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MCB Zs Values
Max Let-Through Energy (kA
2
s)
at PSCC
Max Z
s
(ohms)
I n
3kA
6kA
10kA 0.2 - 1s
sec
5 sec
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
30o
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
Earth loop impedance (Z
s
) values for MCBs and MCCBs
Below are the maximum permissible values of Zs to obtain
disconnection for compliance with BS 7671:2008 Amendment 3
Protection
Devices