ANSI/IEEE C37.010-1979
(Includes Supplement ANSI/IEEE C37.010d-1984)
(Revision of ANSI/IEEE C37.010-1972)
An American National Standard
IEEE Application Guide for
AC High-Voltage Circuit Breakers
Rated on a Symmetrical Current Bass
Sponsor
Switchgear Committee of the
IEEE Power Engineering Society
Approved May 29,1975
Reaffirmed October 20, 1988
Secretariat
Institute of Electrical and Electronics Engineers
National Electrical Manufacturers Association
Approved December 12,1975
Reaffirmed February 2, 1989
American National standardsInstitute
ANSlllEEE C37.010d-1984 approved March 11, 1982 by the IEEE Standards Board
Approved April 29, 1983 by the American National Standards Institute
Published by
The Institute of Electrical and Electronics Engineers, Inc
345 East 47th Street, New York, NY 10017
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@Copyright 1979 by
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Foreword
I
(This Foreword is not a part of ANSI/IEEE C37.010-1979, American National Standard Application Guide for AC
High-Voltage Circuit Breakers Rated o n a Symmetrical Current Basis.)
This standard is a guide to aid in the selection and application of AC high-voltage circuit breakers
on electrical power systems. Described in the guide are the general application conditions, application considerations, and short-circuit considerations necessary to apply alternating current circuit
breakers.
This standard is a revision of ANSI/IEEE C37.010-1972. This revision contains substantive revisions due to the efforts of a working group on revisions to C37.010-1972, and editorial changes as
a result of general power circuit breaker consolidation efforts.
Three substantive
ns are included. The first involves deletion of a Procedure Chart, from
Section 5.3,page 26,
e previous standard. This simplified procedure for calculating short-circuit
current was useful in only a few cases, and was somewhat confusing for anyone attempting t o use it.
The second substantive revision is an expansion of the statements in 5.3.2 involving the general
terms “one transformation” and “two or more transformations” to recognize large external impedances such as long lines or station service transformers. Previously, “one transformation” in the
form of a station service transformer dictate the use of factors from Figs 7 and 8. However, because
this relatively large impedance limits the ac component decay, the wording on page 34 specifies the
use of factors from Fig 9 for both three-phase and line-to-ground faults.
The third substantive revision involves the rotating machine reactance table of multipliers on page
40. The multipliers under the heading “Closing and Latching Duty” were previously specified in a
section of Note 4 which is deleted. The substantive change involves the placing of all motors above
1000 hp with speeds 1800r/min and below into the 1.5 multiplier category. See chart below.
3600
M = 3.0
t
,,
M = 1.5
A
Y
II
MULTIPLIER
50
250
- 3.0
500
M - 1.5
750
IOC Y
o
0
RATING - HP
Based on the ac short-circuit time constants of these larger, slower-speed machines and the fact that
circuit breakers are available with rated interrupting times of 5 cycles or less, the 1.5 multiplier will
give conservative results, whereas the previously shown 3.0 multiplier would not.
The remainder of the changes are editorial. They involve updating the examples and clarification
of the material in the guide. In addition, a number of changes were made as part of the general consolidation of power circuit breaker standards.
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The development of standards for the rating, testing, and manuf
breakers began almost simultaneously with the application of the
power supply systems.
A number of engineering and manufacturers trade organizations were int
-voltage circuit breakers as well as other types of electrical equipment
ard requirements for capabilities, sizes, and testing procedures.
AIEE’, the National Electric Light Association (NELA), the Electric
NEMA - the National Electrical Manufacturers Association), the As
ing Companies (AEIC), and the Edison Electric Institute (EEI).
s up to 1940, these organizations adopted and pub1
cerning rating, testing, and other requirements for hi
In 1941 a unified series of standards for circuit breakers, based
NEMA, w
blished for trial use by the American Standards Associa
the first American Standard for high-voltage circuit breakers. In 19
approved American Standard with the familiar C37 number iden
g, preferred sizes, testing, and application of circuit
dards was revised and supplemented by additional se
basic group of American Standards for high-voltage circuit breakers. At
group of standards included:
NSI C37.4-1953
AC Power Circuit Breakers (included definitions, ra
test requirements)
ANSI C37.5-1953
Methods for Determining the RMS Value of a Sinusoidal Current Wave
and Normal-Frequency Recovery Voltage, and for Simplified Calculation
of Fault Currents
ANSI C37.6-1953
Schedules of Preferred Ratings for Power Circuit Breakers
ANSI C37.7-1952
Interrupting Rating Factors for Reclosing Service
ANSI C37.8-1952
Rated Control Voltages and their Ranges
ANSI C37.9-1953
Test Code for Power Circuit Breakers
ANSI C37.12-1952
Guide Specifications for Alternating Current Power Circuit Breakers
Under these original standards, the basis of the interrupting rating was established by 6.11 of
ANSI C37.4-1953 as the highest current t o be interrupted at the specified operating voltage and
was the “. . . rms value including the dc component at the instant of contact separation as determined from the envelope of the current wave.” Since this standard based the interrupting rating on
the total current including dc component at the instant of contact separation, it has
as the “Total Current Basis of Rating.”
For circuit breaker application, a simplified method was avail
iplying factors for use with the system symmetrical fa
tal rms current which could be present at contact separation. This c
choose the required circuit breaker rating from those listed in
revisions. The factors recognized typical system characteristics an
‘AIEE (American Institute of Electrical Engineers) merged with IRE (Institute of Radio Engineers) January 1, 1963
to form the joint organization IEEE (Institute of Electrical and Electronics Engineers).
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In 1951, the AIEE Switchgear Committee began to give consideration to the development of a
circuit breaker rating method based on symmetrical interrupting currents. This work was initiated
with the goal of:
(1) Simplifying application where high-speed relaying and fast clearing circuit breakers are used
(2) Bringing American standards into closer agreement with accepted international standards
(IEC-International Electrotechnical Commission) t o avoid confusion on rating differences
(3) Requiring that circuit breakers are proven to demonstrate a definite relationship between
asymmetrical interrupting capability and symmetrical ratings
During the course of this work, principally in a working group of the AIEE Power Circuit Breaker
Subcommittee, numerous reports of the proposals on the new rating, testing, and application
methods were made to the industry as a whole through committee sponsored papers a t AIEE meetings in 1954, 1959, and 1960. Suggestions made in discussions were considered by the working
group and incorporated where practicable. The principal change from the 1953 “Total Current”
standard was in the basis of rating. 4.5.1 of ANSI C37.04 established the Rated Short Circuit Current as “the highest value of the symmetrical component of the.. . short-circuit current in rms
amperes, measured from the envelope of the current wave at contact separation, which the circuit
breaker is required t o interrupt at rated maximum voltage. . .”. Certain related capabilities were also
required, including operation under specified conditions of asymmetry based on typical circuit
characteristics and circuit breaker timing. This rating structure became known as the Symmetrical
Current Basis of Rating as compared to the previous Total Current Basis of Rating. However, as the
new ratings were developed, it became apparent that changes from the older t o the newer standard
could not occur overnight due to requirements for rerating and retesting of many PCBs. It was,
therefore, decided to retain both rating structures, with the understanding that all new circuit
breaker developments would be directed toward the symmetrical siandards. The circuit breakers
based on the total current standards would be transferred t o the new standards as work progressed
in rerating programs. This transfer is being carried out and ANSI C37.6 and ANSI C37.06 have
been revised accordingly a number of times.
The symmetrical current group of standard sections was published in 1964 and was given ANSI
C37.04, C37.05, C37.06, etc, designations. These sections and the corresponding 1953 sections
were :
Total Current Standard
Symmetrical Current Standard
Subject
ANSI C37.4
ANSI C37.03
ANSI C37.04
ANSI C37.04a
Definitions
Rating Structure
ANSI C37.5
ANSI C37.05
Measurement of Voltage
and Current Waves
ANSI C37.6
ANSI C37.06
ANSI C37.06a
Preferred Ratings
ANSI C37.7
ANSI C37.07
Reclosing Factors
ANSI C37.8
(included in
ANSI C37.06)
Control Voltages
ANSI C37.9
ANSI C37.09
ANSI C37.09a
Test Code
ANSI C37.5
(Section 3)
ANSI C37.010
Application Guide
(expansion of material
previously in C37.5)
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Sections .04a, .06a, and .09a, also issued in 1964, were addenda concerned with supplemental
dielectric capability requirements.
In ANSI C37.06-1964 and subsequent revisions prior to 1971, circuit breaker symmetrical current
i n t e r r u p w ratings were derived from ratings in ANSI C37.6-1961 by a relationship following a
middle ground position between the total (asymmetrical) current of the former rating method and
the full range of related requirements of the new rating method. For a given breaker this derivation
was expressed by the formula:
rated short circuit current
where
= 1,961
nominal voltage
rated maximum voltage
11961
=
interrupting rating in amperes appearing in ANSI C37.6-1961
F
=
0.933 for 3 cycle breakers
0.955 for 5 cycle breakers
1.0 for 8 cycle breakers
Rated short circuit current was tabulated for rated maximum voltage rather than for nominal
voltage as had been the case under the total current basis of rating.
It was stressed that this derivation was for the numerical conversion only and that a
breaker, designed and tested under the total current basis of rating, could not be assu
these capabilities under the symmetrical current basis of rating without approval of the manufacturer.
In the revision of ANSI C37.06 published in 1971, several simplifications were introduced, including the use of a new method for selection of intempting current ratings for outdoor circuit breakers
121 kV and above. Values for rated short circuit current were chosen fr
the R-10 preferred
discontinued. Also
number series, and the use of a reference nominal 3-phase MVA identificatio
the rated voltage range factor K was changed to unity, 1.O,to simplify rating and testing procedures.
In the intervening years since the official publication of the primary sections of the symmetrical
basis of rating standard for high-voltage circuit breakers, a number of revisions, additions, and
improvements have been developed and published. Many of these additions were in subject areas
of major importance in the rating, testing, and application of circuit breakers and were published as
complete standards containing appropriate definitions, rating performance criteria, rating numbers,
test procedures, and application considerations. This was done to avoid delay in publication and the
necessity of reprinting other existing standards as each of these was completed. The result has been
the publication of a substantial number of individual supplementary standards. The basic subject
areas considered in these supplementary standards, and their initial publication dates, are shown
below:
ANSI 637.071-1969
Requirements for Line Closing Switching Surge Control
ANSI C37.072-1971
Requirements for Transient Recovery Voltage
ANSI C37.0721-1971
Application Guide for Transient Recovery Voltage
ANSI C37.0722-1971
Transient Recovery Voltage Ratings
ANSI C37.073-1972
Requirements for Capacitance Current Switching
ANSI C37.0731-1973
Application Guide for Capacitance Current Switching
ANSI C37.0732-1972
Preferred Ratings for Capacitance Current Switching
ANSI C37.074-1972
Eequirements for Switching Impulse Voltage Insulation Strength
ANSI C37.076-1972
Requirements for Pressurized Components
ANSI C37.078-1972
Requirements for External Insulating
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ANSI C37.0781-1972
Test Values for External Insulation
ANSI C37.079-1973
Method of Testing Circuit Breakers When Rated for Out-of-Phase
Switching
A goal of work recently completed, and reprehented by the 1979 publication of these standards,
has been the editorial incorpo
the supplementary standards listed above into the proper
primary standards documents.
breakers rated on a
metrical current basis, the consolidated standards sections are:
ANSI/IEEE C37.04-1979
Rating Structure
ANSI C37.06-1979
Preferred Ratings and Related Required Capabilities
ANSI/IEEE C37.09-1979
Test Procedure
ANSI/IEEE C37.010-1979
Application Guide - General
ANSI/IEEE C37.011-1979
Application Guide - Transient Recovery Voltage
ANSI/IEEE C37.012-1979
Application Guide - Capacitance Current Switching
The present ANSI C37.05, Measurement of Current and Voltage Waves, is incorporated into
ANSI/IEEE C37.09; ANSI C37.07, Interrupting Capability Factors for Reclosing Service, is incorporated into ANSI/IEEE C37.04, ANSI C37.06, and ANSI/IEEE C37.09. Definitions which have
been in C37.03-1964 are now in ANSI C37.100-1972.
Standards are presently being developed in a number of additional subject areas, which will be
initially published as supplementary standards and incorporated into the primary subject document
at some future date. Included among these subjects are requirements for current transformers, a
guide for synthetic testing, sound level measurements, and seismic capability requirements.
For circuit breakers still rated on a total current basis, as listed in ANSI C37.6, the existing
standards ANSI C37.4, ANSI C37.6, ANSI C37.7, and ANSI C37.9 will continue to be applicable.
Documents pertaining to guide specification and control schemes, which apply to both groups of
ratings, are included in the ANSI C37 series as shown below:
ANSI C37.11-1972
Requirements for Electrical Control on AC High-Voltage Circuit Breakers
Rated on a Symmetrical Current Basis and a Total Current Basis
ANSI C37.12-1969
Guide Specifications for AC High-Voltage Circuit Breakers Rated on a
Symmetrical Current Basis and a Total Current Basis
Periodic review of all these standards takes place through the normal ANSI procedure that
standards are reaffirmed, revised, or withdrawn within no more than five year intervals from the
original publication date.
Suggestions for improvement gained in the use of this standard will be welcome. They should be
sent to the
American National Standards Institute
1430 Broadway
New York, NY 10018
The basic data included in this consolidated document is the result of contributions made by
many individuals over many years. At the time of approval, however, the American National
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Standards Committee on Power Switchgear, 637, which reviewed and approved this standard, had
the following personnel:
J. P. Lucas, Secretary
C. L. Wagner, Chairman
J. E. Beehler, Executive Vice-chairmanof High- Voltage Switchgear Standards
W. E. Laubach, Executive Vice-chairmanof Low-Voltage Switchgear Sthndards
Wilson, Executive Vice-
irman of IEC Activities
Organization Represented
Name of Representative
Association of Iron and Steel Engineers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electric Light & Power Group . . . . . . . . . . . . . . . . . . . . . .
,.................
.
Institute of Electrical and Electronies Engineers
trical Manufacturers Association.
Testing Laboratory Group.
H. G. Frus
K. D. Hendrix
F. R. Solis
............................
......................
.................................
....
......
Tennessee Valley Authority. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
U.S. Department of the Army . . . . . . . . . . . . . . . . . . . . . . . . .
.......
.......
U.S. Department of the Interior, Bureau of Reclamation. . . . . . . .
U.S. Department of the Navy, Naval Facilities Engineering Command . . . . . . . . . . . . . .
The personnel of the C37 Subcommittee on High Voltage Circuit Bre
approved this document were as follows:
H. F. White
M. J. Beachy (Alt)
C. A. Mathews (Alt)
R. A, McMaster (Alt)
D. C. Musgrave (Alt)
A. P. Colaiaco
R. W. Dunham
D. G. Portman
G. A. Wilson
W. R. Wilson
E. J. Huber
R. A. Naysmith
R. W. Seelbach (Alt)
R. C. St. Clair
R. H. Bruc
E.M.Tom
D. M. Hannemann
hich reviewed and
F. G. Schaufelberger, Chairman
J. J. Fayed, Secretary
J. E. Beehler
D. 0. Craghead
M. A. Durso
C. J. Dvorak
R. E. Friedrich
R. D. Hambrick
D. R. Kanitz
W. E. Laubach
G. N. Lester
F. W. Smith
D. L. Swindler
W. R. Wilson
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The basic documents used for the consolidation sections of this standard were prepared by various
working groups of the Power Circuit Breaker Subcommittee of the IEEE Switchgear Committee.
