Possibly coming soon to an electrical distributor near you is the arc-fault circuit interrupter (AFCI). Although they may seem like just another new industry acronym to remember, AFCIs solve a specific problem. As defined in proposals for the 1999 NEC, an AFCI is a device that provides protection from effects of arc faults by recognizing characteristics unique to arcing, and then deenergizing the circuit upon detection of an arc fault. Its basic application is protection of 15A and 20A branch circuits in single- and multi-family residential occupancies.
Is the product really necessary? Absolutely-since more than 40,000 fires annually are attributed to electrical causes in fixed residential distribution systems, where equipment and wiring are not generally accessible to home owners. Similarly, electrical appliances, extension cords, and heating and cooking equipment cause more than 100,000 fires. The culprit is typically overheating from current flow or electric arcs in damaged or misapplied equipment.
Residential fire causes from electrical equipment According to NFPA data, fires strike indiscriminately-in every room and space of a dwelling. However, the highest incidences are in bedrooms, living rooms, attics, and attached garages. Most of these fires occur in the fixed wiring system, between walls and floors, and in unfinished areas of residences.
The Consumer Product Safety Commission (CPSC) estimates electrical equipment causes 155,100 or 34% of the 451,000 fires in residential structures. A high percentage of these fires in the distribution system and connected cords originate from arcing conditions.
Here is where the AFCI comes in to provide protection. It directly targets arcing occurrences within the fixed wiring and connections of the distribution system and through extension and appliance cords, when damage or improper installation occurs.
What about categories other than the distribution system? Most fires resulting from cooking and heating equipment relate to the heating element and its heating function as opposed to exposed electrical conductors. However, equipment in all categories has cords or wiring plus electrical connections subject to arcing when damage or misuse occurs. Fires caused by arcs in electrical cords of cord-connected appliances are of major concern. Let's look at several recorded occurrences that resulted, or nearly resulted, in a fire.
Shown in the following photo (in original article) is an example of pierced insulation. The Bureau of Fire Prevention in Cedar Rapids, Iowa collected this evidence from a fire occurrence. The three pieces of No. 12 AWG, Type TW copper wire were installed in a length of flexible metallic conduit, which firefighters disconnected after a call reporting heavy smoke. The overcurrent protective device did not open, indicating the current was not high enough or sustained long enough to cause opening. Although there is some melting of insulation on all three wires, a direct short circuit between wires did not occur. Exposed conductors of the red and white wires indicate arcing to the conduit, rather than to each other. Apparently, the tubing had become hot enough to melt the wire insulation. Firefighters discovered burned construction sawdust next to the concealed conduit, the source of smoke and smoldering. An AFCI would have detected sputtering arcs from the wires to the conduit and prevented this heating /smoldering sawdust.
Below is an example (in original article) of wound wires overheated from normal current flow. The wound cord apparently connected to a window air conditioner left unattended for a period of time. Normal current flow through the wound conductor caused significant melting of the insulation, resulting in arcing between conductors. In this case, the unit was unplugged before a fire occurred. The overcurrent protective device did not open because current was normal load current, except for occasional bursts of line-to-neutral arcing. An AFCI would not have prevented the melting of insulation nor potential ignition of the insulation from heating. However, it would have detected bursts of arcing and opened the circuit well before this degree of damage occurred.
The example above shows surface tracking at an outdoor porch lamp. The homeowner turned off the light when smoke, accompanied by a buzzing sound, occurred. There is evidence of arcing on the screw shell in the socket, from the energized tip of the light bulb. There also is evidence of arcing to the grounded mounting screw in the socket base. The arc was apparently tracking along the surface of the bulb's base, so much so that the base eroded from intense heat. If the homeowner did not turn off the circuit, arcing would have extinguished either itself or ignited insulation or other materials on or near the lamp. An AFCI would have detected the line-to-neutral arc or line-to-ground arc and opened the circuit.
In the photos [at right] (in original article), these high-resistance joints caused intense heat at a specific location, with arcing as a secondary effect. A loose connection is an example of a high-resistance joint. A glowing connection is a special case of a high-resistance joint, typically involving dissimilar metals and a loose connection. Extremely high temperatures are associated with glowing connections, sometimes exceeding the melting temperature of conductors at the point of connection. An AFCI will not detect a glowing connection or hot spot, unless arcing is present. However, as the hot spot degrades insulation or mounting materials, arcing conditions frequently result as a secondary effect. An AFCI will detect many of these.
