I *-N8S

* ( '^S'-'CATIONS

NBSIR 81-2221

Experimentally Determined

Performance of Some Residential

Circuit Breakers

Robert W. Beausoliel

William J. Meese

Center for Building Technology

National Engineering Laboratory

U.S. Department of Commerce

National Bureau of Standards

Washington. DC 20234

April 1981

H)C100 I S. DEPARTMENT OF COMMERCE

I U 56 ATIONAL BUREAU OF STANDARDS

81-2221

1981

C.2

ATIONAL BUREAU OF STANDARDS
jirr

lit
NBSIR 81-2221

EXPERIMENTALLY DETERMINED

PERFORMANCE OF SOME RESIDENTIAL

CIRCUIT BREAKERS

HATlONAl* BUBXAH

Of iTA»n)A»»>»

ubbabt

MAY 1 8 1981

/ G

/to. - ^

Robert W. Beausoliel

William J. Meese

Center for Building Technology

National Engineering Laboratory

U.S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

April 1981

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
‘A*. B !•

I

A
ABSTRACT

Laboratory test results show that at low ambient temperatures some

residential-type circuit breakers may not trip at currents up to 140 percent

of rated currents. Under some environmental conditions this may lead to

wiring temperatures that exceed the limitations specified in the National

Electrical Code. The results also show that circuits sometimes open at the

point of short circuits before circuit breakers operate. Ignition of com- bustibles proximate to the point of such short circuits sometimes occurs.

The results indicate the need for a more detailed study of overcurrent

protection performance in the field and under laboratory conditions. Also

needed are the development of more meaningful scientific/mathematical

principles and models on the functional characteristics of circuit breakers.

Key Words: branch circuits; circuit breaker; electrical fire; low ambient

temperature; trip time.

f

ill
SI CONVERSION UNITS

In view of the present accepted practice in this country for building

technology, common U.S. units of measurement have been used throughout this

document. In recognition of the position of the United States as a signatory

to the General Conference on Weights and Measures, which gave official status

to the metric SI system of units in 1960, assistance is given to the reader

interested in making use of the coherent system of SI units by giving conversion factors applicable to U.S. units used in this document.

Mass 1 pound-mass (Ibj^) = 0.453592 Kg

Length 1 inch = 0.0254 meter (m)

Temperature t (Celsius) = 5/9[t(Fahr)-32]

Torque .1 Lbf.in = 0.113 newton meter (N«m)

Time 1 hour = 60 minutes = 3,600 seconds

iv
TABLE OF CONTENTS

Page

1. INTRODUCTION 1

1 . 1 Scope . , 1 1.2 Background 1 1.3 Literature Search 2

1.4 Functional Description of a Thermal Magnetic Circuit

Breaker 2

2. CIRCUIT BREAKER PERFORMANCE WITH SLIGHT TO MODERATE

OVERCURRENTS AT VARIOUS AMBIENT TEMPERATURES 5

2.1 Circuit Breaker/Temperature Test Setup and Test Procedure ... 5

2.2 Test Results for Single Circuit Breakers 7 2.3 Test Results for Fuses 11

2.4 Multi-Circuit Breaker Test Results 20

3. CIRCUIT BREAKER PERFORMANCE UNDER SHORT-CIRCUIT CONDITIONS 24

3.1 Short-Circuit Test Setup and Test Procedure 24

3.2 Test Results 28

4. CONCLUSIONS 42

5. RECOMMENDATIONS 42

6. REFERENCES 43

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1 . INTRODUCTION

The National Electrical Code (NEC) [1]* states that a circuit breaker Is a device designed to open and close a circuit by nonautomatic means through

manual operation of the device and to open the circuit automatically at or beyond a predetermined overcurrent without damage to itself or to protected

electrical circuits when properly applied within its rating. An overcurrent

is defined as any current in excess of the rated current of equipment conductors. Overcurrents may result from overloads, short circuits, or ground

faults [1]

.

1.1 SCOPE

This investigation presents results from laboratory testing of residential

type, single-pole, 20 A, thermal-magnetic circuit breakers. Standard 20 A

plug fuses were tested for comparison with circuit breaker performance.

1.2 BACKGROUND

The need for circuit breaker research became apparent during recent investigations of temperatures produced by self-heating of wiring when surrounded by

thermal insulation [2]. During that investigation it was recognized that

circuit breakers do not quickly open circuits that are subjected to overcurrents. Circuit breakers are widely utilized in buildings because of the

simplicity of resetting following operation on overloaded circuits without

replacement, as in the case of fuses.

The Underwriters’ Laboratories Standard, UL 489, for molded-case circuit

breakers requires circuit breakers to open circuits within one hour at

135 percent of their rated current [3]. The UL standard requires this onehour performance with an ambient temperature of 25“C (77®F). The standard

does not include criteria for lower ambient air temperatures. The National

Electrical Manufacturers Association (NEMA) standard for molded-case circuit

breakers considers circuit breaker operation at under 0°C ambient temperature

as unusual service conditions that should be brought to the attention of the

manufacturer [4]. The standard does not present details concerning circuit

breaker performance at 0°C or below.

A further impetus for circuit breaker research was provided during NBS

laboratory short circuit experimentation with branch circuit wiring systems.

It was observed that circuit breakers did not always open during short

circuits. It was found that the circuits opened at a contact interface

such as that between the circuit conductor and a shorting nail. The opening

of the circuit in this manner was often accompanied by the expulsion of

molten metal particles from the nail and/or conductor interface. Such

performance might cause ignition of proximate combustibles. Neither the

literature nor the circuit breaker standards appear to address this type

of circuit breaker performance per se.

* References cited at end of text.

1
1 . 3 LITERATURE SEARCH

A search of the technical literature revealed a UL Bulletin of Research [5]

which addresses ambient temperature effects on circuit breakers. UL research

was carried out because field reports to UL indicated that overcurrent devices

sometimes failed to open circuits when subjected to moderate overloads and

low ambient temperatures. The UL Bulletin as listed in reference [5] showed

that both circuit breakers and plug fuses of 15 A rating oftentimes did not

open the test circuits when tested at 135 percent of rated current at ambient

temperatures of 32, 0, -10, -20, and -30“F. The UL Bulletin does not have

a conclusion section per se [5]; however, the abstract states the following:

"These findings confirm a view previously held that thermally

operated devices are affected by the ambient temperature at

which they are operated, and this should be taken into

account in actual practice."

The UL circuit breaker and plug fuse tests were carried out over thirty years

ago; NBS found no recent tests reported in the literature. The NBS tests

presented in this report were carried out to gain insight into the low

temperature performance of modern residential single-pole circuit breakers.

The literature search did not reveal studies concerning residential circuit

breaker performance during short circuits. The literature contained very

little information concerning the scientific/mathematical principles of

residential circuit breakers; mathematical models for these breakers were

not found in the literature.