The personnel of the subcommittee at the time of consolidation were:
G. N. Lester, Chairman
H. W. Anderl
J. E. Beehler
D. M. Benenson
L. E. Brothers
R. G. Colclaser
J. C. Coon
C. F. Cromer
C. R. Cusick
A. Dupont
C. J. Dvorak
J. D. Finley
R. E. Friedrich
T. F. Garrity
W. F. Giles
K. I. Gray
G. P. Guaglione
R. D. Hambrick
G. R. Hanks
W. C. Huening, Jr
P. L. Kolarik
S.R. Lambert
D. M. Larson
W. E. Laubach
M. J. Maier
J. A. Maneatis
R. A. McMaster
G. J. Meinders
G. L. Nuss, Jr
I. E. Olivier
G. 0. Perkins
J. G. Reckleff
H. K. Reid
A. B. Rishworth
W. N. Rothenbuhler
F. G. Schaufelberger
H. N. Schneider
E. F. Solorzano
C. J. Truax
E. F. Veverka
C. L. Wagner
D. R. Webster
A. C. Wert
G. A. Wilson, Jr
W. R. Wilson
B. F. Wirtz
C. E. Zanzie
The Working Group members of the IEEE Power Circuit Breaker Subcommittee who developed
the substantive revision were:
P. L. Kolarik, Chairman
H. W. Anderl
W. C. Huening, Jr
G. L Nus,J r
H. 0. Simmons, Jr
C. J. Truax
C. L. Wagner
The Working Group of this subcommittee responsible for the editorial consolidation work on this
standard consisted of:
N. E. Reed
W. N. Rothenbuhler
F. G. Schaufelberger
When the IEEE Standards Board approved this standard on September 4,1975, it had the following membership :
Warren H. Cook, Vice Chairman
Joseph L. Koepfinger, Chairman
Sava I. Sherr, Secretary
Jean Jacques Archambault
Robert D. Briskman
Dale R. Cochran
Louis Costrell
Frank Davidoff
Jay Forster
Irvin N. Howell, Jr
Stuart P. Jackson
Irving Kolodny
William R. Kruesi
Benjamin J. Leon
Anthony C. Lordi
John P. Markey
Donald T. Michael
Voss A. Moore
William S. Morgan
William J. Neiswender
Gustave Shapiro
Ralph M. Showers
Robert A. Soderman
Leonard Thomas
Charles L. Wagner
William T. Wintringham
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Contents
SECTION
1.
................................................................
Relatedstandards ................................................
15
15
..............................................................
General Application Conditions ............................................
3.1
Usual Service Conditions ...........................................
Unusual Service Conditions .........................................
3.2
Application Consideration ................................................
Maximum Voltage for Application ....................................
4.1
4.2
Voltage Range Factor ..............................................
4.3
Frequency ......................................................
Continuous Current ...............................................
4.4
Rated Dielectric Strength ...........................................
4.5
Standard Operating Duty ...........................................
4.6
4.7
Interrupting Time .................................................
4.8
Permissible Tripping Delay ..........................................
4.9
ReclosingTime ...................................................
4.10 Short-circuit Rating ...............................................
4.11 Transient Recovery Voltage Rate .....................................
4.12 Load Current Switching Capability and Life ............................
15
Scope
1.1
2.
3.
4.
Purpose
4.13
4.14
4.15
4.16
4.17
4.18
4.19
4.20
5.
6.
PAGE
Capacitance Current Switching .......................................
Lineclosing .....................................................
Conditions of Use with Respect to the Out-of-Phase Switching Current Rating
Shunt Reactor Current Switching .....................................
Excitation Current Switching ........................................
Mechanical Life ..................................................
Rated Control Voltage .............................................
Fluid Operating Pressure ............................................
....
15
15
15
16
17
17
17
17
23
23
24
24
25
26
27
27
27
27
29
29
29
29
29
30
Short-circuit Considerations ..............................................
5.1
System Short-circuit Currents .......................................
5.2
Selection of Applicable Circuit Breaker Ratings
5.3
Methods for Calculating System Short-circuit Currents ....................
5.4
Electrical Quantities Used
30
30
32
32
39
References
43
..........................
..........................................
............................................................
FIGURES
Fig 1
Fig 2
Fig 3
Fig 4
Fig 5
Fig 6
Current .
Time Relationship to Determine Short-Time Load-Current Capability
of High-Voltage Circuit Breakers
OperatingTime ..................................................
Examples of Reclosing Capability for Some Usual Reclosing Duty Cycles Shown
Graphically
Power Circuit Breaker Design Requirements
System Illustrating Use of Simplified Method of Short-circuit Calculation .....
Positive-Sequence Reactances for System Shown in Fig 5
.....................................
22
24
......................................................
26
31
33
33
............................
..................
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FIGURES
Fig 7
Fig 8
Fig 9
Fig 10
Fig 11
Fig 1 2
Fig 13
Fig 1 4
Fig 15
Fig 1 6
Fig 1 7
Fig 18
Fig 19
PAGE
Zero-Sequence Reactance for System Shown in Fig 5 . . . . . . . . . . . . . . . . . . . . .
Three-phase Fault Multiplying Factors Which Include Effects of AC and DC
Decrement ......................................................
Line-to-Ground Fault Multiplying Factors Which Include Effects of AC and DC
Decrement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Three-phase and Line-to-Ground Fault Multiplying Factors Which Include
Effects of DC Decrement Only . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
System Illustrating Use of the E / X Method With Adjustment for AC and DC
........ .
Decrements . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Positive-Sequence Impedance for System Shown in Fig 11(Br
Zero-Sequence Impedance for System Shown in Fig 11(Brea
Positive-Sequence Impedance for System Shown in Fig 11(B
Zero-Sequence Impedance for System Shown in Fig 11 (Breaker B) . . . . . . . . . .
System Illustrating large Short-circuit Contribution from Motors . . . . . . . . . . . .
...........
X / R Range for Power Transformers . , . . . . . . . . . . . . . . . . .
............
X / R Range for Three-phase Induction Motors . . . . . . . . . . .
X / R Range for Small Solid Rotor and Salient Pole Generators and Synchronous
Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
34
35
36
37
37
37
38
38
40
42
42
42
TABLES
Table 1
Table 2
Table 3
Table 4
Table 5
Summary of Temperature Limitations for Circuit Breaker
for Various Ambient Temperatures . . , .
Ratios of (la/lr)
Typical Thermal Time Constants . . . . . . . . . . . . . . . . . . .
Range and Typical Values of X / R Ratios of System Compo
Equivalent System X / R Ratio at Typical Locations , . . . .
.
...... .
19
............
. . .. . . . ... . .
41
ents
21
APPENDIX
Appendix A Basis for E / X Method Corrected for AC and DC D
of Short-circuit Currents . . . . . . . . . . . . . . . . . . . . . . .
Al.
A2.
A3.
A4.
A5.
A6.
A7.
. . . . . . . . . 45
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . 45
Application Methods . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . .. . . . 45
The Effects of AC Component Decrement . . . . . . . . . . . . . . .
. . . . . . . . . . . 46
Derivation of E / X Multipliers . . . . . . . . . . . . . . . . . . . . . . . . .
. . . * . . . . . . . . 49
Longer Contact Parting Time . . . . . . . . . . . . . . . . . . . . . . . . .
Accuracy of Proposed E / X Multipliers . . . . . . . . . . . . . . . . . . . . . . . . . .
References to the Appendix . . . . . . . . . . . . . . . . . . . . .
APPENDIX FIGURES
Fig A
Fig A2
Fig A3
Fig A4
Symmetrical and Total Current Decrement; Three-phase Short Circuit; 107
.. . . . . . . . . . 46
MVA 3600 r/min Conductor-Cooled Turbine Generator . .
Symmetrical and Total Current Decrement Three-phase
Circuit with
Generator and System Contribution . . . . . . . . . . . . . . . . . . . . . .
. . . . . 47
Symmetrical and Total Current Decrement Three-phase S
it; Repreenerators ;
sentative 95 t o 200 MVA Conductor-Cooled 3600 r/min
ults at High Side Terminals of Gener
... . . . . .
mmetrical and Total Current Decr
sentative 35 to 65 MVA 3600 r/min Turbine Generators . . . . . . . . . . . . . . . . . . .
.
.
e
.
.
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PAGE
APPENDIX FIGURES
Fig A5
Relationship of ( I w m /Isym)mcd to X / R for Several Breaker Contact Parting
Times ..........................................................
Illustration of Accuracy of Fault Determination Hydro Generation; ThreePhase Fault at High Side Terminals of Station Step-up Transformers .........
Relationship Between X / R Ratio and AC Decrement from Accurate Fault
Calculation ......................................................
E / X Multiplier for Equivalent Symmetrical Amperes for Actual X / R (Breaker
Capability Curve Corresponds to X / R of Approximately 15) . . . . . . . . . . . . . . . .
Relationship of Iwm/Isyrn
to X / R for Several Breaker Contact Parting Times
(AC Decrement Included) ..........................................
Three-phase Fault Multiplying Factors Which Include Effects of AC and DC
Decrement ......................................................
Line-to-Ground Fault Multiplying Factors Which Include Effects of AC and
DCDecrement ...................................................
Three-phase and Line-to-Ground Fault Multiplying Factors Which Include
Effects of DC Decrement Only. ......................................
Breaker Asymmetrical Capability .....................................
Illustration of Accuracy of Fault Determination .........................
Illustration of Accuracy of Fault Determination, Single Line-to-Ground Fault
[Conventional Cooled Generator (3600-rlmin) Range 95-200 MVA] . . . . . . . . .
Illustration of Accuracy of Fault Determination (107 MVA Turbine Generator) .........................................................
Fig A6
Fig A7
Fig A8
Fig A9
Fig A10
Fig A l l
Fig A12
Fig A13
Fig A14
Fig A15
Fig A16
48
48
48
50
50
50
51
52
52
52
53
53
APPENDIX TABLES
Table A 1
Calculated Symmetrical Amperes
.....................................
46
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A n American National Standard
IEEE Application Guide for
AC High-Voltage Circuit Breakers
Rated on a Symmetrical Current Basis
Supplement C37.01Od-1984 is indicated by a
revision bar in the margin.
I
1. Scope
2. Purpose
This application guide applies to the ac highvoltage circuit breakers rated in accordance
with the methods given in ANSI/IEEE C37.04
1979, Schedules of Rating Structure for AC
High-Voltage Circuit Breakers, and listed in
ANSI C37.06-1979, Preferred Ratings and Related Required Capabilities for AC High-Voltage
Circuit Breakers Rated on a Symmetrical
Current Basis. Circuit breakers rated and
manufactured to meet other standards, or used
in' switchgear assemblies, should be applied in
accordance with application procedures adapted
to their specific ratings or applications.
This guide is intended for general use in the
application of circuit breakers. Familiarity
with other American National Standards applying to circuit breakers is assumed, and
provisions of those standards are indicated
herein only when necessary for clarity in
describing application requirements.
3. General Application Conditions
3.1 Usual Service Conditions. Service conditions for circuit breakers are defined in ANSI/
IEEE C37.04-1979 (see 4.1). These conditions
specify limits in altitude and in ambient
temperature.
3.1.1 Provision for System Growth. Power
system facilities must be increased from time
to time to serve larger loads. This usually
results in higher values of short-circuit current.
Therefore, liberal allowance for expected
future increase in short-circuit current is
advisable.
3.1.2 System Design. Methods of limiting
the magnitude of short-circuit currents or
reducing the probability of highcurrent
faults by system design are outside the scope
of this guide. Such methods should be considered where short-circuit currents approach
the maximum capability of the circuit breakers.
Many station design features may depend on
available circuit breaker ratings. System design
should take into account the necessity for
circuit breaker inspection and maintenance.
1.1 Related Standards
When the following American National
Standards referred to in this document are
superseded by a revision approved by the
American National Standards Institute, Incorporated, the revision shall apply.
ANSI/IEEE C37.04-1979, Ra
for AC High-Voltage Circuit Bre
ANSI C37.06-1979, Schedules of Preferred
Ratings and Related Required Capabilities for
AC High-Voltage Circuit Breakers Rated on a
Symmetrical Current Basis.
ANSI/IEEE C37.09-1979, Test Procedure for
AC High-Voltage Circuit Breakers.
ANSI/IEEE C37.5-1979, Guide for Calculation of Fault Currents for Application of AC
High-Voltage Circuit Breakers Rated on a Total
Current Basis.
ANSI/IEEE C37.24-1971, Guide for Evaluating the Effect of Solar Radiation on Outdoor
Metal-Clad Switchgear.
ANSI/IEEE Std 21-1977,Requirements and
Test Code for Outdoor Apparatus Bushings.
ANSI C37.100-1972, American National
Standard Definitions for Power Switchgear.
3.2 Unusual
Service
Conditions. Unusual
service conditions are listed in 4.2 of ANSI/IEEE
C37.04-1979 and include those conditions listed
below. Special installation, operation, and
15
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ANSIlIEEE
C37.010-1979
APPLICATION GUIDE FOR AC HIGH-VOLTAGE
maintenance provisions should be considered
where these conditions are encountered.
3.2.1 Applications at Altitudes Above 3300
f t (IO00 m). Rating corrections for altitudes
above 3300 ft (1000 m) are listed in Table 1
of ANSI/IEEE C37.04-1979.
not designed for use in explosive atmospheres.
For this type of service, special consideration
should be given in conjunction with applicable
regulatory bodies so that acceptable equipment
is selected.
3.2.5 Exposure
Abnormal Vibration,
Shock, or Tilting. S
ard circuit breakers are
designed for mounting on substantially level
structures free from excessive vibration, shock,
or tilting. Where any of these abnormal conditions exist, recommendations for the particular
application should be obtained from the
manufacturer.
3.2.6 Seasonal or Infrequent Use. Equipment stored or deenergized for long periods
should be protected against accelerated deterioration. Before energizing for service, operating
performance and insu on security should
be checked.
EXAMPLE: Consider an outdoor oil circuit breaker
having a rated maximum voltage of 38 kV, a continuous current rating of 1200 A, a rated short-circuit
current of 2 2 000 A at 38 kV, a maximum symmetrical
interrupting capability of 36 000 A, and a rated interrupting time of 5 cycles. Assume that this breaker is
to be operated at an altitude of 7000 ft (2100 m).
The operating voltage and the normal-frequency recovery voltage should be [interpolating between the
values given for 5000 and 10 000 ft (1500 and
3000 m ) ] :
38 kV
[
0.95
- (0.95 - 0.80)::701(
1E)]
= 33.8 kV.
Rated withstand voltages should be multiplied by the
same factor. The continuous current must be similarly
interpolated:
0.99
plication Consi~e~ation
- (0.99 - 0.96)
In the application of circuit breakers to
electrical systems,
attention must be
given to many items
hnical importance to
assure that a misapplication does not occur. In
the usual application, the principal function of
the circuit breaker is to
provide a means for the
circuit current. However, it may be used for
frequent load, exciting current, or capacitivecurrent switching. In some cases, switching
requirements may be the determining factor in
selection rather than the requirements of
short-circuit current i
ption. Special
attention must also be
to applications
where frequent operations are essential.
In the selection of a
tion must be given t
the present needs for
for current-carrying capability. These considerations include the possibility of connected
circuit changes, such as additions of supplemental cooling means for transformers
nection of multiple cable or overhead
ossible future transfer of the
m its initial postion to some
= 1174 A.
The rated short-circuit current, related required capabilities, and the rated interrupting
time are not affected by altitude.
3.2.2 Exposure to Damaging Fumes, Vapor,
Steam, Oil Vapors, Salt Air, Hot and Humid
Climate
(1) Provison may be necessary to avoid
condensation on all electrical insulation and
current-carrying parts
(2) Bushings with extra creep distance may
be required
(3) In cases where particular exposure
represents a hazard to insulation security,
special maintenance including insulator washing may be necessary
(4) Materials resistant to fungus growth may
be required.
