New Products for Integrated Electrical Systems/Questions and Answers/Resources necdigest Seen in necdigest TM NFPA s Official NEC Magazine The Voice of Authority www.necdigest.org Feb/March 2002 Volume 1 by Dr. Joseph Engel Arc-Fault Circuit Interrupters Bringing a new level of electrical protection into the home Each year in the United States, residential electrical fires result in more than 700 deaths, 3,000 injuries and $700 million in property damage 1. A number of these fires begin with little warning, kindled by sputtering arc faults in damaged or deteriorated wiring. The arc fault creates a spark, generates heat, and eventually ignites nearby combustible material. necdigest.org feb/mar 2002 necdigest
Due to the development of advanced, affordable circuit interrupter technology and the recently updated NEC code, however, fires and deaths attributable to arc faults may soon be on the decline. Building On Conventional Breaker Safety Conventional residential circuit breakers prevent fires by automatically opening the circuit before conductors or the insulation that shields them can be damaged by excessive and dangerous temperatures. The response times of conventional circuit breakers are determined solely to protect against circuit overload 2 or overcurrent. However, the response is inadequate for protecting against the fire hazards associated with arcs, the temperatures of which can exceed 6000 o C. Arcs also have an extremely short duration, which cannot typically be detected by conventional breakers. The challenge, therefore, has been to improve circuit protection by identifying the presence of arcing faults and responding to them fast enough to prevent fire. Modern electronics has answered this challenge with Arc-Fault Circuit Interrupters (AFCIs), devices that recognize the unique current signatures associated with arcing faults and act to interrupt the circuit before the temperature of combustibles can rise to hazardous levels. Common Wiring Hazards Hazards in residential wiring systems develop from either high-energy arcing (parallel) or high-resistance (series) faults. 3 High-energy arcing faults are caused by a failure of the insulation between conductors at different potentials that are located anywhere on the circuit. Insulation can fail over time due to extended exposure to moisture, heat, or extraordinarily high voltages. Insulation can also be accidentally cut or damaged by nails, staples or other materials. The age of the wiring or physical abuse can also be contributing factors. Once the insulation is compromised, heat from an arc melts the conductors in the wire and the electricity seeks a new path. As it develops so does a sporadic or intermittent arc fault. Under these conditions, conventional circuit breakers may not trip either magnetically, since peak currents are too low, or thermally, since the duration of the current is too short to cause a thermal element to trip. If the breaker should trip, it will only do so after an unacceptable delay. A high-resistance fault is not an arc fault initially, but it can become one over time. A layer of copper or aluminum oxide that forms at connections, such as at wire nuts, receptacle terminals, or plugs, creates this type of fault. The coating replaces the low resistance path with one of high resistance and begins to generate heat. Broken conductors can also create high-resistance faults. Regardless of the cause, local I 2 R power dissipation can overheat and pyrolyse the insulation leading to a parallel arcing fault. Like the situations that create highenergy faults, conventional breakers will not always detect the situation and trip. The magnitude of parallel faults in residential circuits can be estimated from Curve A of Figure 1, which is derived from a UL Report 4 prepared for the Electronics Industries Alliance. The curve shows the distribution of Estimated Short Circuit Current Available (Amperes) vs. Percentage of Circuits Having Estimated Short Circuit Current Available Figure 1 The distribution of available fault currents, at 15A household receptacles Curve (A), and at the end of six feet of #18 appliance wire plugged into those receptacles, Curve (B). 2