1.4 FUNCTIONAL DESCRIPTION OF A THERMAL-MAGNETIC CIRCUIT BREAKER

Residential circuit breakers are usually of the molded-case type. A

molded-case circuit breaker is one that is assembled as an integral unit in

a housing of molded insulating materials [6]. The typical molded-case

circuit breaker is equipped with both time-delay and Instantaneous tripping

devices [6 ]

.

Time-delay tripping has Inverse time characteristics. This means that as

the overload increases, the tripping time decreases. This results in the

following performance.

a. The circuit breaker accepts short-duration overloads for motor

starting

.

b. During a large overcurrent or a short circuit, the circuit breaker

is intended to trip rapidly to protect circuit conductors and

electrical insulation from overheating.

The NEC defines several terms applying to circuit breakers [1]. These are as follows

2
a. Adjustable: A qualifying term Indicating that the circuit

breaker can be set to trip at various values of current and/or

time within a pre-determined range.

b. Instantaneous Trip: A qualifying term indicating that no delay is

purposely introduced in the tripping action of the circuit breaker.

c. Inverse Time: A qualifying term Indicating there is purposely

Introduced a delay in the tripping action of the circuit breaker,

which delay decreases as the magnitude of the current increases.

d. Nonad justable: A qualifying term indicating that the circuit

breaker does not have any adjustment to alter the value of current

at which it will trip or the time required for its operation.

e. Setting: The value of current and/or time at which an adjustable

circuit breaker is set to trip.

Most residential circuit breakers of 15 A and 20 A size are single pole,

inverse time, and nonad justable. Residential circuit breakers of this size

usually operate by thermal-magnetic means. The operation of a thermal magnetic breaker is depicted in figure 1 [7], which shows the release or tripping

element. The element consists of a thermal, bimetallic element to which a magnetic plate has been added. During slight overcurrents, the magnetic

force produced by the magnetic plate is so slight that it contributes little

force to trip the circuit breaker, and the circuit breaker trips primarily

because of the force generated by the heating of the bimetallic element.

With high short-circuit currents, the magnetic tripping force predominates.

3
n Overload

Figure 1. Principle of thermal-magnetic breaker [7]

4
2. CIRCUIT BREAKER PERFORMANCE WITH SLIGHT TO MODERATE OVERCURRENTS AT

VARIOUS AMBIENT TEMPERATURES

The purpose of the investigation reported upon in this section is to determine

whether or not circuit breakers will trip during overcurrents when the circuit breaker is located in cool ambient air, such as might exist in a basement

.

2.1 CIRCUIT BREAKER/TEMPERATURE TEST SETUP AND TEST PROCEDURE

The test setup shown in figure 2 consisted of the following;

a. The four-circuit breaker load center was of a type suitable for

service entrance usage. The 20 A circuit breakers were wired with

0^12 AWG solid copper TW insulated wlre^

.

b. The measuring junction of a copper/constantan (type T) thermocouple

formed from 0.020-inch diameter thermocouple wire was placed in

contact with the circuit breaker wire binding screw.

c. The thermocouple output was read on a digital voltmeter.

d. The reference junction of the thermocouple was maintained at 32®F

in a Dewar containing a mixture of ice and water.

The millivolt thermocouple readings were converted to temperature in degrees

Fahrenheit based on NBS Monograph 125 [8]

.

e. Low voltage current was Impressed on the circuit breaker or

fuse by means of a current controller, a motor-driven auto transformer, and a 5 KVA step-down transformer having a 120 V primary

winding and a 12 V secondary winding.

f. Current was measured by recording the voltage drop across a 0.001 ohm + 0.05%, 500 A, AC shunt.

g. The digital voltmeter was connected to a double-pole double-throw

toggle switch for measuring either circuit breaker temperature or current

.

h. The circuit breaker or fuse and load centers were placed in their

normal wall-mounted position within a small environmental chamber

having a temperature range capability of +350°F to -120°F with

a specified control tolerance of + 1/4°F. The work space was

16 inches wide, 12 inches high, and 11 inches deep. The chamber

had its own temperature indicator.

^ TW wire insulation has a 60° C maximum operating temperature rating

5
- DOUBLE-POLE DOUBLE-THROW TOGGLE SWITCH

(THERMOCOUPLE OR SHUNT TO VOLTMETER)

TRANSFORMER BUILDING POWER SUPPLY

Figure 2. Test setup for temperature performance of breakers and fuses

6
1. Electric timers were used for timing overcurrent device trips. In case of very fast trips a storage-type oscilloscope was connected

across the AC shunt mentioned In (f) above.

The test procedure consisted of the following:

a. Single-pole, 20 A, molded-case circuit breakers of two manufacturers

were used In this work. (See table 1). Three of these breakers had

been used In other tests Involving several hundred short circuits

with currents In excess of 500 A rms (root mean square), with a building supply voltage of 118 volts. Other breakers were new (as

received from the vendor)

.

b. The autotransformer was adjusted for proper current setting.

c. When the temperature established within the environmental chamber

was measured on the wire binding screw of the circuit breaker, the

current was Impressed on the breaker and the timer was started.

When the breaker tripped, the timer was shut off, recording the time

to trip.

The test setup of figure 2 was for only one circuit breaker or one fuse. It

Is conceivable that a residence may have only one circuit operating at a given time. However, circuit-breaker load centers usually Involve combinations of breakers, some carrying current and some not. Current-carrying

breakers and wiring contribute heat to the circuit breaker load center, which

elevates the temperatures of other breakers. The test setup of figure 3 was

used for determining the Influence of heat from additional breakers on breaker

tripping performance. The load center containing three breakers was mounted

In the chamber shown In figure 2. The load center was Identical to that used

In figure 2. The setup of figure 3 uses an additional autotransformer, stepdown transformer, 0.001 ohm AC shunt and digital meter to measure and control

current In the circuit breaker specimens #5 and y/6. Circuit breaker specimens

and #6 were operated at 0-18 A In order to supply additional heat and

observe whether breaker //I would trip when subjected to 27 A with relatively

low ambient air temperature. Temperature was measured only on breaker #1.

The voltage drops across circuit breakers and currents through them were

used to Indicate the heating effect of each breaker.

2.2 TEST RESULTS FOR SINGLE CIRCUIT-BREAKERS

Circuit breaker specimen # 1 was tested over the temperature and current

range shown In table 2. Since circuit breaker #1 did trip satisfactorily

over the range of temperature and currents of table 2, the breaker was subjected to other currents and temperatures, presented In table 3. As this

test shows, the breaker usually did not trip. Run number 6 was a check on breaker performance at 135 percent of rated current similar to the calibration test In UL Standard 489 [3]. The breaker passed this calibration test.

