AFCI - What Its All About
By Mike Holt for EC&M magazine, back to basics.
Arcing is defined as a luminous discharge of electricity
across an insulating medium. Electric arcs operate at temperatures of between 5,000 and
15,000°F and expel small particles of molten or burning materials from the center. Higher
current arcs are more likely to cause a fire because of the higher energy in the arc.
Greater current will melt more of the conductor metal and therefore expel more molten
particles. The volume of hot, ionized gas emitted increases proportionally with energy.
Arcing faults can occur in one of two ways, series arcing faults
or parallel arcing faults, but the most dangerous of these is the parallel arc. A series
arc can occur when the conductor in series with the load is unintentionally broken. Examples
might be a frayed conductor in a cord that has pulled apart or a loose connection to a
receptacle or in a splice. A series arc is load limited, such that arc current cannot
be greater than the load the conductor serves. Current with an arc in series has a lower
rms value than current without the arc due to extinction and re-ignition. Typically, series
arcs do not cause enough heat to create a fire.
Parallel arcing faults either occur in two ways, either a short
circuit or a ground fault.
Short-Circuit Arc. A short circuit arc might occur if the wire insulation is cut by a
staple or a cord is cut by a metal table placed on it. The current flow of a short-circuit
arc is only limited by the system impedance and the impedance of the fault itself. A ground
fault arc can occur only when a ground path is present, and this fault can be cleared
by either GFCI or AFCI protection device. The rms current value for parallel arc faults,
will be considerably less than that of a solid fault, and a typical 15 A might not clear
this fault before a fire is ignited.
To protect against fires, the NEC requires AFCI protection of the branch circuit
wiring in dwelling unit bedrooms, see 210.12. In addition, UL 1699 contains the requirement
for listing AFCI devices. Each type of AFCI protection device is intended to protect different
aspects of the branch circuit and extension wiring.
- Branch/Feeder AFCI - This device is installed
at the origin of a branch circuit or feeder, such as at a panelboard, to provide AFCI
parallel arc protection of the branch or feeder circuit wiring. This device also protects
against a parallel arc in the cord sets and power supply cords. This is the device
that is required by the NEC.
- Outlet Circuit AFCI - This device, which
is installed at a branch circuit outlet, provides AFCI parallel protection of the
cord sets and power-supply cords plugged into the outlet. It does not provide AFCI
protection on feed-thru branch circuit conductors.
- Combination AFCI - This device provides
AFCI parallel protection for branch circuit wiring, cord sets and power-supply cords,
downstream of the device. Typically this device is a receptacle.
UL 1699 requires testing of the AFCI through a rigorous set
of tests for arc detection ability, unwanted operation tests (to avoid nuisance operation),
and operation inhibition tests. The operation inhibition tests assure that the AFCI will
detect an arc even though it may be connected electrically in series or parallel with
loads that might attenuate, mask or otherwise tend to hide the arc signal.
If you have any feedback, let me know, Mike@MikeHolt.com.
Why is AFCI protection only required for branch circuit
The NEC Code panel want the industry to gain experience with these devices in bedroom
circuits so that in the future their usage might be expanded to other rooms and facilities
that could benefit by the added protection to provide.
Studies have shown that over 60 percent of fires are from causes
in the fixed wiring, switches, receptacle outlets and lighting fixtures that are part
of the fixed electrical system of a residence. This data soundly supports the present
NEC language that requires AFCI protection at the branch.
The following proposals were all rejected:
- The U. S. Consumer Product Safety Commission request that
existing 125-volt, single-phase, 15-and 20-ampere lighting and appliance branch circuit
be individually protected by an arc-fault circuit interrupter when the service equipment
- Lighting not be AFCI protected, because light may be needed
when the AFCI device operates and cord wiring extends from receptacle outlets, not
from lighting outlets.
- Extend AFCI protection for branch circuit for guest rooms
of motels and hotels .
- Permit the AFCI receptacle outlet type or AFCI breaker
to provide the required protection.
- Omit AFCI protection for the smoke detector circuit conductors.
- Delete the AFCI requirement completely.