3.2.3 Exposure to Excessive Dust or Abrasive, Magnetic, or Metallic Dust
(1) Totally enclosed nonventilated equipment or compartments may be necessary
(2) Where
current-carrying
equipment
designed for ventilated operation is enclosed in
a nonventilated compartment, derating may be
necessary.
3.2.4 Exposure to Explosive Mixtures of
Dust or Gases. Standard circuit breakers are
In the United States, two types of circuit
breakers are available for use at most voltage
ratings. These types come under the general
classification of oil and oilless power circuit
16
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CIRCUIT BREAKERS RATED ON A SYMMETRICAL CURRENT BASIS
ANSIIIEEE
C37.010-1979
without derating so that none of their temperature limits is exceeded
(3) As far as temperature limits are concerned, any high-voltage circuit breaker may be
used with cables having an 85OC or lower
temperature limit, provided that the temperature limit of the cable insulation is not
exceeded. Among the methods of accomplishing
this are:
(a) Operate the breaker at a continuous
current sufficiently below its rating
(b) Use a section of oversize conductors or
a special terminal connector ahead of the cable
to reduce the temperature at the cable terminal
(See IEEE Std 55-1953, Guide forTemperature
Correlation in the Connection of Insulated
Wire and Cables to Electric Equipment.)
(4)The possible adverse effect on or by
closely associated equipment operating at a
higher or a lower temperature than the breaker
should be examined
(5) It is recognized that when a circuit
breaker is properly selected for continuous
current operation, it may also be used for
starting such equipment as motors, synchronous
condensers, and cold loads on an infrequent
basis; under this condition, the continuous
current rating may be exceeded without
causing damage to the circuit breaker.
4.4.2 Rated Continuous Current for Capacitor Banks. When a high-voltage circuit breaker
is used in a circuit supplying static capacitors,
the continuous current rating should be selected
to include the effects of:
(1) Operation at voltages below and up to
1 0 percent above the capacitor rated voltage
(2) The positive tolerance in capacitance of
static capacitors (- 0+15 percent)
(3) The additional heating caused by
harmonic currents
(4) The effect of grounded or nongrounded
neutral connection of the capacitor.
In the absence of specific information, it will
usually be conservative to use 1.25 times the
nominal capacitor current at rated capacitor
voltage for nongrounded neutral operation or
1.35 times the nominal current for grounded
neutral operation.
4.4.3 Load Current Carrying Capability
Under Various Conditions of Ambient
Temperature and Load
4.4.3.1 General. Circuit
breakers
are
designed for normal application in accordance
breakers, and the preferred ratings available are
listed in ANSI C37.06-1979. The choice of
breaker type may be influenced by such considerations as attended versus nonattended
operation, contaminated atmosphere, location,
maintenance, ease of handling the interrupting
medium, and cost of the installation.
4.1 Maximum Voltage for Application. The
operating voltage and the normal-frequency
recovery voltage should not exceed the rated
maximum voltage, since this is the upper limit
for operation.
4.2 Voltage Range Factor. The voltage range
factor, K, is published in ANSI C37.06-1979.
In most cases, the voltage range is within the
range commonly used. (See 4.10.)
4.3 Frequency. Rated frequency for circuit
breakers is 60 Hz.
At slightly lower frequencies, such as 50 Hz,
the main current-carrying parts will be
adequate, but ac control devices may not be
suitable for the application.
At 25 Hz, the continuous current ratings in
amperes are higher and are listed in ANSI
C37.06-1979.
Interruption of fault currents on systems
operating at frequencies other than rated
frequency may require modification of
mechanisms to change speed of opening or may
require a change in interrupting ratings of the
circuit breakers.
In any case, special consideration should be
given to applications at frequencies differing
from 60 Hz.
4.4 Continuous Current. Circuit breakers are
designed for normal application where the
sustained load current does not exceed the
rated continuous current, the altitude above
sea level is 3300 f t (1000 m) or less, and the
ambient temperature does not exceed 4OOC.
Rated continuous current of a circuit breaker
should not be exceeded except for short
periods such as in starting of motors or synchronous condensers or energizing cold loads
or for conditions covered by 4.4.3.
4.4.1 Normal Operation. The conditions to
be followed when making an application are:
(1) Breakers designed for installation in
enclosures may be so used without any further
derating
(2) Breakers designed for use in open rooms
and outdoors cannot be used in enclosures
17
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ANSI/IEEE
C37.010-1979
APPLICATION GUIDE F ~ AC
R HIGH-VOLTAGE
a should be provi
with 4.1 and 5.4 of ANSIPEEE C37.04-1979,
where the sustained load current does not exontinuous current, the altitude
less, and
above sea level is 3300 f
the ambient temperature
ed 4OoC.
The rated continuous current is based on the
maximum permissible total temperature limitations of the various parts of the circuit breaker
when it is carrying rated current at an ambient
temperature of 4OOC. Th otal temperature
of these parts under service conditions depends
both on the actual load current and the actual
ambient temperature. It is, therefore, possible
to operate at a current higher than rated continuous current when the ambient temperature
is less than 4OoC, provided that the allowable
total temperature limit is not exceeded. Similarly, when the ambien
than 4OoC, the current
than rated continuous c
temperatures within all0wable limits.
on at a current higher than rated
C
us current will usually cause the
allowable temperature limitations to be
exceeded and should be avoided except in
these instances:
peribds, such as in the starting
(1) For
of motors
nchronous condensers, or when
energizing cold loads. Generally, the time
duration of this type of current increase is
short enough that it does not raise temperatures significantly.
(2) When operating at an ambient temperature below 4OoC, as covered in 4.4.3.2.
(3) For short periods following operation at
a current less than that permitted by the
existing ambient temperature, as covered
in 4.4.3.3.
The method of calculating
allowable
current at an ambient temperature above 4OoC
is given in 4.4.3.2. For some high ambient
temperature conditions, it may not be practical
to reduce current sufficiently to keep the total
temperatures within their allowable limits.
Forced ventilation will help in many cases.
Temperature limits of associated equipment
such as cables and current transformers must
be considered. Effect of solar radiation must
also be considered. (See ANSI/IEEE C37.241971, Guide for Evaluating the Effect of Solar
Radiation on Outdoor Metal-Clad Switchgear.)
Where ambient temperature is be
minimum limit of -30°C, special
may be required, and additional heater capacity
bility Based on
The continuous
where
Omax
=
allowable h
NOTE: The temperature rise of a current-carrying
part is proportional to ah exponential value of the
current flowing through it.
although the exponent may have
depending on breaker design and componen
of 1/1.6 to 1/2,0.
that is suitable for this application guide.
follows :
Circuit Breaker
Component
joints, etc
Bushings
Curkent transfor
Number
C37.04-1979
C76.1-1976
18
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Section
ANSI/IEEE
C37.010-1979
CIRCUIT BREAKERS RATED ON A SYMMETRICAL CURRENT BASIS
The values of temperature limitations
specified in these various standards are summarized in Table 1.In order that none of these
limitations be exceeded when the load current
is adjusted t o the value permitted by the actual
ambient temperature, the values for Om= and
8, should be determined as follows:
(1) If the actual ambient is less than 4OoC,
the component with the highest specified
valyes of allowable temperature limitations
should be used for determining Omax and 8,.
(2) If the actual ambient is greater than
4OoC, the component with the lowest specified
values of allowable temperature limitations
should be used for determining Om, and Or.
The use of these values in the calculation will
result in an allowable continuous current which
will not cause the temperature of any part of
the circuit breaker to exceed the permissible
limits.
NOTE: Although circuit breakers are designed to
carry rated current continuously while exposed t o an
ambient temperature of 4OoC, most circuit breakers in
service are subjected t o a much less severe ambient
temperature condition which varies with time of day
and season of year. The average outdoor air temperature during any 24 h period is usually 5 to 10°C lower
than the maximum, and the maximum is usually less
than 4OoC. This is one important reason for the long
life which circuit breakers historically have demonstrated. Continuous operation at rated current and at
an ambient temperature of 4OoC likely would result in
a life somewhat shorter than that usually experienced.
The user is cautioned that this guide is intended to
cover situations where the circuit breaker is used at its
total temperature limitations infrequently (a few
occasions in the expected lifetime), for relatively short
periods of time (perhaps a few days).
The manufacturer should be consulted if operation
at maximum allowable temperature is required at
frequent intervals or over extended periods of time.
ay not be necessary to include the
extreme values of highest and lowest allowable
temperature limitations, as listed in Table 1,in
Table 1
Summary of Temperature Limitations for
Circuit Breaker Components
Component Description
Temperature
Rise, OC
Limit of
Total
Temperature, OC
0,
0max
10
50
40
80
onnected to 85OC
45
85
Hottest spot temperature of parts where they
contact oil; silver (or equal) contacts in oil;
silver (or equal) conducting joints in oil
50
90
Silver (or "equal) contacts in air; silver (or
equal) conducting joints in air; hottest spot of
bushing conductor o r of bushing metal parts in
contact with Class A insulation or with oil;
hottest spot winding t
rature 5 5 ' ~rise of
current transformers.
65
105
7
External surfaces not accessible t o an operator
in the normal course of his duties
70
110
8
Hottest spot winding temperature of 80°C
dry-type current transformers
110
150
1
Circuit breaker parts handled by the operator
in the normal course of his duties
2
Copper contacts, copper-to-copper conducting
joints, external surfaces accessible to the
operator in the normal course of his duties,
external terminal connected t o bushing
3
4
insulated cable
5
19
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ANSI/IEEE
C37.010-1979
APPLICATION GUIDE
FOR AC HIGH-VOLTAGE
Table 2
*For limiting current, use lowest 0, and Omax.
?For limiting current, use highest Or and Omax.
estimated from Table 2 or may be calculated
directly from the lormula.
this determination. The lowest value is for
circuit breaker parts handled by the operator in
the normal course of his duties, and the highest
value generally is for external surfaces of a
circuit breaker, not accessible to an operator in
the normal course of his duties. In many cases,
overheating of these parts beyond the specified
allowable limitations will not impair circuit
breaker performance or expected life. If these
parts are neglected, a higher value of permissible
continuous load current can be obtained from
the calculation. In most cases, this higher
value of current may safely be used. However,
in some circuit breaker designs there may be
gaskets or other components located near such
inaccessible external surfaces which would be
damaged if overheated. The use of 80°C rise
dry-type current transformers would place a
further limit on the allowable current for
operation at an ambient of less than 40°C.
Evaluation of the breaker design should be
made to determine whether or not these
extreme values of temperature limitation
should be included. In case of doubt, the
manufacturer should be consulted.
Table 2 lists the calculated values of l a / I r for
each specified temperature limit for the various
componen
e circuit breaker over a
range of
ent temperatures. The
allowable current in any given situation can be
EXAMPLE 1: Consider an oil circuit breaker with
copper-to-copper contacts, American National Standard
bushings, and 55OC rise current transformers. When
operating at an ambient temperature of 2
current could be increased by a factor correspo
with the highest temperature
e components
according to Table 2, the factor of
increase is 1.12 times rated curre
the current must be reduced by a factor which corresponds t o the component with the lowest temperature
20
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ANSI/IEEE
C37.010-1979
CIRCUIT BREAKERS RATED ON A SYMMETRICAL CURRENT BASIS
current can be increased by a factor of 1.12, corresponding to the temperature rise limitations of
65OC of the silver-to-silver contacts and the current
transformers.
If the ambient temperature were 5OoC, the reduction
factor would be 0.87 corresponding to the temperature
rise limitation of 45OC of the connected cable.
4.4.3.3.I De termination of Allowable Time
for I, by Use of Fig 1 . One method for
determining the allowable duration of current
Is is shown in Fig 1. The values of time
obtained from the figure are given in “time
constant units.” The actual time is determined
by multiplying the time constant units by the
proper thermal time constant listed in Table 3.
Current transformers that form part of a
circuit breaker assembly or that are normally
considered a part of the breaker installation
will usually have heating characteristics similar
to those of the circuit breaker. The continuous
current permitted by the formula will not
cause overheating of a current transformer
connected to a ratio tap corresponding to rated
current of the circuit breaker. However, if a
lower ratio tap is used, the expected secondary
current must be evaluated in terms of the
continuous thermal rating factor of the current
transformer. The effect of the secondary current
on apparatus connected to the current transformer terminals should also be considered.
4.4.3.3 Short-Time Load Current Capability. When a circuit breaker has been operating
at a current level below its allowable continuous
load current I,, it is possible to increase the
load current for a short time to a value greater
than the allowable current without exceeding
the permissible temperature limitations. The
length of time that the short-time load current
Is can be carried depends on these factors:
(1) The magnitude of current Is to be carried
(2) The magnitude of initial current I i carried
prior to application of Is
(3) The thermal-time characteristics of the
circuit breaker
The time duration of the short-time current
may be calculated directly or may be obtained
by simple use of Fig 1as described below. The
time duration of the current Is determined in
this manner will not cause the total temperature
limits of the circuit breaker to be exceeded,
provided that these requirements are fulfilled :
(1) The circuit breaker, and in particular the
main contacts, shall have been well maintained
and in essentially new condition
(2) The value used for the current Ii is the
maximum current carried by the breaker
during the 4 h period immediately preceding
the application of current Is
(3) At the end of the time period, the
EXAMPLE: A 1200 A oil circuit breaker with silverto-silver contacts and American National Standard
bushings is operating at 1000 A (Zi = 1000 A). It is
desired to increase the current to 1600 A ( I , = 1600 A)
for a short time. The maximum ambient temperature
expected is 25’C. For how long a time can the circuit
breaker carry this current?
The allowable continuous current is determined
according to 4.4.3.2. Since the ambient is less than
4OoC, the highest temperature rise limitation of 65’C
is used, the temperature rise limit of the bushing. From
Table 2, the ratio of 1.12 is determined for the ambient
temperature of 25’C. Therefore, Z, = 1.12 x 1200 =
1344 A.
The ratio (Ii/Is)is (1000/1600) = 0.625. The corresponding value ti from Fig 1 is 0.58 time constant
units. The ratio (Za/Zs) is (1344/1600) = 0.84. The
corresponding value ta from Fig 1is 1.31 time constant
units. The period of time for which the excess current
can be carried is t,
ti = 1.31 0.58 = 0.73 time
constant units. The actual time is obtained by
multiplying this value by the thermal time constant of
0.5 h taken from Table 3 (0.73 x 0.5 h ) = 0.365 h or
about 22 min. After this time, the current must be
reduced to no more than 1344 A in order to prevent
overheating of the bushings.
-
-
4.4.3.3.2 Determination of Allowable
Time for Is by Direct Calculation. The allowable duration of current Is may be calculated
directly. While the chart method is quicker and
simpler than hand calculation of these equations, the increasing availability of digital computers makes it desirable to provide the details
of these calculations. These equations may be
easily written in a computer language such as
Fortran so that many sets of conditions may be
analyzed quickly. The program also should
then be immediately available for use at a
Table 3
Typical Thermal Time Constants
Circuit Breaker
Listed in
ANSI C37.06-1979
Typical Time
Constant
r, hours
Table 1
current Is is reduced to a value which is no
Table 2
Table 3
Table 4
Table 5
greater than the cunent l a
(4) The value of current I, is limited to a
maximum value of two times rated current I,.
21
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ANSI/IEEE
APPLICATION GUIDE FOR AC HIGH-VOLTAGE
C37.010-1979
9
-
8
x
x
9
?
