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necdigest

ew Products for Integrated Electrical Systems/Questions and Answers/Resource

eb/March 2002 Volume 1

Arc-Fault InterruptersBringing a new level of electricalprotection into the home

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Each year in the United States, residential electrical fires result in more than 700deaths, 3,000 injuries and $700 million in property damage1. A number of thesefires begin with little warning, kindled by sputtering arc faults in damaged or dete-riorated wiring. The arc fault creates a spark, generates heat, and eventually

ignites nearby combustible material.

necdigest.org feb/mar 2002 necdigest

FPAs Official NEC

Magazine The Voice of Authority wwww.necdigest.org

by Dr. Joseph Engel

Seenin

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Estimated Short Circuit Current Available (Amperes) vs.Percentage of Circuits Having Estimated Short Circuit Current Available

Figure 1 The distribution of available fault currents, at 15A household receptacles Curve (A), and at the end of six feet of #18appliance wire plugged into those receptacles, Curve (B).

Due to the development of advanced, affordable circuit

interrupter t echnology and the recently updatedN EC

code, however, fires and deaths att ributable to arc faults

may soon be on the decline.

Building On Conventional Breaker SafetyConventional residential circuit breakers prevent fires by

automatically opening the circuit before conductors orthe insulation that shields them can be damaged by

excessive and dangerous temperatures. The response

times of conventional circuit breakers are determined

solely to protect against circuit overload 2 or overcurrent.

However, the response is inadequate for protecting

against the fire hazards a ssociated with arcs, the temper-

atures of which can exceed 6000oC.

Arcs also have an extremely short dura tion, which

cannot typically be detected by conventional breakers.

The challenge, therefore, has been to improve circuit

protection by identifying the presence of arcing faults

and responding to them fast enough to prevent fire.Modern electronics has answered this challenge with

Arc-Fault Circuit Interrupters (AFCIs), devices that rec-

ognize the unique current signatures associated with arc-

ing faults and act to interrupt the circuit before the tem-

perature of combustibles can rise to hazardous levels.

Common Wiring HazardsHazards in residential wiring systems develop from

either high-energy arcing (parallel) or high-resistance

(series) faults.3 High-energy arcing faults are caused by a

failure of the insulation between conductors at different

potentials that a re located anywhere on the circuit.

Insulation can fail over t ime due to extended exposure

to moisture, heat, or extraord inarily high voltages.

Insulation can also be accidentally cut or damaged by

nails, staples or other materials. The age of the wiring

or physical abuse can a lso be contributing factors.

Once the insulation is compromised, heat from an arc

melts the conductors in the wire and the electricity seeksa new path. As it develops so does a sporad ic or inter-

mittent arc fault. Under these conditions, conventional

circuit breakers may not trip either magnetically, since

peak currents are too low, or thermally, since the dura-

tion of the current is too short to cause a thermal ele-

ment to trip. If the breaker should tr ip, it will only do

so after an unacceptable delay.

A high-resistance fault is not an arc fault initially, but

it can become one over time. A layer of copper or alu-

minum oxide that forms at connections, such as at wire

nuts, receptacle terminals, or plugs, creates this type of

fault. The coating replaces the low resistance path withone of high resistance and begins to generate heat.

Broken conductors can also create high-resistance faults.

Regardless of the cause, local I2R power dissipation

can overheat and pyrolyse the insulation leading to a

parallel arcing fault. Like the situations that create high-

energy faults, conventional breakers will not always

detect the situation and trip.

The magnitude of parallel faults in residential circuits

can be estimated from Curve A of Figure 1, which is

derived from a UL Report 4 prepared for the Electronics

Industries Alliance. The curve shows the distribution of

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Figure 2 I Typical current waveforms observed when

a carbon-steel blade cuts through 16 AWG SPT-2 cord.

The available current is 100A.

The challenge has been to improve circuit protection by

identifying the presence of arcing faultsand responding to them fast enough to prevent fire.

aavailable fault currents at household receptacles. The X-

axis displays the estimated short circuit currents available;

the Y-axis shows the percentage of circuits that have partic-

ular values of short circuit current available or higher.

The data applies to bolted faults with the line and neu-

tral securely clamped together. For 15A receptacles, the

available fault levels are 75A RM S or higher. Only half have

available levels of 250A or higher. These relatively low shortcircuit levels are due to the impedance of fixed premise

wiring. The impact of wire impedance is further emphasized

by Curve B, which shows the available fault currents with

six feet of #18 appliance wire plugged into the receptacles.

