, closets, and hallways.
When discussing what an AFCI is, beginning with what it isn’t is helpful—namely, it is not a ground-fault circuit interrupter (GFCI). GFCIs first appeared in the NEC in the 1960s and are now used in bathrooms, kitchens, garages, unfinished basements, and outdoor receptacles.
The difference between AFCIs and GFCIs is quite simple: A GFCI protects people from shock if parts of an electric appliance or a tool become energized due to a ground fault, while an AFCI protects branch circuit wiring from arcing faults that could start an electrical fire.
Arc faults occur when current flows in an unintended path, and the heat generated at the point of the arc, which may reach 10,000F (5,538C), creates burning particles that can ignite key structural components of a home. They typically occur either accidentally, as when a nail punctures a wire during installation or maintenance of the electrical system, or when the wire or insulation deteriorates due to age and abuse.
There are two types of arc faults. A parallel arc occurs when current travels from one circuit conductor to another. If a nail or screw pierces the two current-carrying conductors at the same point, for example, the current will travel between the conductors, which are now slightly gapped. A series arc, on the other hand, occurs when a single conductor is severed and current continues to flow in the gap between the two ends. A combination AFCI device detects both parallel and series arcs.
Essentially, a combination AFCI uses its built-in electronic processing technology to monitor a circuit for both dangerous and normal arcing conditions, based on known behaviors of electrical arcs. If the AFCI deems the signal to be a dangerous arc, it will open the circuit, thus removing the arcing condition.
How much do they cost?
Arc-fault circuit interruption has its roots in a 1993 proposal by the Electronic Industries Association and the U.S. Consumer Product Safety Commission (CPSC) to reduce the trip level of circuit breakers to help prevent arc faults.1 In conjunction with Underwriters Laboratories, Inc. (UL), the CPSC did a study that found that AFCIs were the most promising technology to address the problem. Although the NEC did not address AFCIs until 1999, the study prompted electrical equipment manufacturers to accelerate the development of their own solutions, which began appearing on the market in the late 1990s.
The 1999 edition of the NEC required AFCIs in all bedroom receptacle circuits, and the 2002 edition extended coverage to all 120-volt bedroom circuits, including lighting and smoke alarm circuits. The 2005 edition required that combination AFCIs be installed on bedroom circuits in new homes. Expanding the use of combination AFCIs from bedroom circuits to circuits in other areas in the 2008 edition is a natural progression, but some in the industry voiced concerns about the cost of implementing such a requirement.
As a result of numerous states adopting the 2008 NEC, studies have shown the cost increase for AFCIs to be extremely conservative. For example, the Ohio Chapter of the International Association of Electrical Inspectors completed a study in 2008 that estimated the cost impact to be less than 20 cents per square foot (0.09 square meters).2 The study researched the cost for a combination AFCI device and multiple receptacle types from a local distributor, then applied those costs to three home types:
- 900-square-foot (84-square-meter) home: $160.18, or 18 cents per square foot (0.09 square meters)
- 1,700-square-foot (158-square-meter) home: $205.27, or 12 cents per square foot (0.09 square meters)
- 2,100-square-foot (195-square-meter) home: $241.36, or 11 cents per square foot (0.09 square meters)
To some, those cost estimates might seem high in light of the fact that AFCIs do not have the “cool factor” possessed by other features of a new home, such as decorative lighting or granite countertops. However, AFCIs can prevent home electrical fires, which according to the National Electrical Manufacturers Association, continue to be a major concern. An estimated 24,200 home structure fires per year from 2002 to 2005, inclusive, involved electrical distribution or lighting equipment. These fires resulted in 320 civilian deaths, 830 civilian injuries, and $698 million in property loss per year.3
Moving beyond electrical distribution and lighting equipment, electrical failures were cited as contributing to 53,600 home structure fires per year during the same period, or one in every seven home fires. These electrical failures accounted for 510 civilian deaths per year.
Unfortunately, projecting AFCIs’ life- and property-saving potential is difficult, as will be isolating their impact after usage becomes widespread. Statistically, tracking fires that don’t happen is impossible. For example, GFCIs have been around for years, and electrocution deaths have declined for most or all of that period. However, many factors played into that, making quantifying the role that GFCIs played difficult.
Discussing AFCI requirements and the 2008 NEC is impossible without discussing nuisance tripping, which occurs when a circuit breaker opens a circuit unnecessarily because it detected a condition that was not an actual fault. The history of GFCIs shows that this can be a challenge, but AFCIs should have fewer problems, partly because they have benefitted from the GFCI experience.
AFCIs have been on the market for about eight years, and, though combination AFCIs have been on the market for nearly two years, the technology has been extensively tested. AFCI manufacturers learned valuable information by placing AFCIs in numerous homes to face daily usage conditions that would result in improvements to the technology before placing them on the market. Manufacturers also learned many lessons developing GFCIs, including radio frequency immunity and improved electronics. The technology has matured to the point where AFCIs are better programmed to discern a dangerous arc from a normal arc or even normal operating conditions.
The AFCI technology has other benefits, such as serving as a means of quality control for electrical contractors. If a homeowner experiences AFCI tripping, it could mean that a mistake was made during the installation process that can be rectified or an appliance may be malfunctioning and causing the breaker to trip. A homeowner should be made aware that when an AFCI trips due to an improper installation or malfunctioning appliance, the device is doing its job protecting the home from the hazard it was designed to address.
Who supports AFCI protection?
Numerous organizations in the electrical industry, including the International Brotherhood of Electrical Workers, the Independent Electrical Contractors, the National Electrical Contractors Association, UL, and the electrical inspection community all support combination AFCI expansion to enhance electrical safety. The National Association of State Fire Marshals endorses the installation of combination AFCI protection on all circuits in the home, not just those required by the 2008 edition of the NEC, while the CPSC endorses the installation of combination AFCIs in new construction and during the installation of new services in older homes.
A number of states across the country are now enforcing AFCI protection in accordance with their adoption of the 2008 NEC (see sidebar, opposite page). The bottom line for state and local code committees that are currently debating expanding the use of combination AFCIs in their own 2008 NEC adoption process is that the device can keep parallel and series arc faults from starting a fire. Astute code adopters will realize that lives are at stake—not only homeowners’ lives, but the lives of firefighters responding to a fire.
Alan Manche is director of Industry Standards for Schneider Electric, of which Andy Haun is director of Electronics and Software Engineering.
- Underwriters Laboratories, Inc., Technology for Detecting and Monitoring Conditions That Could Cause Electrical Wiring System Fires, prepared for the CPSC, September 1995.
- International Association of Electrical Inspectors, Ohio Chapter, Let the Code Decide: Understanding the Cost Impact of the 2008 NEC, March 2008.
- John R. Hall, Jr., Home Structure Fires Involving Electrical Distribution or Lighting Equipment, NFPA Fire Analysis and Research Division, March 2008.