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An Arc Fault Circuit Interrupter (AFCI) is an advanced circuit breaker that breaks the circuit when it detects a dangerous electric arc in the circuit it protects to prevent electrical fires. An AFCI selectively distinguishes between a harmless arc incidental to normal operation of switches, plugs, and brushed motors, versus a potentially dangerous arc that can occur, for example, in a lamp cord which has a broken conductor.
Contents
- Operating principle
- Electrical code requirements
- Limitations
- Interference with power line networking
- References
AFCI breakers have been required for circuits feeding electrical outlets in residential bedrooms by the electrical codes of Canada and the United States since the beginning of the 21st century; the US National Electrical Code has required them to protect most residential outlets since 2014, and the Canadian Electrical Code has since 2015.
Arc faults are one of the leading causes for residential electrical fires. Each year in the United States, over 40,000 fires are attributed to home electrical wiring. These fires result in over 350 deaths and over 1,400 injuries each year.
Conventional circuit breakers only respond to overloads and short circuits, so they do not protect against arcing conditions that produce erratic, and often reduced current. An AFCI is selective so that normal arcs do not cause it to trip. The AFCI circuitry continuously monitors the current and discriminates between normal and unwanted arcing conditions. Once detected, the AFCI opens its internal contacts, thus de-energizing the circuit and reducing the potential for a fire to occur.
Operating principle
The electronics inside an AFCI breaker detect electrical current alternating at characteristic frequencies, usually around 100 kHz, known to be associated with wire arcing, which are sustained for more than a few milliseconds. A combination AFCI breaker provides protection against parallel arcing (line to neutral), series arcing (a loose, broken, or otherwise high resistance segment in a single line), ground arcing (from line, or neutral, to ground), overload protection and short circuit protection.
AFCI receptacles contain electronic components to monitor a circuit for the presence of dangerous arcing conditions. Based upon an established threshold in the sine wave, the AFCI can be triggered to quickly react and de-power a circuit if dangerous arcing is detected.
When installed as the first outlet on a branch circuit, AFCI receptacles provide series arc protection for the entire branch circuit. They also provide parallel arc protection for the branch circuit starting at the AFCI receptacle. Unlike AFCI breakers, AFCI receptacles may be used on any wiring system regardless of the panel.
Electrical code requirements
Starting with the 1999 version of the National Electrical Code in the United States, and the 2002 version of the Canadian Electrical Code in Canada, the national codes require AFCIs in all circuits that feed outlets in bedrooms of dwelling units. As of the 2014 NEC, AFCI protection is required on all branch circuits supplying outlets or devices installed in dwelling unit kitchens, family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, laundry areas, and similar rooms and areas. They are also required in dormitory units. This requirement may be accomplished by using a "combination type" breaker—a specific kind of circuit-breaker defined by UL 1699—in the breaker panel that provides combined arc-fault and overcurrent protection or by using an AFCI receptacle for modifications/extensions, as replacement receptacles or in new construction, at the first outlet on the branch. Not all U. S. jurisdictions have adopted the NEC's AFCI requirements so it is important to check local code requirements.
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.
AFCI receptacles are an alternative solution to AFCI breakers. These receptacles are designed to address the dangers associated with both types of potentially hazardous arcing: parallel and series. AFCI receptacles offer the benefit of localized test and reset with such buttons located on the face of the device. This is very convenient and saves a journey to the breaker panel.
In 2002, the NEC removed the word "receptacle", leaving "outlets", with the effect that lights and other wired-in devices such as ceiling fans within bedrooms were added to the requirement. The 2005 code made it clearer that all outlets must be protected despite discussion in the code-making panel about excluding bedroom smoke detectors from the requirement. "Outlets" as defined in the NEC includes receptacles, light fixtures and smoke alarms, among other things. Basically, any point where electricity is used to power something is an outlet.
As of January 2008, only "combination type" AFCIs meet the NEC requirement. The 2008 NEC requires the installation of combination-type AFCIs in all 15 and 20 ampere residential circuits with the exception of laundries, kitchens, bathrooms, garages, and unfinished basements, though many of which require GFCI protection. The 2014 NEC adds kitchens and laundry rooms to the list of rooms requiring AFCI circuitry, as well as any devices (such as lighting) requiring protection.
Limitations
AFCIs are designed to protect against fires caused by electrical arc faults. While the sensitivity of the AFCIs helps in the detection of arc faults, these breakers can also indicate false positives by identifying normal circuit behaviors as arc faults. For instance, lightning strikes provide voltage and current profiles that resemble arc faults. This nuisance tripping reduces the overall effectiveness of AFCIs. Research into advancements in this area is being pursued.
Also, AFCIs provide no specific protection against glowing connections also known as a High Resistance Connection, excess current, high line voltages or low line voltages. AFCI circuit breakers include a standard inverse-time circuit breaker. Glowing connections occur when relatively high current exists in a relatively large resistance object. Heat comes from power dissipation. This energy, when dissipated in a small junction area, can generate temperatures above 1000 °C (1800 °F) and can ignite most flammable materials.
Bad wiring junctions can occur in utilization equipment, cords, or in-situ wiring and especially in a defective switch, socket, plug, wiring connection and even at the circuit breaker or fuse panels. Terminal screws loosened by vibration, improper tightening or other causes offer increased resistance to the current, with consequent heating and potential thermal creep, which will cause the termination to loosen further and exacerbate the heating effect. In North America, high resistance junctions are sometimes observed at the terminations of aluminum wire circuits, where oxidation has caused increased resistance, resulting in thermal creep. No technology located in a circuit breaker or fuse panel could detect a high-resistance wiring fault as no measurable characteristic exists that differentiates a glow fault from normal branch circuit operation. Power Fault Circuit Interrupters (PFCI) located in receptacles are designed to prevent fires caused by glowing connections in wiring or panels. From the receptacle a PFCI can detect the voltage drop when high current exists in a high resistance junction. In a properly designed and maintained circuit, substantial voltage drops should never occur. Proper wire terminations inside utilization equipment, such as appliances, and cords prevent high-resistance connections that could lead to fires.
An AFCI does not detect high line voltage due to an open neutral in a multiwire branch circuit. A multiwire branch circuit has both energized wires of a 120-240V split phase service. If the neutral is broken, devices connected from a 120 V leg to the neutral may experience excess voltage, up to twice normal.
AFCIs do not detect low line voltage. Low line voltage can cause electromechanical relays to repeatedly turn off and on. If current is flowing through the load contacts it causes arcing across the contacts as they open. The arcing can oxidize, pit and melt the contacts. This process can increase the contact resistance, superheat the relay and lead to fires. Power fault circuit interrupters are designed to prevent fires from low voltage across loads.
AFCIs are also known to be sensitive (false tripping) to the presence of radio frequency energy, especially within the so-called high frequency spectrum (3-30 MHz) which include legitimate Citizens Band and Amateur Radio operations. Sensitivities and mitigation have been known since 2013.
Interference with power line networking
AFCIs may interfere with the operation of some power line communication technologies.