Ω

Zs Values

Maximum earth fault loop impedance values for MCBs and fuses from BS 7671 Tables 41.2–41.6.

MCB Type B (Table 41.2)

MCB Type B — Max Zs Values

Rating (A)	Max Zs 0.4s (Ω)	Max Zs 5s (Ω)
6	7.67	12.88
10	4.60	7.73
16	2.87	4.83
20	2.30	3.86
25	1.84	3.09
32	1.44	2.41
40	1.15	1.93
50	0.92	1.54
63	0.73	1.23

BS 7671: Table 41.2 Open Calculator →

MCB Type C (Table 41.3)

MCB Type C — Max Zs Values

Rating (A)	Max Zs 0.4s (Ω)	Max Zs 5s (Ω)
6	3.83	6.44
10	2.30	3.86
16	1.44	2.41
20	1.15	1.93
25	0.92	1.54
32	0.72	1.21
40	0.57	0.96
50	0.46	0.77
63	0.37	0.61

BS 7671: Table 41.3

MCB Type D (Table 41.4)

MCB Type D — Max Zs Values

Rating (A)	Max Zs 0.4s (Ω)	Max Zs 5s (Ω)
6	1.92	3.22
10	1.15	1.93
16	0.72	1.21
20	0.57	0.96
25	0.46	0.77
32	0.36	0.60
40	0.29	0.48
50	0.23	0.39
63	0.18	0.31

BS 7671: Table 41.4

BS 88 Fuses

BS 88 Fuses — Max Zs Values

Rating (A)	Max Zs 0.4s (Ω)	Max Zs 5s (Ω)
6	8.89	14.0
10	5.33	8.57
16	2.80	4.18
20	1.78	3.04
25	1.41	2.10
32	1.09	1.63
40	0.86	1.18
50	0.55	0.87
63	0.43	0.62
80	0.32	0.47
100	0.25	0.36

BS 7671: Table 41.6

BS 3036 Fuses

BS 3036 Fuses — Max Zs Values

Rating (A)	Max Zs 0.4s (Ω)	Max Zs 5s (Ω)
5	10.9	18.3
15	3.43	5.58
20	2.44	3.83
30	1.48	2.39
45	0.96	1.41
60	0.58	0.96
100	0.27	0.46

BS 7671: Table 41.5

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