Now only half have fault magnitudes of 200A or higher at

the end of the appliance wire.

Conventional residential circuit breakers have instanta-

neous trip levels in the range of 125-200A rms. With ref-

erence to Curve B of Figure 1, this means that bolted

faults, and their associated sine waves, would cause

instantaneous tripping in 50-85% of residential circuits.

For the remaining 15-50% of the circuits, the breakerwould tr ip in response to the heating of the bimetal.

For the case of arcing faults, however, these same

residential circuit breakers, with trip levels in the range125-200A RMS, would respond instantaneously in a

much lower p ercentage of circuits due to two character-

istics of arcing faults:

their arcing voltage of about 50V introduc significantimpedance into low voltage 125V circuits, reducing the

current amplitude

parallel arcing faults are sputtering in nature as indicatedin the oscillogram of Figure 2. The reduced amplitude

pulses are intermittent and of short duration.

Instead of functioning normally, conventional

breakers would actually trip instantaneously in an even

lower percentage of circuits than indicated in Figure 1.

Further, a conventional breaker might never trip due to

bimetal heating because the RMS current level associated

with the intermittent sputtering arc could be less than

the RM S breaker rating.

Where Faults Are FoundHigh-energy parallel arcing and high-resistance series

faults occur at numerous locations throughout thehome, including in-wall and in-room wiring.

The causes of in-wall faults include wire insulation

physically damaged by staples or nails or environmen-

tally damaged by lightning or moisture. An exampleof an in-wall high-resistance series fault is a loose

receptacle terminal connection that can cause local

heating and damage to the wire insulation and/or the

receptacle.

Ground faults are common as in-wall wiring

includes grounded conductors. The ground fault, if

detected soon enough, can prevent the fault from

escalating into a high-energy arcing fault. In-room

faults are typically caused by the abuse of cords and

plugs, (e.g. a chair resting on a cord) or wear and

tear over extended periods of time (e.g. a loose plug-

receptacle connection).

The AFCI DifferenceAFCIs offer two major advantages over conventional

breakers:

1. The electronic instantaneous trips for an AFCI

for arcing faults are set to trip with PEAK arcing

currents as small as 50 amperes. This is possible

because AFCIs can distinguish between the signa-

tures of pa rallel arcing faults from switching and

steady state currents associated with normal electri-

cal loads. Further, AFCIs will not nuisance trip

with transient sinusoidal load currents havingPEAK values of 300 amperes or more (such as

occur when a freezer compr essor tu rns) or

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Typical Residential Wiring

Figure 3 I Division of residential wiring into four zones. Zone 0 is associated with the meter, meter socket and service cable, Zone 1

with the loadcenter and the fixed premise wiring, Zone 2 with the wiring between the receptacles and the loads, and Zone 3 with the

appliances and other loads.

transient current pulses such as those that occur when

switching loads, or when an incandescent lamp burns

out. An AFCIs ability to distinguish a low amplitude

arcing current from a high amplitude normal current,

an intelligent instantaneous tr ip, is one of an AFCIs

important features.

2. AFCIs can include ground fault protection. It is recog-nized that high-resistance series faults at wire termina-

tions and old aluminum wiring are hazards. In fact, the

hazards of old aluminum branch circuit wiring on 15-

and 20-ampere branch circuits are well known. Just as

hazardous, but less recognized, are glowing contacts.

They are associated with todays copper wire and

modern wiring devices, receptacles and switches.

A loose wire-to-receptacle terminal connection can

overheat and create a so-called glowing contact.

Eaton Corporation funded a UL Special Services

Investigation that determined such hazards could bemitigated by an AFCI circuit breaker providing both

arcing and ground fault protection. The glowing con-

tact has the potential to eventually melt the wire insu-

lation and the receptacle itself. Typically a line-to-neu-

tral arcing fault or a line-to-ground o r neutra l-to-ground

ground fault will develop. An AFCI circuit breaker

employing both a rc and ground fault interrupters will

respond to these conditions by tripping and deenergizing

the branch circuit feeding the receptacle.

Rigorous TestingThere are four major manufacturers currently producing

AFCIs: Siemens, Square D, GE and Eaton Corp. Of

these, Eaton Corp.s Cutler-Hammer business unit first

filed for a patent on AFCI technology on September 26,

1991. Their design passed ULs draft standard tests inNovember/December 1996 and became the first com-

mercially available AFCI on September 30, 1997.

