Arc-fault circuit interrupter
Encyclopedia
An Arc Fault Circuit Interrupter (AFCI) is a circuit breaker
designed to prevent fires by detecting a non-working (i.e., non-intended/non-useful) electrical arc
and disconnecting the power before the arc starts a fire
. An AFCI should, but may not always, distinguish between a working arc that may occur in the brushes of a vacuum cleaner, on operation of a light switch, the insertion or removal of a plug into an electrical receptacle, or during the operation of other household devices and a non-working arc that can occur. For example, a lamp cord that has a broken conductor in the cord from overuse.
Arc faults in a home are one of the leading causes for household 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 through the AFCI. AFCIs use unique current sensing circuitry to discriminate between normal and unwanted arcing conditions. Once an unwanted arcing condition is detected, the control circuitry in the AFCI trips the internal contacts, thus de-energizing the circuit and reducing the potential for a fire to occur. An AFCI should not trip during normal arcing conditions, which can occur when a switch is opened or a plug is pulled from a receptacle.
AFCIs resemble a GFCI/RCD (Ground-Fault Circuit Interrupt/Residual-Current Device)
in that they both have a test button although it is important to distinguish between the two. GFCIs and RCDs are designed to protect against electrical shock while AFCIs are primarily designed to protect against arcing and/or fire.
in Canada
(CSA Standard C22.1), the national codes require AFCIs in all circuits that feed outlets in bedroom
s of dwelling units. This requirement is typically accomplished by using a kind of circuit-breaker (defined by UL
1699) in the breaker panel that provides combined arc-fault and overcurrent
protection. Not all US jurisdictions have adopted the AFCI requirements of the NEC as written.
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.
The advanced electronics
inside an AFCI breaker detect sudden bursts of electric current
in millisecond
s; long before a standard circuit breaker
or fuse
would trip. A "combination AFCI breaker" will provide 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.
In 2002 the NEC removed the word "receptacle", leaving "outlets", with the effect for lights and other wired-in devices such as ceiling fans within bedrooms of dwellings are added to the requirement. The 2005 code made it more clear 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, amongst other things.
As of January 2008 only "combination type" AFCIs will 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.
Also, AFCIs provide no specific protection against "glowing" connections, excess current, high line voltages or low line voltages. (Excess current is protected against in all currently marketed AFCI devices that are designed to mount in circuit breaker panels, as all of these AFCI devices incorporate the function of a standard inverse-time circuit breaker and add to it the AFCI function.) Glowing connections occur when relatively high electric 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 degrees Celsius 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, see "Coefficient of expansion" in aluminium wire
, 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 premise 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.
Excess current can heat entire lengths of wire. Thermal circuit breakers are designed to protect against excess current through the permanent circuit wiring. However excess current through the smaller wires in equipment can exist at levels below the trip thresholds of a circuit breaker. It is the responsibility of the equipment designer or producer to ensure that these smaller wires are protected against overload, as no device installed as part of a permanent building wiring system can do this independently.
High line-voltage creates excess power and heat in utilization devices such as heaters, light bulbs, appliances, motors and other electronics. In extreme cases, this heat can ignite fires. One common source of high line voltage occurs from a 'neutral' path opening within a multi-wire branch circuit. A three-conductor cable supplying two 120V circuits using two ungrounded conductors ('hot legs') at 240V to each other, and one grounded conductor ('neutral') at 120V to each 'hot', may deliver 240V to connected 120V-rated loads if the ungrounded conductor is opened because of a failed connection or worker error. When the grounded conductor breaks or opens, the utilization equipment 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 normal power and heat under load. During such overheating, some utilization equipment can reach self-ignition temperature in less than 10 minutes. Power fault circuit interrupters (PFCIs) are designed to prevent fires caused by 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 relay
s to repeatedly turn off (relay opens) and on (relay closes again). If current is flowing through the load contacts it will cause arcing across the contacts when 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.
technologies.
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...
designed to prevent fires by detecting a non-working (i.e., non-intended/non-useful) electrical arc
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...
and disconnecting the power before the arc starts a fire
Fire
Fire is the rapid oxidation of a material in the chemical process of combustion, releasing heat, light, and various reaction products. Slower oxidative processes like rusting or digestion are not included by this definition....
. An AFCI should, but may not always, distinguish between a working arc that may occur in the brushes of a vacuum cleaner, on operation of a light switch, the insertion or removal of a plug into an electrical receptacle, or during the operation of other household devices and a non-working arc that can occur. For example, a lamp cord that has a broken conductor in the cord from overuse.
Arc faults in a home are one of the leading causes for household 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 through the AFCI. AFCIs use unique current sensing circuitry to discriminate between normal and unwanted arcing conditions. Once an unwanted arcing condition is detected, the control circuitry in the AFCI trips the internal contacts, thus de-energizing the circuit and reducing the potential for a fire to occur. An AFCI should not trip during normal arcing conditions, which can occur when a switch is opened or a plug is pulled from a receptacle.
AFCIs resemble a GFCI/RCD (Ground-Fault Circuit Interrupt/Residual-Current Device)
Residual-current device
A Residual Current Device is a generic term covering both RCCBs and RCBOs.A Residual-Current Circuit Breaker is an electrical wiring device that disconnects a circuit whenever it detects that the electric current is not balanced between the energized conductor and the return neutral conductor...
in that they both have a test button although it is important to distinguish between the two. GFCIs and RCDs are designed to protect against electrical shock while AFCIs are primarily designed to protect against arcing and/or fire.
