n
New Products for Integrated Electrical Systems /Questions and Answers
ni
ee/Resources
t
s
e
ig
d
c
ne
S
and the loads, and Zone 3 is associated with the appliances 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.
necdigest
Residential Fires
Breakdown by Zone
Improved Safety
AFCIs represent a significant step forward in electrical
safety. The Consumer Products Safety Commission
(CPSC) has reported that more than 35% of all electrical 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 parallel 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 improvement 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.
TM
®
NFPA’s Official NEC Magazine The Voice of Authority
w
www.necdigest.org
• by Dr. Joseph Engel
Figure 4 I Percentage of electrical fires associated with
the Zones defined in Figure 3.
Eaton’s 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-Hammer
business provides customer-driven solutions that serve the changing needs of the industrial, utility,
light commercial, residential, and OEM markets. To learn more about Eaton’s innovative Cutler-Hammer
products visit www.cutler-hammer.eaton.com.
Eaton Corporation is a global $7.3 billion diversified industrial manufacturer that is a leader
in fluid power systems; electrical power quality, distribution and control; automotive engine air
management 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 more
about Eaton Corporation visit www.eaton.com.
AFCIs and the NEC ®
Six AFCI proposals were submitted for the 1999
National Electrical Code ®, four from two manufacturers
and two from the Electronics Industries Alliance. The
code was subsequently updated to require the installation 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.
Arc-Fault
Circuit
Feb/March 2002 Volume 1
Interrupters
Bringing a new level of electrical
protection into the home
References
[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 Number NC233, 94ME78760) for the
Consumer Product Safety Commission (Contract
Number CPSC-C-94-1112), September 1995
1000 Cherrington Parkway
Moon Township, PA 15108
www.cutler-hammer.eaton.com
1-800-525-2000
[4] “An Evaluation of Circuit Breaker Trip Levels,” Fact
Finding Report Prepared by Underwriters Laboratories
for the Electronic Industries Association under UL
Project 92ME51901, October 25, 1993
RE00402001E
June 2002
Printed in U.S.A.
[5] Memorandum from the United States Consumer
Products Safety Commission, 1992 Estimated Fire
Losses involving Electrical Equipment
Each year in the United States, residential electrical fires result in more than 700
deaths, 3,000 injuries and $700 million in property damage1. A number of these
fires begin with little warning, kindled by sputtering arc faults in damaged or deteriorated wiring. The arc fault creates a spark, generates heat, and eventually
ignites nearby combustible material.
5
necdigest.org feb/mar 2002 necdigest
Due to the development of advanced, affordable circuit
interrupter technology and the recently updated NEC ®
code, however, fires and deaths attributable to arc faults
may soon be on the decline.
potentials that are located anywhere on the circuit.
Insulation can fail over time due to extended exposure
to moisture, heat, or extraordinarily 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 also be contributing factors.
Once the insulation is compromised, heat from an arc
melts the conductors in the wire and the electricity seeks
a new path. As it develops so does a sporadic or intermittent arc fault. Under these conditions, conventional
circuit breakers may not trip either magnetically, since
peak currents are too low, or thermally, since the duration of the current is too short to cause a thermal element to trip. If the breaker should trip, 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 aluminum 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 with
one 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 highenergy 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 Report4 prepared for the Electronics
Industries Alliance. The curve shows the distribution of
Building On Conventional Breaker Safety
Conventional residential circuit breakers prevent fires by
automatically opening the circuit before conductors or
the 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 overload2 or overcurrent.
However, the response is inadequate for protecting
against the fire hazards associated with arcs, the tempero
atures of which can exceed 6000 C.
Arcs also have an extremely short duration, 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 recognize the unique current signatures associated with arcing faults and act to interrupt the circuit before the temperature of combustibles can rise to hazardous levels.
Common Wiring Hazards
Hazards 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
Estimated Short Circuit Current Available (Amperes) vs.
Percentage of Circuits Having Estimated Short Circuit Current Available
available fault currents at household receptacles. The Xaxis displays the estimated short circuit currents available;
the Y-axis shows the percentage of circuits that have particular values of short circuit current available or higher.
The data applies to “bolted faults” with the line and neutral securely clamped together. For 15A receptacles, the
available fault levels are 75A RMS or higher. Only half have
available levels of 250A or higher. These relatively low short
circuit 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 instantaneous trip levels in the range of 125-200A rms. With reference 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 breaker
would trip in response to the heating of the bimetal.
