IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 29, NO. 1, JANUARYFEBRUARY 1993
195
All Amperes Are Not Created Equal: A Comparison
of Current Ratings of High-Voltage Circuit Breakers
Rated According to ANSI and IEC Standards
Baldwin Bridger, Jr., Fellow, IEEE
Abstract- The internationalization of the electrical power
equipment market has required engineers to apply equipment
designed and built to unfamiliar standards. For high-voltage
circuit breakers, the two best-known standards are the ANSI
and the IEC standards. Since ratings with similar names have
different meanings in the two series of standards, it is vital
that application engineers understand the differences.This paper
examines the continuous current, interrupting, short-time, and
closing current ratings and compares these ratings in the two
series of standards.
INTRODUCTION
T
HERE ARE TWO well-known systems of standards used
to describe the rating and testing of high-voltage (including medium-voltage) circuit breakers. The first set of standards
is that issued by the American National Standards Institute
(ANSI) and generally used in North America [1]-[3]. The
second set is that issued by the International Electrotechnical
Commission (IEC) and generally used in Europe [4], [ 5 ] .
Other parts of the world may use either or both of these sets
of standards. The purpose of these standards is to establish
definitions, test procedures, and rating structures to allow the
comparison of one circuit breaker with another on an equitable
basis.
In addition to the rating and testing standards, the ANSI C37
series of standards includes several standards [6]-[8] designed
to help an application engineer choose the proper rating of
circuit breaker for the particular set of system conditions likely
to be encountered at the installation site in question. These
conditions may vary from the standard conditions upon which
the rating and testing standards are based.
The electrical power equipment business is becoming
increasingly internationalized, with both equipment suppliers
and users operating on a worldwide basis. Because of this,
application engineers familiar with one of these two sets of
circuit breaker standards may find themselves required to apply
circuit breakers rated and tested according to the other set of
standards.
Of course, it is possible for a circuit breaker manufacturer
to test a breaker to both sets of standards and establish
Paper ICPSD 91-2, approved by the Power Systems Protection Committee
of the IEEE Industry Applications Society for presentation at the 1991
Industrial and Commercial Power Systems Conference, Memphis, TN, May
6-9. Manuscript released for publication April 29, 1992.
The author is with Powell Electrical Manufacturing Co., Houston, Tx
11211.
IEEE Log Number 9204194.
ratings in both the ANSI and the IEC systems, but this
is unlikely to happen for at least two reasons. First, the
required testing is very expensive. Since both the ANSI and
the IEC standards require testing to demonstrate conformity
with assigned ratings, there is no good way to use a single
set of tests to demonstrate ratings that conform with both
sets of standards. The testing of a single rating of mediumvoltage circuit breaker (of the type used in 15-kV metal-clad
switchgear) can cost upwards of $50 000 or even more if the
manufacturer has to purchase test laboratory services for all
of the required testing. Regardless of which set of standards is
used for the original testing, most manufacturers find it difficult
to justify the added expense of a second round of testing. The
second reason is that a design that is optimized for the rating
structure of one set of standards is seldom optimal for the
other set, leading to a reduction in ratings when moving from
one set of requirements to the other.
There are differences between the two sets of standards in
the definitions of various ratings. These differences, if not
understood by the application engineer, can lead to errors in
the application of circuit breakers. This paper will examine
the various current ratings assigned to high-voltage circuit
breakers, including continuous current, short-circuit current,
short-time current, and closing current, comparing the ANSI
and IEC rating methods and suggesting ways of converting
circuit breaker ratings from one system to the other. The effects
of dc offset, voltage range factors, and interrupting time on the
rating structures will also be examined.
CONTINUOUS CURRENT
Both ANSVIEEE C37.04 [l] (in Section 5.4) and IEC 694
[5] (in Section 4.4.1) define the continuous current rating of
a circuit breaker as the rms current the breaker can carry
continuously without exceeding certain temperature rise limitations. Both standards base the limiting temperature rises on
a maximum ambient of 40" C. Each standard includes tables
of allowable temperature rise for various parts of a circuit
breaker, and these values of allowable temperature rise are
different in the two standards.