0
a9
0
h
.*E
.f=
Y
E
ga
3
?
k
0
.-E
0
?f
0
'9
0
'9
z
s-
0
0
L
c
c
-
0
0
0
22
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CIRCUIT BREAKERS RATED ON A SYMMETRICAL CURRENT BASIS
future time. These are the equations used to
calculate the time duration ts for a short-time
load current Is :
Y = (emax
4.5 Rated Dielectric Strength
4.5.1 Low-Frequency Withstand Voltage.
Circuit breakers are tested to withstand, for
1 min at normal frequency, a voltage that
exceeds the rated maximum voltage. This test
provides a margin of safety for normal
deterioration, for minor contamination, and
for normal voltage surges encountered in service.
Test values are listed in rating tables in ANSI
C37.06-1979. These apply to new circuit breakers tested at the factory. See 1.3 of ANSI/IEEE
C37.09-1979 for tests after delivery.
In some types of circuit breakers, dielectric
withstand voltage may be reduced below a safe
value by loss of air or gas pressure caused by a
stuck valve or other malfunctioning device.
Automatic isolation of the breaker should be
considered in such cases.
4.5.2 Impulse Withstand Voltage. The basic
impulse insulation levels (BIL) of circuit
breakers are specified in rating tables in ANSI
C37.06-1979 for various breaker ratings. In
applying circuit breakers it is necessary to
make certain that the insulation levels of all
facilities at a terminal are properly coordinated.
Since this is a matter of system insulation
coordination, it is outside the scope of an
application guide for circuit breakers.
The purpose of impulse voltage withstand
tests is to demonstrate that the insulation level
of the circuit breaker, when coordinated with
suitable protective devices, will protect against
dielectric failure or flashover under lightning
surge or equivalent conditions.
When surge arresters are installed on the bus
or on transformers and not on each circuit
breaker, the surge voltage at the breaker can
exceed that at the arresters, the amount of the
excess depending upon the steepness of the
wave front and the distance from the circuit
breaker to the surge arresters. When the circuit
breaker is in the open position, an incoming
surge voltage may be doubled by reflection at
the open terminals. Selection of too low an insulation level for circuit breakers, if not individually protected by arresters, may result in
an excessive number of operations of local or
remote breakers in clearing faults caused by
external flashovers.
- 40°C) (Ii/Ir)1.8
allowable hottest spot total temperature from Table 1, in degrees Celsius
actual ambient expected (between
-30°C and 6OoC), in degrees Celsius
initial current carried prior to application of I s , in amperes (the maximum
current carried by the breaker during
the 4 h period immediately preceding
the application of current I s )
short-time load current, in amperes
rated current, in amperes
thermal time constant of the circuit
breaker from Table 3, in hours
permissible time for carrying current
Is at ambient 19, after initial current
Ii time is in same units of hours as r.
NOTE: These equations are derived in the following
manner :
Let
O s = total temperature, in degrees Celsius, that
would be reached if current I, were applied
continuously at ambient ea
O i = total temperature, in degrees Celsius, due to
continuous current Ii at ambient ea
total temperature, in degrees Celsius, at some
time t after current is raised from Zi to I,
et=
Then
et = (es - ei) (1 - E - ~ / T ) + ei
Let
et = Omax
;solve for t .
Then
where
ei = (ema - 4OoC) (
I ~ / I +, ea
~
and
e,
=
(e,,,
-
ANSI/IEEE
C37.010-1979
4OoC) ( Z , / Z ~ ) ~ . +
~ ea.
For the special case where the initial current is zero,
the equations in this form are useful. By substitution
and further manipulation, the equations given above in
terms of Y are obtained; these are convenient to use in
the more usual case where the initial current is not
zero.
4.6 Standard Operating Duty. Power circuit
breakers are rated for current interrupting
ability on the basis of a standard operating
duty. (See 5.6 of ANSIIIEEE C37.041979.)
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ANSI/IEEE
APPLICATION GUIDE
C37.010-1979
If the actual duty cycle application is different from the standard operating duty, refer to
5.10.2.6 of ANSI/IEEE C37.04-1979, and to
Fig 1 of ANSI C37.06-1979,for rating factors
for reclosing service and for examples illustrating the use of the rating factors.
FOR AC HIGH-VOLTAGE
instability. For low values of current, these
considerations are less important.
The rated interrupting times of specific circuit
breaker ratings are given in the tables of ANSI
C37.06-1979. Fast interrupting speeds may be
significant where system stability is critical or
where line conductor damage is an important
consideration.
Fig 2 shows the sequence of events in the
course of a circuit interruption and reclosure.
4.7 Intempting Time. The rated interrupting
time of a circuit breaker is the time between
trip circuit energization and power arc interruption on an opening operation, and is used to
classify breakers of different speeds.
The rated interrupting time adjusted by
factors included in 5.7 of ANSI/IEEE C37.041979 indicates the length of time to circuit
interruption of power current after trip initiation. Note that the rated interrupting time may
be exceeded at low values of current and for
close-open operations; also, the time for interruption of resistor current for interrupters
equipped with resistors may exceed the rated
interrupting time. The increase in interrupting
time on close-open operation may be important
from the standpoint of line damage or possible
4.8 Permissible Tripping Delay. The rated per-
missible tripping delay Y constitutes a thermal
limit which should not be exceeded for a close,
carry, and interrupt sequence at K times rated
short-circuit current. The permissible tripping
delay T , in seconds, may be determined for
currents lower than K times rated short-circuit
current by use of the formulas given in 5.8 of
ANSI/IEEE C37.04-1979. The rigorous method
is generally not required for application unless
the approximate method of the section produces a result close to the tripping delayaproposed for use.
Fig 2
Operating Time
EXTINCTION OF
ARC ON
PRIMARY CONTACTS
INlTlATION OF
SHORT -CIRCUIT
RESISTOR CIRCUIT
COMPLETED ON RECLOSURE
I
I
I
I
PARTING OF
SECONDARY
ARCING CONTACTS
ENERGIZATION
OF TRIP CIRCUIT
EXTINCTION OF
ARC SHUNTING
RESISTOR CURRENT
PARTING OF
PRIMARY ARCING
CONTACTS
t
I
f
f
f
PRIMARY ARCING
CONTACTS M A K E
T
I
_
+
(*I RECLOSING TIME IS T H E TIME INTERVAL BETWEEN ENERGIZATION OF THE
TRIP CIRCUIT AND MAKING OF THE PRIMARY ARCING CONTACTS. WHERE LOW
OHMIC RESISTORS ARE USED, MAKING OF T H E RESISTOR CONTACT ON RECLOSURE MAY BE MORE SIGNIFICANT.
24
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CIRCUIT BREAKERS RATED ON A SYMMETRICAL CURRENT BASIS
1
K (Rated Short-circuit Current)
T =Y
L
opened. Synchronous motors and static capacitors included in the load will tend to prolong
the period of arcing. On tie lines, dead time on
the circuit is the time interval between interruption of current by the last breaker t o clear
and making of the contacts on the first breaker
to reclose.
A dead time on the circuit of at least 8 cycles
is normally required at 115-138 kV for breakers
without resistors across the interrupters. The
required dead time is greater for higher voltages.
Where resistors of low ohmic value are used
across the interTpters, the necessary dead time
may be affected by the current flowing through
the resistors. Investigations in this area are
being made.
Fig 2 illustrates the reclosing time whieh is
defined in ANSI C37.100-1972. Figure 1 of
ANSI C37.06-1979 gives factors to be applied
to the interrupting capabilities of circuit
breakers for reclosing duty cycles other than
the standard operating duty. Examples of the
use of these factors are given below.
1
-1
1,
where
Y
I,,
=
=
aconstant
the quotient E / X , in amperes.
For the values of Y , see ANSI C37.06-1979
(1.1.3).
EXAMPLE : Consider an outdoor circuit breaker
having a rated short-circuit current of 22 000 A and a
voltage range factor K of 1.65. Assume that I s is
20 000 A and Y is 2. Then the permissible tripping
delay is:
T = 2(
1.65x 22000
20 000
)
ANSI/IEEE
C37.010-1979
= 6.6s.
This limit applies to short-circuit current and
does not apply to load current, motor starting
current, or similar service. The aggregate
tripping delay on all operations within any
30 min period must not exceed the time obtained from the above formula.
4.9 Reclosing Time. High-speed reclosing may
be applied on radial lines to minimize the
effect of line outages. In most cases, reclosing
within l/2 s from incidence of a fault will
prevent any adverse effect of the circuit outage
on residential and commercial customers. In
many cases, industrial customers can modify
their equipment to eliminate most of the
adverse effects of momentary outages if highspeed reclosing is employed.
High-speed reclosing has been used successfully on tie lines where opening a circuit
separates a portion of the system from the
remainder. In this case, it is necessary to
reclose before the rate of change or magnitude
of the phase angle of voltages across the open
circuit has reached a value beyond the capability
of the circuit to restore synchronism following
reclosure. In many applications on loop and
grid systems, high-speed reclosing is used to
avoid instability, to improve voltage conditions, and to minimize the effects of line
outages.
Several definitions of dead time for a circuit
breaker are given in ANSI C37.100-1972.Before
a circuit can be successfully reenergized, there
must be sufficient dead time in the circuit
Example 1. Determine the symmetrical interrupting capability of a circuit breaker when
used on a duty cycle of 0 + Os + CO + 15s
+ CO + 60s + CO on a system operating at
23 kV. The breaker rated short-circuit current
is 15 000 A at a rated maximum voltage of
38 kV on a standard duty cycle of CO + 15s +
CO.
(1) The symmetrical interrupting capability
at 23 kV is 15 000 AX (38/23) = 24 800 A.
(2) (From Fig 1 of ANSI C37.06-1979.) The
calculating factor d, from Fig 1 for reducing
the symmetrical interrupting capability is 4.0
at 24 800 A.
(3) (From 5.10.2.6 of ANSI/IEEE C37.041979.) Total reduction factor D is 3 X 4.0 =
12.0
D
= 4(4
+ o = 12
- 2) + 4 ( 1 515- 0 ) + 4 (15-15)
15
The reclosing capability factor is thus R
= 100 - 12.00 = 88.0 percent.
(4) The symmetrical interrupting capability
for this duty cycle at 23 kV is 24 800 A X 0.88
= 21 800 A.
(5) This circuit breaker may be used on this
duty cycle at 23 kV on any circuit where the
calculated system short-circuit current does not
exceed 21 800 A after correction for X / R , if
necessary.
breaker for the arc path at the fault to become
deionized. On a radial line where the load includes a large motor component, arcing may be
sustained after the breaker at the source is
25
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ANSI/IEEE
APPLICATION GUIDE FOR AC HIGH-VOLTAGE
C37.010-1979
0
I-
00
o
z
[r
0
&
z
90
Duty Cycles Shown Graphically
Example 2. Determine the symmetrical interrupting capability of a circuit breaker when
used on a duty cycle 0 + 0 s + CO + 5 s + C O
on a system operating at 28 kV. The breaker
rated short circuit is 22 000 A at a rated maximum voltage of 38 kV on a standard duty cycle
of C O + 15 s + CO.
(1) The symmetrical interrupting capability
at 28 kV is 22 000 A X (38/28)= 29 900 A.
(2) (From Fig 1 of ANSI C37.06-1979.)The
calculating factor dl from the reclosing capability curve is 4.9 at 29 900.
(3) (From 5.10.2.6 of ANSI/IEEE C37.041979.) The total reduction factor using the
formula for this duty cycle
where
(5) This breaker may be used on this duty
cycle at 28 kV on any circuit where the calculated system short-circuit current does not exceed 26000 A, after corr
necessary.
Examples of other reclo
some reclosing duty cyc
ically in Fig 3.
4.10 Short-Circuit Rating. In the application of
circuit breakers, it is necessary that none of the
short-circuit current capabilities of a circuit
breaker be exceeded. These capabilities are
derived from rated short-circuit current and are
described in 5.10 of ANSI/IEEE C37.04-1979.
dl = 4.9
n = 3
tl
= 0
tz =
5
+4.9 0 5 - 5 1
D = 4.9 (3 - 2)+ 4.9 (15-0)
15
15
times the rated max
metrical interrupting c
.
= 13.1
U
"
(4) The symmetrical interrupting capability
= 29 900 A X 0.869 X 26 000 A at 28 kV and
1
OpeEating Voltage
1
except that, f
required symmetrical interrupting capability is
the above duty cycle.
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CIRCUIT BREAKERS RATED ON A SYMMETRICAL CURRENT BASIS
be taken to protect breakers against opening
where normal current zeros are not obtained.
The precaution includes:
15 percent higher but in no case greater than K
times the rated short-circuit current.
EXAMPLE: Consider an indoor oilless circuit breaker
having a rated short-circuit current of 37 000 A at the
rated maximum voltage of 15 kV and K=1.30. The
symmetrical interrupting capability at an operating
voltage of 13.2 kV is
37000 X
ANSI/IEEE
C37.010-1979
(1) Extended relay times
(2) Special X/R ratios
(3) Circuit breaker location to avoid this
objectionable degree of asymmetry
(:::;)
- =42050A.
For examples of calculations of system
short-circuit currents for both 3-phase and
line-to-ground faults, see 5.3 and 5.4.
4.10.3 Service Capability. Refer to 5.19.3.3 of
ANSI/IEEE C37.04-1979 for the service capability of the breaker and for the specified breaker
condition following certain performances.
The required capabilities of the circuit
breaker should not be exceeded, even though
only one interrupting operation may be
imposed.
4.11 Transient Recovery Voltage Rate. See
ANSI/IEEE C37.011-1979, American National
Standard, Application Guide for Transient
Recovery Voltage for AC High-Voltage Circuit
Breakers Rated on a Symmetrical Current Basis.
4.12 Load Current Switching Capability and
Life (Repetitive Operation). Careful attention
must be given to ANSI C37.06-1979.
Motor starting duty may require closing the
circuit against inrush currents many times
greater than the running current. This will not
be limiting on a circuit breaker having a continuous current rating at least equal to the
maximum running current of the motor. [See
5.12(3) of ANSI/IEEE C37.04-1979.1
Special circuit breakers may be required for
applications involving highly repetitive operations such as arc furnace switching and plugging, jogging, or reversing of motors.
4.13 Capacitance Current Switching. See
ANSI/IEEE C37.012-1979, American National
Standard Application Guide for Capacitance
Current Switching for AC High-Voltage Circuit
Breakers Rated on a Symmetrical Current Basis.
At an operating voltage of 11.5 kV which is less than
1/K times rated maximum voltage, the symmetrical
interrupting capability is limited to 37 000 K = 37 000
x (1.30) = 48 000 A.
4.10.2 Asymmetrical Requirements. A circuit
breaker having adequate symmetrical intermpting capability will have adequate capability to
meet all of the related short-circuit requirements unless there is a significant contribution
from motor load or unless the XIR ratio is
greater than approximately 15. This will result
in a slower decrement rate for the asymmetrical
current than that incorporated in the rating
structure. (See Fig 2 of ANSI/IEEE C37.04
1979.)
If the actual opening time of the circuit
breaker is less than the assumed value (1.0
cycles for a 2cycle breaker, 1.5 cycles for a
3-cycle breaker, 2.5 cycles for a 5cycle breaker,
and 3.5 cycles for an 8-cycle breaker), the
breaker is required to have a symmetrical interrupting capability corresponding to the actual
minimum contact parting time in accordance
with Fig 2 of ANSI/IEEE C37.04-1979. Therefore, it is not necessary to allow a margin in
application for the possibility of breaker opening faster than normally expected. Note that
any combination of symmetrical and directcurrent components is permissible within the
limitations covered in 5.10.2.2 of ANSI/IEEE
C37.04-1979.
If the actual relay time is less than 0.5 cycle,
this fact should be taken into consideration
when
calculating required asymmetrical
capability.
