Arc-fault circuit interrupters (AFCIs) are sometimes confused with similar appearing ground-fault interrupters, GFCIs, which protect against electric shock. Rather than protecting against electric shock, AFCIs protect buildings and occupants from electrical fire. Internal electronics and operating principles of the two devices are entirely different.
A GFCI protects the occupant from electric shock by detecting the imbalance in the amount of current flowing in the hot conductor and the amount of current flowing in the return conductor. In accordance with Kirchoff’s electrical current law, the number of electrons that flows into any junction and the current that flows out is always the same. So that excess must go somewhere, and there is the possibility that the fault path is through a human body to ground. This type of fault is detected by the GFCI, and in response it instantly opens the circuit, interrupting the hazardous fault current.
The AFCI performs the same action, but rather than a ground fault, what is detected is the arc fault.
There are two quite different types of arc faults. A parallel arc fault is a partial fault involving the hot (usually black wire) conductor and the return conductor (usually white wire). This color coding is the same for wires enclosed in metal raceway and in Type NM (Romex) cable. The parallel arc fault typically occurs when a cable is pierced by a nail or screw that strikes both current-carrying conductors or partially grounds out the hot wire. The key word here is partially. If a total fault connection is made, the shunt causes the over-current circuit interrupter to open and the AFCI is not needed. However, there may be sufficient current to create a local rise in heat but not enough current to trip the breaker. Thus, a partial electrical fault may be more subtle and dangerous than a total short.
A partial series electrical fault may be still more insidious. In this case, that errant screw pierces but does not sever one of the current-carrying conductors. There is a continuous accumulation of heat but never a current overload. Only an AFCI guards against this condition.
Another source of a series arc fault in premises wiring is a loose connection. In feeder and branch circuit wiring, splices and terminations in the initial installation or in any subsequent rework must be tightened to the correct torque, not beyond. A torque screwdriver permits the user to conform torque values for each wire gauge, and this is essential in getting durable connections. Over-tightening also introduces a hazard, because the threads may be damaged, actually making a permanent loose connection.
Most wire conductors are cylindrical, and when they first touch, it is along a single radial line. This splice or termination lacks sufficient ampacity due to the limited area of contact and there is the possibility that the electrical resistance will cause a heat rise that could ignite combustible material, setting the stage for a full-scale fire. By torqueing the splice or termination by the correct amount, two or more wires in a connector or a single wire in a lug or under a screw will deform slightly so the connection has good ampacity and will not heat up. The object in torqueing an electrical connection is to produce the correct amount of deformation and also so there is enough friction to keep the connection from backing off because of vibrations or heat variations.
In the large number of splices and terminations in a building’s wiring system it is possible that one of them will be under- or over-torqued or subsequently damaged, and it is this situation that the AFCI addresses. Its inner working are a little more complex than the GFCI. The GFCI simply makes a continuous comparison of the amount of current flowing through the two circuit conductors, while the AFCI continuously monitors the current waveform in the circuit, looking for unique anomalies that signify an arc fault.
A series arc fault characteristically exhibits an irregular, sputtering frequency averaging 100 kHz bursts, which may be intermittent. The space between the severed conductors is comprised of the conductive metal, air and sometimes insulation fragments. Typically, such a mix conducts in a limited fashion. When voltage is first applied, there is no appreciable current. But if the voltage is sufficiently high, the material between the conductors abruptly breaks down, or in atomic terms, ionizes. Electrons are driven out of their orbits, attracted by the powerful charge. They then become charge carriers, and the now-ionized atoms also become charge carriers, completing the higher-current circuit.
A conventional circuit breaker, also incorporated in the AFCI, contains thermal and magnetic sensors that interrupt the power source, typically at the entrance panel, on the detection of a hazardous situation that could impose a load on downstream wiring greater than its ampacity.
AFCI electronics are more complex than either a conventional circuit breaker or a GFCI. The load current conductor and load neutral conductor, attached to the AFCI body, pass through an internal 30-mA ground-current sensor coil. Current from this coil is first amplified, then goes to a logic block. The logic block also receives input from the load current sensor via an arc signature filter and second amplifier. Thermal sensor, magnetic sensor and logic block are all connected to the circuit interrupter and current through the device is interrupted when thermal sensor, magnetic sensor or logic block activates.
A test circuit, governed by a pushbutton on the outside of the AFCI body that can be pressed by the user, connects to the arc signature filter, amplifier block and logic block to activate the device so that it can interrupt power for test purposes. A reset button is provided to restore power.
Like GFCIs. AFCIs are available in breaker as well a receptacle format, providing downstream protection but not upstream protection. The output of an AFCI, when fed to any downstream receptacle, allows that device to also protect against arc faults.
The installer must note, however, that the first AFCI in a given branch circuit does not protect the wiring that supplies it, and such protection is required for that wiring beginning at the point where it enters the room. Therefore, these conductors must be enclosed (from the point of entry to the first AFCI) in metal raceway or metal-clad cable. The usual choice is Type EMT or Type MC.
AFCIs in both breaker and receptacle formats are equipped with test and reset buttons. Like other electronic devices that interrupt power in the event of hazard, AFCIs are subject to false alarms. This is a troubling phenomenon for two reasons. It may lead the user to believe that an actual alarm is false, so that it is ignored, and it can fail to operate as required in the event of an actual hazard. Such erratic operation may be caused by electrical storms, where the extremely fast rise and fall times are high-frequency components induced in the electrical system, and also by laser printers and old brush motors including vacuum cleaners that exhibit excessive arcing. Other causes are CB and amateur radio operating in the 3-30 MHz range.
AFCIs are not sensitive to low voltage, which however can cause electromechanical relays to cycle excessively, causing arcing and ultimate heating of the contacts, leading to fire.