Arc Fault

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Arc Fault Circuit Interrupters
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Residential fires caused by electricity occur
more than 40,000 times each year in the U.S.
alone.
40 are due to arcing faults!
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Yearly statistics for U.S. residential electrical
fires (necdigest.org)
700 deaths
3,000 injuries
700 million property damage
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Division of residential wiring into zones
Zone 0
Zone 1
Zone 2
Zone 3
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36
12
50
of residential fires in each zone
AFCI protection
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Fire locations (in descending order)

• bedrooms
• living rooms
• kitchens
• closet/storage areas
• garages
• bathrooms
• laundries
• halls
• dining rooms

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The National Electrical Code requires arc fault
protection for the circuits supplying receptacle
outlets in the bedrooms of new homes.
The AFCI has been called the first major advance
in home electrical system protection since the
ground fault circuit interrupter was introduced
in the late 1960's.
Arc heat can exceed 6000 degrees Celsius and
ignite building components such as framing,
flooring, or paneling. Frayed line cords can
ignite furniture, rugs or draperies.
Common causes of electrical arcing faults
include broken down insulation due to age or
lightning strikes, poor electrical workmanship,
and home owner abuse.
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Circuit Breaker Review
A circuit breaker can be designed to trip on ?
Current faults (oldest technology) ? Ground
faults ? Arc faults (newest technology) ? All
of the above
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Armature
Contact points
A magnetic circuit breaker trips when the current
in the circuit (and coil) is large enough to move
the armature.
Coil
Spring
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Armature
Contact points
The trip time is small and the trip current
is much higher than the rated current.
Coil
Spring
Residential breakers trip magnetically in the
range of 125 to 200 A rms.
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Percentage of circuits having short
circuit current available versus short circuit
current
UL data for bolted faults (hot to neutral dead
short)
of circuits
Fault at the end of 6 feet of 18 appliance wire
Conclusions

1. All circuits can supply 75 A.
2. Half can supply 255 A.
3. Half can supply 203 A with appliance cord.
4. Magnetic tripping will not occur for all dead
   shorts.

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A thermal circuit breaker trips when the current
in the circuit is high enough to heat and warp
(bend) the bi-metal element.
Bi-metal element
Contact points
Spring
Latch
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The trip time is long and the trip current is
only a little more than the rated current.
Bi-metal element
Contact points
Spring
Latch
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A thermal-magnetic breaker provides better
protection fast circuit interruption for severe
overloads (magnetic action) and eventual circuit
interruption for modest overloads (thermal
action). This allows loads like motors to come
on-line without tripping the breaker.
Running current 18 A Starting current 50 A
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The thermal-magnetic breaker provides adequate
protection from many kinds of faults even some
caused by a lack of common sense!
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However, a thermal-magnetic breaker cannot
prevent fires in all cases. Here, a frayed
line cord is arcing at an average current of 15
amperes. The breaker does not trip.
The heat is intense! 15 A x 120 V 1.8 kW
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Typical arc waveform
60 A
40 A
20 A
0 A
AFCIs will trip on this waveform. Other breakers
will not because the rms value of the fault
current is too low to produce a thermal trip and
the peak value is too low to produce a magnetic
trip.
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AFCIs use an electronic circuit that looks for an
arc signature. The trick is to differentiate
the arc signature from the high-frequency noise
associated with thyristor light dimmers, sparking
motor commutators, and other high-frequency
sources. Also, the peak starting current
demanded by motors must not cause tripping.
Arc signature
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Test Requirements Definition
Examples Operational Denotes condition
that the The AFCI device Efficacy device
trips when arc fault is trips when arc fault
detected. detected.
Parallel Arcing Denotes an arc fault
between Damaged insulation Faults the line
and neutral on fixed wiring conductors
in parallel with or extension cords. the
loads in the circuit.
Series Arcing
An arc fault in series with A
corroded Faults the load. Such arcs
result or loose terminal. from loose
connections, or severed wires.
Time and heat can cause a series fault to become
a parallel fault.
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Arcing to Ground Denotes an unintentional If a
wall plug or arcing fault between a
part switch was not operating
normally at some installed properly or
potential-to-ground and where connections
ground. became loose.
Operation Denotes the
concealment of The device should Inhibition an
arc fault by the normal trip even if the arc
operation of circuit fault occurs on
a components such as circuit that has lamp
switching devices, lamp dimmers, RF line
dimmers, RF filters, long filters, or long
wire lengths, etc. use of wire lengths.
Faults equipment such as drills should not
be fans, or other devices. masked by
drills or
fans.
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Unwanted Tripping Denotes a tripping function
Device should not in response to an
arcing trip when switching condition that is
not an arc devices, drills, fault but a
condition that hairdryers, lamp occurs as
part of the normal dimmers, or motors
operation of circuit are operated.
components.
Currently UL489 allows for a product to be
labeled as "mitigating the effects of arcing
faults." A UL Standard (UL1699)
specifically defines an arc fault circuit
interrupter.
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