Refer to Note 4 of 5.4.1 for the treatment of
the motor contribution where it affects the
initial asymmetrical current. Methods of allowing for slow decrement of the asymmetrical
current are discussed in 5.3 and the sections
that follow it.
In some applications, particularly on
generator buses, a condition may occw where
over 100 percent asymmetry is obtained [ 8 ] .
Where this condition exists, precautions should
4.14 Line Closing (Line Closing SwitchingSurge
Factor). When a circuit breaker is closed to energize an overhead transmission line, it produces
on the power system a transient overvoltage,
the crest of which is called the line closing
switching surge maximum voltage. Circuit
breakers which have been specifically designed
to control such voltage to be less than a specified limit are assigned a rating called the rated
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ANSI/IEEE
C37.010-1979
APPLICATION GUIDE FOR AC HIGH-VOLTAGE
line closing switching surge factor, This rating
designates that the circuit breaker is capable of
controling line closing switching surge voltages,
so that there is a probability of at least 98 percent of not exceeding the rated line closing
switching surge factor, when switching the
standard reference transmission line from the
standard reference power source.
Establishment of the rated line closing switching surge factor is based on closing of the circuit
breaker governed by random time energization
of the device which initiates closing of all three
phases.
Random closing of the circuit breaker will
produce line closing switching surge maximum
voltages which vary in, magnitude according to
the instantaneoug value of the source voltage,
the parameters of the connected system, and t o
the time differences between completion of the
circuit path in each phase. These variations will
be governed by the laws of probability so that
the highest and the lowest possible overvoltages
will occur very infrequently. In recognition of
this and of the economy associated with not
designing for the worst case, the rated line closching surge factor is based o
at the circuit breaker wil
ing switching surge factor equal to, or
less than, the rated factor 98 percent or more
of the times it is closed, and never more than
1.2 times the rated factor.
4.14.1 Statis
lysis o f the Number of
Allowable Occ
in Excess of the Rated
Value. Any device with which there is a mathematically constant probability of 2 percent of
the occurrence of an event will usually
different test results th
a series of 50 tests. In
resent case, if the
event is defined as the line closin'g switching
surge factor exceeding the rated factor, and a
large number of test series consisting of 50
tests each were conducted, in 36.4 percent of
the repeated test series, of 50 tests each, the
rated factor would not be exceeded; in 37.1
factor of a circuit breaker design will be based
on a great number of tests made by the manufacturer on a simulated syst
by a user to perfor
will be done in acco
ANSI/IEEE C31.09-1979,
cent of the tests to have a li
surge factor in excess of the rated factor. The
probability of obtainin
no more than one exce
bility (26.5 perc
tests each will
simulated tests
made with a m
bility of the circuit
within known statis
Alpha
=
of exceeding rated factor for unacceptable circuit breaker
5 percent =
ceptance
the series the rated value would be
once; in 18.6 percent, twice; in 6.1
times; in 1.5 percent, 4 times; and
in 0.3 percent, 5 times (from the binomial
formula). Thus a circuit breaker with a perned and constant 2 percent probeeding the rated switching surge
factor would do so four or more times in about
2 percent of repeated 'te eries of 50 tests each.
It is expected that the rated switching surge
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CIRCUIT BREAKERS RATED ON A SYMMETRICAL CURRENT BASIS
4.14.2 Type of Power System to Which the
ANSI/IEEE
C37.010-1979
the duties specified in 4.15.9 of ANSI/IEEE C37.091979.
(2) The requirements of this standard cover the great
majority of applications of circuit breakers intended for
switching during out-of-phase conditions. Several circumstances would have to be combined to produce a
severity in excess of those covered by the tests of this
standard and, as switching during out-of-phase conditions is rare, it would be uneconomic to design circuit
breakers for the most extreme conditions.
Where frequent out-of-phaseswitching operations are
anticipated, or where for other reasons out-of-phase
switching is a matter of importance, the user should
consider actual system recovery voltages. A special
circuit breaker, or one rated at a higher voltage, may
sometimes be required. As an alternative solution, the
severity of out-of-phase switching duty is reduced in
several systems by using relays with coordinated
impedance-sensitive elements to control the tripping
instant, so that interruption will occur either substantially after or substantially before the instant the phase
angle is 180'.
Rated Line Closivg Switching Surge Factor
Applies. The rated line closing switching surge
factor applies to a power system where the circuit breaker connects an overhead transmission
line directly to a power source. The factor applies specifically to a power system with characteristics of the standard reference power system as described in 4.14.1 of ANSI/IEEE
C37.09-1979 and also to those actual power
systems whose characteristics are not greatly
different from those of the standard reference
power system.
The transmission line is open at the receiving end and is not konnected to such terminal
apparatus as a power transformer although it
may be connected to an open circuit breaker or
disconnect switch.
The system does not include shunt reactors,
potential transformers, series capacitors, shunt
capacitors, lightning arresters, or any similar
apparatus unless specified in Table 8 of ANSI
C37.06-1979.
Applications where a circuit breaker is used
on the low-voltage side of a transformer to
energize a line on the high-voltage side of the
transformer are being studied for possible
future inclusion in this standard.
It is anticipated that any actual power system
which deviates too greatly from the standard
reference power system may require that a
simulated study be made of it in order to determine the actual line closing switching surge
factor to be expected.
4.16 Shunt Reactor Current Switching. Circuit
breakers may be applied to switch shunt reactors. Because of the high transient recovery
voltage rate produced by switching shunt reactors, the arcing time of some circuit breakers,
under that condition, may be significantly
longer than it would be for other types of
switching duty. This longer arcing time may
require more frequent maintenance. (See 5.16
of ANSI/IEEE C37.04-1979.)
4.17 Excitation Current Switching. Circuit
breakers used for switching exciting current including energizing and deenergizing of feeder
regulators and transformers may require special
consideration because of switching frequency
or severity.
4.18 Mechanical Life. Circuit breakers are d e
signed to operate satisfactorily for the number
of operations specified in Table 6 of ANSI
C37.06-1979.
4.15 Conditions of Use with Respect to the
Out-of-Phase Switching Current Rating. The
conditions of use with respect to the out-ofphase switching current rating are :
(1) Opening and closing operations carried
out in conformity with the instructions given
by the manufacturer for the operation and
proper use of the circuit breaker and its auxiliary equipment; closing operations should be
limited to a maximum out-of-phase ingle of
90' whenever possible [see Note (2) below]
(2) Grounding condition of the neutral of
the power system corresponding to that for
which the circuit breaker has been tested
(3) Frequency within f 2 0 percent of the
rated frequency of the circuit breaker
(4) Absence of a fault on either side of the
circuit breaker
4.19 Rated Control Voltage. The successful
performance of circuit breakers depends upon
maintenance of control voltage within the
standard limits as shown in Table 8 of ANSI
C37.06-1979. The voltage at the control terminals of the circuit breaker should approximate
the rated control voltage under loaded conditions and must not be less than the minimum
specified in Table 8 of ANSI C37.06-1979,
even under minimum expected battery voltage
conditions.
Batteries, battery chargers, control transformers, etc, should be selected considering
this limit and taking into account the line drop
encountered in the control buses, leads, relay
series coils, the condition and charge of the
NOTES: (1) All circuit breakers having an assigned outof-phase switching current rating are able to perform
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ANSI/IEEE
C37.010-1979
APPLICATLON GUIDE FOR AC HIGH-VOL
battery, and the volt-ampere burden on the
control transformers. The possibility of
simultaneous closing and tripping of two or
more power circuit breakers should be considered in selecting the control power supply.
Control current
free operation may be
quite high. The
are especially critical
where lower control voltages
V
- (24-48
.
subjected b t h e following types of faults:
Where emergency service, communication
loads, indicating lights, r similar power
requirements must be supplied from a control
battery, these represent a sustained load
requirement for the battery and charger and
should be include
fixing the size of
ment,
4.20 Fluid Operating Pressure. Circuit breakers
operated by fluid pressure are designed to
operate over a pressure range prescribed by the
manufacturer. Limit switches to control compressor operation, to indicate low pressure, and
to prevent circuit breaker operation below
minimum pressure are normally provided. A
suitable power sou
r compressor operation
is required.
euit Considerations
circuit breakers h*ave 1
urrents. One of the
ts of circuit breaker
application is the ' determination of the
maximum short-circuit duty imposed on the
breaker.
Different methods of determining system
short-circuit currents have been published. In
general, more complex calculations give
improved accuracy. The application engineer
should select the method in acco
accuracy. Ordinarily, the meth
5.3.2will be conservative.
5.1.1 Sh-ort-circuit Tests. The most accurate
ation of short-circuit current may be
trolled and instrus. Sometimes such
ade to test new equipment or system
arrangements, but are not
a means of determining
circuit breakers on a p
most cases, selection
precedes completion of new facil
5.1.2 Types and
Circuits.
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ANSI/IEEE
C37.010-1979
CIRCUIT BREAKERS RATED ON A SYMMETRICAL CURRENT BASIS
1
z
z!
kiii
Va f
BREAKER WITH I-CYCLE CONTACT PARTINGTIME
I
:‘IC 2
\
BREAKER WITH 1-1/2 CYCLE CONTACT
PARTING TIME )t
PARTING TIME Y
CI
W >
?!!!
r I
v)c
*
I I
1.1
:*
IME
QlL
:k 1.0
0 0
2wcc5
a w
CL
I /2
I
2
3
4
CIRCUIT BREAKER CONTACT PARTING TIME-CYCLES
( SUM OF 1/2 CYCLE TRIPPING DELAY P U S
THE OPENING TIME OF T H E CIRCUIT BREAKER )
(*The circuit breaker required asymmetrical interrupting capability for any time up to
the permissible tripping delay. See 4.7.)
Fig 4
Power Circuit Breaker Design Requirements
Circuit breakers are designed to interrupt
satisfactorily with the asymmetrical and
symmetrical current relationships shown in
Fig 4. When a breaker interrupts a short-circuit
current, the critical current value is that existing at the time of primary arcing contact parting. The curve of Fig 4 is designed t o specify
the required asymmetrical capability of any
circuit breaker based on %-cycleminimum relay
time plus the circuit breaker opening time. Once
this asymmetrical capability is so established
(see 5.10.2.2 of ANSI/IEEE C37.04-1979),it
remains fixed for any contact parting time
within the permissible tripping delay period.
(See 5.8 of ANSI/IEEE C37.04-1979.For
example, if a circuit breaker has an opening
time of 1/2 cycle, the minimum contact parting
time is 2 cycles. From Fig 4, the circuit breaker
has an asymmetrical current capability of 1.2
at 2 cycles, where the required symmetrical
capability is 1.0.
The more exact type of calculation (see 5.3.2)
evaluates the decay in the ac and the dc components of the short-circuit current. When the
fault is close, electrically, to a major element
of generation, the ac decay will be appreciable
during the first few cycles.
The ac decay may not be significant in system locations that are electrically remote from
generation. This may be so even in auxiliary
systems fed directly from the generator terminals, but through a relatively high impedance
reactor or transformer.
In cases where high X/R ratios are encountered and some tripping delay in excess of
l/2 cycle is used, advantage may be taken of
the system decay in either or both of the dc
and ac components of short-circuit current.
In locations where such advantage is taken of
the decay in the ac and dc components and
where the circuit breaker is applied at about
1/K times the rated maximum voltage, special
care must be exercised to assure that the circuit
breaker capability at l/2 cycle (closing and
latching capability) is not exceeded. This is
1.6 times K times the rated short-circuit current. Also, in the presence of motor load,
closing on a short-circuit may be critical, in
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ANSI/IEEE
C37.010-1979
APPLICATION GUIDE FOR AC HIGH-VOLTAGE
which case the critical current may be that of
the first major current crest. Proper provision
for this condition is assured by making a
closing and latching duty calculation using the
rotating machine reactances indicated in 5.4.1.
In any case, neither the required symmetrical
nor the required asymmetrical interrupting
current capability of a circuit breaker should
be exceeded at the time of primary arcing
contact separation.
be substituted for E / X in the descriptions
which follow.
5.3.1 EfXSimplified Method. In many cases of
short-circuit current calculations a simple E/X
computation (E/X,) for 3-phase faults or
3E/(2X1 + X,) for single line-to-ground faults)
will provide adequate accuracy for circuit
breaker application.
NOTE: See 5.4 for definitions of electrical quantities.
The results of the E / X simplified procedure
may be compared with 100 percent of the
circuit breaker symmetrical interrupting
capability where it is kno
tem X/R ratio (Xl,lRl f
or (2X1 +X0)/(2R1+R,) f
ground faults is 15 or less.
5.2 §election of Applicable Circuit Breaker
Ratings. With the short-circuit current duties
at the circuit breaker location available
(see 5.3), the most severe of these should be
used in selecting the desired breaker rating
from the tables of preferred ratings in ANSI
(337.06-1979. The proper table for indoor or
outdoor, oil or oilless breaker types should be
chosen. One or more interrupting and continuous ampere ratings are available in most voltage
classes.
NOTE : For simplification, negative-sequence reactance
X z is assumed equal to positive-sequence reactance X I .
A similar assumption is made for resistance values Rz
and R I .
The E/X simplified procedure may be used
without determining the system R if E/Xdoes
not exceed 80 percent of the symmetrical
interrupting capability of the breaker. If it is
desired to use a breaker where the current
determined in this case exceeds 80 percent of
the
breaker’s
symmetrical interrupting
capability, a more exact method of calculation
such as that described in 5.3.2 should be used
to check the adequacy of the circuit breaker.
5.3 Methods for Calculating System ShortCircuit Currents. Various methods and devices
are in use for the calculation of short-circuit
current. Rather rigorous methods involving
step-by-step application of ac and dc decrements have been devised and used for faults at
generating stations. These same methods are
sometimes used for faults at other locations,
but the application of the methods is more
difficult and less satisfactory as the impedance
networks became more complex.
A simplified method is described in 5.3.1.
This method requires only an E/X calculation
up to certain limits which represent the degree
of accuracy of the method.
A more accurate method is described in 5.3.2.
This method gives results approximating those
obtained by more rigorous methods. In using
it, it is necessary first to make an E/X current
calculation. Then it is necessary to adjust the
E/X current value for both the ac and the dc
decay which depend upon the circuit conditions. This method also provides for the
possibility of including both ac and dc decays
where relay time delay in excess of the l/2 cycle
minimum is utilized. This method should provide results having an accuracy commensurate
with the usually available short-circuit characteristics of electrical equipment and systems.
In those instances where it is desired to use
impedances instead of reactances for determining short-circuit current magnitudes, E/Z may
NOTE: In many cases, calculation will show the X/R
ratio to be less that 15. In such cases, no further
calculation is required since EIX can be used up to
100 percent of breaker rati
The more exact procedure should also be used
if a single line-to-ground fault supplied
predominantly by generators, at generator
voltage, exceeds 70 percent of the circuit
breaker symmetrical interrupting capability for
single line-to-ground fa
EXAMPLE OF E / X SIMP
(1) General: Consider the system shown in Fig 5.
Faults on both sides of breaker A in Fig 5 and of all
other circuit breakers should be considered, assuming
in each case that the breaker in question is the last
breaker to clear the fault. However, upon careful
observation of the system involved it may be apparent
that a fault on one side of the breaker gives a higher
fault current than does a fault on the other side. This is
the case for breaker A in the system shown in Fig 5.
Therefore, currents have been calculated for a fault on
only one side at positi
ver, in case of doubt,
fault currents for fau
sides of the breaker
should be calculated.
line-to-ground faults
on tie lines, the con
the remote breaker
32
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ANSI/IEEE
C37.010-1979
CIRCUIT BREAKERS RATED ON A SYMMETRICAL CURRENT BASIS
I
132 K V
.
