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

Quick Lookup

Device Type

Rating

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	3.64	2.91
10	2.19	1.75	2.19	1.75
16	1.37	1.10	1.37	1.10
20	1.09	0.87	1.09	0.87
25	0.87	0.70	0.87	0.70
32	0.68	0.54	0.68	0.54
40	0.55	0.44	0.55	0.44
50	0.44	0.35	0.44	0.35
63	0.35	0.28	0.35	0.28
80	0.27	0.22	0.27	0.22
100	0.22	0.18	0.22	0.18
125	0.17	0.14	0.17	0.14

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	1.82	1.46
10	1.09	0.87	1.09	0.87
16	0.68	0.54	0.68	0.54
20	0.55	0.44	0.55	0.44
25	0.44	0.35	0.44	0.35
32	0.34	0.27	0.34	0.27
40	0.27	0.22	0.27	0.22
50	0.22	0.18	0.22	0.18
63	0.17	0.14	0.17	0.14
80	0.14	0.11	0.14	0.11
100	0.11	0.09	0.11	0.09
125	0.09	0.07	0.09	0.07

BS 88 Fuses (gG)

Rating (A)	0.4s Max Zs (Ω)	0.4s Test 80% (Ω)	5s Max Zs (Ω)	5s Test 80% (Ω)
6	8.09	6.47	12.84	10.27
10	4.85	3.88	7.04	5.63
16	2.57	2.06	3.97	3.18
20	1.67	1.34	2.77	2.22
25	1.37	1.10	2.19	1.75
32	0.99	0.79	1.75	1.40
40	0.77	0.62	1.28	1.02
50	0.57	0.46	0.99	0.79
63	0.43	0.34	0.77	0.62
80	0.29	0.23	0.55	0.44
100	0.21	0.17	0.39	0.31
125	0.17	0.14	0.31	0.25

BS 1361 Fuses

Rating (A)	0.4s Max Zs (Ω)	0.4s Test 80% (Ω)	5s Max Zs (Ω)	5s Test 80% (Ω)
5	9.92	7.94	15.57	12.46
15	3.12	2.50	4.75	3.80
20	1.62	1.30	2.66	2.13
30	1.09	0.87	1.75	1.40
45	0.55	0.44	0.90	0.72

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

• 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.