The draft standard testing involved parallel tests using

A) a guillotine with a carbon-steel blade across the con-

ductors of NM-B cable and two conductor Type SPT-2

flexible cord and B) sputtering arcs across a cut in their

insulation that had been conditioned through formation

of a carbon bridge. Series fault tests were also conducted

with NM-B fixed premise wiring with the AFCI respond-

ing to the ground fault current resulting from arcing at a

broken conductor without ignition of surrounding mate-

rial. The test also demonstrated resistance to unwantedtripping, including motor starting and dimmers, and

resistance to operation inhibition including filters.

Figure 3 depicts typical residential wiring divided

into four zones. Zone 0 is associated with the meter,

meter socket, and service cable. Zone 1 is associated

with the loadcenter and the fixed premise wiring. Zone

2 is associated with the wiring between the receptacles

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and the loads, and Z one 3 is associated with the appli-

ances and other loads. A good AFCI with ground fault

protection will mitigate against parallel arcing and series

high-resistance faults in zones 1 through 4.

Improved Safety

AFCIs represent a significant step forward in electricalsafety. The Consumer Products Safety Commission

(CPSC) has reported that more than 35% of all electri-

cal wiring fires are associated with the fixed wiring.5

The Task Force of the NEMA Molded Case Circuit

Breaker Section also analyzed fire statistics provided by a

major insurance company. Figure 4 shows the percentage

of electrical fires associated with the various Zones. This

figure indicates that many fires are associated with Zone

1, with statistics that are similar to those reported by

CPSC. In addition to detecting and interrupting potentially

dangerous parallel arcs in Zone 1, the AFCI detects paral-

lel arcs in Zones 2 and 3. Also series faults are mitigated,as they tend to escalate either into a parallel arcing fault or

a ground leakage fault. This is a major safety improve-

ment over conventional circuit breaker technology.

It must be noted, however, that AFCIs will mitigate

the effect of arcing faults but will not eliminate them

completely. Even under optimum conditions there will

always be at least one arcing half cycle and, in certain

environments, this could cause ignition at high currents.

AFCIs and the NEC

Six AFCI proposals were submitted for the 1999

N ational Electrical Code, four from two manufacturers

and two from the Electronics Industries Alliance. The

code was subsequently updated to require the installa-

tion of AFCI protection on all branch circuits supplying

15A or 20A single-phase 125V outlets installed in

dwelling unit bedrooms as of January 1, 2002.

[1] 1999, National Fire Data Center

[2] Section 240-1 (FPN) of the 1996 National Electric Code

[3] Technology for Detecting and Monitoring Conditions

That Could Cause Electrical Wiring System Fires,

report Prepared by Underwriters Laboratories (UL

Project N umber NC233, 94ME78760) for the

Consumer Product Safety Commission (Contract

Number CPSC-C-94-1112), September 1995

[4] An Evaluation of Circuit Breaker Trip Levels, Fact

Finding Report Prepared by Underwriters Laboratories

for the Electron ic Industries Association und er UL

Project 92M E51901, October 25, 199 3

[5] Memorandum from the United States ConsumerProducts Safety Commission, 1992 Estimated Fire

Losses involving Electrical Equipment

References

Residential FiresBreakdown by Zone

Figure 4 I Percentage of electrical fires associated with

the Zones defined in Figure 3.

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1000 Cherrington ParkwayMoon Township, PA 15108www.cutler-hammer.eaton.com1-800-525-2000

RE00402001EJune 2002Printed in U.S.A.

Eatons Cutler-Hammer business is a worldwide leader in electrical control, power distribution,and industrial automation products and services. Through advanced product development,world-class manufacturing methods, and global engineering services and support, the Cutler-Hammerbusiness provides customer-driven solutions that serve the changing needs of the industrial, utility,light commercial, residential, and OEM markets. To learn more about Eatons innovative Cutler-Hammerproducts visit www.cutler-hammer.eaton.com.

Eaton Corporation is a global $7.3 billion diversified industrial manufacturer that is a leaderin fluid power systems; electrical power quality, distribution and control; automotive engine airmanagement and fuel economy; and intelligent truck systems for fuel economy and safety.Eaton has 49,000 employees and sells products in more than 50 countries. To learn moreabout Eaton Corporation visit www.eaton.com.

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