A Series of Faults: The emergence and advancement of AFCIs

Knob-and-tube wiring on a residential renovation site | Shutterstock / Alessandro Cancian
Knob-and-tube wiring on a residential renovation site | Shutterstock / Alessandro Cancian
Published On
Mar 15, 2021

Knob-and-tube wiring was used in buildings as a source of electrical energy from the 1880s. From the late 1930s–1950s, nonmetallic sheathed cable (type NM cable) and armored cable (type AC cable) were installed without an equipment grounding conductor or a reduced-size equipment grounding conductor. As such, most electrical systems in buildings did not require a branch circuit equipment grounding conductor and ungrounded receptacles were installed.

In the 1999 National Electrical Code , 210-12(A) defined an AFCI as a device intended to provide protection from the effects of arc faults by recognizing the characteristics unique to arcing faults and functioning to de-energize the circuit when an arc fault is detected.

The first AFCI protection was in dwelling unit bedrooms, and subsequent editions required it for most 125V, single-phase, 15A and 20A branch circuits throughout the house. Before any existing branch circuits can be protected in older homes, a thorough understanding of AFCIs and their operation must be determined. New requirement 210-12(B) is based on the installation of AFCI protection for all branch circuits that supplied 125V, single-phase, 15A and 20A receptacle outlets.

Since people sleep in bedrooms at night, Panel 2 decided to make bedrooms a first step until AFCI protection could prove itself and become more accepted by the electrical industry and the public. In the 2002 NEC , AFCI protection was expanded to cover any outlets located within bedrooms, including smoke alarms.

Later NEC editions required all 125V, single-phase, 15A and 20A branch circuits supplying outlets, as well as devices such as switches for security lighting outside, to be AFCI-protected. Early AFCI circuit breakers were called branch/feeder AFCIs and these specialized breakers recognized electrical arcs called parallel electrical arcs.

A parallel electrical arc often occurred due to staples supporting NM cables that were driven too deeply into a wood stud, causing a low- or high-resistant fault between the hot conductor and the neutral conductor in the cable. When the electrical arc in these circuits reached a peak fault current level of approximately 75A and the arcing signature was recognized, the circuit breaker would trip, protecting the circuit. These devices were designed and tested to not trip from common arcs, such as arcs caused by switches with contacts opening and closing or motor-starting capacitors with centrifugal contacts.

When these AFCI circuit breakers were being developed, the manufacturers and UL tested them with all different types of electrical arcing components to ensure the circuit breakers would not react with false tripping. In addition, AFCI circuit breakers were designed to trip at a minimum level of 30 milliamps (mA) of current as ground-fault protection. Where ground-fault circuit interrupter protection for personnel with a ground-fault trip value of 4 to 6 mA or more was required, based on 210.8(A), a combination AFCI/GFCI device was also available.

AFCI and GFCI devices required a neutral connection through the circuit breaker device for the solid-state devices to properly sense the arcs in AFCI-protected circuits and ground faults for GFCI-protected circuits. These AFCI/GFCI circuit breakers were called dual-function circuit breakers since the breakers were providing dual protection.

Since the early branch/feeder AFCIs only provided protection for parallel arc faults, manufacturers developed AFCI circuit breakers that would provide parallel arc-fault protection, as mentioned in previous paragraphs, as well as series arc-fault protection. These were appropriately named combination-type AFCIs (not to be confused with the dual-function circuit breakers mentioned earlier). This type was required as of the 2005 NEC .

A series arc fault is one that occurs in a conductor where the conductor has a gap, such as where a cord has been damaged internally, and there is a resultant arc from one side of the gap to the other. This series fault could occur at a point of connection of the conductor to a receptacle or a switch terminal or within an extension cord installed under a rug. These series faults could cause high heat at the point of the arc, resulting in a fire. These combination-type AFCI devices provide protection for these circuits.

Knob-and-tube wiring branch circuits or ungrounded branch circuits in older homes may not operate properly for this series or parallel-fault current protection. Exercise care when installing extensions to these circuits or where replacing services or panelboards supplying power for these types of circuits, since 210.12 would require this type of protection for these circuits.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com.

Stay Informed Join our Newsletter

Having trouble finding time to sit down with the latest issue of
ELECTRICAL CONTRACTOR? Don't worry, we'll come to you.