The photo [top] (in original article) shows a basement receptacle used for 15 years for connection to a water distiller. Although the ground connection is present, it never connected to ground because it is an extension of a two-wire circuit. Apparently, there was a high-resistance connection leading to surface tracking from the ground bracket to each of the receptacle jaws. A line-to-neutral arcing fault eventually occurred, and the house filled with smoke. Eventually, the circuit breaker opened the circuit. A 234-in. wood board, on which the receptacle attached, appeared blackened but self extinguished.
The photo [bottom] (in original article), on page 62 (in original article), shows an attachment plug connected to a space heater. Again, a high-resistance connection caused degradation of insulation of the plastic housing and insulation on the cord. You can see how the cord insulation melted away approaching a direct wire-to-wire (line-to-neutral) arcing fault where the wires come together. An AFCI will not detect a high-resistance connection and would not have prevented damage to the plug housing. However, it would have detected a line-to-neutral arcing fault as the damage increased.
These are just a few of the circumstances that could lead to fire. Fortunately, these occurrences were arrested before a major fire developed, and we're able to see the cause in progress.
Below is a list of conditions in which arcing frequently results and may lead to fire. *Loose or improper connections. *Frayed or ruptured appliance or extension cords. *Pinched or pierced insulation on construction wire or appliance or extension cords, such as from staples or other fasteners. *Cracked insulation on wire or cords from age, heat, corrosion, or bending stress. *Overheated wire or cords. *Damaged appliances in which support or insulation for energized electrical parts is impaired. *Wire or cords touching vibrating metal.
Contrasting AFCI protection with overcurrent and GFCI protection OCPDs have been an integral part of distribution systems for so long that end users seldom have reason to think of the extensive protection they provide. They provide thermal protection for circuit components as might arise from overloads, and they provide short-circuit protection. These devices are already detecting and interrupting many of the occurrences that involve hazardous arcs when they are overcurrent conditions. They respond to a predetermined time-current characteristic; they do not have intelligence to respond to conditions below the characteristic, nor are they intended to, even if the condition is a continuously burning arc.
GFCIs provide another level of protection, primarily people against shock. A GFCI opens the circuit whenever leakage to ground is above 6mA and will detect arcing conditions when they are line-to-ground arcs in the part of the circuit protected. They cannot detect line-to-neutral or series arcs.
An AFCI targets arcing conditions that may cause fire (not detected by an OCPD). These presently undetected situations contribute to today's fire statistics. For maximum performance, the AFCI must be able to detect all arcs-series, line-to-neutral, and line-to-ground.
One of the great differences between an AFCI and OCPD or GFCI is in the various conditions an AFCI must detect or distinguish as non-hazardous. An OCPD responds to a single time-current characteristic. A GFCI responds to a single 6mA criteria. An AFCI must distinguish between a variety of potentially hazardous arcing conditions and safe energy. Enormous progress has been made recently by manufacturers of AFCIs and engineers/scientists developing the standard in understanding the variety of conditions an AFCI must respond to in order to be effective. A reflection of that understanding is in the variety of tests and conditions under the three categories of the draft standard. These address the majority of arcing conditions known to lead to fire.
What are the requirements for an AFCI?
Now that we have some idea of the problem, let's find out what an AFCI must do to deal with the problem?
A task force of the National Electrical Manufacturers Association (NEMA), with participation by UL, developed a comprehensive draft standard for AFCIs. The draft is undergoing reviews within NEMA before its expected transfer to UL. Once UL has the standard, it must complete reviews, editorial work, and approvals before it becomes a formal standard.
In the interim, before availability of the formal standard, devices may be UL "Classified for Mitigating the Effects of Arcing Faults." The only products to be so classified to date are circuit breakers containing the AFCI function in addition to the normal overcurrent protection function. They are labeled "Underwriters Laboratories Inc. Listed Circuit Breaker Also Classified for Mitigating the Effects of Arcing Faults." Requirements in the draft standard provide a guide.
The list of tests is extensive; but the tests specific to the AFCI function are in categories: efficacy (arc detection), unwanted tripping, and operation inhibition.