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 faults
and responding to them fast enough to prevent fire.
For the case of arcing faults, however, these same
residential circuit breakers, with trip levels in the range
125-200A RMS, would respond instantaneously in a
much lower percentage of circuits due to two characteristics of arcing faults:
physically damaged by staples or nails or environmentally damaged by lightning or moisture. An example
of 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 plugreceptacle connection).
• their arcing voltage of about 50V introduc significant
impedance into low voltage 125V circuits, reducing the
current amplitude
in 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 RMS breaker rating.
2
Rigorous Testing
There 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 UL’s draft standard tests in
November/December 1996 and became the first commercially available AFCI on September 30, 1997.
The draft standard testing involved parallel tests using
A) a guillotine with a carbon-steel blade across the conductors 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 responding to the ground fault current resulting from arcing at a
broken conductor without ignition of surrounding material. The test also demonstrated resistance to unwanted
tripping, 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
2. AFCIs can include ground fault protection. It is recognized that high-resistance series faults at wire terminations and old aluminum wiring are hazards. In fact, the
hazards of old aluminum branch circuit wiring on 15and 20-ampere branch circuits are well known. Just as
hazardous, but less recognized, are “glowing contacts.”
They are associated with today’s 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.”
• parallel arcing faults are sputtering in nature as indicated
Figure 1 The distribution of available fault currents, at 15A household receptacles Curve (A), and at the end of six feet of #18
appliance wire plugged into those receptacles, Curve (B).
respond to these conditions by tripping and deenergizing
the branch circuit feeding the receptacle.
transient current pulses such as those that occur when
switching loads, or when an incandescent lamp burns
out. An AFCI’s ability to distinguish a low amplitude
arcing current from a high amplitude normal current,
an “intelligent instantaneous trip,” is one of an AFCI’s
important features.
The AFCI Difference
AFCIs 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 signatures of parallel arcing faults from switching and
steady state currents associated with normal electrical loads. Further, AFCIs will not nuisance trip
with transient sinusoidal load currents having
PEAK values of 300 amperes or more (such as
occur when a freezer compressor turns) or
Where Faults Are Found
High-energy parallel arcing and high-resistance series
faults occur at numerous locations throughout the
home, including “in-wall” and “in-room” wiring.
The causes of “in-wall” faults include wire insulation
3
Eaton Corporation funded a UL Special Services
Investigation that determined such hazards could be
mitigated by an AFCI circuit breaker providing both
arcing and ground fault protection. The “glowing contact” has the potential to eventually melt the wire insulation and the receptacle itself. Typically a line-to-neutral arcing fault or a line-to-ground or neutral-to-ground
ground fault will develop. An AFCI circuit breaker
employing both arc and ground fault interrupters will
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.
4
Due to the development of advanced, affordable circuit
interrupter technology and the recently updated NEC ®
code, however, fires and deaths attributable to arc faults
may soon be on the decline.
potentials that are located anywhere on the circuit.
Insulation can fail over time due to extended exposure
to moisture, heat, or extraordinarily 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 also be contributing factors.
Once the insulation is compromised, heat from an arc
melts the conductors in the wire and the electricity seeks
a new path. As it develops so does a sporadic or intermittent arc fault. Under these conditions, conventional
circuit breakers may not trip either magnetically, since
peak currents are too low, or thermally, since the duration of the current is too short to cause a thermal element to trip. If the breaker should trip, 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 aluminum 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 with
one 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 highenergy 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 Report4 prepared for the Electronics
Industries Alliance. The curve shows the distribution of
Building On Conventional Breaker Safety
Conventional residential circuit breakers prevent fires by
automatically opening the circuit before conductors or
the 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 overload2 or overcurrent.
However, the response is inadequate for protecting
against the fire hazards associated with arcs, the tempero
atures of which can exceed 6000 C.
Arcs also have an extremely short duration, 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 recognize the unique current signatures associated with arcing faults and act to interrupt the circuit before the temperature of combustibles can rise to hazardous levels.
Common Wiring Hazards
Hazards 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
Estimated Short Circuit Current Available (Amperes) vs.