(0.400 + 0.200 )
26 KV
(0.050+ 0.020)
-y-
a
\
Total X I =
/
(0.050 + 0.020) +
- 0.070 x
0.50 = o.06,
0.070 + 0.50
SYSTEM
EQUIVALENT
OPEN
Base voltage = 132 kV
Base current = 437 A
The value of operating voltage corresponding to the
highest typical operating voltage at the fault point is
134 kV or 1.015 per unit.
Fig 5
System Illustrating Use of Simplified Method
of Short-circuit Calculation
X,
1.015
Isc = -X 437
0.061
=
7270 A.
(3) Single Line-to-Ground Fault Calculations. Consider the system shown in Fig 7.
:0050
TotalXo =
x,:0020
(0.030 + 0.20) (0.20) = o.107
(0.030 + 0.20) + (0.20)
Since Xo is greater than X I , the single line-to-ground
fault need not be considered.
(4) Selection of Breaker. A circuit breaker is t o be
selected from the preferred rating schedules in ANSI
C37.06-1979. The load current requirement is 600 A
and the standard duty cycle is used.
Consider a breaker which has a rated maximum
voltage of 145 kV, a continuous current rating of
1200 A, a rated short-circuit current of 20 000 A at
145 kV, a maximum symmetrical interrupting capability of 20 000 A, and a voltage range factor K of
1.0. For a three-phase fault the symmetrical interrupting capability of the breaker is 20 000 A at the
134 kV operating voltage.
Since the three-phase short-circuit current (7270 A)
is less than 80 percent of the symmetrical interrupting
capability (16 000 A), this breaker is adequate for the
service required. There is a large margin for growth
which may be important in selecting the breaker for a
new application.
SYSTEM
OPEN
Fig 6
Positive-Sequence Reactances for System
Shown in Fig 5
x, =o033
8--
5.3.2 E / X Method With Adjustment for AC
and DC Decrements. For greater accuracy than
that given by the E/X simplified method described in 5.3.1, the following procedure
should be used.
The technical basis for this procedure is
presented in the Appendix. The procedure
involves steps for applying factors to the E / X
calculation. These factors depend upon the
point on the system at which the short circuit
occurs,and upon the system X / R ratio as seen
from that point.
For determination of the system X / R ratio it
should be noted that there is no completely
accurate way of combining two parallel circuits
with different values of X / R into a single
circuit with one value of X / R . The current
from the several circuits will be the sum of
several exponentially decaying terms, usually
with different exponents, while that from a
Fig 7
Zero-Sequence Reactance for System
Shown in Fig 5
closed may sometimes produce a fault current through
the breaker higher than that occurring when the remote
breaker is open. In such a case, the single line-toground fault current will not exceed the 3-phase fault
current except for a very unusual combination of
impedances.
(2) Three-phase Fault Calculations. In the system
shown in Fig 6, per-unit reactances are indicated
adjacent to generators, transformers, and lines. Base
apparent power = 100 MVA. Nominal voltage is used
as base a t all levels.
33
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ANSI/IEEE
C37.010-1979
APPLICATION GUIDE FOR AC HIGH-VOLTAGE
BREAKER
' 0
TIME
I
io
I 1
I
I/--
'
1.2
13
14
13
14
I O
I I
12
13
IO
I I
I2
13
FACTORS FOR E / X A M P E R E S
NOTE: Consideration has been given to extending Che curves o
1.0, but since there are possibilities of nonconservative application in both symmetrical and asymmetrical current
determinations in this area, the curves have not been extended. Further studies within these zones are recommended
for future revisions of this application guide.
Fig 8
Three-phase Fault Multiplying Factors
Which Include Effects of AC and DC Decrement [See 5.3.2(1)]
single circuit contains just one such term.
Investigation has shown [2] that for practical
proportions, the procedure of reducing the
reactance to a single value with complete
disregard for the resistances and reducing the
resistance to a single value with complete
disregard for the reactmces gives, in general,
more accurate results than any other reasonably
simple procedure (including the phasor representation used at system frequency). In addition, the error for practical cases is on the
conservative side. For these reasons, this is the
recommended procedure.
In cases where an E/Z calculation is made
it is acceptable to substitute Z/R for X I R
provided that the R is obtained from a separate
resistance disregarding
The factors taken from Figs 8, 9, or 10 should
be applied to the E / X calculation so that the
ac and dc decrements are properly included in
the final result.
The following procedure is usually conservative :
. The E / X current should be multiplied by a factor from
times the generator per-unit sub-
34
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ANSI/IEEE
C37.010-1979
CIRCUIT BREAKERS RATED ON A SYMMETRICAL CURRENT BASIS
1.0
1.1
1.2
1.3
1.4
IO
1.1
1.2
1.3
1.0
1.1
1.2
1.3
MULTIPLYING FACTORS FOR E/X AMPERES
Fig 9
Line-to-Ground Fault Multiplying Factors
Which Include Effects of AC and DC Decrement [See 5.3.2(1)]
transient reactance on a common
system megavolt-ampere base
It will be noted that the maximum correction
factor obtained from Figs 8, 9, and 1 0 in most
practical applications is approximately 1.25.
This 1.25 factor forms the basis for establishing 80 percent of circuit breaker capability as a
limit for application of the simplified method
in 5.3.1 where XIR is unknown.
In breaker applications, relays slower than
one-half cycle are frequently used. In some
cases, consideration could be given to utilize
this relay time to reduce the fault current at
contact parting time to avoid or postpone
replacement of circuit breakers. Figs 8, 9, and
10 include curves for breakers of typical speeds
for longer contact parting t i e s to aid in
checking the adequacy of circuit breakers with
contact parting times longer than the'normal
minimum. The breakers must still meet the
closing and latching duty.
(2) The factors of Fig 1 0 include only the
effects of dc decay. The E / X current should be
multiplied by a factor from Fig 10 for a threephase or a line-to-ground fault if the shortcircuit current it fed predominantly from
generators through:
(a) Two or more transformations or
external to the
(b) A per-unit
generator that is equal to or exceeds
1.5 times the generator per-unit
subtransient reactance on a common system megavolt-ampere base
The resulting product must not exceed the
symmetrical interrupting capability of the
circuit breaker being considered.
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ANSI/IEEE
C37.010-1979
APPLICATION GUIDE FOR AC HIGH-VOLTAGE
I30
I20
I IO
IOC
9c
8C
U
\
x 7c
0
5
6C
U
5c
4(
3c
2c
IC
C
~
3
1.1
1.2
1.3
1.4
1.5
1
1.1
1.2
1.3
1.4
1.0
1.2
1.1
1.3
1.0
1.1
1.2
1.3
MULTIPLYING FACTORS FQR E / X AMPERES
Fig 10
Three-phase and Line-to-Ground Fault Multiplying Factors
Which Include Effects of DC Decrement Only [See 5.3.2(
the highest
t is 46.8 kV
EXAMPLE OF E / X METHOD WITH ADJUSTMENT
FOR AC AND DC DECREMENTS:
(1) Gen
nsider the system shown in Fig 11.
Since it
arent that faults on the line side of
oduce higher fault current through
breakers A an
the breaker than does a fault on the bus side, currents
been calculated on only one side of each breaker.
se of doubt, fault currents for faults on both sides
of the breaker should be calcualted.
ulation of Breaker A
wn in Fig 12, per-unit
nt t o generators, transformers, and lines. Base apparent power = 100 MVA.
Nominal voltage is used as base at all levels.
(0.150 + 0 . 0 7 0 )
- (0.150 + 0.070
TotalR, =
(0.020 + 0.120)
+ 0.020 + 0.120)
25
(3) Single Line-toBreaker A (Case 1).
Fig 13.
Total X o =
(0.070)
10.070
+
(0.030 + 0.300) = o.0578
0.030 + 0.300)
0143
= 0.0856
55 = 16 720.
(0.0017 i0.0015) (0.0005 + 0.0120) = o.oo255
(0.0017 + 0.0015 + 0.0005 + 0.0120)
Accurate Calculation (C
Base voltage = 46 kV
Base current = 1255 A
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for More
ase fault:
ANSI/IEEE
C37.010-1979
CIRCUIT BREAKERS RATED ON A SYMMETRICAL CURRENT BASIS
SYSTEM
EQUIVALENT
A
GENERATOR
46 K V
+4
345KV
Fig 11
System Illustrating Use of the E / X Method with
Adjustment for AC and DC Decrements
X i = 0.070
R , = O 0005
Fig 12
Positive-Sequence Impedance for System
Shown in Fig 11 (Breaker A)
R o = 0 03
Fig 13
Zero-Sequence Impedance for System
Shown in Fig 11 (Breaker A)
From Fig 9, the single line-to-ground short-circuit
current should be multiplied by 1.13 t o assure conservative breaker application. The current to be compared with the breaker symmetrical interrupting
capability for single line-to-ground faults is 1 6 720
1.13 = 1 8 890 A.
(5) Selection of Breaker (Case 1). A circuit breaker
is t o be selected from the preferred rating schedules of
ANSI C37.06-1979. The load current requirement is
700A and the standard duty cycle is used.
Consider an outdoor oil circuit breaker with a rated
maximum voltage of 48.3 kV, a continuous current
rating of 1200 A, a rated short-circuit current of
17 000 A at 48.3 kV, a voltage range factor K of 1.21,
Consider a 5-cycle breaker with a contact parting
time of 3 cycles and a normal duty cycle. Since the
breaker is on the bulk system, is only one transformation away from generation, and the X/R ratio is greater
than 15, the breaker duty should be multiplied by a
factor from Fig 8 t o assure conservative breaker
application. This factor is 1.05. The current t o be
compared with the breaker symmetrical interrupting
capability is 1 4 910 x 1.05 = 15 660 A.
For a single line-to-ground fault:
2x1 + X , - 2 X 0.0856 + 0.0518 = 35.1
2 X 0.00255 + 0.00143
2R1 + Ro
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ANSI/IEEE
C37.010-1979
APPLICATION GUIDE FOR XC HIGH-VOLTAGE
I
X , = O 070
R,(,,)=O 0015
X I = 0 020
__
R I (ac) = O O l L O
x , = 0 I20
R,
= 0 0005
,
x,=0.200
IR(oc) = O . O O y
1
Fig 14
Positive-Sequence Impedance for System
Shown in Fig 11 (Breaker B)
Xo
= O 030
= 0 300
Ro : O 0 3 0
X,
X,
R,
= 0070
=00015
II
oj
D
R, = O 0 3 0
!--i
Fig 15
Zero-Sequence Impedance for System
Shown in Fig 11 (Breaker B)
NOTE: X may be either X I or 2X1 + X o and R may
be either R I o r 2R1 + Ro as shown in 5.4.
(6) Three-phase Fault Calculation of Breaker B
(Case 2). Consider the system shown in Fig 14.
(b.150 + 0.010 +
= (0.160
-
)'
(0.020)
+ 0.200 = 0.219
+ 0.070 + E)+
2
(0.020)
(0.0017 + 0.0015+ o.0120)
(0.0005)
2
At Rated
Fault
The value of voltage corresponding t o the highest
typical operating voltage at the fault point is 34 kV or
0.986 per unit.
At
X/R Adjusted Maximum Operating
Voltage Voltage
E/X Factor E/X
3-phase 1 4 9 1 0 1.05
single
line-toground 1 6 7 2 0 1.13
15660
18890
I
17 000
17540
19550
20 170
= 0.0047
Base voltage = 34.5 kV
Base current = 1670 A
Symmetrical
Interrupting
Capability
Calculated
Short-Circuit Current
i
0.0040
Is, = 0'986
-X 1670 = 7520
0.219
A
(7) Single Line-to-Ground Fault Calculation of
Breaker B (Case ~ 2 ) .Consider the system shown in
Fig 15.
1
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ANSI/IEEE
CIRCUIT BREAKERS RATED ON A SYMMETRICAL CURRENT BASIS
(0.070 +
x, =
(0.070
'9)+
X I is the positivesequence reactance. (Equivalent to X as used in this standard.)
X , is the negative-sequence reactance. (In the
simplified method of fault calculation, X2 is
assumed equal to X , .)
X , is the zero-sequence reactance which may
be obtained from design data, by calculation,
or by test.
X,!J' is the subtransient direct-axis reactance
of a synchronous machine or locked-rotor
reactance of an induction machine. This is
always a positive-sequence reactance.
Xi is the transient direct-axis reactance of a
synchronous machine. This is always a positivesequence reactance.
R is the corresponding lowest value of system
resistance as viewed from the fault point
(determined with X assumed 0) with the
resistances of the system components determined as specified in 5.4.2.Instead of calculating R , an estimate of the system X / R ratio
may be determined as shown in Table 5.R may
be either R I or 2R, + R o depending on
whether three-phase or single line-to-ground
currents are being calculated.
R is the positive-sequence resistance.
R 2 is the negative-sequence resistance.
Ro is the zero-sequence resistance which may
be obtained from design data, by calculation,
or by test.
L is inductance in henrys. L = X / 2 n f (where
f is system frequency).
C is capacitance.
2 is impedance.
I is current.
I, is the calculated symmetrical short-circuit
current.
K is the voltage range factor (see 5.2 of ANSI/
IEEE C37.04-1979and ANSI C37.06-1979.
Tdc is the direct-current time constant for
the' circuit involved in the short circuit being
calculated (see 5.1.3).
S is the ratio as shown in Fig 4 and 5.10.2.2
of ANSI/lEEE C37.04-1979.
(0.030)
0 300
+ -)+2
0.200 = 0.226
(0.030)
(0.0015+0 0,300)
(0.0010)
+ 0.0040 = 0.0049
Ro
= (0.0015+O
o r ) + (0.0010)
0*986
=
(0.219x 2) + 0.226 x 1670 = 7440 A
( 8 ) Factors Applicable to E/X Calculation for More
Accurate Calculation (Case 2). For a three-phase fault:
Xi - 0.219
R1
0.0047
= 49.0
Consider a 38 kV 5-cycle breaker with a minimum
relay time of 1.5 cycles resulting in contact parting
time of 4 cycles and standard duty cycle. Since the
breaker is on the subtransmission system, is remote
from generation by more than one transformation, and
the X/R ratio is greater than 15, the breaker duty
should be multiplied by a factor from Fig 1 0 for a
conservative breaker application. This factor is 1.20.
The current t o be compared with the breaker symmetrical interrupting capability is 1.20 X 7510 = 9012 A
for a 3-phase fault. For a single line-to-ground fault:
2x1 + X o
2R1 + Ro
-- (2X 0.219)+ 0.226
(2 X 0.00447)+ 0.0044
C37.010-1979
,
= 47.8
From Fig 10, the single line-to-ground short-circuit
current should also be multiplied by 1.19.The current
t o be compared with the breaker symmetrical interrupting capability for single line-to-ground faults is
1.19 X 7440 = 8850 A.
A breaker would be selected as described in (5).
Selection of Breaker (Case 1).
5.4 Electrical Quantities Used. E is the lineto-neutral value corresponding to the highest
typical operating voltage which occurs^at the
circuit breaker location.
X is the corresponding lowest value of
system reactance (determined with R assumed
0) as viewed from the fault point with all
rotating machines represented by the appropriate reactances as specified in 5.4.1. It may
be either X , or 2 X , + X o according to whether
three-phase of single line-to-ground currents
are being calculated.
5.4.1 Rotating Machine Reactances. Basically, initial short-circuit current of rotating
machines is determined by the machine subtransient reactances. For the simplified and
more accurate methods of short-circuit current
calculation, the following reactances are
NOTE: Load reactances are neglected in this guide
pince they are usually large with respect t o the series
reactance and have little effect on the magnitude of
deed :
short-circuitcurrent.
