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.
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