TechTopics No. 23

Circuit breaker ratings - type GMI circuit breakers

TechTopics No. 4 discussed the changes in the ratings structure for medium-voltage circuit breakers used in metal-clad switchgear. This issue of TechTopics includes more detailed versions of the ratings tables for type GMI circuit breakers, both for the “constant MVA” and the “constant kA” ratings.

 

The structure of ratings for these circuit breakers is defined in the following standards:

Table 1: Standards
Standard
"Constant MVA" ratings
"Constant kA" ratings
Title
ANSI/IEEE C37.04
1979
1999
Rating structure for AC high-voltage circuit breakers
ANSI C37.06

1979

1987

1997

2000

AC high-voltage circuit breakers rated on a symmetrical current basis–preferred ratings and related required capabilities
ANSI/IEEE C37.09
1979
1999
Test procedure for AC high-voltage circuit breakers rated on a symmetrical current basis
ANSI/IEEE C37.010
1979
1999
Application guide for AC high-voltage circuit breakers rated on a symmetrical current basis

The 1999-2000 revisions comprise the first major structural change to the circuit breaker rating standards since the change from the total (asymmetrical) current basis of rating to the symmetrical current basis of rating in 1964. The 1964 rating structure reflects a “constant MVA” ratings basis over a range of operating voltages, and reflects the air-magnetic interruption technology then common. At the maximum design voltage, interrupting capacity is limited by the ability of the circuit breaker to withstand the transient recovery voltage across the circuit breaker contacts following interruption. As the operating voltage is reduced, the interrupting capability increases, as the air-magnetic arc chutes can more readily handle the reduced transient recovery voltage. 

 

Finally, as voltage is decreased further, a limit is approached at which the contacts cannot absorb further increases in heat during interruption. The maximum design voltage was designated as “V,” and the range (the “constant MVA” region) over which the interrupting current capability increases as voltage decreases is defined in terms of voltage range factor “K.” The voltage V/K defines the associated lower limit of voltage. 

 

In the range of V/K to V, the interrupting current varies so the product of voltage and interrupting current is a constant. Thus, interrupting MVA is constant over the range from V/K to V.

 

The “constant MVA” rating structure does not conform to the physics of modern interrupting techniques. For today’s vacuum interrupters, the interrupting capability of the vacuum interrupter does not increase significantly as the operating voltage is decreased from rated maximum design voltage. The 1999-2000 revisions to the standards recognized this by changing the voltage range factor (K) to equal 1.0, which effectively removes it from the rating structure.

 

Because there is a huge installed base of circuit breakers that are rated to the old standards, we expect that new circuit breakers and switchgear will be available with the old “constant MVA” ratings for many years. “Constant MVA” circuit breakers must be designed, rated, and tested to the old standards, as the new standards do not define the full rating structure or test requirements for the “constant MVA” circuit breakers. 

 

Gradually, however, the new “constant kA” circuit breakers and switchgear will become more widely used. The use of the “constant kA” ratings simplifies the application of circuit breakers and switchgear, and also more accurately represents the true physics of modern vacuum interrupters.

 

Type GMI circuit breaker ratings (historic “constant MVA” rating basis)

These ratings are in accordance with the following standards: 

  • ANSI/IEEE C37.04-1999 standard rating structure for AC high-voltage circuit breakers rated on a symmetrical current basis
  • ANSI C37.06-1987 AC high-voltage circuit breakers rated on a symmetrical current basis – preferred ratings and related required capabilities
  • ANSI/IEEE C37.09-1979 standard test procedure for AC high-voltage circuit breakers rated on a symmetrical current basis
  • ANSI/IEEE C37.010-1999 application guide for AC high-voltage circuit breakers rated on a symmetrical current basis.

