Miniature Circuit Breakers Zs Values
In
Max Let-Through Energy (kA2s) at PSCC
Max Zs (ohms)
3kA
6kA
10kA
0.2 - 1s sec
5 sec
Commercial Distribution
Earth Fault Loop impedance (Zs) values for MCBs and MCCBs Below are the maximum permissible values of Zs to obtain disconnection for compliance with BS 7671:2008 Amendment 3
MTN/NBN (B Curve) 6
5.9
10.5
15
7.28
7.28
10
6.5
12.2
21.5
4.37
4.37
16
8.0
17.5
30
2.73
2.73
20
8.8
19.5
34
2.19
2.19
25
10
21
38
1.75
1.75
32
11
24
42
1.37
1.37
40
12.5
29
50
1.09
1.09
50
15
34
61
0.87
0.87
63
16
38
72
0.69
0.69
NCN/HM (C Curve) 0.5
0.01
0.01
0.01
43.7
62.43
1
4.0
7.0
10
21.85
31.21
2
4.0
7.0
10
10.93
15.61
3
5.0
10.0
15
7.28
10.40
4
5.9
10.5
15
5.46
7.80
6
5.9
10.5
15
3.64
5.20
10
6.5
12.2
21.5
2.19
3.12
16
8.0
17.5
30
1.37
1.95
20
8.8
19.5
34
1.09
1.56
25
10
21
38
0.87
1.25
32
11
24
42
0.68
0.98
40
12.5
29
50
0.55
0.78
50
15
34
61
0.44
0.62
63
16
38
72
0.35
0.50
80
-
-
-
1.27
0.39
100
-
-
-
0.22
0.31
125
-
-
-
0.1
0.25
NDN (D Curve) 0.5
0.01
0.01
0.01
21.85
62.43
1
4.0
7.0
10
10.93
31.21
2
4.0
7.0
10
5.46
15.61
3
5.0
10.0
15
3.64
10.40
4
5.9
10.5
15
2.73
7.80
6
5.9
10.5
15
1.82
5.20
10
6.5
12.2
21.5
1.09
3.12
16
8.0
17.5
30
0.68
1.95
20
8.8
19.5
34
0.55
1.56
25
10
21
38
0.44
1.25
32
11
24
42
0.34
0.98
40
12.5
29
50
0.27
0.78
50
15
34
61
0.22
0.62
63
16
38
72
0.17
0.50
80
0.14
0.39
100
0.11
0.31
125
0.09
0.25
Data is subject to errors and technical modifications.
1.84