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ANSI/IEEE
C37.010-1979
APPLICATION GUIDE FOR AC HIGH-VOLTAGE
Using Z = (,!?/Xi) E-tIT': as the expression for the
exponential decay of induction motor symmetrical
current t o a terminal short circuit, the reactance
multiplying factor is E + ~ / T, where t is thz proper time
after initiation of the short circuit and T is the motor
short-circuit time constant. (Both should be in the
same time units.) For example, using manufacturer's
motor data for T ', the reactance multiplying factor for
determining the interrupting duty may
d using
t equal to the circuit breaker minimum
parting
time. For a circuit breaker with a five-cycle rated
interrupting time, t = 3 cycles (0.05 s). For determining the closing and latching duty, t = 0.5 cycles
(0.00833 s) in the reactance multiplying factor
calculations.
Positive Sequence
Reactances for Calculating
5 P e of
Rotating Machine
Interrupting Closing and
Duty
Latching Duty
(Per Unit)
(Per Unit)
All turbogenerators, all
hydro-generators with
amortisseur windings,
and all condensers (See
Note 2)
1.0
Hydrclgenerators without
amortisseur windings
(See Note 2)
0.75Xd
0.75Xd
All synchronous motors
(See Notes 1, 4, and 5)
1.5 Xi
1.0
xi
1.5 X t
1.0
x:
xl;
1.0
x:
POWER SYSTEM
EQUIVALENT
Induction Motors (See
Notes 3, 4, and 5)
Above 1000 hp at
1800 r/min or less
Above 250 hp at 3600
r/min
From 50 t o lo00 hp at
3.0 X:
1800 r/min or less
From 50 to 250 hp
at 3600 r/min
Neglect all 3-phase induction motors
below 50 hp and all single-phase
T""
1.2 x;
lo
I
I
3
INDUCTION WTOR
LOAD
INDIVIDUAL SIZES
50 TO 250 HP
2000
HP
NOTES :
(1) Xl of synchronous rotating machines is the
rated-voltage (saturated) direct-axis subtransient reactance.
(2) Xd of synchronous rotating machines is the
rated-voltage (saturated) direct-axis transient reactance.
(3) X g of induction motors equals 1.00 divided by
per-unit locked-rotor current at rated voltage.
(4) The current contributed t o a short circuit by
induction motors and small synchronous motors may
usually be ignored on utility systems except station
service supply systems and at substations supplying
large industrial loads. At these locations, as well as in
industrial distribution systems or locations close t o
large motors, or both, the current at l / 2 cycle will be
increased by the motor contribution t o a greater degree,
proportionately, than the symmetrical current will be
increased at minimum contact parting time. In these
calculation of '12 cycle current
g the methods of 5.3.1 or 5.3.2
and the appropriate reactance values given above under
the heading "Closing and Latching Duty." A 1.6
multiplying factor should be used for asymmetry and
this result must not exceed the closing and latching
' x,= 7.7
-r
I
HP
2000
HP
Fig 16
System Illustrating Large
Short-circuit Contribution from Motors
f
EXAMPLE: In the syste shown in Fig 16, impedances are per unit on a 100 MVA base.
at 4.15 kV = 1 3
Per-unit react
re indicated adjace
tors and transformers. Nominal voltage is used as base
at all positions. The value of voltage corresponding to
the highest typical operating voltage at the fault point
is 4.16 kV or 1.0 per unit. T
be 1.0 (see 5.3.1).
rated maximum voltage of 15.0
factor K of 2.27, a continuou
1200 A, a rated short-circuit current of 9300 A, and a
maximum symmetrical interrupting capability of
21 000 A at 6.6 kV and below. The symmetrical
interrupting capability of this breaker at 4.16 kV is
21 000 A. The current to be compared with the symmetrical interrupting capability is, for a 3-phase fault
near breaker A:
capability of the circuit breaker being used.
(5) These rotating machine reactance multipliers and
the E/X amperes multipliers of Figs 8 and 9 include the
effects of ac decay. However, the methods for calculation of system short-circuit current described in 5.3.1
and 5.3.2 incorporate sufficient conservatism t o permit
the simultaneous use of a rotating machine reactance
and a E/X amperes multiplier from Fig 8 or 9.
(6) When the contribution of large individual
induction motors is an appreciable portion of the
short-circuit current, substitution for the tabulated
multiplying factors of more accurate multipliers based
on manufacturer's time constant data is appropriate.
l
+
1
l39000 1.87 + 0.10
(3.45) (1.5) + (1.10)
[
1
+ (16.8) (i.0) + 7.7 +
1
(1.5);4.75)]
1
= 11 400 A
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ANSI/IEEE
CIRCUIT BREAKERS RATED ON A SYMMETRICAL CURRENT BASIS
The system R could be calculated and a correction
factor based on X/R determined. However, since this
factor will be 1.00 and since the current determined is
already less than the symmetrical interrupting capability, it is not necessary t o make this refinement.
The symmetrical component of current at l/2 cycle
is:
l3
ing temperature. In setting up the R network
for the calculation of the equivalent X/R ratio
of any system, rotating machine resistance
values obtained from the manufacturer or
through use of the following table should be
adjusted by the applicable rotating machine
reactance multipliers from the table in 5.4.1.
The ranges and typical values of the X/R
ratios of system components may be obtained
from Table 4. An estimate of the total system
equivalent X/R ratio to the point of fault may
be obtained from Table 5.
l
+
1
1.87 + 0.10
3.45 + 1.10
-'1
+ (16.8)(1.2)
1
+
+ 7.7
4.75
C37.010-1979
= 13 5 0 0 A
The symmetrical interrupting capability is adequate.
Since 1.6 times the symmetrical component of the ' / z
cycle current (21 600 A) does not exceed the circuit
breaker closing and latching capability (34 000 A),
the breaker is satisfactory for service at position A
in Fig 16.
Approximate
Resistance
System Components
5.4.2 Resistance of System and Typical X/R
Ratio. For the purpose of determining the
equivalent X/R ratio, it is recommended that
the manufacturer's advice be obtained concerning the resistance value to be used for
important electrical devices. For machines, the
X/R ratio required is a measure of the time
constant of the exponential decay of the dc
component of machine current for a fault at
its terminals. In the absence of manufacturer's
recommendations, the approximate values of
resistance from the table listed are suggested.
In both cases, measured values on rotating
machine should be converted to normal operat-
Turbine generators and
condensers
effective resistance'
Salient pole generators
and motors
effective resistance'
Induction motors
1.2 times the dc armature
resistance
Power transformers
ac load loss resistance (not
including no-load losses or
auxiliary losses)
Reactors
ac resistance
Lines and cables
ac resistance
X2"
'Effective resistance = 2TfTa3
where X p V is the rated voltage negative-sequence reactance and T,3 is the rated voltage generator armature
time constant in seconds. It is usually about 1.2 times
the dc resistance.
Table 4
Range and Typical Values of X / R Ratios of System Components
System Component
Large generators and hydrogen cooled synchronous condensers
Range
Typical Values
40-120
80
Power transformers
see Fig 1 7
Induction motors
Small generators and synchronous motors
see Fig 18
see Fig 19
-
40-120
80
Reactors
Open wire lines
2-16
5
Underground cables
1-3
2
NOTE: Actual values should be obtained if practical.
41
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ANSI/IEEE
C37.010-19 79
APPLICATION GUIDE FOR AC HIGH-VOLTAGE
Equivalent System
(For Quick Approxi
Type of Circuit
Range
(1) Synchronous machines connected directly t o the bus or through reactors
40-1 20
(2) Synchronous machines connected through transformers rated 100 MVA and larger
40-60
Synchronous machines connected through transformers rated 25 t o 100 MVA for
( 4 ) Remote synchronous mach
larger for each three-phase bank,
the total equivalent impedance t o
rmers rated 100 M
e 90 percent or m
30-50
(5) Remote synchronous machines connected through transformers rated
100 MVA for each three-phase bank, where the transformers provide 90 percent or
of the total equivalent impedance t o the fault point
(6) Rqmote synchronous machines connected through other types of circuits, such as:
transformers rated 10 MVA or smaller for each three-phase bank, transmission lines,
distribution feeders, etc
Based on class of transformer, obtain the proper
factor from the table below. Multiply the transformer
megavolt-ampere rating by this factor before using
Fig 17 t o obtain the typical X / R value.
Class
Rating in MVA
1 5 or less
6?
50
Factor
1
2
5
IO
500 1000
100
50
3-PHASE,FOA-POWER TRANSFORMER MVA
(STANDARD IMP
LIMITS)
Fig 18
X / R Range for Three-phase Induction Motors
50
40
and Synchronous Motors
z
5
X
30
70
60
_I
U
U
0
z
>
50
20
[r
2
t-
40
J
a 30
U
a
IO
>- 20
I-
IO
0
50
100
250
500
2500 5000
1000
NAMEPLATE
H
0
1000
10,000
P
2500
5000
1 25000
20000
10000 15000
NAMEPLATE KVA
42
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CIRCUIT BREAKERS RATED ON A SYMMETRICAL CURRENT BASIS
6. References
6.1 References to the Text
[l] SKUDERNA, J. E. The X l R Method of
Applying Power Circuit Breakers. AIEE
Transactions (Power Apparatus and Systems), vol80, June 1959, pp 328-338.
[2] AIEE Committee Report. Calculation of
Electric Power System Short Circuits
During the First Few Cycles. AIEE
Transactions (Power Apparatus and Systems), vol 75, April 1956, Pp120-127.
[12]
[13]
[3] HAHN, W. M., and WAGNER, C. F.
Standard Decrement Curves. AIEE
Transactions, vol 51, 1932, pp 352361.
[14]
[4] WAGNER, C. F., and EVANS, R. D.
Symmetrical Components. New York:
McGraw-Hill Book Company, 1933.
[15]
[ 51 Electrical
Transmission and Distribution Reference Book. East Pittsburgh:
Central Station Engineers of Westinghouse Electric Corporation, 1950.
[6] WAGNER, C. F. Decrement of Short
Circuit Currents. Electrical Journal,
March, April, May, 1953.
[7] KIMBARK, E. W. Power System Stability. New York: Wiley, vol I, 1948;
vol 11, 1950;~01111,1956.
[8] OWEN, R. E., and LEWIS, W. A. Asymmetry Characteristics of Progressive
Short Circuits on Large Synchronous
Generators. IEEE Transactions on Power
Apparatus and Systems, vol PAS-90,
March/April1971, pp 587-596.
[ 91 AIEE Committee Report. Calculated
Symmetrical and Asymmetrical ShortCircuit Current Decrement Rates on
Typical Power Systems. AIEE Transactions (Power Apparatus and Systems),
vol75, June 1956, pp 274-285.
ANSI/IEEE
C37.010-1979
Determining the Transient Recovery
Voltages on Power Systems. AIEE
Transactions (Power Apparatus and
Systems), vol 77, Aug 1958, pp 592604.
HANNA, W. M., TRAVERS, H. A.,
WAGNER, C. F., WOODROW, C. A.,
and SKEATS, W. F. System Short Circuit Currents. AIEE Transactions,
V O 60,1941,
~
pp 877-881.
HUENING, W. C., Jr. Time Variation of
Industrial System Short-circuit Currents
and Induction Motor Contributions.
A IEE Transactions (Industry and General
Applications), vol 74, May 1955, pp 90101.
KNABLE, A. H. Proposed New Breaker
Ratings and Their Effect on Application for Industrial Systems. AIEE
Transactions (Power Apparatus and Systems), vol 80, Feb 1962, pp 957-963.
AIEE Committee Report. A New Basis
for Rating Power Circuit Breakers.
AIEE Transactions (Power Apparatus
and Systems), vol 73, April 1954,
pp 353-367.
[ 161 AIEE
Committee Report. Proposed
Revision of American Standard Alternating-Current Power Circuit Breakers,
C37.4-1953; AIEE Conference Paper
NO CP-59-186,1959.
[ 171 AIEE Committee Report. Simplified
Calculation of Fault Currents. Electrical
Engineering, vol 67, 1948, pp 14331435.
[18] SKEATS, W. F., TITUS, C. H., and
WILSON, W. R. Severe Rates of Rise of
Recovery Voltage Associated with Transmission Line Short Circuits. AIEE
Transactions (Power Apparatus and Systems), vol 76, Feb 1958, pp 12561266.
6.3
References Concerning Line-Closing
Switching Surge Voltage Control Switch
[19] JOHNSON, I. B., SILVA, R. F., and
WILSON, D. D. Switching surges due to
energization or reclosing. Proceedings
American Power Conference, vol XXIII,
1961, pp 729-736.
[20] ARISMUNANDAR, A., PRICE, W. S.,
and McELROY, A. J. A digital computer
iterative method for simulating switching
6.2 General References Concerning ShortCircuit Current Interruption
[lo] DANDENO, P. L., WATSON, W., and
DILLARD, J. K. Transient Recovery
Voltage on Power Systems. AIEE Transactions (Power Apparatus and Systems),
V O ~77, Aug 1958, pp 581-592.
[ll] GRISCOM, S. B., SAUTER, D. M., and
ELLIS, H. M. Practical Method of
43
Authorized licensed use limited to: MINCYT. Downloaded on December 23,2014 at 15:15:43 UTC from IEEE Xplore. Restrictions apply.
in high vol
uit breakers. IEEE
Paper, No 31CP66-112.
~ 2 4 1 SONNENBERG, C. F. Methods of
switching surge testing based on statistical sampling theory. IEEE Paper, No
31C43-C.
surge responses of power transmission
networks. IEEE Transactions, Paper No
63-1031.
[211 LAUBER, T. S. Resistive energization of
ission lines: part I single-phase
part I1 three-phase lines. IEEE
Papers, Nos 31CP65-171 and 31CP65174.
[22] Sampling Procedures and Tables for Inn b y Attributes. MILStd-105D.
r231
ER, C . L. and BANKOSKE, J. W.
Evaluation of surge suppression resistors
[251 WALD, A. Sequential Ana
York: John Wiley & Sons, 19
1261 CALABRO, S.
and Practices.
1962.
New
liability Principles
ork: McGraw-Hill,
[271
and
Y of
Reclosing Transients on a 765 kV
Shunt Compensated
Transmission
Line, IEEE Transactions on Power
Apparatus and Systems, vol 97, July/
Aug, 1978, pp 1447-1457.
44
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d
Appendix
(This Appendix is not a part of ANSI/IEEE C37.010-1979, American National Standard IEEE Application Guide for
AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis.)
Appendix A
Basis for E / X Method Corrected for AC and DC
Decrements in the
Calculation of Short-circuitCurrents
A l . Introduction
A2. Application Methods
The methods used in developing the application guide for the more accurate procedure
described in the main body of this standard
(see 5.3.2) are covered in this Appendix. This
Appendix shows the derivations that permit
the use of a semi-rigorous procedure for
short-circuit current calculations. It avoids
the intricate procedure required in the rigorous
procedure while providing a degree of accuracy
which is within the practical limits of shortcircuit calculation considering the accuracy of
constants which are usually available for such
computations.
Of primary importance to any application
procedure is the method for determining the
value of short-circuit current which a circuit
breaker must interrupt. A method of calculation [ A l l which employs correction factors
for application to E / X short-circuit calculations
has been suggested. The correction factors
proposed to be applied to the E / X values are
based on the system X / R values at the point of
fault. That method does not account for any
decay in the symmetrical component of the
short-circuit current. It, therefore, often gives
overly conservative results, particularly for
faults that are near generation. The application
procedures developed in this Appendix employ
the basic principles proposed by an earlier
paper [ A l l and, in addition, include the
adjustments considered necessary for the ac
decrement effects on the total short-circuit
current value which is to be used in selecting a
circuit breaker.