At the end of the 4.5-hour run #8 In table 3, the current was Increased to

28 A, shown In the table as run //9. At the end of run #9 without the circuit

7
ICE POINT REFERENCE

(FOR OBTAINING

VOLTAGE DROP

ACROSS EACH

CIRCUIT BREAKER)

THERMOCOUPLE ON CIRCUIT

BREAKER SCREW

CIRCUIT BREAKER #5

CIRCUIT BREAKER #6

AUTO TRANSFORMER (FOR

CIRCUITBREAKER NOs 5 & 6)

VOLTAGE DROP PROBES

Figure 3. Multi-circuit-breaker test setup

8
TABLE 1. Circuit Breaker and Fuse Test Specimen

Circuit Breakers: Manufacturer A Manufacturer B Specimen No. Specimen No.

1 (used) 6 (new)

2

.. 7

"

3

" 8

•’

4 (new) 9

5

Fuses: 30 standard 20 A plug fuses of one manufacturer.

TABLE 2. Performance of Circuit Breaker #1 Over Extreme Temperature Range

RUN

AMBIENT AIR TEMPERATURE

AND CIRCUIT BREAKER TRIP TIME - SEC.'

150“F 104°F 77“F 50°F 32°F 0“F -10°F -20°F -30“F -40°F

1

. 70 A

3.85 5.3 6.0 6.8 8.3 8.4 9.4 9.9 12.0 17.0

2 3.89 5.84 6.1 6.6 8.6 9.4 9.8 11.9 11.3 17.8

4

. 60 A

4.7 8.20 9.2 10.4 12.7 14.3 14.7 15.0 16.0 17.5

5 5.8 7.7 8.0 10.7 12.9 14.2 15.5 16.0 17.7 17.0

5

. 50 A

7.2 12.0 14.2 16.5 23.9 26.7 27.6 29.2 27.7 30.0

6 7.1 11.76 14.8 16.9 24.5 27.8 27.8 32.7 28.0 30.0

7

. 40 A

11.9 22.0 30.0 42.7 54.8 81.6 80.3 32.7 99.4 125.8

8 12.0 22.8 28.9 37.4 57.5 105 76.0 193 90.8 126.8

9

. 27 A

59.7 24.24 76.8

10 34.5 34.3 406

9
TABLE 3. Test of Circuit Breaker #1 Over the Current Range

24 A to 29 A Within the Temperature Range

36°F to 77“F

Run #

Ambient

Temperature

°F

Current

Amperes Trip

Run Time

Hours Seconds

1 39 25 No 3.5

2 36 27 No 3.5

3 41 27 No 6.7

4 50 27 No 5.5

5 60 27 No 5.0

6 77 27 Yes 972

7 77 24 No 1.5

8* 61 27 No 4.5

9* 61 28 No IT) •

CM

10* 61 29 Yes 10.8

* consecutive runs

10
II

I

i| breaker tripping, the current was increased to 29 A. As shown, the circuit

!j breaker tripped in 10 seconds.

, Circuit breaker //2 was tested over the temperature range 39®F to 77°F and at

1 currents of 25 A and 27 A, as shown in table 4. The circuit breaker tripped

during all runs.

Circuit breaker #3 was tested over the temperature range 49°F to 77°F and the

current range from 27 A to 40 A. Results are shown in table 5. The circuit

I breaker did not trip at 27 A and 49“F during one run out of three. During the

I one run at 25 A and 49 °F, the circuit breaker did not trip. The circuit

breaker did not trip during the run at 24 A and 59°F. During one run at 25 A

I and 77®F, the circuit breaker tripped; during a following run at 25 A and

78“F the circuit breaker did not trip.

Results for circuit breaker //4 are shown in table 6. The circuit breaker

I did not trip during runs #7 and #9 at 27 A and 45°F ambient temperature,

j However, during run #8 when the current was increased by one ampere to 28 A,

the circuit breaker tripped in 591 seconds.

Circuit breaker specimens #5 and //6 were spot checked at 27 A and 45°F. These

circuit breakers did not trip during 6-hour runs.

Table 7 shows that breaker specimen //7 always tripped at 27 A and 44°F ambient

temperature. At 25 A the circuit breaker tripped during run #4 in about

twenty minutes; however, in repeating run #4 as run //5, the breaker did not

trip.

Results for the test of circuit breaker #8 are shown in table 8. The circuit

breaker had not tripped at the end of run #5 at 25 A and 44°F and at that

time the current was increased to 26 A without tripping the circuit breaker

during an additional three-and one-half hours as run #6.

Table 9 gives results for the test of circuit breaker //9. The circuit breaker

tripped at 26 A and 44°F. The circuit breaker did not trip at 27 A and 32®F.

2.3 TEST RESULTS FOR FUSES

Tables 10 and 11 show results for standard 20 A plug fuses. These were tested

over the temperature range 30°F to 77°F and current range 22 A to 40 A. At

22 A and 23 A and 44“F, fuses did not blow. The 44°F ambient temperature

data of table 10 shows that fuse currents had to be reduced to 22 or 23 A to

prevent fuses from blowing. Comparison of the data in tables 10 and 11 shows

that a reduction in the ambient temperature from 77°F to 31°F almost doubled

the time for a fuse to open the circuit when carrying 27 A.

Tables 7 and 8 show that circuit breakers would not trip when carrying 25 A

when the ambient temperature was 44°F. Fuses in table 10 show better performance than circuit breakers at this temperature. These fuses blew at 24

and 25 A.

11
TABLE 4. Test of Circuit Breaker //2 at 25 A and 27 A

Within the Temperature Range 39°F to 77°F

Run #

Ambient

Temperature

°F

Current

Amperes Trip

Run Time

Seconds

1 59 27 Yes 204.2

2 54 27 Yes 239

3 54 25 Yes 2280

4 39 27 Yes 611

5 39 25 Yes 3540

6 77 25 Yes 410

7 77 25 Yes 481

8 77 25 Yes 285

9 77 27 Yes 100

10 77 27 Yes 231.4

11 77 27 Yes 121,7

12 77 27 Yes 155.1

12
TABLE 5. Test of Circuit Breaker over Current Range

24 A to 40 A and Temperature Range 49®F to 78“F

Run //

Ambient

Temperature

(”F)

Current

(Amperes) Trip

Run Time

(Seconds) (Hours)

1 59 27 Yes 399.2

2 59 24 No 4

3 59 25 Yes 802

4 49 27 No 2

5 49 27 Yes 415

6 49 27 Yes 1009

7 49 25 No 7

8 77 25 Yes 1620

9 78 25 No 2

10 77 27 Yes 180

11 77 27 Yes 444

12 77 27 Yes 180

13 77 40 Yes 20

14 77 40 Yes 15

15 77 40 Yes 23

16 77 40 Yes 23

17 77 40 Yes 22

13
TABLE 6. Test of Circuit Breaker #4 over Current Range

27 A to 40 A and Temperature Range 44°F to 77®F

Run #

Ambient

Temperature

(°F)

Current

Amperes Trip

Run Time

(Seconds) (Hours)

Temperature/

Screw

1 77 27 Yes 950

2 77 27 Yes 1403

3 77 27 Yes 982

4 77 40 Yes 28

5 77 40 Yes 25

6 77 40 Yes 28

7 44 27 No 3.7

8 45 28 Yes 591 91

9 45 27 No 4 95

14
TABLE 7. Test of Circuit Breaker //7 over Current Range

25 A to 40 A and Temperature Range 44“F to 77°F

Run it

Ambient

Temperature

(°F)