available fault currents at household receptacles. The X- axis displays the estimated short circuit currents available; the Y-axis shows the percentage of circuits that have particular values of short circuit current available or higher. The data applies to bolted faults with the line and neutral securely clamped together. For 15A receptacles, the available fault levels are 75A RMS or higher. Only half have available levels of 250A or higher. These relatively low short circuit levels are due to the impedance of fixed premise wiring. The impact of wire impedance is further emphasized by Curve B, which shows the available fault currents with six feet of #18 appliance wire plugged into the receptacles. Now only half have fault magnitudes of 200A or higher at the end of the appliance wire. Conventional residential circuit breakers have instantaneous trip levels in the range of 125-200A rms. With reference to Curve B of Figure 1, this means that bolted faults, and their associated sine waves, would cause instantaneous tripping in 50-85% of residential circuits. For the remaining 15-50% of the circuits, the breaker would trip in response to the heating of the bimetal. Figure 2 I Typical current waveforms observed when a carbon-steel blade cuts through 16 AWG SPT-2 cord. The available current is 100A. The challenge has been to improve circuit protection by identifying the presence of arcing faults and responding to them fast enough to prevent fire. For the case of arcing faults, however, these same residential circuit breakers, with trip levels in the range 125-200A RMS, would respond instantaneously in a much lower percentage of circuits due to two characteristics of arcing faults: their arcing voltage of about 50V introduc significant impedance into low voltage 125V circuits, reducing the current amplitude parallel arcing faults are sputtering in nature as indicated in the oscillogram of Figure 2. The reduced amplitude pulses are intermittent and of short duration. Instead of functioning normally, conventional breakers would actually trip instantaneously in an even lower percentage of circuits than indicated in Figure 1. Further, a conventional breaker might never trip due to bimetal heating because the RMS current level associated with the intermittent sputtering arc could be less than the RMS breaker rating. Where Faults Are Found High-energy parallel arcing and high-resistance series faults occur at numerous locations throughout the home, including in-wall and in-room wiring. The causes of in-wall faults include wire insulation physically damaged by staples or nails or environmentally damaged by lightning or moisture. An example of an in-wall high-resistance series fault is a loose receptacle terminal connection that can cause local heating and damage to the wire insulation and/or the receptacle. Ground faults are common as in-wall wiring includes grounded conductors. The ground fault, if detected soon enough, can prevent the fault from escalating into a high-energy arcing fault. In-room faults are typically caused by the abuse of cords and plugs, (e.g. a chair resting on a cord) or wear and tear over extended periods of time (e.g. a loose plugreceptacle connection). The AFCI Difference AFCIs offer two major advantages over conventional breakers: 1. The electronic instantaneous trips for an AFCI for arcing faults are set to trip with PEAK arcing currents as small as 50 amperes. This is possible because AFCIs can distinguish between the signatures of parallel arcing faults from switching and steady state currents associated with normal electrical loads. Further, AFCIs will not nuisance trip with transient sinusoidal load currents having PEAK values of 300 amperes or more (such as occur when a freezer compressor turns) or 3
transient current pulses such as those that occur when switching loads, or when an incandescent lamp burns out. An AFCI s ability to distinguish a low amplitude arcing current from a high amplitude normal current, an intelligent instantaneous trip, is one of an AFCI s important features. 2. AFCIs can include ground fault protection. It is recognized that high-resistance series faults at wire terminations and old aluminum wiring are hazards. In fact, the hazards of old aluminum branch circuit wiring on 15- and 20-ampere branch circuits are well known. Just as hazardous, but less recognized, are glowing contacts. They are associated with today s copper wire and modern wiring devices, receptacles and switches. A loose wire-to-receptacle terminal connection can overheat and create a so-called glowing contact. Eaton Corporation funded a UL Special Services Investigation that determined such hazards could be mitigated by an AFCI circuit breaker providing both arcing and ground fault protection. The glowing contact has the potential to eventually melt the wire insulation and the receptacle itself. Typically a line-to-neutral arcing fault or a line-to-ground or neutral-to-ground ground fault will develop. An AFCI circuit breaker employing both arc and ground fault interrupters will respond to these conditions by tripping and deenergizing the branch circuit feeding the receptacle. Rigorous Testing There are four major manufacturers currently producing AFCIs: Siemens, Square D, GE and Eaton Corp. Of these, Eaton Corp. s Cutler-Hammer business unit first filed for a patent on AFCI technology on September 26, 1991. Their design passed UL s draft standard tests in November/December 1996 and became the first commercially available AFCI on September 30, 1997. The draft standard testing involved parallel