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Arc-fault circuit Breaker
An arc fault circuit breaker or interrupter (arc
fault circuit breakers) is a circuit breaker
designed to stop fires by sensing non-functional
electrical arcs and disconnect power before the arc
starts a fire. The arc fault circuit breakers should
distinguish between a working arc that may occur in
the brushes of a vacuum sweeper, light switch, or
other household devices and a non-working arc that
can occur, for instance, in a lamp cord that has a
broken conductor in the cord from overuse. Arc
faults in a home is one of the leading causes for
household fires.
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Arc fault circuit breakers look like a GFCI/ circuit
breaker (Ground-Fault Circuit Interrupter in that
they both have a test button, although it is important
to distinguish the difference between the two. GFCIs are designed
to protect people against electrical shock, while arc fault
circuit breakers are primarily designed to protect
against fire.
Starting with the 1999 version of the National
Electrical Code (NEC, also called NFPA 70) in the
United States and the 2002 version of the Canadian
Electrical Code in Canada (CSA Standard C22.1), the
national codes require AFCIs in all circuits that
feed receptacles in bedrooms of dwelling units. This
requirement is typically accomplished by using a
kind of circuit-breaker (defined by UL 1699) in the
breaker panel that provides combined arc-fault and
overcurrent protection. Not all U.S.A. jurisdictions
have adopted the AFCI requirements of the NEC as
written.
The AFCI is intended to prevent fire from arcs. AFCI
circuit breakers are designed to meet one of two
standards as specified by UL 1699: "branch" type or
"combination" type (note: the Canadian Electrical
Code uses different terminology but similar
technical requirements). A branch type AFCI trips on
75 amperes of arcing current from the line wire to
either the neutral or ground wire. A combination
type adds series arcing detection to branch type
performance. Combination type AFCIs trip on 5
amperes of series arcing. Advanced electronics
inside an AFCI breaker detect sudden bursts of
electrical current in milliseconds, long before a
standard circuit breaker or fuse would trip. A
"combination AFCI breaker" will provide protection
against
1. Parallel arcing (line to neutral).
2. Series arcing (a loose, broken, or otherwise high
resistance segment in a single line)
3. Ground arcing (from line, or neutral, to ground)
4. Overload protection (for resistance loads such as
heaters. inductive loads such as motors may require
additional overload protection)
5. Short circuit protection
In 2002, the NEC removed the word "receptacle"
leaving "outlets", in effect adding lights within
dwelling bedrooms to the requirement [debated
interpretation]. The 2005 code made it more clear
that all outlets must be protected, despite code
making panel discussion about excluding bedroom
smoke detectors from the requirement. "Outlets" is
defined in "Article 100 Definitions" of the NEC as
"A point on the wiring system where current is taken
to supply utilization equipment" and this includes
receptacles, light fixtures, and smoke alarms, among
other things.
As of January 2008, only "combination type" AFCIs
will meet the NEC requirement. The 2008 NEC requires
installation of combination-type AFCIs in all 15 and
20 ampere residential circuits with the exception of
laundries, kitchens, bathrooms, garages, and
unfinished basements.
Limitations
AFCIs are designed to protect against fires caused
by electrical arcing faults. However, they provide
no specific protection against "glowing"
connections, excess current, high line voltages, or
low line voltages. For descriptions of electrical
fire causes, see "How Electricity Starts Fires",
John S Robison, P.E., Electrical Line magazine,
November / December 2007 issue, pages 60–70 (http://www.electricalline.com/);
NFPA 921 "Guide for Fire and Explosion
Investigations", 2004 edition; and "Ignition
Handbook", Dr. Vytenis Babrauskas, Fire Science
Publishers, 2003.
Glowing connections occur when relatively high
electric current exists in a relatively large
resistance. Heat comes from power dissipation. Power
(in watts, symbol W) equals the current (in amperes,
symbol A) squared, multiplied by the resistance (in
ohms, symbol Ω). For example, a 60 watt lamp
operating on a 120 V circuit draws 1/2 ampere of
current. An 1800 watt space heater on a 120 V
circuit draws up to 15 amperes. If a bad wiring
junction in a circuit has a resistance of 1 ohm,
then a 60 W lamp will cause it to dissipate 0.25
watt of power (0.5 A * 0.5 A * 1 Ω). In contrast, an
1800 W heater could theoretically cause the bad
wiring junction to dissipate 178 watts (13 A * 13 A
* 1 Ω). Note that the current is less than 15 A
because of the combined resistance of the heater
plus the bad wiring junction. This energy,
dissipated in a small junction area, can generate
temperatures above 1000 degrees Celsius. Those
temperatures can ignite most flammable materials.