Current

(Amperes) Trip

Run Time

(Seconds) (Hours)

1 44 27 Yes 472

2 44 27 Yes 360

3 45 27 Yes 400

4 44 25 Yes 1307

5 44 25 No 2.7

6 77 27 Yes 210

7 77 27 Yes 211

8 77 27 Yes 234

9 77 27 Yes 240

10 77 40 Yes 30

11 77 40 Yes 24

12 77 40 Yes 32

13 77 40 Yes 33

14 77 40 Yes 31

15 77 40 Yes 31

15
TABLE 8. Test of Circuit Breaker #8 over Current Range

25 A to 27 A and Temperature Range 32“F to 4A**F

Run //

Ambient

Temperature

(“F)

Current

(Amperes) Trip

Run

(Seconds)

Time

(Hours)

1 43 27 Yes 561

2 44 27 Yes 515

3 44 27 Yes 619

4 44 25 No 2460

5 44 25 No 4.3

6 44 26 No 3.5

7 32 27 Yes 672

8 32 27 Yes 556

9 32 27 Yes 610

10 32 26 Yes 1.1

16
TABLE 9. Test of Circuit Breaker //9 over Current Range

26 A to 40 A and Temperature Range 32®F to 77°F

Run //

Ambient

Temperature

(°F)

Current

Amperes Trip

Run Time

(Seconds) (Hours)

1 44 26 Yes 1179

2 44 26 Yes 3517

3 32 27 No 6.5

4 77 27 Yes 288

5 77 27 Yes 307

6 77 27 Yes 297

7 77 40 Yes 37

8 77 40 Yes 36

9 77 40 Yes 37

17
TABLE 10. Test of Standard 20 A Plug Fuses over Current Range

23 A to 27 A and Temperature Range 31®F to 44®F

Ambient

Temperature Current Opened Run Time

Fuse # (“F) (Amperes) Circuit (Seconds) (Hours)

1 43 26 Yes 146

2 43 25 Yes 338

3 43 24 Yes 1708

4 44 24 Yes 765

5 44 23 No 2.5

6 44 23 Yes 554

7 44 22 No 5.3

8 44 26 Yes 167

9 44 25 Yes 332

10 44 24 Yes 1530

11 44 23 No 1.4

12 44 23 No 4.2

13 31 27 Yes 93

14 31 27 Yes 84

15 31 27 Yes 77

16 33 27 Yes 83

18
TABLE 11. Test of Standard 20 A Plug Fuses over Current

Range 26 A to 40 A at Room Temperature

Fuse //

Ambient

Temperature

(“F)

Current

(Amperes)

Opened

Circuit

Run Time

Seconds

17 7/ 26 Yes 52

18 77 26 Yes 94

19 77 26 Yes 89

20 77 26 Yes 90

21 77 26 Yes 73

22 77 27 Yes 38

23 77 27 Yes 54

24 77 27 Yes 48

25 77 27 Yes 43

26 77 27 Yes 44

27 77 40 Yes 2

28 77 40 Yes 2.6

29 77 40 Yes 2.7

30 77 40 Yes 2.6

19
2.4 MULTI-CIRCUIT BREAKER TEST RESULTS

Tables 12A, 12B and 12C show results for circuit breaker specimen #1 tested at

27 A and 49®F ambient air temperature in the same load center with circuit

breaker specimens and #6. Specimens #5 and #6 were in series and carried

currents of 0, 5, 10, 15, and 19 A. As shown in the tables, circuit breaker

#1 did not trip when operated four hours and fifteen minutes with circuit

breakers //5 and #6 not carrying current. Circuit breaker #1 did not trip

when the circuit breakers and #6 were operating at 15 A or less. However,

circuit breaker //I tripped consistently when circuit breakers #5 and //6 were

operated at 19 A. The heat generated by circuit breakers 5 and 6 resulted in

a sufficient temperature rise in circuit breaker #1 to cause circuit breaker

//I to trip. The temperature rise at circuit breaker //I was 35®F when the

current in circuit breakers 5 and 6 was Increased by 4 amperes. (See run

//5 in table 12B and run iH in table 12C). The total power generated in the

circuit breakers with 15 A during run #5 of table 12B was 20 Btu per hour

(6 watts) and with 19 A during run #1 of table 12C was 25.6 Btu per hour

(7.5 watts). These heat rates were determined from the voltage drop across

the circuit breakers and the currents given in the tables.

20
Table 12A. Test of

Breakers

Breaker

#5 & //6 at

Installed in an Enclosure

49 °F Ambient Temperature

with

Run Run Time Breaker Voltage

Breaker #1

(Minutes) Number Drop-

(mV)

Current-

(A)

Trip Time - (Minutes)

Screw Tero.peratur<

(“F)

1 252 1 146.3 27.19 Did not Trip 87 — 5 0 0 — —

— 6 0 0 — —

2 256 1 146.0 27.07 Did not Trip 85

ti 5 23.5 4.93 — — •t 6 23.0 4.93 — —

335 1 145.3 27.00 Did not Trip 87

It 5 23.0 5.06 — —

II 6 24.0 5.06 — —

3 337 1 145.0 26.98 Did not Trip 87

It 5 47.9 10.04 — —

II 6 47.5 10.04 — —

389 1 146.1 27.11 Did not Trip 88

II 5 48.0 9.97 — —

6 47.7 9.97

21
Table 12B. Test of Breaker //I Installed in an Enclosure with

Breakers #5 & at 49®F Ambient Temperature

Run Run Time

(Minutes)

Breaker

Number

Voltage

Drop-

(mV)

Current-

(A)

Breaker #1

Wire Binding

Trip Time - Screw Temperature

(Minutes) (®F)

3 409 1 145.8 27.08 Did not Trip 88

II 5 47.8 9.94 — —

" 6 47.5 9.94 -- —

4 412 1 145.9 27.11 Did not Trip 88

•' 5 72.9 14.95 — —

II 6 72.6 14.95 — —

5 466 1 145.4 26.99 Did not Trip 91

II 5 75.3 15.24 — —

II 6 75.1 15.24 — —

491 1 146.1 27.10 Did not Trip 91

•' 5 75.6 15.28 — —

II 6 75.4 15.28

22
Table 12C. Test of Breaker //I Installed in an Enclosure with

Breakers at 49®F Ambient Temperature

Run Run Time

(Minutes)

Breaker

Number

Voltage

Drop-

(mV)

Current-

(A)

Breaker #1

Wire

Trip Time - Screw

(Minutes) (

Binding

Temperature

°F)

1 1 139.8 27.10 30 . .. „ 30 5 96.7 19.05 * 126

6 97.1 19.05 * —

2 1 145.7 27.77 6.7

6.7 5 95.2 19.10 * —

6 96.1 19.10 * —

3 1 147.1 27.29 16.7

16.7 5 95.4 19.06 * —

6 96.3 19.06 *

* Breakers #5 and did not trip
3. CIRCUIT BREAKER PERFORMANCE UNDER SHORT CIRCUIT CONDITIONS

The NEMA standard for molded-case circuit breakers presents data concerning

the maximum available short circuit current in service drops* [4]

The data

indicate a potential for several thousand amperes of short-circuit current

between line and ground. Actual short-circuit current in a particular residence is governed by transformer characteristics, length and wire size, and

resistance of the service drop. Examples of NEMA data are given in table 13.