tests using A) a guillotine with a carbon-steel blade across the conductors of NM-B cable and two conductor Type SPT-2 flexible cord and B) sputtering arcs across a cut in their insulation that had been conditioned through formation of a carbon bridge. Series fault tests were also conducted with NM-B fixed premise wiring with the AFCI responding to the ground fault current resulting from arcing at a broken conductor without ignition of surrounding material. The test also demonstrated resistance to unwanted tripping, including motor starting and dimmers, and resistance to operation inhibition including filters. Figure 3 depicts typical residential wiring divided into four zones. Zone 0 is associated with the meter, meter socket, and service cable. Zone 1 is associated with the loadcenter and the fixed premise wiring. Zone 2 is associated with the wiring between the receptacles Typical Residential Wiring Figure 3 I Division of residential wiring into four zones. Zone 0 is associated with the meter, meter socket and service cable, Zone 1 with the loadcenter and the fixed premise wiring, Zone 2 with the wiring between the receptacles and the loads, and Zone 3 with the appliances and other loads. 4
and the loads, and Zone 3 is associated with the appliances and other loads. A good AFCI with ground fault protection will mitigate against parallel arcing and series high-resistance faults in zones 1 through 4. Residential Fires Breakdown by Zone Improved Safety AFCIs represent a significant step forward in electrical safety. The Consumer Products Safety Commission (CPSC) has reported that more than 35% of all electrical wiring fires are associated with the fixed wiring. 5 The Task Force of the NEMA Molded Case Circuit Breaker Section also analyzed fire statistics provided by a major insurance company. Figure 4 shows the percentage of electrical fires associated with the various Zones. This figure indicates that many fires are associated with Zone 1, with statistics that are similar to those reported by CPSC. In addition to detecting and interrupting potentially dangerous parallel arcs in Zone 1, the AFCI detects parallel arcs in Zones 2 and 3. Also series faults are mitigated, as they tend to escalate either into a parallel arcing fault or a ground leakage fault. This is a major safety improvement over conventional circuit breaker technology. It must be noted, however, that AFCIs will mitigate the effect of arcing faults but will not eliminate them completely. Even under optimum conditions there will always be at least one arcing half cycle and, in certain environments, this could cause ignition at high currents. Figure 4 I Percentage of electrical fires associated with the Zones defined in Figure 3. AFCIs and the NEC Six AFCI proposals were submitted for the 1999 National Electrical Code, four from two manufacturers and two from the Electronics Industries Alliance. The code was subsequently updated to require the installation of AFCI protection on all branch circuits supplying 15A or 20A single-phase 125V outlets installed in dwelling unit bedrooms as of January 1, 2002. References [1] 1999, National Fire Data Center [2] Section 240-1 (FPN) of the 1996 National Electric Code [3] Technology for Detecting and Monitoring Conditions That Could Cause Electrical Wiring System Fires, report Prepared by Underwriters Laboratories (UL Project Number NC233, 94ME78760) for the Consumer Product Safety Commission (Contract Number CPSC-C-94-1112), September 1995 [4] An Evaluation of Circuit Breaker Trip Levels, Fact Finding Report Prepared by Underwriters Laboratories for the Electronic Industries Association under UL Project 92ME51901, October 25, 1993 [5] Memorandum from the United States Consumer Products Safety Commission, 1992 Estimated Fire Losses involving Electrical Equipment 5
n Eaton s Cutler-Hammer business is a worldwide leader in electrical control, power distribution, and industrial automation products and services. Through advanced product development, world-class manufacturing methods, and global engineering services and support, the Cutler-Hammer business provides customer-driven solutions that serve the changing needs of the industrial, utility, light commercial, residential, and OEM markets. To learn more about Eaton s innovative Cutler-Hammer products visit www.cutler-hammer.eaton.com. Eaton Corporation is a global $7.3 billion diversified industrial manufacturer that is a leader in fluid power systems; electrical power quality, distribution and control; automotive engine air management and fuel economy; and intelligent truck systems for fuel economy and safety. Eaton has 49,000 employees and sells products in more than 50 countries. To learn more about Eaton Corporation visit www.eaton.com. 1000 Cherrington Parkway Moon Township, PA 15108 www.cutler-hammer.eaton.com 1-800-525-2000 RE00402001E June 2002 Printed in U.S.A.