Arc-Fault Circuit Interupters
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.
ARC-FAULT CIRCUIT INTERRUPTERS (AFCIs)
An Arc-Fault Circuit Interrupter (AFCI) is a circuit breaker designed to prevent fires by detecting an unintended electrical arc and disconnecting the power before the arc starts a fire. An AFCI should distinguish between a harmless arc that may occur in the brushes of a vacuum cleaner, an 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 an undesirable arc that can occur, for example, in a lamp cord that has a broken conductor in the cord.
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 monitor 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 Interrupter/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 fire.
Electrical Code Requirements
Starting with the 1999 version of the National Electrical Code (NFPA 70) in the United States and the 2002 version of the Canadian Electrical Code in Canada (CSA Standard C22.1), the national cods require AFCIs in all circuits that feed outlets in bedrooms of dwelling units. This requirements is typically accomplished by using a type of circuit breaker 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 fires from arcs. AFCI circuit breakers are designed to meet one of two standards as specified by UL 1699: “branch” type or “combination” type. 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 milliseconds; 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 include 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 on combination-type AFCIs in all 15 and 20 ampere residential circuits with the exception of laundries, kitchens, bathrooms, garages, and unfinished basements.
AFCIs are designed to protect against fires caused by electrical arc faults. While the sensitivity of 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 exists in a relatively high 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, 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 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 wiring 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 120 volt circuits using two ungrounded conductors (“hot legs”) at 240 volts to each other, and one grounded conductor (“neutral”) at 120 volt to each “hot”, may deliver 240 volts to connected 120 volt rated loads if the grounded 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 volts 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 relays to repeatedly turn off (relay opens) and on (relay closes again). If current is flowing through the load contacts it will cause arcing in 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.
(Credits Attributed to Wikipedia, April 26th, 2012)
Posted: to Safety Notices on Sun, Jun 1, 2014
Updated: Sun, Jun 1, 2014