The table gives currents in a service drop which was shorted at various

distances from distribution transformers [4]. Similar currents could occur

in short circuits within service entrance equipment. Therefore, a circuit

breaker must be designed to interrupt large short-circuit currents without

damage to Itself or other elements of the building system. Circuit breakers

used in these investigations were rated to interrupt 10,000 A.

The American Electricians’ Handbook Indicates that there is no easy calculation

or accurate predictive approach available for determining short-circuit

currents at various points in a given installation [7]. There are unknown

factors that must be determined before accurate calculations can be made

[7]. One such unknown is circuit Impedance.

Within a residence, large short-circuit currents could occur in branch

circuits at locations close to the load center. Because of branch circuit

conductor resistance, short-circuit currents are smaller at locations farther

from the load center. The short-circuit current, line to ground, at the end

of the branch circuit in figure 4 would be about 490 A (calculated from Ohm's

law.) Of course, short circuits at other positions in the branch could have

higher currents. Certain motors or capacitors operating during the time of

the short circuit would add some current to the fault and Increase the shortcircuit current. Based on the above calculations, the magnitudes of shortcircuit currents generated under laboratory conditions as presented in this

section were chosen as representative for general purpose residential branch

circuits

3.1 SHORT-CIRCUIT TEST SETUP AND TEST PROCEDURE

The laboratory experiments presented in this section were conducted to

determine if short circuits caused by nail penetration would be cleared by

overcurrent protection devices before ignition of combustibles could occur.

The short-circuit tests used the assembly shown in figure 5 to create the

short-circuit. This figure represents the nominal 2" x 4" wooden wall stud.

The nail shown in figure 5 was driven through the stud into the cable. The

* Service drop: the overhead service conductors from the last pole or other aerial support to and Including the splices, if any, connecting

to the service-entrance conductors at the building or other structure

[1]. See the service drop in figure 4.

24
Table 13. Symmetrical Single-Phase Short-Circuit Current in Amperes with

120 Volts Line to Ground at Various Distances in //2 AWG 3-Wire

Cable from Various Residential Type Distribution Transformers

Rated 120/240 Volts

Distance From

Transformer

(Feet)

Short-Circuit Current, Amperes

Transformer Size

25 kVA 37.5 kVA 100 kVA

0 8780 13830 35187

5 6927 10904 20845

10 6023 8843 14156

25 4268 5497 7031

50 2835 3318 3792

75 2114 2367 2593

100 1683 1838 1969

25
I—

75

FEET

OF

#2

AWG

ALUMINUM

TRANSFORMER

/

SERVICE

DROP

&

SERVICE

ENTRANCE

/

CABLES-0.000266

OHMS/FOOT

z

^ a os o Si

osCO 5

Figure 4. Short circuit in a residence
CABLE

UNDER

TEST

Figure 5. Short circuit specimen

27
nail made contact (short-circuited) between a current-carrying conductor and

grounding conductor. The cotton indicator was used to demonstrate possible

ignition of nearby combustibles. The stud appears to be particularly vulnerable to penetration by nails. This vulnerability is considered by article

300-4 (a) (1), "Bored Holes,” of the NEC [1] which requires that holes in

studs for cables be protected against nail or screw penetrations. That

portion of the article concerning studs reads as follows: "... holes in

studs for cable-type wiring methods shall be bored so that the edge of the

hole is not less than 1-1/4 inches from the nearest edge of the stud or shall be protected from nails and screws by either a steel plate or bushing

at least 1/16-inch thick and of appropriate length and width Installed to cover the area through which nails or screws might penetrate the Installed

cable.

After installing the short-circuit specimen in the test setup shown in

figure 6, the specimen resistance was measured using a kelvin bridge. The

resistance of the short could be determined by subtracting the known resistance of specimen wiring and test setup wiring. The initial test specimen

temperature was recorded. Surgical cotton, used as an ignition Indicator,

was packed in the stud hole around the wiring, as shown in figure 5.

Activation of the control switch powered the short; the following information

was obtained:

a. Whether or not the circuit breaker tripped and number of cycles

of current to trip (time of trip) , recorded on the oscilloscope

b. Peak current and voltage - also line voltage, recorded on the

oscilloscope.

c. Whether or not cotton was ignited.

d. Temperature on the cable near the short.

e. Whether or not the circuit opened at the shorted nail (if not,

the final resistance of the circuit was measured).

3.2 TEST RESULTS

Tables 14A-14H (see pages 30-37) show short circuit results for both

nonmetallic-sheathed cable, type NM, and armored cable, type AC. Results

show the circuit breaker did not operate during 27 of 64 short-circuit tests.

During those tests when the breaker did not trip, the circuit was opened at

the conductor and nail interface by "explosive" melting of interface material

during the short circuit. Temperature rise above ambient room temperature on the cable near the penetrating nail was about 20®F. The following were

also observed from tables 14A-14H:

a. Maximum peak short-circuit currents were 1000 A, the test results

in (specimen #2).

b. As shown by specimen #59, initial resistances were sometimes as high as 4.27 ohms. Usually the relatively high resistance was

unstable. Resistance variations of + 0.5 ohm were observed.

c. The cotton indicators ignited ten times, and in one case the

wooden stud was also Ignited.

28
Figure 6. Short circuit test setup

29
TABLE

14A.

Short

Circuit

Test

Results

with

Simulated

2x4

Stud

Section

and

//12d

Common

Nail

vO o vO <)• Cl O X X 8 X o o\ r-l 00

00

X >cr

X OS • VO • o • o OS

in o in r-4 CM X 8 X

in os o X oc X • r- oc • o • • o 00

Cl 00

m

vO o m CM X X X o o so X es 8 X 00 • Cl VO • o • • O 00 CJS c- o o CO

cs

X o r- 1—1 m X t-H X 8 X 00 • 00 c^ • o • • o os CM

r- 00 o 00 1—1

X 8 X o o '3- X CVJ X 00 • r-- c- • o • •

r-4 00 00

r>i Cl

cno o m cn

X

CM o 1^ i-H CO X vO X X rH o • m Cl • • rH o • • f-4 o Os CM

Cl 00

00o o m 00 OS CM

CM o o r- 00

CN

X CM

XX

X o • • • fH o CN o

00o

CM

•—1

X 1 X

X

X 1 1 1 1 1 j o y

CO (4

O § 3 y

T3 JS 4-1 (4 y 1 o 3 3

x: (4 4J

4-1

y 1 CO

y 3

COy 00 > •o o. V4 • y c y < 3 3 B y o JC s CO y 1-1 1 o y a

:z CO 4-1 CO CO CO (3 1 V4

y

H (*i 6 pn

y 1 COy o y o y o COy 3 4Jy 0 y o

(3 i-Hy

y •H

X

X z X S3

u (3 CO

O 1-1 CO

y

H

y •H D- CO CO

y

y 3 1 (4 1 1 B

c3

r-H

Xy 1-1

a t-i CO

yy 1-1

y

•H iH

H

X CO o y 4-t (4 6 S 1H (4 1—1 y CO y a r-l 1-1 iH 1-1 1-1 1-1 y •o c_> 4-1 1 iH •o y Ui na pd 4-1 S O CJ H r-l 3 !2 3

o.