Percentage of Circuits Having Estimated Short Circuit Current Available
available fault currents at household receptacles. The Xaxis displays the estimated short circuit currents available;
the Y-axis shows the percentage of circuits that have particular values of short circuit current available or higher.
The data applies to “bolted faults” with the line and neutral securely clamped together. For 15A receptacles, the
available fault levels are 75A RMS or higher. Only half have
available levels of 250A or higher. These relatively low short
circuit 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 instantaneous trip levels in the range of 125-200A rms. With reference 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 breaker
would trip in response to the heating of the bimetal.
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 faults
and responding to them fast enough to prevent fire.
For the case of arcing faults, however, these same
residential circuit breakers, with trip levels in the range
125-200A RMS, would respond instantaneously in a
much lower percentage of circuits due to two characteristics of arcing faults:
physically damaged by staples or nails or environmentally damaged by lightning or moisture. An example
of 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 plugreceptacle connection).
• their arcing voltage of about 50V introduc significant
impedance into low voltage 125V circuits, reducing the
current amplitude
in 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 RMS breaker rating.
2
Rigorous Testing
There 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 UL’s draft standard tests in
November/December 1996 and became the first commercially available AFCI on September 30, 1997.
The draft standard testing involved parallel tests using
A) a guillotine with a carbon-steel blade across the conductors 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 responding to the ground fault current resulting from arcing at a
broken conductor without ignition of surrounding material. The test also demonstrated resistance to unwanted
tripping, 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
2. AFCIs can include ground fault protection. It is recognized that high-resistance series faults at wire terminations and old aluminum wiring are hazards. In fact, the
hazards of old aluminum branch circuit wiring on 15and 20-ampere branch circuits are well known. Just as
hazardous, but less recognized, are “glowing contacts.”
They are associated with today’s 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.”
• parallel arcing faults are sputtering in nature as indicated
Figure 1 The distribution of available fault currents, at 15A household receptacles Curve (A), and at the end of six feet of #18
appliance wire plugged into those receptacles, Curve (B).
respond to these conditions by tripping and deenergizing
the branch circuit feeding the receptacle.
transient current pulses such as those that occur when
switching loads, or when an incandescent lamp burns
out. An AFCI’s ability to distinguish a low amplitude
arcing current from a high amplitude normal current,
an “intelligent instantaneous trip,” is one of an AFCI’s
important features.
The AFCI Difference
AFCIs 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 signatures of parallel arcing faults from switching and
steady state currents associated with normal electrical loads. Further, AFCIs will not nuisance trip
with transient sinusoidal load currents having
PEAK values of 300 amperes or more (such as
occur when a freezer compressor turns) or
Where Faults Are Found
High-energy parallel arcing and high-resistance series
faults occur at numerous locations throughout the
home, including “in-wall” and “in-room” wiring.
The causes of “in-wall” faults include wire insulation
3
Eaton Corporation funded a UL Special Services
Investigation that determined such hazards could be
mitigated by an AFCI circuit breaker providing both
arcing and ground fault protection. The “glowing contact” has the potential to eventually melt the wire insulation and the receptacle itself. Typically a line-to-neutral arcing fault or a line-to-ground or neutral-to-ground
ground fault will develop. An AFCI circuit breaker
employing both arc and ground fault interrupters will
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.
4
Due to the development of advanced, affordable circuit
interrupter technology and the recently updated NEC ®
code, however, fires and deaths attributable to arc faults
may soon be on the decline.
potentials that are located anywhere on the circuit.
Insulation can fail over time due to extended exposure
to moisture, heat, or extraordinarily 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 also be contributing factors.
Once the insulation is compromised, heat from an arc
melts the conductors in the wire and the electricity seeks
a new path. As it develops so does a sporadic or intermittent arc fault. Under these conditions, conventional
circuit breakers may not trip either magnetically, since
peak currents are too low, or thermally, since the duration of the current is too short to cause a thermal element to trip. If the breaker should trip, 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 aluminum 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 with
one 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 highenergy 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 Report4 prepared for the Electronics
Industries Alliance. The curve shows the distribution of
Building On Conventional Breaker Safety
Conventional residential circuit breakers prevent fires by
automatically opening the circuit before conductors or
the 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 overload2 or overcurrent.
However, the response is inadequate for protecting
against the fire hazards associated with arcs, the tempero
atures of which can exceed 6000 C.
