AP01301001E (1)

Effective September 2009
Application Paper AP01301001E
Determining power circuit breaker
interrupting ratings
Overview
All of Eaton’s Magnum姠 DS line of power circuit
breakers are 100% rated and tested to applicable
industry standards, including ANSI C37.13,
C37.16, C37.17, C37.50; NEMA姞 SG3; UL姞 1066;
and CSA姞 22.2.
When correctly applying a circuit breaker to a
system that has different %pf and X/R values from
those used in the circuit to test the breaker, one
should take into account the system’s X/R ratio
and rms symmetrical current to arrive at an adjust-
ed rms symmetrical current that will be compared
to the breaker’s tested interrupting ratings, among
other considerations.
Breaker tested conditions
There has been a lot of confusion in the industry
as to how to properly select the interrupting rating
of a breaker with so many different current values
to consider. For the purposes of selecting the
interrupting rating of a circuit breaker, one
should only concern oneself with the rms
symmetrical current rating of the system, along
with the system %pf or X/R ratio and the circuit
breaker interrupting rating. Other current values,
such as rms asymmetrical current and peak
asymmetrical currents, are dependent on the
%pf or X/R ratio and can thus be removed from
consideration. Figure 1 shows the relationship
between the various current values.
The Magnum DS product line contains circuit
breakers rated at varying levels of interrupting
rating, from 42 kAIC to 100 kAIC at 600V and up to
200 kAIC at 480V. The interrupting ratings shown
are rms symmetrical current values tested with a
%pf of 15, which corresponds to an X/R ratio of 6.6.
Examples of the relationship between these
different current values are shown in Tables 1
and 2. These tables convert the tested X/R ratio
for Eaton’s Magnum DS breakers with their
varying rms symmetrical interrupting ratings
and their equivalent rms and peak asymmetrical
interrupting ratings. Calculations were
based on a 60 Hz system and t=1/2 cycle
(ANSI/IEEE姞 C37.13.1990/10.14).
Table 1. Magnum DS Asymmetrical
Multiplication Factor
%pf X/R rms Multiplication Peak Multiplication
15 6.59 1.3308 2.2923
Table 2. Magnum DS Asymmetrical Currents
Circuit
Breaker rms Symmetrical rms Asymmetrical Peak Asymmetrical
MDS4 42,000 55,894 96,277
MDS5 50,000 66,540 114,615
MDS6 65,000 86,502 149,000
MDS8 85,000 113,118 194,846
MDSC 100,000 133,080 229,230
Figure 1. Structure of an Asymmetrical Current Wave
Total Current—a Wholly Offset Asymmetrical Alternating Wave
Scale of Current Values
rms Value of Total Current
Direct Component—the Axis
of Symmetrical Wave
Time in Cycles
of a 60 Hz Wave
Alternating Component Symmetrical Wave
rms Value of Alternating Component
3.0
2.5
2.0
1.5
1.0
0.5
–0.5
–1.0
–1.5
–2.0
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Eaton Corporation
Electrical Sector
1111 Superior Ave.
Cleveland, OH 44114
United States
877-ETN-CARE (877-386-2273)
Eaton.com
© 2009 Eaton Corporation
All Rights Reserved
Printed in USA
Publication No. AP01301001E / Z9004
September 2009
PowerChain Management is a registered
trademark of Eaton Corporation.
All other trademarks are property of their
respective owners.
Application Paper AP01301001E
Effective September 2009
Determining power circuit breaker
interrupting ratings
Figure 2 shows the relationship between the X/R ratio and the
multiplication factors used to calculate the circuit breaker rms
and peak asymmetrical currents. Their interdependency allows us
to narrow our consideration to the %pf or X/R ratio and the system
rms symmetrical currents in determining circuit breaker
interrupting ratings.
Selecting breakers with different %pf
To properly size a circuit breaker for a system that has a %pf greater
than or equal to 15 (X/R ratio of less than 6.59), one should simply
ensure that the rms symmetrical interrupting rating of the breaker
is sufficiently large enough to cover the rms symmetrical current
of the system.
In systems where the %pf is less than 15 (X/R ratio greater than 6.6),
an adjusted rms symmetrical current must be determined by applying
a multiplying factor (MF) to the system’s calculated rms symmetrical
current. This adjusted rms symmetrical current must then be
compared to the breaker’s tested rms symmetrical currents. The
breaker selected must have an rms symmetrical interrupting rating
that is larger than the adjusted rms symmetrical current. Table 3
shows the adjusted system rms symmetrical currents at various
%pf and X/R ratios at varying system rms symmetrical currents.
For example, consider a system that has an rms symmetrical
current of 60 kA, with %pf of 7.1 and an X/R ratio of 14. We can
determine from Figure 2 or mathematical calculations that this
system has an rms asymmetrical current of 90.5 kA and a peak
asymmetrical current of 153 kA; however, these figures are not used
in selecting the circuit breaker interrupting rating. Because the %pf
is less than 15 (X/R ratio is greater than 6.6), the adjusted system
rms symmetrical current must be determined. Using Table 3, one
can apply the multiplication factor of 1.111 to the system’s rms
symmetrical current to arrive at an adjusted rms symmetrical current
of 66.6 kA. A circuit breaker that has an interrupting rating greater
than this value must be selected, such as the 85 kAIC Magnum DS
breaker (MDS8).
For further information, please consult Eaton’s Consulting
Application Guide or visit www.eaton.com/consultants.
Figure 2. Relation of X/R Ratio and Asymmetrical Multiplication Factor
Based Upon: rms Asym – DC2 + rms Sym2 with DC Value Taken at Current Peak
Circuit X/R Ratio (Tan Phase)
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2.0
1.9
1.8
1.7
1.6
1.5
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.41 1.5 2 3 5 7 9 15 25 40 60 80 1002.5 6 8 10 20 30 50 70 904
rms Multiplication Factor =
rms Maximum
Asymmetrical
rms Symmetrical
Peak Multiplication Factor =
Peak Maximum
Asymmetrical
rms Symmetrical
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Table 3. Adjusted rms Symmetrical Currents
%pf
4.0 5.0 5.5 6.2 7.1 8.3 10.0 12.4 15.0 20.0
X/R 25.0 20.0 18.0 16.0 14.0 12.0 10.0 8.0 6.6 4.9
Multiplying factor (MF) 1.162 1.145 1.136 1.125 1.111 1.093 1.069 1.035 1.000 1.000
System rms
symmetrical
current
40,000 46,488 45,814 45,449 45,001 44,440 43,716 42,746 41,383 40,000 40,000
50,000 58,110 57,268 56,811 56,252 55,550 54,644 53,432 51,729 50,000 50,000
60,000 69,732 68,722 68,174 67,502 66,660 65,573 64,119 62,074 60,000 60,000
80,000 92,976 91,629 90,898 90,003 88,880 87,431 85,491 82,766 80,000 80,000
100,000 116,220 114,536 113,623 112,504 111,100 109,289 106,864 103,457 100,000 100,000