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ElectronicsElectronics

Principles & ApplicationsPrinciples & ApplicationsSeventh EditionSeventh Edition

Arc FaultCircuit Interrupters

Charles A. Schuler

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©2008 The McGraw-Hill Companies, Inc. All rights reserved.

Residential fires caused by electricity occur morethan 40,000 times each year in the U.S. alone.

40%are due

to arcingfaults!

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Yearly statistics for U.S. residentialelectrical 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

2 % 36 % 12 % 50 %% of residential fires in each zone

AFCI protection

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

Fire locations (in descending order):

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The National Electrical Code requires arc fault protectionfor the circuits supplying receptacle outlets in the bedroomsof new homes.

The AFCI has been called the first major advance in homeelectrical system protection since the ground fault circuitinterrupter was introduced in the late 1960's.

Arc heat can exceed 6000 degrees Celsius and ignitebuilding components such as framing, flooring, orpaneling. Frayed line cords can ignite furniture, rugsor draperies.

Common causes of electrical arcing faults includebroken 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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20A

CoilSpring

ArmatureContactpoints

A magnetic circuit breakertrips when the current in thecircuit (and coil) is largeenough to move the armature.

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©2008 The McGraw-Hill Companies, Inc. All rights reserved.

20A

CoilSpring

ArmatureContactpoints

The trip time is smalland the trip current ismuch higher than therated current.

Residential breakerstrip magnetically inthe range of 125 to

200 A rms.

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©2008 The McGraw-Hill Companies, Inc. All rights reserved.

Amperes

% o

f ci

rcu

its

Percentage of circuits having short circuitcurrent available versus short circuit current

Fault at the end of 6 feetof #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.

UL data for “bolted faults”(hot to neutral dead short)

1

234

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20A

Spring

Latch

Bi-metalelement

Contactpoints

A thermal circuit breaker trips when thecurrent in the circuit is high enough toheat and warp (bend) the bi-metal element.

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©2008 The McGraw-Hill Companies, Inc. All rights reserved.

20A

Spring

Latch

Bi-metalelement

Contactpoints

The trip time is long and thetrip current is only a littlemore than the rated current.

The time associatedwith the thermal

tripping of breakers isspecified in UL 489.

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©2008 The McGraw-Hill Companies, Inc. All rights reserved.

A thermal-magnetic breaker provides better protection:fast circuit interruption for severe overloads (magneticaction) and eventual circuit interruption for modestoverloads (thermal action). This allows loads like motorsto come on-line without tripping the breaker.

Running current: 18 AStarting current: 50 A

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©2008 The McGraw-Hill Companies, Inc. All rights reserved.

The thermal-magnetic breaker providesadequate protection from many kinds offaults; even some caused by a lack ofcommon sense!

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©2008 The McGraw-Hill Companies, Inc. All rights reserved.

However, a thermal-magnetic breakercannot prevent fires in all cases.

Here, a frayed line cord is arcing at anaverage 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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Arc

cu

rren

t

Time (20 ms/div)

0 A

20 A

40 A

60 A

Typical arc waveform

AFCIs will trip on this waveform. Other breakers will not becausethe 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.

AFCItrip

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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 deviceEfficacy device trips when arc fault is trips when arc fault

detected. detected. Parallel Arcing Denotes an arc fault between Damaged insulationFaults 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 corrodedFaults 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 shouldInhibition 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 "mitigatingthe effects of arcing faults." A UL Standard (UL1699) specificallydefines an arc fault circuit interrupter.

©2008 The McGraw-Hill Companies, Inc. All rights reserved.

©2008 The McGraw-Hill Companies, Inc. All rights reserved.