y < cO c y (3 XI

y

y 1-1 > 3 X

X

CO (K

X

y •H o 4-1O

y

a r-l 3 4-JM iM r-l o y fH 4-1 4J 1-1 4J M CO a CO y -i<i u 3 4-i Ul 3 (4 B &. c XI •H 4-1 13 4-> o y •H C (-1 cO « o y 1-1 3 3 3 E X o CO c o 00 O y l-i lH •H y y f I—1 3 y iH y < H X CJ M CJ M CJ CO P3 H U 0l4 (^ CO CJ M :s (>1 X oin

II

<uu3

)-i a;i"

0)

4-1 c

(U§om

cno

*0

0) JCu

«0 4J

<ce

(UBo

<ua • CO ^<0 o <u 0 (3k

CO

0) 4J

CJ *-1 c o

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4J

CD vO

•H VD

CD »-4 0)^ II 4J 0) •H tlO ;] co y 4J

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4-1

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4) *H H i-t

30
iIfii

(it>1i4

n) !ac01oo

•o

CNJ

T3C

COCo

•H

4-1 0

<U CO•o3

4-t CO

«o-X

CM

•3

(U

4-> CO iH1

•H

CO4=

CO 4J rH3

CO

Pi

CO

<u H3o

•H

CJ

V4o

x:CO

•cf W

hJ

PC<H

CS CO o m CO

vO

m

X

CM o 1"' ^H

r-4

X 1—4

X

X 1—4 Ov • 00 ov • • o • • o o r>. C'

00 o r>. f" o o

in CM

X 8 o o orH

X f—4 X

X

X vO 1—4 • VO CO . o • • o CJV f—4

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CM n.

O VO

'O' cn CM

O rH

1—4

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XX

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cy> VO

o^ CO o in <3- CO «<r X X

CM o rH

1—1 X •<r

X 1-^ 00 • 00 'd' • . o • * o o nf'' 00 o 1—4 St- o VO '3-

CNJ

XX

CM o t-^ rH C"

1—4

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X 1—4 CJv • t'' VO • • o • o o VO

00

rH o in 00

1—4

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f—4

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X 8 X CJv • r-4 00 . o . • o OV in

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CM

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CM o rHm

CT4

X CO

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4= 3 >-t O g 3 3

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a

0) 1 3 3 o 3 o 3 o 3 3 3 4J 3 0 3 0 3 rH 3 •H X X X 4J 3 3 o .H 3 3 H

(U Xi •H 3 3 3 3 3 3 1 M 1 1 s

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iHX Pi 3 o 3 4-t 1-4

a S 1—4

M iH o 3 H •3 3 a fH •H .H •H .H -H

(U •3 CJ 4-J 1—1

•3 3 •3 Pi 4-1

g o CJ

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4J 4-1 •H 4J

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31

Test

Circuit

Resistance

(No

Specimen

Attached)

Equaled

0.1135

ohms,

Ambient

Temperature

was

75°F,

Line

Voltage

166

Volts

Peak
Table

14C.

Short

Circuit

Test

Results

with

Simulated

2x4

Stud

Section

and

//12d

Common

Nail

<?s CO o m '0-

Sj- o X

X

CM o rs rS

CM

X CM

X »—< o • VO tjs • • o • • o O CJs sr

rs 00 o os rS CO X o C?s o

CN

XX

X 8 X VO • 00 CO • o . • CM rsrs 00 o m CO

CM in

XX o os o

CM

X

m

XX 8 • 00 1 • o • 1 o rs

rs o rs

Mt CO o VO CO rs

r—

CM

X

X

CM o rs rS rs

CM

X

X o • m • • r-4 o • O o 00rs O o

vO ?“S

O •sT

XX

CM 1 ! 1 CM X t—

X rS j i I CO CO . • • • O o 00

rs 00 o

o^ o o sr

CTx

X

X o OS o

1^

X St

X

X 8 00 • CO 00 • o • • o 00rs 00 o o

SO o m CM rs VO

oc CM CM o rs. rs 00 X X X X 1*4 OS • • • o o o

r“4 o

sr sr o vO CM CO X

CM o rs rS

X i-S

X

X r—

OS • m rs . • o • • O o o m

00 00

(0E cu x: CO is o g 3 cu .2 4-t is

0) 1 o CO 3

is 4-) <u 1 CO cu to

to tj oo > 73

a is •

(U c cu < G

G

a (U o to o •H 1 O cu a

:z to 4-> (0 CO CO c T3 1 is

O

H fa

a fa 1 CO (U o (U o (U o to <U 3 4-1 CU o cu o G rH CJ •H X 2 X X is 4J G CO O -H CO <U H

0) •C •H tu CO CO (U (U 3 1 is 1 1 E 6

iHa <U •H (X is CO o cu •rH (U •H iH X Pi CO O <u U is a 3 rH is rH U to H •a CU (X rH "H •H •H •iH iH

(U "X} c_> 4-1 fH T3 (U is TO Pi 4-t

aO CJ

H rH to !3 to

a (U < AJ to c (U c Ji cu (U •H < > to Si !Z CO u S tu •H o 4-1 O o a rH G 4J is •H rH o <u 1—

4J 4J .w 4J V4 to

a to CJ is to 4J is to is E a C .H 4-1 c 4.4 o cu c is cO to o (U •H to G to c X o to C o W) o o is t-i •H •H (U (U s: 1—

3 <U •H (U <: H CJ M o M CJ CO PQ H u p4 (X CO CJ M S: fa

32

Test

Circuit

Resistance

(No

Specimen

Attached)

Equaled

0.1135

ohms.

Ambient

Temperature

was

75°F,

Line

Voltage

166

Volts

Peak
TABLE

14b.