Efficacy tests. These tests demonstrate the AFCI's basic functionality in detecting arcs. With arcing test currents from 5A up through the expected instantaneous operating current for an overcurrent protective device, the AFCI is required to open the circuit before conditions most expected to lead to fire ignition have occurred.
Unwanted tripping tests. Here, the AFCI is subjected to a series of tests with equipment or loading conditions that could look like an unwanted arc to some forms of AFCI devices. There are six loading conditions, with multiple tests under each condition. Several appliance or equipment loads are applied under each condition using adverse circuit and operational situations. New equipment and equipment with service life is used. The six conditions are inrush current (tungsten lamps, motors), normal operation arcing (switches, plugs, brush motors), non-sinusoidal waveform (electronic dimmers, shop tools, computers), cross talk (arc current on an adjacent circuit), and multiple loads (branch loaded to full load with various equipment).
Operation inhibition tests. This series of tests evaluates whether the AFCI can distinguish an unwanted arc even in the presence of other loads or conditions in the circuit that might attenuate, hide, or disguise the arc signal. This test sequence is perhaps the most demanding of the AFCI, because the device must find the arc among a variety of circuit loading and noisy conditions in the following four categories: 1)Tests for masking (a wide variety of loading conditions); 2)EMI filter tests (multiple filters and configurations) 3)Line impedance tests (several configurations including arc faults to a grounded water pipe); and 4) Minimum voltage (operation at 85% of rated voltage). The arc generator and criteria are the same for these tests as in the efficacy tests.
It's the author's understanding these test conditions are applied to AFCIs under evaluation by Underwriters Laboratories Inc. for products submitted to mitigate the effects of arcing faults. Because of the variety of conditions in present circuits, from hobby arc welders to computers to heavy shop tools, the earliest AFCI devices and the industry standard may not be sensitized to detect all conditions below the circuit rating of 15A or 20A. There's been little field experience from any source in describing the variety of normal conditions or hazardous arcing conditions. However, the AFCI meeting the requirements drafted will detect and clear the circuit with conditions described for the EIA and many other conditions understood to potentially lead to fire.
NEC proposals There are three proposals in the National Electrical Code Committee Report on Proposals for the 1999 revision cycle: Proposals 2-128, 2-129, and 2-130. Each proposes an AFCI be required in each 15A and 20A branch circuit for lighting, receptacle outlets covered by Sec. 210-52(a) through (h), and receptacle outlets in basements and attached garages. The almost universal coverage of 15A and 20A branches arises from the reality that fires start in every room and space of a residence.
In the May 1997 EC&M Electrical Code Watch column, Senior Editor Fred Hartwell correctly presented voting status of the proposals as 7 to 5 favoring, with an 8 to 4 vote required for acceptance. He also summarized negative vote comments. Below is an abbreviated response to some of the issues.
Availability of AFCIs for all panelboards. Just as all circuit breaker manufacturers offer GFCI circuit breakers, it' a reasonable guess that all will also offer AFCIs. Products other than circuit breakers may also be available.
Inadequate testing. This could relate to either field experience or to certification testing, such as for UL Listing. One company (Square D Co.) has had units in field tests since 1993 gathering data to support a solid design for a commercial product. Not only is the draft standard comprehensive, but it also has given the nine NEMA member manufacturers participating in its development significant exposure to the range of conditions the product must face.
Use European RCD concept instead of AFCI. The European residual current device (RCD) is typically a main unit in a panelboard with residual ground-fault protection similar to a GFCI, but with sensitivity set at 30mA or 100mA, instead of 6mA for a GFCI. First, this concept does not provide arc fault protection; it provides only ground fault protection. Second, when an RCD detects a fault, it opens the main and turns off every circuit in the residence. There is a companion 1999 NEC proposal (Number 2-264) that would permit AFCI protection to be located in a feeder rather than in each branch.
Line side or service entrance arcs. The concern here is potential nuisance operation when lightning or other disturbances cause line side arcing. One of the unwanted operation tests in the draft standard directly addresses this issue by testing with an arc on an adjacent circuit, and demonstrating nuisance operation will not occur.
We now have the technology to detect arc faults, one of the most damaging causes of fires in residences. Prototypes have been in the field since 1993, and a comprehensive standard has been drafted directly addressing fire causes in residences. There is every reason to begin immediately improving the safety of homes by requiring the installation of AFCIs.