Max Zs Tables

Maximum earth fault loop impedance values for protective devices

BS 7671:2018+A2:2022 Tables 41.2, 41.3, 41.4

0.4s Disconnection

Final circuits (sockets, lighting)

5s Disconnection

Distribution circuits

80% Rule

Testing limit shown in blue

MCB Type B (BS EN 60898)

Rating (A)	0.4s Max Zs (Ω)	0.4s Test 80% (Ω)	5s Max Zs (Ω)	5s Test 80% (Ω)
6	7.28	5.82	7.28	5.82
10	4.37	3.50	4.37	3.50
16	2.73	2.18	2.73	2.18
20	2.19	1.75	2.19	1.75
25	1.75	1.40	1.75	1.40
32	1.37	1.10	1.37	1.10
40	1.09	0.87	1.09	0.87
50	0.87	0.70	0.87	0.70
63	0.69	0.55	0.69	0.55
80	0.55	0.44	0.55	0.44
100	0.44	0.35	0.44	0.35
125	0.35	0.28	0.35	0.28

MCB Type C (BS EN 60898)

Rating (A)	0.4s Max Zs (Ω)	0.4s Test 80% (Ω)	5s Max Zs (Ω)	5s Test 80% (Ω)
6	3.64	2.91	7.28	5.82
10	2.19	1.75	4.37	3.50
16	1.37	1.10	2.73	2.18
20	1.09	0.87	2.19	1.75
25	0.87	0.70	1.75	1.40
32	0.68	0.54	1.37	1.10
40	0.55	0.44	1.09	0.87
50	0.44	0.35	0.87	0.70
63	0.35	0.28	0.69	0.55
80	0.27	0.22	0.55	0.44
100	0.22	0.18	0.44	0.35
125	0.17	0.14	0.35	0.28

MCB Type D (BS EN 60898)

Rating (A)	0.4s Max Zs (Ω)	0.4s Test 80% (Ω)	5s Max Zs (Ω)	5s Test 80% (Ω)
6	1.82	1.46	3.64	2.91
10	1.09	0.87	2.19	1.75
16	0.68	0.54	1.37	1.10
20	0.55	0.44	1.09	0.87
25	0.44	0.35	0.87	0.70
32	0.34	0.27	0.68	0.54
40	0.27	0.22	0.55	0.44
50	0.22	0.18	0.44	0.35
63	0.17	0.14	0.35	0.28
80	0.14	0.11	0.27	0.22
100	0.11	0.09	0.22	0.18
125	0.09	0.07	0.17	0.14

BS 88 Fuses (gG)

Rating (A)	0.4s Max Zs (Ω)	0.4s Test 80% (Ω)	5s Max Zs (Ω)	5s Test 80% (Ω)
16	2.42	1.94	4.80	3.84
20	1.84	1.47	3.70	2.96
25	1.61	1.29	3.20	2.56
32	1.04	0.83	2.10	1.68
40	0.87	0.70	1.70	1.36
50	0.68	0.54	1.40	1.12
63	0.54	0.43	1.10	0.88
80	0.42	0.34	0.84	0.67
100	0.34	0.27	0.67	0.54
125	0.27	0.22	0.54	0.43
160	0.21	0.17	0.42	0.34
200	0.17	0.14	0.34	0.27

BS 1361 Fuses

Rating (A)	0.4s Max Zs (Ω)	0.4s Test 80% (Ω)	5s Max Zs (Ω)	5s Test 80% (Ω)
5	10.90	8.72	21.80	17.44
15	3.12	2.50	6.20	4.96
20	2.42	1.94	4.80	3.84
30	1.61	1.29	3.20	2.56
45	1.04	0.83	2.10	1.68

BS 3036 Semi-enclosed Fuses

Rating (A)	0.4s Max Zs (Ω)	0.4s Test 80% (Ω)	5s Max Zs (Ω)	5s Test 80% (Ω)
5	9.58	7.66	18.40	14.72
15	2.76	2.21	5.22	4.18
20	2.14	1.71	4.14	3.31
30	1.26	1.01	2.64	2.11
45	0.83	0.66	1.66	1.33

Important Notes

• 80% Rule: When testing, measured Zs should be ≤80% of tabulated value to account for conductor resistance at operating temperature.
• 0.4s: Required for socket outlets and circuits supplying portable equipment (Reg. 411.3.2.2).
• 5s: Permitted for distribution circuits and fixed equipment in certain conditions (Reg. 411.3.2.3).
• Values assume Cmin = 0.95 and nominal voltage U₀ = 230V.
• For TT systems, RCD protection is typically required regardless of Zs values.