Short

Circuit

Test

Results

with

Simulated

2x4

Stud

Section

and

#12d

Common

Nail

cs CO o in CO

VO in

XX

CM o rs pH

pH XI pH

X pH 0\ . 00 as . • o . . o o rs rs rs 00 o rs o o pH

m CM 8 o o o

pH X! pH

XX

X

X vO pH . vO CO . o . . o CTi pH rs 00

CO si- CM rs o VO St

'd- CO

XX

CM o n. pH

pH

X pH

X pH Ov . CM in . . o . . o o 00 CO

rs 00

CTv VO

C^ CO o m St

CO

X

CM o rs pH

pH

X

X

X pH c» . 00 St . . o . . o o rs St

rs 00 o pH

pH sr o vO .O- CM St X

CM o rs. pH rs

pH

X pH

X

X i-H C3V . rs VO . . o . o c VO

00

pH

O in 00

pH in

X o CO o

pH

X pH

X

X 8 X GO . pH 00 . o . . o OV 00

rs ov

pH

<3v o m rs o CM X 8 X o rs o

pH

X pH

X

X C3V • pH 00 . o . . o Dv in

rs c^ o CO

in c O in CO

CM

X

CM

O r-s rH

m

X CO

X

X pH 00 • rs • . • o . CO o o 00 00

rs i cu

CO 3 o S 3 cu .c >H

(U 1 o cO 3

IH 4H

01 1 CO 0) CO

CO o CIO > TJ & IH . 0) c 0) <3 c 3 B cu o CO o hH 1 O o; a p CO s 4-1 CO CO CO c •o 1 IH U Eh Ph B Ph

0) 1 CO 01 o cu o cu o CO (U 3 4J 0) o a; 0 G pH CJ .H X !S X X 4J G CO O hh CO 0) H

(U ,o •H 0) CO CO

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01 XI

uU pH •o 0)

u 'O PC 4-1

B

O CJ

H pH CO CO P. 0) < p CO G (U G JG (U 01 .pH < > CO Z Z

CO

u S 01 •H

O 4JO

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G 4H Vh hH pH o <u pH 4-1 4-> •H 4J P cO D- cO O pH »H CO 4J 3 CO >H B o- C .Q •H 4-) G 4J o <U •pH c u CO CO O 0) hH cO 3 CO u X o (0 G O bO O o U •H hH (U 0) JG pH G CU •H CU <3 H u M U M CJ CO pq H CJ pH Ph CO CJ M !S pH 2;

33 i

Test

Circuit

Resistance

(No

Specimen

Attached)

Equaled

0.1135

ohms,

Ambient

Temperature

was

75°F,

Line

Voltage

166

Volts

Peak
Table

14E.

Short

Circuit

Test

Results

with

Simulated

2x4

Stud

Section

and

#12d

Common

Nail

in

T—

as

O o CO o CM X X

rH o O r.. sr XI rH X rH • r- CO • • o • o o rH

oc o o CO CO o o sf

as cs

XX

CM o fH CO X! X rH • 00 in • • o . • o o CO

00

CO

r~i o o CO

00 Cvj

X 8 X

in o o

CO

X

X \o • t". . o • o CM

00 o o CO h.

as

X

X o Os o i>- CO X X 8 X VO • as • o • CM o

00

IT) m o m CO

vo c?\

X o fH as

CO

X CO

X

X 8 X as • • o> • o VO • o 1^

00

as

s£> o o sJm CM X X o 00 O

CO

X rHXX 8 00 • 1 . o 1 oo CM CO Os o in

.O' in

XX

rH o f.. CO X rH X rH CO • m 1 • • o • 1 o o vO oin o o rH CO rH m 00 o

CO

X CO

X

X

X 8 X vO • m 1 « o . 1 o VO

COS 0) JZ CO M o g 3 <u 03 £ 4J iH

<u 1 o CO 3

V4 4J 4J (U 1 CO (U CO «0 CJ 00 > 03 a P4 • (U c (U <3 13 tz Q 01 o x: CO o •H 1 o Oi a A w s 4-1 CO CO CO 13 03 1 iH o H pH 0 Ph

(U 1 CO (U o (U o (U o cO 01 3 4-» (U

O 0) 0 c iH o •H X X 25 X !3 4-1 C CO (=> *H CO 0) H

(U •H (U CO CO <U (U 3 1 iH 1 1 E <0 fH cx (U .H a iH CO u <u •H (U •H o rH X 0i CO o <u 4J P* S & ?H P4 rH O CO H 03 0) tx 1—I "H •H •H •H *H

(U TJ o 4-i iH 03 (U V4 03 Pi 4J

B o c_>H rH cO tS CO

a 0) < CO c (U tz (U (U •H < > CO is is w s <u .H o 4-1 o u (X f-H 3) 4J iH •H rH o 0) iH 4-1 4-J •H 4-1 u CO (X cO O M ^ u CO 4H M CO iH a. c .fH 4J c 4J o <u .H (3 P4 CO cO O <U •H cO 3 CO X o CO c o 00 o o Vh iH •rH •H 01 <U 43 rH fZ (U •H OJ H o M C_) M u CA PQ H pH O Ph u M IS pH is

34

Test

Circuit

Resistance

(No

Specimen

Attached)

Equaled

0.1135

ohms.

Ambient

Temperature

was

75®F,

Line

Voltage

166

Volts

Peak
Table

14F.

Short

Circuit

Test

Results

with

Simulated

2x4

Stud

Section

and

i^l2d

Common

Nail

CO

m

CO IT) o m CO

CM

X

CO o 1-4

X

X

X r-4 00 . C3V • . o • • o o VO Mf

r- CO

'O' MT

lO CO

CM

X

X

CM

X 1—c X r-4 1 1 1 If) -O’ • . 1 1 1 • • o o o SJ- CO CO

'O' C3V <» CM o uo CO

CM CM o 1—4 rX i-H X X X r-4 Ov • r- . . o • o o CM00

vO

IT) Ov o m CO c- »

1 1—I o r«. 1-4

X

XX

X f-t cr> • ov • . o • o o oCO o o CM

iO UO

O

X o X

X

X 8 X 1^ . ov . o • • vO CM

CO

CM

VO CM o o O X X

CO o r>. 1-4

X CM

X 1-4 VO . m 00 • • o • • O o vO iH r^ CO

rH CO

fH c^ o O CO

CM

XX

fH o r>. r-4

X tH

X ?H . ov CO • • o • • o o VO 1-4 CO m CO o CO o o

CM 1-4 o f-4

X

X

X

X 1-4 • CO • • o « • o o c- CM

r-N CO I cu CO d o e 3 cu T3 42 4J d

OJ 1 O CO 3

x:

d 4-1 4J <u 1 CO (U CO

CJ 60 > •d Cu

d

(Ud CU <3dd

B CU x: CO o •H 1 o (U a

Ui s CO CO CO

d "d 1 d o H Id

B Pd

(U 1 ^ CO CU o cu o 0) o 0) 3 4J cu o (U o

1—1 o •H

X tzX z X z » 4J

d CO pa *H CO CU

H

.C •H <U CO CO (U cu 3 1 d 1 1 s

rH Ou cu •t4 (Xd CO cu cu •H CU iH X Pi CO o cu 4-1 d B Is 1-4 d -H

0}

H T3 (U

a r—i *H •H •H •H t4 T3 o 4J iH cu d •d X 4-1 B o o H r-4 CO IS CO

(U •<

m « CO

d cud 42 cu cu •H < > CO Z !3

>-l (u •H o 4-1 o o a iH

d 4J

d •H t-4

O <U rH 4-1 •H 4J

d CO d. CO o 4«5

d CO 4J

d cO

d

B

a c x> •H 4-1

d 4-)

O (U •H

d

d CO CO

O CU •H CO

d CO

C

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35

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Specimen

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Equaled

0.1135

ohms,

Ambient

Temperature

was

75“F,

Line

Voltage

166

Volts

Peak
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36

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Equaled

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Ambient

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166

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Peak
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37

Test

Circuit

Resistance

(with

no

Specimen

Attached)

Equaled

0.1135

ohms.