Arcs also have an extremely short duration, 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 recognize the unique current signatures associated with arcing faults and act to interrupt the circuit before the temperature of combustibles can rise to hazardous levels.
Common Wiring Hazards
Hazards 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
Estimated Short Circuit Current Available (Amperes) vs.
Percentage of Circuits Having Estimated Short Circuit Current Available
available fault currents at household receptacles. The Xaxis displays the estimated short circuit currents available;
the Y-axis shows the percentage of circuits that have particular values of short circuit current available or higher.
The data applies to “bolted faults” with the line and neutral securely clamped together. For 15A receptacles, the
available fault levels are 75A RMS or higher. Only half have
available levels of 250A or higher. These relatively low short
circuit 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 instantaneous trip levels in the range of 125-200A rms. With reference 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 breaker
would trip in response to the heating of the bimetal.
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 faults
and responding to them fast enough to prevent fire.
For the case of arcing faults, however, these same
residential circuit breakers, with trip levels in the range
125-200A RMS, would respond instantaneously in a
much lower percentage of circuits due to two characteristics of arcing faults:
physically damaged by staples or nails or environmentally damaged by lightning or moisture. An example
of 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 plugreceptacle connection).
• their arcing voltage of about 50V introduc significant
impedance into low voltage 125V circuits, reducing the
current amplitude
in 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 RMS breaker rating.
2
Rigorous Testing
There 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 UL’s draft standard tests in
November/December 1996 and became the first commercially available AFCI on September 30, 1997.
The draft standard testing involved parallel tests using
A) a guillotine with a carbon-steel blade across the conductors 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 responding to the ground fault current resulting from arcing at a
broken conductor without ignition of surrounding material. The test also demonstrated resistance to unwanted
tripping, 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
2. AFCIs can include ground fault protection. It is recognized that high-resistance series faults at wire terminations and old aluminum wiring are hazards. In fact, the
hazards of old aluminum branch circuit wiring on 15and 20-ampere branch circuits are well known. Just as
hazardous, but less recognized, are “glowing contacts.”
They are associated with today’s 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.”
• parallel arcing faults are sputtering in nature as indicated
Figure 1 The distribution of available fault currents, at 15A household receptacles Curve (A), and at the end of six feet of #18
appliance wire plugged into those receptacles, Curve (B).
respond to these conditions by tripping and deenergizing
the branch circuit feeding the receptacle.
transient current pulses such as those that occur when
switching loads, or when an incandescent lamp burns
out. An AFCI’s ability to distinguish a low amplitude
arcing current from a high amplitude normal current,
an “intelligent instantaneous trip,” is one of an AFCI’s
important features.
The AFCI Difference
AFCIs 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 signatures of parallel arcing faults from switching and
steady state currents associated with normal electrical loads. Further, AFCIs will not nuisance trip
with transient sinusoidal load currents having
PEAK values of 300 amperes or more (such as
occur when a freezer compressor turns) or
Where Faults Are Found
High-energy parallel arcing and high-resistance series
faults occur at numerous locations throughout the
home, including “in-wall” and “in-room” wiring.
The causes of “in-wall” faults include wire insulation
3
Eaton Corporation funded a UL Special Services
Investigation that determined such hazards could be
mitigated by an AFCI circuit breaker providing both
arcing and ground fault protection. The “glowing contact” has the potential to eventually melt the wire insulation and the receptacle itself. Typically a line-to-neutral arcing fault or a line-to-ground or neutral-to-ground
ground fault will develop. An AFCI circuit breaker
employing both arc and ground fault interrupters will
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.
4
n
New Products for Integrated Electrical Systems /Questions and Answers
ni
ee/Resources
t
s
e
ig
d
c
ne
S
and the loads, and Zone 3 is associated with the appliances 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.
necdigest
Residential Fires
Breakdown by Zone
Improved Safety
AFCIs represent a significant step forward in electrical
safety. The Consumer Products Safety Commission
(CPSC) has reported that more than 35% of all electrical 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 parallel 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 improvement 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.
TM
®
NFPA’s Official NEC Magazine The Voice of Authority
w
www.necdigest.org
• by Dr. Joseph Engel
Figure 4 I Percentage of electrical fires associated with
the Zones defined in Figure 3.