 

 

Table 2: Type GMI circuit breaker ratings (historic “constant MVA” rating basis)
 
Measured parameter
Units
Circuit breaker type 5-GMI-250
Circuit breaker type 5-GMI-350
Circuit breaker type 7-GMI-500
Circuit breaker type 15-GMI-500
Circuit breaker type 15-GMI-750
Circuit breaker type 15-GMI-1000
General
Nominal voltage class
kV
4.16
4.16
7.2
13.8
13.8
13.8
General
Nominal three-phase MVA class9
MVA
250
350
500
500
750
1,000
Rated values
Rated voltage
Maximum design voltage (V)2
kV rms
4.76
4.76
8.25
15.0
1.50
1.50
Rated values
Rated voltage
Voltage range factor (K)3
----
1.24
1.19
1.25
1.30
1.30
1.30
Rated values
Insulation levels
Withstand voltage levels
Power frequency
kV rms
19
19
36
36
36
36
Rated values
Insulation levels
Withstand voltage levels
Lightning impulse (BIL)
kV crest
60
60
95
95
95
85
Rated values
Rated current
Continuous4
A rms

1,200

2,000

1,200

2,000

3,000FC4

4,000FC4

1,200

2,000

3,000FC4

4,000FC4

1,200

2,000

1,200

2,000

3,000FC4

4,000FC4

1,200

2,000

3,000FC4

4,000FC4

Rated values
Rated current
Short-circuit (at rated maximum design voltage) (I)5, 6, 10
kA rms sym
29
41
33
18
28
37
Rated values
Rated current
Interrupting time
Cycles
5
5
5
5
5
5
Rated values
Rated current
Permissible tripping delay (Y)
Sec
2
2
2
2
2
2
Related required capabilities
Current
Rated maximum design voltage (V) divided by K (=V/K) 
kV rms
3.85
4.0
6.6
11.5
11.5
11.5
Related required capabilities
Current
Maximum sym. interrupting (K x I)7
kA rms sym
36
49
41
23
36
48
Related required capabilities
Current

Short-time current (K x I)

(three seconds)

kA rms
36
49
41
23
36
48
Related required capabilities
Closing and latching (momentary)
Asymmetrical (1.6 x K x I)8 
kA rms

58

78 opt.1

78

66

77 opt.1

37

58 opt.1

58

77 opt.1

77
Related required capabilities
Closing and latching (momentary)
Peak (2.7 x K x I)8
kA peak

97

132 opt.1

132

111

130 opt.1

62

97 opt.1

97

130 opt.1

130

Footnotes:

  1. High closing and latching (momentary) rating available for special application.
  2. Maximum voltage for which the circuit breaker is designed and the upper limit for operation.
  3. K is the ratio of the rated maximum design voltage to the lower limit of the range of operating voltage in which the required symmetrical and asymmetrical interrupting capabilities vary in inverse proportion to the operating voltage.
  4. 4,000FC indicates that fan cooling is included in the switchgear structure for this rating. The circuit breaker for the 3,000 A rating may be located in the upper cell or in the lower cell of a vertical section. The circuit breaker for the 4,000 A rating must be located in the upper cell of the vertical section. 4,000 A rating is not available in outdoor equipment. Some models use fan cooling for 3,000 A.
  5. To obtain the required symmetrical interrupting capability of a circuit breaker at an operating voltage between 1/K times rated maximum design voltage and rated maximum design voltage, the following formula shall be used: Required symmetrical interrupting capability = rated short-circuit current (I) x [(rated maximum design voltage)/(operating voltage)] For operating voltages below 1/K times rated maximum design voltage, the required symmetrical interrupting capability of the circuit breaker shall be equal to K times rated short-circuit current.
  6. Within the limitations stated in ANSI/IEEE C37.04-1979, all values apply to polyphase and line-to-line faults. For single-phase-to-ground faults, the specific conditions stated in clause 5.10.2.3 of ANSI/IEEE C37.04-1979 apply.
  7. Current values in this row are not to be exceeded even for operating voltage below 1/K times rated maximum design voltage. For operating voltages between rated maximum design voltage and 1/K times rated maximum design voltage, follow footnote 5 above.
  8. Current values in this row are independent of operating voltage up to and including rated maximum design voltage.
  9. “Nominal 3-Phase MVA Class” is included for reference only – this information is not listed in ANSI C37.06-1987.
  10. Standard duty cycle is CO – 15 s – CO.