Electrical code requirements
Starting with the 1999 version of the National Electrical Code (NFPA 70) in the United States (US), and the 2002 version of the Canadian Electrical CodeCanadian Electrical Code
The Canadian Electrical Code, CE code, or CSA C22.1 is a standard published by the Canadian Standards Association pertaining to the installation and maintenance of electrical equipment in Canada....
in Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
(CSA Standard C22.1), the national codes require AFCIs in all circuits that feed outlets in bedroom
Bedroom
A bedroom is a private room where people usually sleep for the night or relax during the day.About one third of our lives are spent sleeping and most of the time we are asleep, we are sleeping in a bedroom. To be considered a bedroom the room needs to have bed. Bedrooms can range from really simple...
s of dwelling units. This requirement is typically accomplished by using a kind of circuit-breaker (defined by UL
Underwriters Laboratories
Underwriters Laboratories Inc. is an independent product safety certification organization. Established in 1894, the company has its headquarters in Northbrook, Illinois. UL develops standards and test procedures for products, materials, components, assemblies, tools and equipment, chiefly dealing...
1699) in the breaker panel that provides combined arc-fault and overcurrent
Overcurrent
In electricity supply, overcurrent or excess current is a situation where a larger than intended electric current exists through a conductor, leading to excessive generation of heat, and the risk of fire or damage to equipment. Possible causes for overcurrent include short circuits, excessive load,...
protection. Not all US jurisdictions have adopted the AFCI requirements of the NEC as written.
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.
The advanced electronics
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
inside an AFCI breaker detect sudden bursts of electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
in millisecond
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
s; long before a standard circuit breaker
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...
or fuse
Fuse (electrical)
In electronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit...
would trip. A "combination AFCI breaker" will provide 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.
In 2002 the NEC removed the word "receptacle", leaving "outlets", with the effect for lights and other wired-in devices such as ceiling fans within bedrooms of dwellings are added to the requirement. The 2005 code made it more clear 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, amongst other things.
As of January 2008 only "combination type" AFCIs will 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.
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, identifying normal circuit behaviors as arc faults. For instance, lightning strikes to the outside of an airplane provide voltage and current profiles that resemble arc faults. This nuisance tripping reduces the overall effectiveness of AFCIs. Research into advancements in this area are being pursued.Also, AFCIs provide no specific protection against "glowing" connections, excess current, high line voltages or low line voltages. (Excess current is protected against in all currently marketed AFCI devices that are designed to mount in circuit breaker panels, as all of these AFCI devices incorporate the function of a standard inverse-time circuit breaker and add to it the AFCI function.) Glowing connections occur when relatively high electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
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 degrees Celsius 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, see "Coefficient of expansion" in aluminium wire
Aluminium wire
Aluminum wire is a type of wiring used in houses, power grids, and airplanes. Aluminum provides a much better conductivity to weight ratio than copper, and therefore is used in power wiring of some aircraft....
, 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 premise 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.
Excess current can heat entire lengths of wire. Thermal circuit breakers are designed to protect against excess current through the permanent circuit wiring. However excess current through the smaller wires in equipment can exist at levels below the trip thresholds of a circuit breaker. It is the responsibility of the equipment designer or producer to ensure that these smaller wires are protected against overload, as no device installed as part of a permanent building wiring system can do this independently.
High line-voltage creates excess power and heat in utilization devices such as heaters, light bulbs, appliances, motors and other electronics. In extreme cases, this heat can ignite fires. One common source of high line voltage occurs from a 'neutral' path opening within a multi-wire branch circuit. A three-conductor cable supplying two 120V circuits using two ungrounded conductors ('hot legs') at 240V to each other, and one grounded conductor ('neutral') at 120V to each 'hot', may deliver 240V to connected 120V-rated loads if the ungrounded conductor is opened because of a failed connection or worker error. When the grounded conductor breaks or opens, the utilization equipment 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 normal power and heat under load. During such overheating, some utilization equipment can reach self-ignition temperature in less than 10 minutes. Power fault circuit interrupters (PFCIs) are designed to prevent fires caused by 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 relay
Relay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...
s to repeatedly turn off (relay opens) and on (relay closes again). If current is flowing through the load contacts it will cause arcing across the contacts when 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.
Interference with power line networking
AFCIs may interfere with the operation of some power line communicationPower line communication
Power line communication or power line carrier , also known as power line digital subscriber line , mains communication, power line telecom , power line networking , or broadband over power lines are systems for carrying data on a conductor also used for electric power transmission.A wide range...
technologies.
External links
- Underwriters Labs summary on UL 1699 standard on AFCIs
- www.AFCIsafety.org – AFCI Educational Web site
- CPSC AFCI Fact Sheet
- AFCIs: Emerging Technology Solutions for Fires Resulting from Electrical Arcing from IAEI News: The Magazine
- EFCI / OFCI on Popular Science
- http://www.ul.com/regulators/afci/categories.cfm 6 types of AFCIs are listed by Underwriters Laboratory