Arc Fault Circuit Interrupters

What Is the AFCI and Where Might You Use One at Your Radio Station?
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What Is the AFCI and Where Might You Use One at Your Radio Station?

What is this new circuit breaker, the Arc Fault Circuit Interrupter? How is it better than the CBs we have?

Let me tell you a little story.

Recently I was working in a live branch panel, adding breakers. The curve of the incoming supply wires from the conduit entrance to the input lugs was a little in the way of snapping in one of the new CBs onto the panel rails. My big trusty screwdriver "persuader" should have been able to move those #4s just enough to allow me to do this.

Crack! I drew an arc that startled me, and I'm not easy to startle, being a veteran of many massive "Arcs of the Covenant."

The electrician who wired the box apparently had changed his mind midstream. He had started to cut the supply wires to length but left another 4 inches on them instead.

I had caught the little, hidden, bare section of wire on the backside where he had started his cut. My screwdriver connected with this bare spot, shorting the wire to box ground.

This quick arc destroyed an otherwise excellent Craftsman screwdriver (not covered under the lifetime Craftsman warranty). But did it trip the 60-amp supply breaker? No, because the overload was neither long enough nor sufficient in intensity to get over the "trip curve" of the standard main supply breaker.

Had the breaker been a Ground Fault Interrupter (GFI) or one of the new AFCIs, would it have tripped? Yes, in a heartbeat plus a smidge.

Trip curve

A standard CB has what is known as a trip curve. When current passing through the circuit breaker exceeds a design value, it will open the supply feed. When and how fast this happens is graphed by the trip curve.

A standard CB normally does this in two ways.

The first is a thermal trip, in which a control element is heated. The rate of heat increase and final temperature are set by the current flowing through the CB.

For example, say you have a standard 20 amp CB in your panel supplying some racks. Around 20 amps, plus or minus about an amp, the thermal control portion of the circuit breaker starts noticing that you have reached the design value. Look at the bogus trip curve graph shown in the illustration. At 21 amps, this hypothetical CB will take 10 minutes to trip; at 25 amps, about a minute; at 30 amps, about 10 seconds.

The second action is magnetic. A heavy overload (design value of two to 10 times rated control current) instantaneously trips the CB. This typically is a solenoid design with a snap spring released to open the CB quickly and definitively. Note the small avalanche point on the graph.

In the incident I described, my arc didn't reach this instantaneous current point and was not long enough to trip the CB thermally.

My personal guess is that 90 percent of all CBs in the universe have these characteristics.


How is a GFI, like the one in your bathroom, different? Generally a GFI senses the current flowing down one supply wire and, reciprocally, the current flowing back on the other.

If the currents are equal (balanced), power is only being consumed in the load and going nowhere else. If the currents are unequal, a portion of the current is flowing someplace else - back to the generator via another path, normally through ground.

Modern GFIs use a variation of an op amp circuit in high amplification, CMR, differential mode. When the sensed current differential, impressed on the two inputs, exceeds a certain small window, this pseudo op amp goes into maximum gain, tripping open the GFI. That test button on the front actually places a tiny resistor between the high side line to ground (the neutral, normally), causing a microcurrent fault to ground. The resistor value is selected to be a current drain just above the design trip value, usually between 4 and 20 mas.

Out in the big wide world of electrical design and contracting, two types of GFI devices exist. One is the familiar GFI described, which simply opens the circuit when a ground fault appears. The other is a "GFI circuit breaker." This is a composite device that will open the circuit like a standard CB with overcurrent as well as when a ground fault appears. You will see these more complicated devices at disconnects for hot tubs, service entrances at 480 volt three phase above 1000 amps and similar situations.


So we come to the AFCI. What's the diff? Now we must shift gears and think small.

The standard CB senses overcurrent and behaves like a fuse; the GFI senses fault current that is finding its own path back to the generator.

The AFI senses current, sometimes way below the trip point of the standard CB and not sensed by a GFI, that is being dissipated in an arc essentially between the supply wires. The generator (normally the power company) and a general protection CB would just see this as a part of the load. An AFCI device looks at the waveform of the current flow and opens the circuit when that waveform resembles an arcing fault. (UL 943 is the standard for ground-fault circuit-interrupters and UL 1699 is the standard for arc-fault circuit-interrupters.)

Is this arcing dangerous?

Because most of the new NEC requirements mandate the use of the AFI in homes, let's start there. The Consumer Product Safety Commission informs us that 145,000 residential fires occur each year. (Do you know where your home fire extinguishers are?)

Some of these fires, especially those of unknown origin, are caused by undetected, usually minor arcs.

According to the CPSC, 13,000 preventable electrical fires claim more than 700 human lives, 6,700 injuries and $1.2 billion in personal property each year.

These arcs can occur in defective switchfields in appliances such as hair dryers or dishwashers; in defective cord sets of small electric appliances that are always plugged in like can openers; etc. Subtle arcs can occur in everyday tasks as pedestrian as driving a picture frame nail through Romex in an interior wall or placing a drywall screw placed during repair work even into armored cable such as Greenfield. Arcs exhibit different current flow/consumption patterns than most loads in a house or radio station. Most of the available AFI CBs have embedded microprocessor control to make this subtle differentiation and trip.

In the 2002 NEC, the primary mandate for AFI is in the bedroom area. But as prices drop and the finesse of AFIs improves over the years, we can expect to see them mandated in more areas of home and business.

The average price at the moment for a 15 or 20 amp single-pole AFI to retrofit into your CB panel is about $35. A QO type Square-D 15 amp AFCI sold at Home Depot in my area in December for $31.

An immediate location where I see immense value for the AFI is your repair shop. I suggest an AFI CB in the panel, followed by a downstream GFI to protect every outlet where you might come into contact with AC, such as the workbench circuit. If you plug in a device and either of the two, GFI or AFI, trips, you know there are serious AC side problems within that box. These trips mean the same thing at home if you plug in your favorite boombox from the '70s and the AFI trips.

If you happen to plug in yourself, there's a good chance you might preserve your life if one of the two interrupters trips quickly enough.


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