Bad wiring junctions can occur in utilization
equipment, cords, or premise wiring (especially in
defective switch, socket, plug, or wire connection,
even at circuit breaker or fuse panels). High
resistance junctions are commonly observed in
improperly terminated aluminum wire junctions. No
technology located in a circuit breaker or fuse
panel could detect a high-resistance wiring fault,
as no measurable characteristic exists that
differentiates a glow fault from normal branch
circuit operation. Power Fault Circuit Interrupters
(PFCI) located in receptacles are designed to
prevent fires caused by glowing connections in
premise wiring or panels. From the receptacle, a
PFCI can detect the voltage drop when high current
exists in a high resistance junction. In a properly
designed circuit, substantial voltage drops should
never occur. Proper wire terminations inside
utilization equipment (e.g. appliances, lamps,
heaters) and cords prevent high-resistance
connections that can lead to fires.
Excess current can heat entire lengths of wire.
Thermal circuit breakers are designed to protect
against excess current through the permanent circuit
wiring. However, excess current through the smaller
wires in utilization equipment can exist, at levels
below the trip thresholds of a circuit breaker.
Overload fault circuit interrupters (OFCI) are
designed to protect against excess current drawn by
utilization equipment. OFCIs must be located within
receptacles. Both thermal circuit breakers and OFCIs
are required to prevent fire ignition from excess
current.
High line-voltage creates excess power and heat in
utilization devices such as heaters, light bulbs,
appliances, motors, and electronics. In extreme
cases, this heat can ignite fires. One extreme
source of high line voltage occurs from a neutral
path opening within a two-leg 120 V residential
electrical system. When a neutral wire breaks or
opens, the utilization voltage can almost double to
over 200 V with large leg-to-leg load imbalances.
This extreme situation can result in almost four
times the power and heat in loads. With such
overheating, some loads can reach self-ignition
temperature in less than 10 minutes. Power fault
circuit interrupters (PFCIs) are designed to prevent
fires caused by excess voltage across loads.
Voltage-trip circuit breakers detect excess line
voltages, but are unable to detect sub-circuit open
neutral conditions.
Low line voltage can cause electro-mechanical relays
(on-off switches) to repeatedly turn off (relay
opens) and on (relay closes again). If current is
flowing through the load contacts, it will cause
arcing across the contacts when they open. The
arcing can oxidize, pit and melt the contacts. This
process can increase the contact resistance,
superheat the relay and lead to fires. Power fault
circuit interrupters are designed to prevent fires
from low voltage across loads.
New
Electrical Code Requirements as of The 2008 National
Electrical Code 2008
NEC—210.12
Arc-Fault Circuit-Interrupter Protection.
(B) Dwelling Units. All 120-volt, single phase,
15- and 20-ampere branch circuits supplying outlets
installed in dwelling unit in family rooms, dining
rooms, living rooms, parlors, libraries, dens, sun
rooms, recreation rooms, closets, hallways, or
similar rooms or areas shall be protected by a
listed arc-fault circuit interrupter, combination
type installed to provide protection of the branch
circuit.
FPN: For information on types of arc-fault
circuit interrupters, see UL 1699-1999, Standard for
Arc-Fault Circuit Interrupters.
Exception No. 1: Where RMC. IMC, EMT or steel
armored cable, Type AC, meeting the requirements of
250.118 using metal outlet and junction boxes is
installed for the portion of the branch circuit
between the branch circuit overcurrent device and
the first outlet, it shall be permitted to install a
combination AFCI at the first outlet to provide
protection for the remaining portion of the branch
circuit.
The action in
these proposals deleted the phrase “supplying
outlets” and the word “bedrooms” now requiring AFCI
protection for all 120-volt, single-phase, 15- and
20-ampere branch circuits in family rooms, dining
rooms, living rooms, parlors, libraries, dens, sun
rooms, recreation rooms, closets, hallways, or
similar rooms or areas. In addition, the last
sentence, permitting branch/feeder AFCI devices
until January 1, 2008, has been deleted, and the
effect is to now require only listed combination
AFCI branch circuit protection.
