What is an Arc Fault Circuit Interrupter or AFCI?

The “AFCI” is an arc fault circuit interrupter. AFCIs are newly-developed electrical devices designed to protect against fires caused by arcing faults in the home electrical wiring.

Arcing faults: a series arc occurs in electrical wiring when there is a small gap or break in a conductor. a parallel arc occurs when a small gap or break which permits current to flow to ground (a ground fault) or between the hot and neutral wires (a short circuit). Arcing faults, especially parallel arcing faults, lead to overheating and a fire hazard even if no shock hazard is present.

AFCIs are an important safety addition to homes in part because they address an additional type of electrical fault that can cause a fire and one which may not be detected and interrupted by a conventional circuit breaker, nor by a ground-fault circuit interrupter (GFCI's).

What Types of AFCIs are Available? 

UL in January 2002 described various types of AFCIs which we summarize here. The first three types of AFCI's, Branch Feeder AFCIs, Outlet Circuit AFCIs, and Combination AFCIs are the three most basic types of arc fault detectors and are important definitions for the home owner or home inspector to understand:

1. Branch Feeder AFCIs - basically a special circuit breaker to be installed in the electrical panel and which will protect all of the devices on an individual electrical branch circuit. (15A or 20A 125V single phase wiring.)
2. Outlet circuit AFCIs - a device installed right in the branch circuit receptacle box. As with GFCIs this device may also protect wiring which is connected "downstream" electrically from the device itself.
3. Combination AFCIs - this device combines the function of Branch Feeder AFCIs and Outlet Circuit AFCIs and will protect power cords plugged into receptacles protected by the AFCI.
   
   The following are additional types of AFCI's described by Underwriters Laboratories.
   
4. Outlet Branch Circuit AFCIs - this device, which sounds to us just like #2 above, is installed as the first outlet in a string of electrical receptacles on a single circuit; it protects the downstream receptacles.
5. Portable AFCIs - these devices can be plugged into a conventional electrical receptacle and provide one or more outlets into which additional devices can be connected. This device would be used by contractors working on a building to obtain additional electrical safety when using power tools.
6. Cord-AFCIs - this device is like the portable AFCI #5 above, but may be incorporated into the permanent power cord of a device or appliance so that when it is plugged into an electrical receptacle the appliance is protected from arc faults.
7. Leakage Current Detection and Interruption LDCIs - these devices are built into a device or appliance and detect current leakage from the device's electrical cord. If a hand held hair dryer contained this device and its damaged cord was dangled in a sink filled with water, the LDCI would detect the current leakage and would shut off the device.

How does an Arc Fault Circuit Interrupter (AFCI) Work?

Conventional circuit breakers only respond to overloads and short circuits; so they do not protect against arcing conditions that produce erratic current flow. An AFCI is selective so that normal arcs do not cause it to trip.

The AFCI circuitry continuously monitors current flow 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.

Presently, AFCIs are designed into conventional circuit breakers combining traditional overload and short-circuit protection with arc fault protection. AFCI circuit breakers (AFCIs) have a test button and look similar to ground fault circuit interrupter (GFCI) circuit breakers.

Some designs combine GFCI and AFCI protection. Additional AFCI design configurations are anticipated in the near future.

It is important to note that AFCIs are designed to mitigate the effects of arcing faults but cannot eliminate them completely. In some cases, the initial arc may cause ignition prior to detection and circuit interruption by the AFCI.

The AFCI circuit breaker serves a dual purpose – not only will it shut off electricity in the event of an “arcing fault”, but it will also trip when a short circuit or an overload occurs. The AFCI circuit breaker provides protection for the branch circuit wiring and limited protection for power cords and extension cords. Single-pole, 15- and 20- ampere AFCI
circuit breakers are presently available.

Where should Arc Fault Circuit Interrupters (AFCIs) be used?

The 1999 edition of the National Electrical Code, the model code for electrical wiring adopted by many local jurisdictions, requires AFCIs for receptacle outlets in bedrooms, effective January 1, 2002. Although the requirement is limited to only certain circuits in new residential construction, AFCIs should be considered for added protection in other circuits and for existing homes as well.

Older homes with aging and deteriorating wiring systems can especially benefit from the added protection of AFCIs. AFCIs should also be considered whenever adding or upgrading a panel box while using existing branch
circuit conductors.

We've seen that arcing of any type can result in burned debris on wire surfaces which causes an increase in electrical resistance and thus overheating at that point. Arcing was examined earlier in detailed studies of the aluminum electrical wiring fire hazard at connections in the wire. Arcing of any type, whether it is the micro-fretting type of arcing that occurs with aluminum wire or possibly larger arcing across a gap or short in a copper wire.

Electricity Basics for Homeowners

Working safely and efficiently with your home wiring and appliances is easier if you understand what electricity is and how it works.
Electricity generating stations: Electric utility generating stations convert either fossil fuel (coal, oil), hydroelectric energy (flowing water), or atomic energy (nuclear reactors) to electrical energy.

The electric energy generated at a power station or electric utility is transported by wires to the factories, offices, schools, and homes that use it.

What Makes Electricity "Flow" in an Electrical Circuit

The short answer is "potential" which is a confusing synonym for "volts" or electrical pressure between an electrical power source (which is pushing at 120V or 240V) and earth (which is sitting at zero V) .

Why do We Need Two Wires for an Electrical Circuit in a Building? 

Electricity is generated by causing all the free electrons in a conductor to move in the same direction.This creates a surplus of electrons in the atoms of one wire at the output of a generator and a shortage of electrons in the atoms of a second output wire from the generator.
When an electrical device is connected to these two wires, electrons will move along an electrical path through the device in order to restore the natural balance.

Why are Some Electrical Wires Called Hot, Neutral, or Ground Wires?

 

Hot or Live electrical wires refer to wires that are connected to a source of electrical power. By convention the hot or live wire in residential circuits is usually black or red - but be careful, someone may have mis-wired or used the wrong color wire. If you touch a live wire and are also connected to the earth (standing on it, touching a water pipe, etc) chances are good that you'll receive an electrical shock, potentially a fatal one.
But beware: the neutral wire and even a ground wire (discussed below) are carrying electrical current in some conditions, and are also potential sources of electrical shock, especially if the electrical wiring in a building is defective.
Ground means simply Mother Earth or something connected to earth, such as a cold water pipe in your home or a copper rod driven onto the ground outside your house near where electric power enters.
Literally, electricity in buildings flows from a power source to the earth. In some countries what is called "ground" in the U.S. is called "earth". Buildings with safe electrical wire contain one or more local connections to earth, wired from the electrical panel, through a grounding electrode (a ground rod), to the earth. The white or neutral wire is connected to this ground in the electrical panel. (A second "ground" connection in electrical panels connects back to a remote ground or earth connection provided (somewhere) by the utility company.))

Ground, and wires connected to ground, can accept electrons or give them up as necessary to cause current to flow between ground and a point at which a shortage or surplus exists.
While both grounding wires and neutral wires are connected to ground, there is a difference in the job each performs in electrical wiring. The job of the ground wire is to provide a path to ground for electric energy when faults occur in the primary power wiring or in electrical devices.  
Grounding wires may have green insulation or be bare (no insulation).
Neutral wires: The job of the white wire (neutral) is to provide the normal path for return current flow to the source when no wiring faults exist. The term hot wire refers to the wire with black or red insulation. This is the wire that causes cur­rent to flow between it and the neutral wire (or grounding wire if a fault occurs. 

 

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