Type GMI circuit breaker ratings (new “constant kA” rating basis)

These ratings are in accordance with the following standards: 

  • ANSI/IEEE C37.04-1999 standard rating structure for AC high-voltage circuit breakers
  • ANSI/IEEE C37.06-2009 AC high-voltage circuit breakers rated on a symmetrical current basis – preferred ratings and related required capabilities
  • ANSI/IEEE C37.09-1999 standard test procedure for AC high-voltage circuit breakers rated on a symmetrical current basis
  • ANSI/IEEE C37.010-1999 application guide for AC high-voltage circuit breakers rated on a symmetrical current basis.
Table 3: Type GMI circuit breaker ratings (new “constant kA” rating basis)
 
Units
Circuit breaker type 5-GMI-31
Circuit breaker type 5-GMI-40
Circuit breaker type 5-GMI-50
Circuit breaker type 7-GMI-40
Circuit breaker type 15-GMI-20
Circuit breaker type 15-GMI-25
Circuit breaker type 15-GMI-31
Circuit breaker type 15-GMI-40
Circuit breaker type 15-GMI-50
Maximum design voltage1
kV
4.76
4.76
4.76
8.25
15.0
15.0
15.0
15.0
15.0
Voltage range factor (K)2
----
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
Withstand voltage levels power frequency
kV rms
19
19
19
36
36
36
36
36
36
Withstand voltage levels lightning impulse (BIL)
kV crest
60
60
60
95
95
95
95
85
95
Continuous3
A rms

1,200

2,000

1,200

2,000

3,000FC3

4,000FC3

1,200

2,000

3,000FC3

4,000FC3

1,200

2,000

3,000FC3

4,000FC3

1,200

2,000

1,200

2,000

1,200

2,000

1,200

2,000

3,000FC3

4,000FC3

1,200

2,000

3,000FC3

4,000FC3

Short-circuit (at rated maximum design voltage) (I)4,5
kA rms sym
31.5
40
50
40
20
25
31.5
40
50
Interrupting time
ms cycles
83/5
83/5
83/5
83/5
83/5
83/5
83/5
83/5
83/5
Permissible tripping delay (Y)
Sec
2
2
2
2
2
2
2
2
2
Maximum sym. interrupting (K x I)
kA rms sym
31.5
40
50
40
20
25
31.5
40
50
% dc component
%
47
47
47
47
47
47
47
47
47

Short-time current (I)

(three seconds)

kA rms
31.5
40
50
40
20
25
31.5
40
50
Closing and latching (momentary) asymmetrical (1.55 x I)
kA rms

49

62
78
62
31
39
49
62
78
Closing and latching (momentary) peak (2.6 x I)
kA peak

82

104
130
104
52
65
82
104
130

Footnotes:

  1. Maximum voltage for which the circuit breaker is designed and the upper limit for operation.
  2. K is listed for informational purposes only. For circuit breakers rated on a “kA basis,” the voltage range factor is 1.0.
  3. 4,000FC indicates that fan cooling is included in the switchgear structure for this rating. The circuit breaker for the 3,000 A rating may be located in the upper cell or in the lower cell of a vertical section. The circuit breaker for the 4,000 A rating must be located in the upper cell of the vertical section. 4,000 A rating is not available in outdoor equipment. Some models use fan cooling for 3,000 A.
  4. All values apply to polyphase and line-to-line faults.
  5. Standard duty cycle is O – 0.3 s – CO – 15 s – CO.

Should you have any questions about this issue of TechTopics or any of our products, solutions, or services, please contact your local Siemens sales representative for more information.