Allowable Temperature Rise
Table I lists a number of the parts of circuit breakers and the
allowable temperature rises under the two standards. It can be
seen from this table that the IEC standard is less stringent
than the ANSI standard in several areas. A point-by-point
0093-9994/93$03.00 0 1993 IEEE
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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 29, NO. 1, JANUARYFEBRUARY 1993
196
TABLE I
A
SELECTED TEMPERATURE RISE VALUES
Circuit Breaker Component
Allowable Temperature
Rise
ANSI(1)
IEC(2)
Contacts, silver or silver plated, in air
65” C
65’ C
Connections, silver or silver plated, in air
65’ C
75O
c
C
B
X
Insulating material and metal parts in
contact with insulation
Class A
550
c
60’ C
Class B
80‘ C
90’ C
NOTES: (1) Refer to ANSYIEEE C37.04 [l], Tables 2, 3, and 4; (2)
Refer to IEC 694 [ 5 ] ,Table V.
comparison of the test results with either standard would be
necessary to determine whether or not a circuit breaker meets
the temperature rise requirements of that standard.
is the envelope of current
Fig. 1. Short circuit current wave. *}
wave, BX is the normal zero line, CC’ ??the displacement of current wave
zero line at any instant, DD’ is the rms value of the ac component of current
at any instant, measured from CC’, EE’ is the instant of contact separation
(initiation of the arc), IMCis the making current, IAC is the peak value of
ac component of current at instant EE’, k is the ms value of the ac
Jz
component at instant EE’, IDC is the dc component of current at instant
o athe percentage value of the dc component.
EE’, and l D ~ A ~ l is
Effect of an Enclosure
Section 4.4.1(2) of ANSUIEEE C37.09 requires enclosed
breakers to be tested in their enclosures. In North American
practice, breakers designed for use in enclosures are seldom
if ever used without enclosures. Section 6.3.2 of IEC 694
requires the circuit breaker to be mounted in all significant
respects as in service, including all normal covers of any part
of the switching device. Since, however, European practice
may involve the use of the same circuit breaker in both open
and enclosed installations, it is common practice to test and
rate the circuit breaker without an enclosure. The nameplate
continuous current rating may be for open-air application. It
may be necessary to derate the circuit breaker for enclosed use.
No general rule for derating can be given; the manufacturer
needs to be consulted in each case.
Effect of Frequency
Section 5.3 of ANSVIEEE C37.04 specifies a rated frequency of 60 Hz for circuit breakers. Section 4.3 of IEC 694
specifies a rated frequency of either 50 or 60 Hz. Section 5.4
of ANSUIEEE C37.04 states that the rated continuous current
is at the rated frequency. Section 6.3.2 of IEC 694 calls for
continuous current tests to be made at rated frequency with
a tolerance of +2% and -5%. Section 4.3 of ANSVIEEE
C37.09 [3] states “Tests demonstrating current carrying ability
which are not made at rated frequency may need correction
factors because the heat released varies with the frequency
of the current, the relation depending on dc resistance, skin
effects, eddy currents, and hysteresis losses.” No guidance is
given for making these corrections, but it is known that higher
frequencies cause higher heat losses. Therefore, a continuous
current carrying test made at 50 Hz may give temperature
rise results that are less than would be experienced if the
same apparatus were tested at 60 Hz. Since the standards
give no mathematical model for correcting these test results,
the continuous current test should be made at the highest
frequency at which the circuit breaker will be applied.
SHORT CIRCUIT
CURRENT
The short circuit current a circuit breaker can interrupt is
probably its most important single rating. The current that
determines this rating is the current at the moment of contact
separation. Before examining the standards to see how this
current is defined in each of the two standards systems, let us
review the basics of short circuit current wave forms.
Review of Basics
Fig. which is taken from Fig. of IEC 56 ,41, shows a
typical short circuit current waveform and defines the various
component
of this wave. At the moment of initiation
of a short circuit, the ac current wave, which is normally
symmetrical about the zero axis (BX in Fig. l), is offset by
some value, creating a waveform that is symmetrical about
another axis (CC’ in Fig. 1). The degree of asymmeQ is a
function of several variables, including the parameters of the
power system up to the point of the short circuit and the point
on the ac wave at which the short circuit was initiated. In a
three-phase circuit, there is usually one phase that is offset
significantly more than the other two phases.