A new exception
has been added permitting RMC, IMC, EMT or steel AC
cable to protect the branch circuit to the first
outlet with the 6-foot length deleted.
Award-Winning Combination Type
Arc Fault Circuit Interrupter from Siemens is Now
Available
Meets requirements of National Electrical Code®
update effective Jan. 1, 2008
ATLANTA -- Siemens Energy & Automation, Inc. today
announced that its full residential line of
award-winning Combination Type Arc Fault Circuit
Interrupters (AFCIs) is now available. Recently
named "Equipment of the Year" by Independent
Electrical Contractors (IEC), Siemens' AFCIs meet
all requirements of the current and proposed codes.
The Combination Type AFCIs have been listed by
Underwriters Laboratories Inc. (UL®) and are
available for use on 120-volt, single-phase, 15- and
20-ampere branch circuits.
The new Combination Type AFCI device is equipped
with Siemens patented unique LED trip indicators,
which will help electricians and home owners
pinpoint the type of problems that may cause the
device to trip. The LEDs located near the handle of
the circuit breaker can distinguish whether the trip
was caused by arcing faults, arcing to ground or
over current conditions.
"Arc faults are extremely dangerous because they
generally occur in wiring that is behind drywall or
in attics, going undetected until a fire breaks out,
which is why Siemens has been investing in research
and development and partnering with industry
innovators to develop arc fault technology," said
Dennis Sadlowksi, president and CEO of Siemens
Energy & Automation. "Siemens ability to be the
first to market with the Combination Type AFCI
technology is a testament to our commitment to
providing our customers with the latest and most
advanced technology."
Siemens Combination Type AFCI device is the most
compact on the market. As a result, it is
retrofitable for all Siemens, Murray, I-T-E, and
Crouse-Hinds load centers and meter load center
combinations. Additionally, Siemens "INSTAWIRE(TM)"
feature included in all Siemens load centers saves
time during installation.
Combination Type AFCIs, which protect against all
three possible types of arc fault, improve on the
protection offered by the branch/feeder AFCIs
currently permitted by the 1999-2002 NEC®, as well
as the 2005 NEC®, until January 1, 2008.
Branch/feeder AFCIs protect only against
line-to-ground and line-to-neutral arcs, known as
parallel or high-energy arcs.
Combination Type AFCIs, which will be required by
2005-2008 NEC starting Jan. 1, 2008, protect against
both of these arc faults plus series arcs, which can
be caused by broken conductors, loose screws and a
host of other invisible faults.
The arc fault circuit breakers is intended to
prevent fire from arcs to ground and works at a
higher threshold (30 mA) than the GFCI/RCD
(Ground-Fault Circuit Interrupt/Residual-Current
Device) implementations protecting against the
safety hazard of electric shock (which operate at 6
mA). However, advanced electronics inside an arc
fault circuit breakers breaker detect sudden bursts
of electrical current in milliseconds, long before
they would trip a regular overcurrent circuit
breaker or fuse. Combined devices are available
which trip at the lower, 6 mA threshold of a true
GFCI/RCD.
In 2002, the NEC removed the word "receptacle"
leaving "outlets", in effect adding lights within
dwelling bedrooms to the requirement [debated
interpretation]. The 2005 code made it more clear
that all outlets must be protected, despite code
making panel discussion about excluding bedroom
smoke detectors from the requirement.
Beginning Jan 2008, only "combination type" arc
fault circuit breakers' will meet the NEC
requirement. These can protect cords as well as
wiring.
Limitations
Even arc fault circuit breakers, however, do not
provide protection against all of the possible
circuit faults that could ignite a fire. In
particular, they provide no special protection
against so-called "glow faults" where a relatively
low-resistance short circuit draws a modest amount
of current (within the trip limits of the circuit
breaker) but heats the localized area of the fault
to red heat. Glow faults also can occur where a
connection in series with a load suddenly develops a
high resistance; this might be the result of a
now-defective switch, socket, plug, or wire
connection (series faults are also commonly observed
in aluminum wire junctions). No practical circuit
breaker could detect either such fault as there is
no measurable characteristic that any circuit
breaker could employ to distinguish a glow fault
from the normal operation of a branch circuit.
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