AFCIs resemble a GFCI/RCD (Ground-Fault Circuit Interrupt/Residual-Current Device) in that they both have a test button, though it is important to distinguish between the two. GFCIs are designed to protect against electrical shock, while AFCIs are primarily designed to protect against fire.
The AFCI is intended to prevent fire from arcs. AFCI circuit breakers are designed to meet one of two standards as specified by UL 1699: "branch" type or "combination" type (note: the Canadian Electrical Code uses different terminology but similar technical requirements). A branch type AFCI trips on 75 Amperes of arcing current from the line wire to either the neutral or ground wire. A combination type adds series arcing detection to branch type performance. Combination type AFCIs trip on 5 Amperes of series arcing. Advanced electronics inside an AFCI breaker detect sudden bursts of electrical current in milliseconds, long before a standard circuit breaker or fuse would trip. A "combination AFCI breaker" will provide protection against
In 2002, the NEC removed the word "receptacle" leaving "outlets", in effect adding lights within dwelling bedrooms to the requirement [debated interpretation]. The 2005 code made it more clear that all outlets must be protected, despite code making panel discussion about excluding bedroom smoke detectors from the requirement. "Outlets" is defined in "Article 100 Definitions" of the NEC as "A point on the wiring system where current is taken to supply utilization equipment" and this includes receptacles, light fixtures, and smoke alarms, among other things.
Beginning January 2008, only "combination type" AFCIs will meet the NEC requirement. The 2008 NEC requires installation of combination-type AFCIs in all 15 and 20 amp residential circuits with the exception of laundries, kitchens, bathrooms, and garage, and unfinished basements.
AFCIs are designed to protect against fires caused by electrical arcing faults. However, they provide no specific protection against "glowing" connections, excess current, high line voltages, or low line voltages. For descriptions of electrical fire causes, see "How Electricity Starts Fires", John S Robison, P.E., Electrical Line magazine, November / December 2007 issue, pages 60-70 (http://www.electricalline.com/); NFPA 921 "Guide for Fire and Explosion Investigations", 2004 edition; and "Ignition Handbook", Dr. Vytenis Babrauskas, Fire Science Publishers, 2003.
Glowing connections occur when relatively high electric current exists in a relatively large resistance. Heat comes from power dissipation. Power (in Watts, abbreviated W) equals the current (in Amperes, abbreviated Amps or A) multiplied by the current (in Amperes) multiplied by the resistance (in Ohms). For example, a 60 Watt lamp operating on a 120 V circuit draws 1/2 Ampere of current. An 1800 Watt space heater on a 120 V circuit draws up to 15 Amperes. If a bad wiring junction in a circuit has a resistance of 1 Ohm, then a 60 W lamp will cause it to dissipate 0.25 Watt of power (0.5 A * 0.5 A * 1 Ohm). In contrast, an 1800 W heater could theoretically cause the bad wiring junction to dissipate 178 Watts (13 A * 13 A * 1 Ohm). Note that the current is less than 15 A because of the combined resistance of the heater plus the bad wiring junction. This heat dissipated in a small junction area can generate over 1000 degrees C of heat. That amount of heat can ignite most flammable materials. Bad wiring junctions can occur in utilization equipment, cords, or premise wiring (especially in defective switch, socket, plug, or wire connection, even at circuit breaker or fuse panels). High resistance junctions are commonly observed in improperly terminated aluminum wire junctions. No technology located in a circuit breaker or fuse panel could detect a high-resistance wiring fault as there is no measurable characteristic to distinguish a glow fault from the normal operation of a branch circuit. Power Fault Circuit Interrupters (PFCI) located in receptacles are designed to prevent fires caused by glowing connections in premise wiring or panels. From the receptacle, a PFCI can detect the voltage drop when high current rxists in a high resistance junction. In a properly designed circuit, substantial voltage drops should never occur. Proper wire terminations inside utilization equipment (e.g. appliances, lamps, heaters) and cords prevent high-resistance connections that can lead to fires.
Excess current can heat entire lengths of wire. Thermal circuit breakers are designed to protect against excess current through the premise circuit wiring. However, excess current through the smaller gauge wires in utilization equipment can exist, at levels below the trip thresholds of a circuit breaker. Overload Fault Circuit Interrupters (OFCI) are designed to protect against excess current into utilization equipment. OFCIs must be located within receptacles. Both thermal circuit breakers and OFCIs are required to prevent fire ignition from excess current.
High line-voltage creates excess power and heat in utilization devices such as heaters, light bulbs, appliances, motors, and electronics. In extreme cases, this heat can ignite fires. One extreme source of high line voltage occurs from a neutral path opening within a two-leg 120 V residential electrical system. When a neutral wire breaks or opens, the utilization voltage can almost double to over 200 V with large leg-to-leg load imbalances. This extreme situation can result in almost four times the power and heat in loads. With such overheating, some loads can reach self-ignition temperatures in less than 10 minutes. Power Fault Circuit Interrupters (PFCIs) are designed to prevent fires from excess voltage across loads. Voltage-trip circuit breakers detect excess line voltages, but are unable to detect sub-circuit open neutral conditions.
Low line-voltage can cause electro-mechanical relays (on-off switches) to turn off (relay opens) and back on (relay closes again) in a repeating cycle. If there is current through the relay at that time, the opening process can create a parting arc across the relay contacts. The arc can melt, pit and oxidize the contact surfaces. This process can increase the resistance to current, superheat the relay and lead to fires. Power Fault Circuit Interrupters are designed to prevent fires from low voltage across loads.
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