It is convenient to analyze this asymmetrical waveform as
consisting of a symmetrical ac wave superimposed on a dc
current. In Fig. 1, CC’ represents the dc current, and the value
of that current at any instant is represented by the ordinate
of CC’. The dc component of the current normally decays
quite rapidly and reaches an insignificant value within 0.1 s
in most power systems. The rate of decay is a function of the
system parameters. When the initial value of the dc current is
equal to the initial peak value of the ac current, the resulting
waveform is said to be fully offset or to have a 100% dc
component. It is possible, in some power systems, to have an
offset in excess of loo%, which may result in a waveform that
has no current zeros for one or more cycles of the ac power
frequency.
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BRIDGER: ALL AMPERES ARE NOT CREATED EQUAL
197
The ac component of the short circuit current will also decay
at a rate dependant on the system parameters. In general, if the
fault is closer to generators or other large rotating machinery,
the decay will be faster.
...
RICAL INTERRUPTING CAPABlLlM = S X SYMMETRICAL INTER1.6
Short Circuit Current Rating
1.5
The ANSI definition of rated short-circuit current is found in
ANSIOEEE C37.04. Per Section 5.10.1 of this standard, “The
rated short-circuit current of a circuit breaker is the highest
value of the symmetrical component of the polyphase or phaseto-phase short-circuit current in rms amperes measured from
the envelope of the current wave at the instant of primary
arcing contact separation which the circuit breaker shall be
required to interrupt at rated maximum voltage and on the
standard operating duty.” This rated short-circuit current is the
value from which the required symmetrical and asymmetrical
interrupting capabilities are derived.
The IEC definition of rated short-circuit current is found
in Section 4.101 of IEC 56, which reads, in part, as follows:
“The rated short-circuit breaking current is characterized by
two values:
I . the rms value of its ac component, termed “rated shortcircuit current” for shortness, and
2. the percentage dc component.”
From these two definitions, the rated short-circuit current
is seen to be defined as an rms ac current in both sets of
standards. However, the rated short-circuit current does not
define the current the circuit breaker is required to interrupt.
That value is further defined as follows.
In Section 5.10.2.1 of ANSIAEEE C37.04, the required
symmetrical interrupting capability at rated maximum voltage
is defined as being the same as the rated short-circuit current.
There is a variation with voltage, which is discussed further
later in this paper. In Section 5.10.2.2, the required asymmetrical interrupting capability is defined as being the highest value
of the total short-circuit current rms amperes, which includes
the rated short-circuit current plus a dc component.
Section 4.101 of IEC 56 goes on to state that “The circuit
breaker shall be capable of breaking any short-circuit current
up to its rated short-circuit breaking current containing any ac
component up to the rated value and associated with it any
percentage dc component up to that specified....”
This, of course, is an asymmetrical current. Note also that
there is a subtle difference in definition between two very
similar terms. The “rated short-circuit current” is only the ac
component of the asymmetrical “rated short-circuit breaking
current.”
Assigning a Value to the dc Component
Both sets of standards recognize the rapid decay of the
dc component of the short-circuit current and therefore base
their value on the expected time from the initiation of the
fault to contact parting. Both standards add one half of a
cycle of relaying time to the expected operating time of the
circuit breaker to determine the contact parting time. From this
point, however, the method of determining the value of the dc
component varies between the two sets of standards.
2Vl
s
1.4
1.3
1.2
1.1
1 .o
0.5
1.0
,008 0.017
2.0
0.033
3.0
0.050
4.0
0.087
CYCLES
SECONDS
CIRCUIT BREAKER CONTACT PARTING TIME
SUM OF 1/2 CYCLE TRIPPING DElAY PLUS THE OPENING TIME
OF THE INDNIDUAL BREAKER
Fig. 2. Ratio of circuit breaker asymmetrical to symmetrical interrupting
capabilities.
Section 5.10.2.2 of ANSVIEEE C37.04 specifies that the
required asymmetrical interrupting capability shall be the
required symmetrical interrupting capability multiplied by a
factor S , which is determined from Fig. 2 of that standard,
which is reproduced as Fig. 2 of this paper.
The ANSI standard is further complicated by the requirement that the contact parting time be considered as the sum
of the one-half cycle relay delay plus the lesser of the actual
opening time of the particular circuit breaker or 1.0, 1.5,2.5, or
3.5 cycles for breakers having a rated interrupting time of 2, 3,
5 , or 8 cycles, respectively. In many instances, this will require
using a contact parting time less than the actual contact parting
time of the circuit breaker. In addition, note that the times
are all given in cycles, which must be related to the 60-Hz
rated frequency of the circuit breaker under ANSI standards.