Ambient

Temperature

was

75“F,

Line

Voltage

166

Volts

Peak
d. The cotton indicators were scorched 12 times without cotton

ignition.

Figures 7, 8, 9 show oscilloscope records of selected short circuits. These

figures show current and voltage drop in phase indicating, for all practical

purposes, a purely resistive short circuit. Figure 7 shows no apparent

current and voltage wave distortion. However, in figures 8 and 9 (for cases when the circuit breaker did not trip) both current and voltage waves were >. distorted due to sporadic interruption of the current. This may be due to a thermostatting effect that makes and breaks contact between the nail and the

wire, thus opening and closing the circuit. Opening and closing on the

short circuit in this manner may prevent the circuit breaker from tripping.

This is demonstrated by data in figure 9. As shown, the average current

during the first cycle was 530 A rms* . No current pulse occurred during the

next 6 ms. At the end of the 6 ms period, a current pulse of 350 A rms occurred for a period of 4 ms. At the end of the 4 ms pulse, the current

was off for 13 ms. At the end of this off period, a final current pulse of

420 A rms occurred, lasting 8 ms, at which time the circuit opened between

the wire and shorting nail. Placing the current pulse data on the circuit

breaker curves of figure 10, it is seen that a circuit breaker trip was likely during the 530 A pulse and could not occur during the other pulses.

In figure 7, 8, and 9, the current waves have the lowest amplitudes and the

voltage waves have the highest amplitudes.

38
Figure 7. Specimen #3 - circuit breaker

tripped during short circuit

Figure 8. Specimen //6 - circuit breaker did

not trip during short circuit

39
-’'iE

'f^P.

'

't

V:r

IFigure 9. Specimen #19 - circuit breaker did

not trip during short circuit

'I

40
MULTIPLES OF RATED CURRENT IN AMPERES - rms

Figure 10. Manufacturer's curves applicable

to a 20-ampere circuit breaker

TTVm

i mul

10000

41
4 . CONCLUSIONS

The following can be concluded from this 20 A residential circuit breaker

study:

(a) Circuit breakers of the types studied for this report may not trip

when carrying currents as large as 28 A when exposed to ambient

temperature of 61“F or less. The inability of these circuit breakers to always trip during overcurrents could allow home owners to

overload their wiring. Such overloads could cause excessive branch

circuit wire temperatures, particularly when the branch circuits

are surrounded by thermal insulation [2]

(b) During short circuits which may result from penetration of wiring

by nails, circuit breakers of the types studied for this report

may not function to interrupt the current and prevent ignition of

proximate combustibles.

5 . RECOMMENDATIONS

The exploratory data presented in this paper indicate a need for a comprehensive study of overcurrent protection devices. Such a study should

yield information concerning potential for fire in residential buildings

resulting from the inability of overcurrent protection devices to always

open circuits and thereby prevent overheating of fixed wiring. The study

should include:

a. Development of more meaningful scientific/mathematical principles

and models of the functional characteristics of residential overcurrent protection devices.

b. A field study to measure residential circuit breaker temperature

during the winter and during the cooling season.

c. A field study of fires alleged to have been electrically Induced, to

reconstruct and better explain those mechanisms of electrical system

failure that result in fires.

d. Laboratory studies of generic types of residential overcurrent

devices on a scale large enough to establish statistical data

concerning their performance.

42
REFERENCES

[1] National Electrical Code, 1978, NFPA No. 70-1978.

[2] "Exploratory Study of Temperatures Produced by Self-Heating of Residential

Branch Circuit Wiring When Surrounded by Thermal Insulation," Beausollel,

R. W., Meese, W. J. , and Galowin, L. S., NBSIR 78-1477, July 1978.

[3] "Molded-case Circuit Breakers and Circuit-breaker Enclosures," UL 489,

Undervnrlter * s Laboratories, Inc., November 1, 1972.

[4] "Molded-case Circuit Breakers," Standards Publication No. ABl-1975, National

Electrical Manufacturer's Assoc., 155 East 44th St., New York, N.Y. 10017.

[5] "Investigation of Over Current Protection Devices as Affected by Ambient

Temperatures," Bulletin of Research, No. 41, July 1948, Underwriter's

Laboratories, Inc.

[6] "Procedures for Verifying Performance of Molded-case Circuit Breakers,"

Publication No. AB2-1976, National Electrical Manufacturer's Assoc., 155

East 44th St., New York, N.Y. 10017.

[7] "American Electricians' Handbook," John H. Watt Editor, Ninth Edition,

McGraw-Hill Book Company, 1970.

[8] NBS Monograph 125, "Thermocouple Reference Tables Based on the IPTS-68"

by Robert Powell, William J. Hall, Clyde H. Hylnk, Jr. and Larry Sparks,

Cryogenics Division, Institute of Basic Standards, NBS and George W. Burns,

Margaret G. Scroger, and Harmon H. Plumb, Heat Division, Institute for

Basic Standards, NBS, March 1974.

43
U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 81-2221

2. Performing Organ. Report No 3. Publication Date

April 1981

4. TITLE AND SUBTITLE

EXPERIMENTALLY DETERMINED PERFORMANCE OF SOME RESIDENTIAL CIRCUIT BREAKERS

5. AUTHOR(S)

Robert W. Beausoliel and William J. Meese

6. PERFORMING ORGANIZATION (If joint or other than NBS, see Instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, O.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State. ZIP)

SAME

10. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here) Laboratory test results show that at low ambient temperatures some residential-type

circuit breakers may not trip at currents up to 140 percent of rated currents. Under

some environmental conditions this may lead to wiring temperatures that exceed the

limitations specified in the National Electrical Code. The results also show that

circuits sometimes open at the point of short circuits before circuit breakers

operate. Ignition of combustibles proximate to the point of such short circuits

sometimes occurs.

The results indicate the need for a more detailed study of overcurrent protection

performance in the field and under laboratory conditions. Also needed are the

development of more meaningful scientific /mathematical principles and models on the

functional characteristics of circuit breakers.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

branch circuits; circuit breaker; electrical fire; low ambient temperature; trip time.

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

[X_J Uni imited

I I For Official Distribution. Do Not Release to NTIS

f I Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

47

15. Price

$6.50

USCOMM-DC 8043-P80