Eaton’s 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-Hammer
business provides customer-driven solutions that serve the changing needs of the industrial, utility,
light commercial, residential, and OEM markets. To learn more about Eaton’s innovative Cutler-Hammer
products visit www.cutler-hammer.eaton.com.
Eaton Corporation is a global $7.3 billion diversified industrial manufacturer that is a leader
in fluid power systems; electrical power quality, distribution and control; automotive engine air
management 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 more
about Eaton Corporation visit www.eaton.com.
AFCIs and the NEC ®
Six AFCI proposals were submitted for the 1999
National Electrical Code ®, four from two manufacturers
and two from the Electronics Industries Alliance. The
code was subsequently updated to require the installation 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.
Arc-Fault
Circuit
Feb/March 2002 Volume 1
Interrupters
Bringing a new level of electrical
protection into the home
References
[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 Number NC233, 94ME78760) for the
Consumer Product Safety Commission (Contract
Number CPSC-C-94-1112), September 1995
1000 Cherrington Parkway
Moon Township, PA 15108
www.cutler-hammer.eaton.com
1-800-525-2000
[4] “An Evaluation of Circuit Breaker Trip Levels,” Fact
Finding Report Prepared by Underwriters Laboratories
for the Electronic Industries Association under UL
Project 92ME51901, October 25, 1993
RE00402001E
June 2002
Printed in U.S.A.
[5] Memorandum from the United States Consumer
Products Safety Commission, 1992 Estimated Fire
Losses involving Electrical Equipment
Each year in the United States, residential electrical fires result in more than 700
deaths, 3,000 injuries and $700 million in property damage1. A number of these
fires begin with little warning, kindled by sputtering arc faults in damaged or deteriorated wiring. The arc fault creates a spark, generates heat, and eventually
ignites nearby combustible material.
5
necdigest.org feb/mar 2002 necdigest
n
New Products for Integrated Electrical Systems /Questions and Answers
ni
ee/Resources
t
s
e
ig
d
c
ne
S
and the loads, and Zone 3 is associated with the appliances 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.
necdigest
Residential Fires
Breakdown by Zone
Improved Safety
AFCIs represent a significant step forward in electrical
safety. The Consumer Products Safety Commission
(CPSC) has reported that more than 35% of all electrical 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 parallel 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 improvement 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.
TM
®
NFPA’s Official NEC Magazine The Voice of Authority
w
www.necdigest.org
• by Dr. Joseph Engel
Figure 4 I Percentage of electrical fires associated with
the Zones defined in Figure 3.
Eaton’s 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-Hammer
business provides customer-driven solutions that serve the changing needs of the industrial, utility,
light commercial, residential, and OEM markets. To learn more about Eaton’s innovative Cutler-Hammer
products visit www.cutler-hammer.eaton.com.
Eaton Corporation is a global $7.3 billion diversified industrial manufacturer that is a leader
in fluid power systems; electrical power quality, distribution and control; automotive engine air
management 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 more
about Eaton Corporation visit www.eaton.com.
AFCIs and the NEC ®
Six AFCI proposals were submitted for the 1999
National Electrical Code ®, four from two manufacturers
and two from the Electronics Industries Alliance. The
code was subsequently updated to require the installation 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.
Arc-Fault
Circuit
Feb/March 2002 Volume 1
Interrupters
Bringing a new level of electrical
protection into the home
References
[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 Number NC233, 94ME78760) for the
Consumer Product Safety Commission (Contract
Number CPSC-C-94-1112), September 1995
1000 Cherrington Parkway
Moon Township, PA 15108
www.cutler-hammer.eaton.com
1-800-525-2000
[4] “An Evaluation of Circuit Breaker Trip Levels,” Fact
Finding Report Prepared by Underwriters Laboratories
for the Electronic Industries Association under UL
Project 92ME51901, October 25, 1993
RE00402001E
June 2002
Printed in U.S.A.
[5] Memorandum from the United States Consumer
Products Safety Commission, 1992 Estimated Fire
Losses involving Electrical Equipment
Each year in the United States, residential electrical fires result in more than 700
deaths, 3,000 injuries and $700 million in property damage1. A number of these
fires begin with little warning, kindled by sputtering arc faults in damaged or deteriorated wiring. The arc fault creates a spark, generates heat, and eventually
ignites nearby combustible material.
5
necdigest.org feb/mar 2002 necdigest