For purposes of comparison with the IEC standards, which
give times in milliseconds, these times should be converted
to milliseconds.
ANSI standards also require a certain percentage asymmetry
in various test duties of Table 1 of ANSVIEEE C37.09. Some
test duties require a percentage asymmetry of less than 20,
but many of the tests require a percentage asymmetry of
50 to 100. Percentage asymmetry is defined as a percentage of the peak value of the ac symmetrical component,
which makes it equivalent to the percentage dc used in IEC
56.
The IEC standard is somewhat simpler. The rated short
circuit breaking current includes a dc component whose value
is determined by reference to Fig. 8 of IEC 56, which is
reproduced here as Fig. 3 of this paper.
Section 4.101.2 of IEC 56 requires the dc component of the
short-circuit breaking current to be determined using a value of
T equal to the sum of the one-half-cycle relaying time and the
shortest opening time of the circuit breaker obtainable under
any service conditions.
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-
198
IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 29, NO. 1, JANUARYIEBRUARY 1993
Voltage Range Factor K
Time interval from initiation of short-circuit
current
T
Fig. 3. Percentage dc component in relation to time interval
(ms)
7.
Asymmetrical Value of Test Current
The rms value of the asymmetrical current I existing at the
moment of contact part may be calculated as follows (from
Fig. 9 of ANSVIEEE C37.09):
where IAC and IDC are as defined in Fig. 1 of this paper.
Per Section 5.10.2.2 of ANSYIEEE C37.04, the asymmetrical current at the moment of contact part may consist of
any combination of symmetrical and dc components, provided
the symmetrical component does not exceed the required
symmetrical interrupting capability, the degree of asymmetry
does not exceed loo%, and the total short-circuit current does
not exceed the required asymmetrical interrupting capability.
Since the IEC test procedure involves separate specification
of the ac and dc components, (1) may be rewritten as follows,
using the definition of percentage dc given in Fig. 1 of this
paper:
I =
J(5$2+
("";;:c)2
(2)
The ANSI standards include a rated voltage range factor K ,
which is defined in Section 5.2 of ANSI/IEEE C37.04. The
effect of this K factor is to establish a voltage range in which
the breaker's interrupting capabilities are a constant value
of MVA, with current increasing proportionally as voltage
decreases. The numerical values of K are established in
the rating tables of ANSI C37.06 [2]. In the latest (1987)
edition of this standard, all outdoor circuit breakers, including
circuit breakers used in gas-insulated substations, have a rated
voltage range factor of 1. These circuit breakers are listed in
Tables I1 and 111. Indoor oilless circuit breakers, which are
commonly used in metal-clad switchgear, are listed in Table
I. These have assigned rated voltage range factors varying
from 1.0 to 1.65, depending on the rating of the circuit
breaker.
Section 4.101 of IEC 56 states that a standard circuit
breaker will be capable of breaking its rated short-circuit
breaking current at voltages below the rated voltage. The
IEC standards do not include a concept similar to the voltage
range factor, although a footnote in Section 4.101 of IEC 56
allows breakers with proven short-circuit breaking currents at
two different voltages to assign intermediate characteristics
based on a straight line drawn between these two points on
a log-log display. This gives results within a few percent of
those obtained using the formula found in Section 5.10.2.1 of
ANSVIEEE C37.04.
Two things need to be said about the K factor. First, the
concept agrees with the physical reality of oil-blast and airmagnetic circuit breakers. Breakers using these technologies
really do have higher interrupting ability at lower voltages,
and assigning a K factor other than one allows a wider application of a given circuit breaker. However, circuit breakers
using vacuum or SFG puffer interrupters are essentially constant current interrupters up to a limiting maximum voltage;
therefore, a K factor other than one does not match the
physical attributes of circuit breakers using these technologies.
Second, for a known system voltage, the K factor is
unimportant. On any given system, if the voltage decreases,
the available short circuit current will also decrease and
not increase. If a circuit breaker is properly applied at the
maximum system voltage, it will have the necessary short
circuit capability for any lower voltage on that system.
