Arc Fault Circuit Interrupters (AFCIs) are a type of electrical safety device that are designed to detect and interrupt dangerous electrical arcs that can occur in a circuit. These arcs can be caused by a variety of factors, such as damaged or frayed wires, loose connections or even pests that may have chewed through the insulation of a wire. AFCIs work by monitoring the current flowing through a circuit and detecting any abnormal or dangerous arcs that may occur. When an arc is detected, the AFCI quickly interrupts the flow of electricity, preventing a fire from occurring. This article will explore the definition and working principle of AFCIs in more detail.
What is an Arc Fault Circuit Interrupter?
Arc fault circuit interrupter (AFCI) is a new protective device that provides enhanced protection against additional fire hazards known as arc faults. It replaces a standard circuit breaker in the electric service panel. Without AFCIs, arc faults may be hidden from plain view until it is too late.
An arc fault is a dangerous electrical problem caused by damaged, overheated, or stressed electrical wiring or devices. Electrical arcing causes more than 30,000 fires a year, according to the National Fire Protection Association.
The U.S. Department of Housing and Urban Development’s Healthy Homes Report listed the absence of AFCIs among the primary residential hazards associated with burns and fire-related injuries.
These devices are so effective that the 2008 edition of the National Electrical Code now requires that they be used to protect almost every circuit in the home.
At present, the National Electrical Code requires that arc-fault circuit interrupters be used on all 120 volts, single-phase, 15- and 20-ampere circuits installed in dwelling units supplying power to family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, or similar rooms or areas.
The Purpose of an Arc Fault Circuit Interrupter
The purpose of an arc fault circuit interrupter, other than functioning as a common breaker, is to detect series and parallel arcs that could start a fire in the house wiring and extension cords.
A 30 mA to 50 mA (0.030-amp to 0.050-amp) ground-fault detection circuit was added to the breaker to make it more sensitive. Some arc fault circuit interrupters are dual-listed as both AFCIs and GFCIs (Ground fault circuit interrupters) and will trip at around 5 mA. AFCI trips faster than a common breaker, which hopefully will save lives.
What is an arc fault?
An arc fault is an unintentional electrical discharge characterized by a low and erratic current, which can start a fire. Three common types of arcs may occur in your home:
Hot to neutral: The most common example is a too-tight NM connector and a too-tight staple.
Hot to ground: An arc between a wire and ground that isn’t sustained long enough to open the circuit of a common breaker.
Series: A wire that is cut but comes back in contact with itself through use or other movements. Extension and line cords are primary examples.
Regular circuit breakers don’t always trip in these instances because it’s not something they were designed to do. For example, most breakers rely on a certain sustained amount of heat to trip. If the surge is short enough, a tremendous current can flow through the overcurrent device (breaker) without tripping it. (This is why lightning surges of 10,000+ amps can flow through a breaker.) Arcs that are less than the breaker’s overcurrent rating also occur. For example, an arc current of fewer than 20 amps may not trip the breaker, but it can start a fire.
How Does an Arc Fault Circuit Interrupter Work?
The arc-fault circuit interrupter samples the electrical current and uses electronic circuitry to detect the waveforms produced by arc faults created by a nail hitting a cable inside a wall, a cable or cord overheating, a cable or cord being pinched, or a staple being sunk into a cable. A bad waveform is nonperiodic and nonrepetitive and has certain frequencies associated with it. The AFCI will sample all the data and open the circuit if needed.
What makes AFCIs so controversial is that it is very hard to create a do-it-yourself field test to prove that they actually work. Because of this, some wonder if they will work as expected in actual practice. Just remember that AFCIs will not open the circuit for every arc in every situation. They will not eliminate all arc fires, as some advertise they will. They work for specific arcs in specific situations.
There are conditions where arcing in an electric circuit is normal, such as these:
- Turning a light switch on or off
- Switching on or off of a motor relay
- Plugging in an appliance that is already turned on
- Changing a light bulb with the power turned on
- Arcing caused by motors that contain a commutator and brushes
There are two broad categories of electrical circuit failure that produce electrical arcs that are ignition capable. The first, referred to in the trade as “parallel arcs,” are those that involve ground faults to either the wiring method itself or to other grounded objects. The second referred to as “series arcs,” are those that involve a failure in a single conductor to itself. A classic example of this category is a damaged extension cord where the last few strands, with their broken ends in very close proximity, sputter until a fire occurs.
The arc-fault circuit interrupter is designed to be able to distinguish between normally occurring arcs and an arc fault. An arc caused by a toggle switch being used to turn a light on or off will produce a current spike of short duration. An arc fault, however, is an intermittent connection and will generally produce current spikes of various magnitudes and lengths of time.
For an arc-fault circuit interrupter to determine the difference between a normally occurring arc and an arc fault, a microprocessor and other related electronic components are employed to detect these differences.
The AFCI contains current and temperature sensors as well as a microprocessor and nonvolatile (retains its information when power is switched off ) memory. The current and temperature sensors permit the AFCI to operate as a normal circuit breaker in the event of a circuit overload or short circuit. The microprocessor continuously monitors the current and compares the waveform to the information stored in the memory. The microprocessor is monitoring the current for the magnitude, duration, and length of time between pulses, not for a particular waveform. For this reason, some appliances can produce waveforms similar to that of an arc fault and may cause the AFCI to trip. Appliances containing motors that employ the use of brushes and a commutator, such as vacuum cleaners and hand drills, will produce a similar waveform.
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