Two methods of short-circuit calculation are
described in 5.3.1 and 5.3.2 of this standard.
The E / X simplified method and its limitations
are described in 5.3 .l.The method as described
in 5.3.2 is the more accurate method which,
though semi-rigorous so far as accuracy of
results are concerned, is relatively simple to
use. It is recommended for accuracy purposes
that this method be used when the following
conditions occur:
(1) Three-phase and line-to-ground short
circuits exceed 80 percent of the circuit
breaker symmetrical interrupting capability.
(2) Line-to-ground short circuits supplied
predominantly by generators at generator
voltage exceed 70 percent of the circuit breaker
symmetrical interrupting capability for single
line-to-ground faults.
This Appendix gives the supporting technical
data for the procedure decribed in 5.3.2.
A3. The Effects of AC Component
Decrement
The need for proper recognition of the decay
in the symmetrical component of short-circuit
current is clearly evident from the decrement
curve calculations which have been made for
short-circuit applications at or near a source of
generation. Evidence of this nature, including
calculated and test results, is presented in an
AIEE Committee report [A2].
Further evidence is included in the sample
figures in this Appendix. The data are based
upon calculated decrement in both the ac component and in the total short-circuit current for
machines of various manufacture. In determin-
1Numbers in brackets refer to References t o the
Appendix, Section AT.
45
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ANSI/IEEE
637.010-1979
A2
A3
A4
APPLICATION GUIDE FOR AC HIGH-VOLTAGE
107 MVA, 3600 r/min, conductor cooled
95-200 MVA, 3600 r/min, conventional cooled,
composite characteristics
35-65 MVA, 3600 rlmin, composite characteristics
Remote
87
HV
LV
HV
78
68
77
= Fault on generator bus, low voltage.
Dower transformers.
Remote = 'see Fig A 2 for fault location.
ing short-circuit current decrements for these
be taken into a
cases, a three-phase short circuit was assumed.
The single line-to-ground fault condition is
discussed later in this Appendix. The calculated
data obtained by a rigorous calculation
procedure are shown in Table A l .
From the cases presented in Table A l , it is
seen that the symmetrical component of shortcircuit current at 4 cycles after short-circuit
as low as 62 percent of the
e remote fault (see Fig A2)
or for the close to generating station short
circuit, where there is substantial fault cu
contribution from a remote system, the. ac
decrement may be much less. However, the
assumption that regardless of fault location
there will be no ac decrement will result in
overly conservative results.
constants for
A4. ~ e ~ v a tofi oE /~X Multipliers
Fig A5 shows the relationship of fault
current ( I a y m /Isym)nacd (the subscript nacd
indicates that there is assumed to be no ac
decrement) 85 a function of X/R for various
contact parting times. This family of curves
whiph considers only the decay of the dc
component of fault current forms the basis of
a recent application proposal [ A l l . They also
form the starting point for the derivation of
E/X multipliers for this proposal.
Fig A7 illustrates the method used to modify
the curves of Fig A5 so that the decay of the
symmetrical component of fault current will
46
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ANSI/IEEE
C37.010-1979
CIRCUIT BREAKERS RATED ON A SYMMETRICAL CURRENT BASIS
8
?
I-5 $ 6
K Z
L55
CO
Z w 4
wv)
Ka
zz
gl= *
iz- I
n m 3
-AS
I
^
[
“
tI
-
CONSTANTS SAME
FOR ABOVE
TRANSFORMERS
0
I3 BKV
1
2
3
4
5
6
1
8
CONTACT PARTING TIME-CYCLES
Fig A2
Symmetrical and Total Current Decrement
Three-phase Short Circuit with Generator
and System Contribution
1
1
1
1
1
1
1
1
1
1
1
1
Fig A3
Symmetrical and Total Current Decrement
Three-phase Short-circuit Representative
95 to 200 MVA Conductor-Cooled
3600 r/min Turbine Generators
Fig A4
Symmetrical and Total Current Decrement
Three-Phase Short Circuits Representative
35 to 65 MVA 3600 r / h h Turbine Generators
47
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ANSI/IEEE
C37.010-1979
APPLICATION GUIDE FOR AC HIGH-VOLTAGE
5
I
TOTAL~URRENT
’
I
2
I
I
xla
0
c
a
a
0
3
4
CONTACT PARTING TIME-CYCLES
1.0 1.1
1.2 1.3 1.4
1.5 1.6 1.7
Fig A6
Illustration of Accuracy of Fault
Determination Hydro Generation
Three-Phase Fault at High Side Terminals
of Station Step-up Transformers
Fig A5
Relationship Of (Iasym/Isym)nacd to x / R
for Several Breaker Contact Parting Times
ATIONSHIP BETWEEN $RATIO
AC DECREMENT FROM
URATE FAULT C A L C U L b T l O N
I
I
1
ADJUSTMENT FACTORS FOR
AC DECREMENT FROM S A N D
FAULT LOCATION
(b)
A D J U S T M E N T FACTOR FOR
SYMMETRICAL D E C R E M E N l
FAULT LOCATION
Fig A7
Relationship Between X / R Ratio and A
Decrement from Accurate Fault Calculation
48
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5
CIRCUIT BREAKERS RATED ON A SYMMETRICAL CURRENT BASIS
very small to very large apparent power ratings
of various manufacture. If considered alone,
1800 r/min machines would fall above this
band, but when considered in combination
with 3600 r/min units they fall within the band,
and even with 1800 r/min machines alone, the
error in calculated current is negligible at
contact parting times up to 4 cycles. As will be
demonstrated later, the upper curve of this
band is used to determine the E / X multipliers,
since this gives the more conservative answers
in terms of circuit breaker application.
Water
wheel generator characteristics
generally fall somewhat above the band
indicated. Although separate E / X multipliers
could be developed for hydro-generator circuit
breaker application, the typical example shown
in Fig A6 indicates that the multipliers
developed in Fig A8 give adequately accurate
results.
Fig A7(b) shows the decay of the symmetrical component of fault current at various
time intervals after fault initiation. Again,
because of variations in machine and system
consthts, individual points plotted on this
figure result in erratic patterns. In order to
remain on the conservative side, points showing
the least ac decrements were favored to establish
the curves shown in this plot. As far as E / X
multipliers for breaker application are
concerned, a reduction from the initial symmetrical E / X current with time has the same
effect as a reduction in X / R if only dc
decrement was considered. Fig A7( c) establishes
reduction factors that can be applied to (Imym /
Isym)nacd ratios of Fig A5 to obtain this effect.
For a given X / R ratio, the reduction factors
b be applied b the (I,,, /Iwm )nacd ratios of
Fig A5 are obtained from the following relationship :
Reduction Factors =
ANSI/IEEE
C37.010-1979
Step 1: Assume a three-phase fault, an X / R
ratio of 80, and time after fault initiation of
3 cycles. From Fig A5 the ratio
Step 2: Referring to Fig A7 (a) and (b) an
X / R ratio of 80 is related to an ac decrement
of 0.70 per unit at a time of 3 cycles after fault
initiation. (For this particular system the
symmetrical component of fault current has
decayed to 70 percent of the initial E / X value.)
Step 3: Following across to Fig A7 (c) and
the curve labeled 1.5 [the ratio (Iwrn /Isym)nacd
obtained from step 13, a reduction factor of
0.885 per unit is obtained.
Step 4 : The modifier ( I w m / E / X ) ratio for
an X / R of 80 is 0.885 X 1.5 = 1.33. This
establishes one point on the three-phase
modified X / R decrement curves of Fig A9.
Following the above step-by-step procedure,
the family of curves shown in Fig A9 was
determined.
The final step in this derivation was to obtain
E / X multipliers for breaker application. This
was accomplished through the use of the
modified X / R decrement curves of Fig A9 and
the breaker capability curve (see Fig 4,in 5.1.3)
which shows asymmetrical to symmetrical
breaker capability ratios of 1.4, 1.2, 1.1,and
1.0 for minimum breaker contact parting
times of 1, 2, 3, and 4 cycles, respectively.
This knowledge, in conjunction with the
system decrements of Fig A9 form the basis
for E / X multipliers of Fig AS. For example,
assume an X / R of 80 as in the earlier step-bystep analysis which gave a modified system
(Iwm/E/X) ratio of 1,33 for a time corresponding to a minimum breaker contact parting time of 3 cycles. If the E/X (Isym) fault
current calculation is now taken as 1.O per unit
the E / X application multiplier required to
ensure sufficient breaker capability is the ratio
1.33/1.1 or 1.21 (where 1.1 is the breaker
capability factor for B three-cycle contact
parting time). This establishes one point for
Fig A 8 which gives E / X multipliers as a function
of X / R for minimum breaker contact parting
times of from 1 to 4 cycles.
E/X
A step-by-step analysis will aid in understanding the manner in which Fig A7 was used
to modify Fig A5 to account for symmetrical
current decrement.
ZRatios of Fig A5.
49
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ANSI/IEEE
C37.010-1979
APPLICATION GUIDE FOR AC HIGH-VOLTAGE
> I 1 I
l
l
1 I
1 I
I
I
I
'
0
1
0 20 30 40 50 60 70 80 90100 110 I20 130 140 150
RATIO
Curve Corresponds to X I R of
roximately 1 5 )
Fig A8
/A' Multiplier for Equivalent Symmetrical
Amperes for Actual X / R
Fig AIO
Multiplying Factors
DC Decrement [See 5.3.2 (l)]
50
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ANSI/IEEE
C37.010-197 9
CIRCUIT BREAKERS RATED ON A SYMMETRICAL CURRENT BASIS
A5. Longer Contact Parting Time
of the accuracy of the proposed E/X multipliers
for three-phase short circuits.
Curve 1 of Fig A13 shows the necessary
breaker asymmetrical capability as a function
of contact parting time as calculated by the
E / X simplified method (see 5.3.1). The E/X
calculation of symmetrical fault current yields
a value of 10 per unit (ac component). For the
fault indicated, the 80 percent limit applies
and, therefore, the breaker symmetrical capability should be at least 1.25 E / X . Breaker
asymmetrical capabilities indicated in curve 1
of Fig A13 develop from the fact that the
rating structure proposes asymmetrical to symmetrical current ratios of 1.0, 1.1,1.2, and 1.4
for contact parting times of 4 , 3 , 2 , and 1cycle,
respectively.
Curve 2 of Fig A13 shows the necessary
breaker asymmetrical capability as a function
of contact parting time as calculated by the
more accurate method (see 5.3.2). In this
case it is necessary to consider both X and R
networks for breaker selection. The following
sample calculation will aid in the understanding
of this method as used for the determination of
In certain breaker applications, breaker contact parting time in excess of the contact
parting time curves of Fig A8 can be
considered. This may be true whenever relay
operating time exceeds the minimum l/2 cycle
tripping delay assumed in arriving at the basic
rating structure. For example, if a breaker with
a minimum contact parting of 2 cycles is
relayed such that it actually parts contacts
4 cycles after fault initiation, the E/X multiplier
for breaker selection can be reduced to account
for the fault current decay during that 2-cycle
period.
Figs A10, A l l , and ‘A12 have been so
arranged as to show the multiplier to be used
in the selection of breakers relayed for both
minimum and longer contact parting times.
A6. Accuracy of Proposed E / X
Multipliers
Figs A6, A13, and A14 give an illustration
Fig A l l
Line-to-Ground Fault Multiplying Factors
Which Include Effects of AC and DC Decrement [See 5.3.2 (l)]
I30
120
110
100
90
BO
a
2
70
0
4 60
a
50
40
30
BREAKER
20
IO
90
II
I2
I3
14
IO I I 1.2 I 3 1.4
IO
I1
I2
MULTIPLYING FACTORS FOR E/X AMPERES
IS
1.3
1.0
I I
1.2
I3
51
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ANSI/IEEE
C37.010-1979
APPLICATION GUIDE FOR AC HIGH-VOLTAGE
1
1.1
1.2
I.3
I.4
TORS FOR E / X AMPERE S
Fig A12
Three-phaseand Line-To-GroundFault Multiplyi
Which Include Effects of DC Decrement Only [ S
14
13
12
c
II
IO
CATION FACTOR)
9
c 8
7
TRA
EN
6
5
4
3
(I) E K R ASYMMETRICAL CAPABILITY U S I N G
3
SIMPLIFIED APPLICATION PROCEDURE
.
2
(80% APPLICATION FACTOQ)
I
0
I
2
3
0
4
TIME-CYCLES
CONTACT PARTING TIME-CYCLES
Fig A14
Illustration of Accuracy
of Fault Determination
Fig A13
Breaker Asymmetrical Capability
52
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ANSI/IEEE
C37.010-1979
CIRCUIT BREAKERS RATED ON A SYMMETRICAL CURRENT BASIS
17
16
15
14
I3
-12
k
z"
=IC
a
w
n 9
d
5
!i
3 4
23
2
I
0
I
2
3
4
CONTACT PARTING TIME- CYCLES
O
1
2
3
4
5
6
7
8
9
CONTACT PARTING TIME-CYCLES
(Conventional Cooled Generator (3600 rlmin)
Range 95-200 MVA)
(107 MVA Turbine-Generator)
Fig A15
Illustration of Accuracy
of Fault Determination,
Single Line-to-Ground Fault
Fig A16
Illustration of Accuracy
of Fault Determination,
Line-to-Ground Fault at Terminals
the required symmetrical capability of a
breaker which parts contacts 2 cycles after
fault initiation:
1.2 X 10.2 = 12.2 per unit. This establishes
one point of curve 2 of Fig A13. Figs A6
and A14 are constructed similarly.
Figs A15 and A16 give an illustration of the
accuracy of the proposed E/X multipliers for
single line-to-groundshort circuits. These figures
show the short-circuit current versus time
characteristics for short circuits on a highvoltage bus and on a generator bus, respectively.
It will be noted that line-to-groundshort-circuit
currents have less symmetrical decrement than
the three-phase short-circuit currents. The E/X
simplified method should not be used for single
line-to-ground faults in excess of 70 percent of
breaker symmetrical single line-to-ground interrupting capability if supplied predominantly
from generators at generator voltage (see 5.1.2).
Should the calculated line-toground fault at
these locations exceed 70 percent of the circuit
X to point of fault =
R to point of fault =
=
(0.10 + 0.10) (0.20) = o.lo
(0.10 + 0.10 + 0.20)
(0.00104 + 0.005) (0.017)
(0.00104 + 0.005 + 0.017)
0.00447
Equivalent X/R to point of fault = 22.5
Total symmetrical fault current ( E / X ) =
10 per unit.
E/X multiplier from Fig A8 = 1.02.
Breaker required symmetrical capability
(10 X 1.02) = 10.2 per unit.
breaker
capability
which
the
application
engineer desires to use, then it becomes necessary to refer to the hultipliers in Fig A l l to
check the adequacy of the breaker.
Since a three-cycle breaker has an asymmetrical to symmetrical ratio of 1.2, the
breaker required asymmetrical capability is
53
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ANSI/IEEE
C37.010-1979
A7. References to the Appendix
Circuit Current Decrement Rates on
Typical Power Systems. AIEE Transactions (Power Apparatus and Systems),
vol75, June 1956, pp 274-285.
[A31 AIEE Committee
A New Basis
uit Breakers.
s (Power Apparatus
and Systems), vol 73, April 1954,
pp 353-36*7.
[ A l l SKUDERNA, J. E. The X / R Method
of Applying Power Circuit Breakers.
A IEE Transactions (Power Apparatus
and Systems), vol 80, June 1959,
pp 328-338.
[A21 AIEE Committee Report. Calculated
Symmetrical and Asymmetrical Short-
54
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