Standard Operating Duty
(4)
If the value of the ac component is stated in rms terms, this
equation becomes
This is a convenient form for calculating the rated shortcircuit breaking current for a given rated short-circuit current.
The ability of a circuit breaker to interrupt its rated shortcircuit current is a function of how often it is required
to make this interruption. The time between operations is
important in that it gives the circuit breaker time to cool
between interruptions. Both sets of standards specify standard operating duties to fix this variable. The values given
below are for circuit breakers not intended for reclosing
duty.
Section 5.6 of ANSYIEEE C37.04 specifies the standard
operating duty as two close-open operations with a 15-s
interval between them or CO-15 s-CO.
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BRIDGER: ALL AMPERES ARE NOT CREATED EQUAL
Section 4.104 of IEC 56 gives two alternative rated operating sequences. The first is 0-3 min-CO-3
min-CO.
The second is CO-15 s-CO, which the same as the ANSI
requirement. The 0-3 min-CO-3
min-CO duty cycle is
more commonly used in Europe.
199
Substituting in (3,we get
I =I T m s / T q $
+2*
27.5 kA = I,,,/l
(g)'
Test Duties
Both sets of standards list several test duties that must be
performed to demonstrate the ability of a circuit breaker to
meet its short-circuit ratings. The ANSI requirements are listed
in Table 1 of ANSVIEEE C37.09. The IEC requirements are
shown in IEC 56. Section 6.106 covers basic short circuit duties, whereas Sections 6.107 and 6.108 cover more specialized
test duties.
Both test series require testing at the rated short-circuit
current that is both symmetrical and asymmetrical. Both
series require tests at currents ranging from about 10 to
about 60% of the rated short-circuit current, and both tests
require single-phase tests as well as three-phase tests. Beyond
the basic requirements, however, there are many differences.
Duty cycles may vary, as does the degree of asymmetry
required for tests at less than full short-circuit rating. Variations
in transient recovery voltage requirements and in requirements for the condition of the circuit breaker after testing make it very difficult to make an exact comparison
of the severity of tests made under the two different systems.
27.5 kA = I T m S d 1 +2 * (.5)'
27.5 kA = I,,,dIT,,
=
27.5 kA
VCiT
I,,, = 22.45 kA.
From this, it can be seen that the ANSI asymmetrical
test requirements for this circuit breaker rated 25 kA can
be satisfied with a test current of 22.45 kA rms plus a dc
component of 50% or 15.87 kA.Other currents could be used,
provided the combination of ac and dc components meet the
overall specification for total rms current.
Using IEC test procedures, the circuit breaker would be
called on to interrupt its rated I,,,
plus a dc component.
Since the opening time of the circuit breaker is 50 ms, and
the one-half-cycle relaying time is 8.33 ms, T is 58.33 ms.
Referring to Fig. 3, we see that the required dc component is
approximately 25%. From (3,we get
I = I ? - m s / l + 2 * ( 1 0%dc
0)
Example
To compare the two sets of interrupting requirements, let us
consider a circuit breaker with a rated short-circuit current of
25 kA, a rated voltage range factor K of 1, a rated interrupting
time of 5 cycles (83 ms), and an actual opening time of 50 ms.
Both ANSI and IEC standards require symmetrical interrupting tests at rated short-circuit current. See test duty 4 in
Table 1 of ANSVIEEE C37.09 and test duty 4 of Section
6.106.4 of IEC 56.
In accordance with Section 5.10.2.2 of ANSI/IEEE C37.04,
a breaker with a rated interrupting time of 5 cycles is assumed
to have a contact parting time of 3.0 cycles. From Fig. 2,
this breaker is assigned an S factor of 1.1. The highest
asymmetrical currents required by Table 1 of ANSMEEE
C37.09 are S I in test duty 6 and K S I in test duty 7. ( I
is the rated short-circuit current, which is equivalent to I,,,
in (3.)Asymmetry of 50 to 100% is also specified for these
two tests.
Since our example has a rated K of 1, S I = K S I . To
meet the ANSI test requirements, this circuit breaker would
be required to interrupt a maximum short-circuit current of
S I , or 1 . 1 ITmS,with a minimum asymmetry of 50%. This
gives us the following:
I = s * I,,,
I = 1.1 * 25 kA
I = 27.5 kA.
