Transformer Protection
Circuit Breakers


Circuit Breaker Selection Charts for Connection in the Primary Windings of Transformers

Due to the inductive windings of transformers a high inrush current is experienced upon 'switch-on'. Typically this can be 10 -15 x full load current of the transformer (In) and is virtually instantaneous.

To protect supply lines to the primary windings of a transformer the circuit breaker must provide thermal (long time) protection and magnetic (short time) protection without the device tripping when the transformer is switched on.
Circuit breaker characteristic curve MBH132  
Typical 1000VA transformer curve 1ph.  

Single Phase - 240V
Transformers Assumed Transformer Inrush Characteristics = 15 x In
Selection table for protection of primary transformer windings. For information on selection with lower/higher transformer VA, or lower/higher inrush characteristics please consult our Technical Services Department.

Transformer

Circuit Breaker Type

MCCB
F
Frame
(A)

HRC
Fuselink
STD
Fuse (A)

Transformer
(VA) 240V

Primary
(A) 240V

MCB
Type B
(A)

MCB
Type C
(A)

MCB
Type D
(A)

MCCB
G
Frame
(A)

100

0.42

6

-

-

-

-

2

250

1.04

10

6

-

-

-

6

500

2.08

16

10

6

-

-

10

1000

4.17

32

16

10

-

-

16

1500

6.25

40

20

10

16

16

16

2000

8.33

50

32

16

16

16

20

2500

10.42

63

32

16

20

20

20

3000

12.50

-

40

20

32

32

25

3500

14.58

-

50

32

32

32

25

4000

16.67

-

63

32

32

32

32

4500

18.75

-

63

32

40

40

32

5000

20.83

-

-

32

40

40

40

7500

31.25

-

-

-

63

63

50

10000

41.67

-

-

-

80

80

63

12500

52.08

-

-

-

100

100

80

15000

62.50

-

-

-

125

125

100

20000

83.33

-

-

-

-

160

125
 

Three Phase - 415V
Transformers Assumed Transformer Inrush Characteristics = 15 x In

Transformer

Circuit Breaker Type

MCCB
F
Frame
(A)

HRC
Fuselink
STD
Fuse (A)

Transformer
(VA) 415V

Primary
(A) 415V

MCB
Type B
(A)

MCB
Type C
(A)

MCB
Type D
(A)

MCCB
G
Frame
(A)

500

0.69

6

6

-

-

-

6

1000

1.39

10

6

6

-

-

10

1500

2.08

16

10

6

-

-

10

2000

2.78

16

10

6

-

-

10

2500

3.47

16

16

6

-

-

16

3000

4.17

20

16

10

-

-

16

3500

4.86

32

16

10

-

-

16

4000

5.56

32

20

10

16

16

16

4500

6.25

32

20

10

16

16

16

5000

6.94

40

32

16

16

16

20

7500

10.42

63

32

16

20

20

20

10000

13.89

-

50

25

32

32

32

12500

17.36

-

63

32

40

40

40

15000

20.83

-

63

32

40

40

40

20000

27.78

-

-

-

63

63

50

25000

34.72

-

-

-

80

80

63

30000

41.67

-

-

-

80

80

63

35000

48.61

-

-

-

100

100

80

40000

55.56

-

-

-

125

125

100

45000

62.50

-

-

-

125

125

100

50000

69.44

-

-

-

-

160

125

55000

76.39

-

-

-

-

160

125

60000

83.33

-

-

-

-

160

125

65000

90.28

-

-

-

-

200

160

70000

97.22

-

-

-

-

200

160

75000

104.17

-

-

-

-

200

160
N.B. All MCCB thermal and magnetic adjustments are assumed to be set at maximum where applicable. For fuselinks, some degree of overloading allowed. For specific examples contact our Technical Services Department.


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