I = 25 kA/I
+2 *
'
(g)'
I = 25 k A \ / l + 2 * (.as)'
I = 25 k A d G T f %
I = 26.52 kA.
From this, it is seen that an IEC test using an ac component
of 25 kA and the required percentage dc of 25% calls on
the circuit breaker to interrupt a total current of only 96.4%
of the current required by the ANSI test. To meet the ANSI
requirements for total asymmetrical current would require a
minimum dc component of 32.4%. Referring to Fig. 3, we
find that this value corresponds to a contact parting time of 50
ms, which is exactly the contact parting time assigned to our
breaker under the ANSI standards.
Summary of Short Circuit Current Considerations
Both ANSI and IEC standards define rated short circuit
current in terms of the rms symmetrical value of an ac current.
The definitions are the same, and the ratings are directly
comparable.
Both ANSI and IEC standards require asymmetrical
interrupting capability. The requirements differ. In general,
the ANSI standards require higher total current than the IEC
standards, at least partly because the ANSI standards require
assignment of an artificially short opening time to most circuit
breakers.
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200
The standard operating duty differs between the two standards. ANSIflEEE C37.04 defines the standard operating duty
as CO-15 s-CO. This is one of two standards listed in IEC
56, but breakers tested to IEC standards more commonly use
the other cycle 0-3 min-CO-3
min-CO.
The exact requirements of individual test duties differ
between the two standards in such areas as the percentage
asymmetry for a particular test current level, transient recovery
voltage, etc.
Because of these factors, it is extremely difficult to make a
direct comparison of the severity of the tests required by the
two sets of standards.
SHORTTIME CURRENT
Section 5.10.2.5 of ANSUIEEE C37.04 defines the shorttime current as any short-circuit current whose maximum crest
value does not exceed 2.7K times the rated short circuit
current and whose rms value over the required test period
does not exceed K times rated short-circuit current. The circuit
breaker is required to carry this current for a period of 3 s.
Section 4.5 of IEC 694 defines the rated short-time withstand current as the rms value of the current a mechanical
switching device can carry in the closed position during a
specified short time. Section 4.6 requires this current to have a
peak value of 2.5 times the rated short-time withstand current.
Section 4.7 gives the standard rated duration of the short-time
current as 1 s but also says that if a time greater than 1 s
is necessary, a value of 3 s is recommended. Section 4.5 of
IEC 56 requires the rated short-time withstand current to be
equal to the rated short-circuit breaking current, which is the
asymmetrical short-circuit current. This would seem to imply
that the current over the time period should have an integrated
value equal to the asymmetrical value of the breaking current,
but a careful reading of Section 6.5.2 of IEC 694, which
specifies the test current, shows that the required current is
the same as that required by the ANSI standard.
Ignoring the K factor, the two differences between the
standards are the difference in the peak current required and
the duration of the current flow. The peak current is required
by ANSI to be 2.7 times the rated short-circuit current, but IEC
only requires 2.5 times this current. The duration of current
flow is specified as 3 s by ANSI but only 1 s by IEC.
CLOSINGCURRENT
Section 5.10.2.4 of ANSI/IEEE C37.04 requires that a
circuit breaker close and, immediately thereafter, latch against
a current whose peak value does not exceed 2.7K times the
rated short-circuit current. It must then carry a current equal
to K times the rated short-circuit current for a time delay of 2
s and then interrupt this current. This capability is proved by
Test Duty 11 of Table 1 of ANSUIEEE C37.09.
As a matter of interest, it should be noted that the value
of this closing and latching current, as given in Tables 1-3
of ANSI C37.06-1987 [2], is stated in peak kiloamps and is
equal to 2.7K times the rated short-circuit current. In earlier
editions of ANSI C37.06, this value was stated in rms terms
and was equal to 1.6K times the rated short-circuit current.
The two values are equivalent, and the actual performance of
the circuit breaker was not required to change.
Section 4.103 of IEC 56 defines the rated short-circuitmaking current as the peak current of the first major loop
of the short-circuit current and requires it to be 2.5 times
the rms value of the ac component of the rated short-circuit
breaking current. The ability to close against this current is
tested by Test Duty No. 4, as described in Section 6.106.4 of
IEC 56. This test duty also includes full short-circuit current
interruption. There is no requirement similar to the ANSI
requirement for latching and carrying a current for 2 s before
interrupting.
CONCLUSIONS
Rating and testing of high-voltage circuit breakers are
very complex procedures, and this paper merely scratches
the surface by looking at some of the basic current ratings
and the tests required to demonstrate these ratings. Similar
analysis could be done on voltage ratings, specialized current
ratings such as capacitor switching, and other required ratings
of circuit breakers.
In the areas examined, we find that similar terms do not
necessarily mean the same thing in ANSI and IEC standards.
In many cases, it is difficult to relate the two standards to
each other because the differences are not easily compared
or quantified. As a general statement, however, it seems that
the ANSI standards require more stringent performance of a
circuit breaker for equivalent ratings. Some examples include
the following:
1. Continuous Current: Several of the allowable temperature rise limits in the ANSI standard are lower by 5 or
10" C than the comparable limits in the IEC standard. If
the circuit breaker is applied in an enclosure, its enclosed
rating should be verified. In addition, if the continuous
current test is run at 50 Hz rather than 60 Hz, the
resulting temperature rise may be lower.
2. Short Circuit Current: For asymmetrical interrupting
tests, the degree of asymmetry and the total short-circuit
breaking current may be lower under the IEC standards
than under the ANSI standards. In addition, it appears
that the ANSI standard operating duty of CO-15 s-CO
is more severe than the commonly-used IEC standard
min-CO.
operating duty of 0-3 min-CO-3
3. Short Time Current: The ANSI requirement of a test
duration of 3 s is much more severe than the standard
IEC test duration of 1 s, and the ANSI requirement for a
peak current of 2.7 times the rated short-circuit current is
more severe than the IEC requirement for a peak current
of 2.5 times the rated short-circuit current.
4. Closing Current: The ANSI requirement for latching
against the rated closing current, carrying rated shortcircuit current for 2 s, and then interrupting that current
is not present in the IEC standards. In addition, the same
difference exists in the ratio of peak current to rated
short-circuit current as in the short-time current rating.
Any engineer who is responsible for applying a high-voltage
circuit breaker rated and tested in accordance with unfamiliar
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201
BRIDGER. ALL AMPERES ARE NOT CREATED EQUAL
standards should study those standards to understand exactly
what the ratings mean in terms of actual performance of the
circuit breaker. Since most breaker applications are not made
at the limit of the breaker’s ratings, it should be possible to
find a circuit breaker that will fit the application regardless
of the set of standards with which it complies. A little care
exercised at the start of a project will pay dividends at the end.
REFERENCES
111 IEEE Standard Rating Structure for ac High-Voltage Circuit Breakers
Rated on a Symmetrical Current Basis, ANSI/IEEE Std. C37.04- 1979,
IEEE Product no. SH06288.
[2] American National Standard for Switchgear-uc High-Voltage Circuit
Breakers Rated on a Symmetrical Current Basis-Preferred Ratings and
Related Required Capabilities, ANSI Std. C37.06-1987, Amer. Nat.
Stds. Inst. Inc, New York.
[3] IEEE Standard Test Procedure for ac High-Voltage Circuit Breakers
Rated on a Symmetrical Current Basis, ANSIflEEE Std. C37.09-1979,
IEEE Product No. SH06312.
High-Voltage Alternating-Current Circuit-Breakers, IEC Std. 56, Int.
Electrotech. Comm., Geneva, 4th ed., 1987.
Common Clauses for High-Voltage Switchgear and Control Gear Standards, IEC Std. 694, Int. Electrotech. Comm., Geneva, 1st ed., 1980.
IEEE Application Guide for AC High-Voltage Circuit Breakers Rated
on a Symmetrical Current Basis, ANSVIEEE Std. C37.010-1979, IEEE
Product No. SH06569.
IEEE Application Guide for Transient Recovery Voltage for AC HighVoltage Circuit Breakers Rated on a Symmetrical Current Basis,
ANSVIEEE Std. C37.011-1979, IEEE Product NO. SH07005.
IEEE Application Guide for Capacitance Current Switching for AC
High- Voltage Circuit Breakers Rated on a Symmetrical Current Busis,
ANSVIEEE Std. C37.012-1979, IEEE Product No. SH06957.
Baldwin Bridger, Jr. (M’50-SM’59-F’88), For photgraph and biography
please see the Officers in the Industry Applications Society section of this
issue on page 2.
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