Max Zs table BS 7671:2018+A4:2026 — MCB, RCBO & fuse values + 80% rule

Look up max Zs

Pick the protective device and rating. Values are for U₀ 230 V, Cmin 0.95 (TN systems).

Protective device

Rating In (A)

Disconnection time

Max Zs (BS 7671 table)—

80% value — compare your measured reading—

—

Your measured Zs (Ω)

Check against BS 7671 and the manufacturer's data; this is a guide, not a design tool.

Max Zs tables — BS 7671:2018+A4:2026

Tabulated values from Tables 41.2, 41.3 and 41.4 (U₀ 230 V, Cmin 0.95), with the 80% figure for checking a reading taken at normal room temperature. Amendment 4 did not change these tables.

MCBs (BS EN 60898) and RCBOs (BS EN 61009-1) — maximum Zs (Ω), Tables 41.3 / 41.4
In (A)	Type B	Type C	Type D 0.4 s	Type D 5 s
3	14.57	—	—	—
6	7.28	3.64	1.82	3.64
10	4.37	2.19	1.09	2.19
16	2.73	1.37	0.68	1.37
20	2.19	1.09	0.55	1.09
25	1.75	0.87	0.44	0.87
32	1.37	0.68	0.34	0.68
40	1.09	0.55	0.27	0.55
45	0.97	—	—	—
50	0.87	0.44	0.22	0.44
63	0.69	0.35	0.17	0.35
80	0.55	0.27	0.14	0.27
100	0.44	0.22	0.11	0.22
125	0.35	0.17	0.09	0.17

MCBs and RCBOs — 80% values for measured Zs at ambient temperature (Ω)
In (A)	Type B	Type C	Type D 0.4 s	Type D 5 s
3	11.66	—	—	—
6	5.82	2.91	1.46	2.91
10	3.50	1.75	0.87	1.75
16	2.18	1.10	0.54	1.10
20	1.75	0.87	0.44	0.87
25	1.40	0.70	0.35	0.70
32	1.10	0.54	0.27	0.54
40	0.87	0.44	0.22	0.44
45	0.78	—	—	—
50	0.70	0.35	0.18	0.35
63	0.55	0.28	0.14	0.28
80	0.44	0.22	0.11	0.22
100	0.35	0.18	0.09	0.18
125	0.28	0.14	0.07	0.14

Type B and C: the same figure applies for 0.4 s and 5 s. The 3 A and 45 A ratings are Type B only.

BS 88-3 cartridge fuse (system C, domestic) — maximum Zs (Ω)
In (A)	0.4 s	80%	5 s	80%
5	9.93	7.94	14.6	11.68
16	2.30	1.84	3.90	3.12
20	1.93	1.54	3.20	2.56
32	0.91	0.73	1.60	1.28
45	0.57	0.46	1.00	0.80
63	0.36	0.29	0.68	0.54
80	—	—	0.51	0.41
100	—	—	0.38	0.30

BS 88-2 gG fuse (systems E and G) — maximum Zs (Ω)
In (A)	0.4 s	80%	5 s	80%
2	33.1	26.48	44.0	35.20
4	15.6	12.48	21.0	16.80
6	7.80	6.24	12.0	9.60
10	4.65	3.72	6.80	5.44
16	2.43	1.94	4.00	3.20
20	1.68	1.34	2.80	2.24
25	1.29	1.03	2.20	1.76
32	0.99	0.79	1.70	1.36
40	0.75	0.60	1.30	1.04
50	0.57	0.46	0.99	0.79
63	0.44	0.35	0.78	0.62
80	—	—	0.55	0.44
100	—	—	0.42	0.34
125	—	—	0.32	0.26
160	—	—	0.27	0.22
200	—	—	0.18	0.14

BS 3036 semi-enclosed (rewireable) fuse — maximum Zs (Ω)
In (A)	0.4 s	80%	5 s	80%
5	9.10	7.28	16.8	13.44
15	2.43	1.94	5.08	4.06
20	1.68	1.34	3.64	2.91
30	1.04	0.83	2.51	2.01
45	0.56	0.45	1.51	1.21
60	0.40	0.32	1.07	0.86
100	—	—	0.51	0.41

BS 1362 plug-top / fused spur fuse — maximum Zs (Ω)
In (A)	0.4 s	80%	5 s	80%
3	15.6	12.48	22.0	17.60
13	2.30	1.84	3.64	2.91

How to use the max Zs values

Zs is the earth fault loop impedance of the circuit: the path from the fault, through the protective conductor, back through the supply transformer and the line conductor. It has to be low enough that a fault current flows big enough to operate the protective device within the required disconnection time.

0.4 s or 5 s?

0.4 s (Table 41.1, TN system, 230 V): final circuits up to 63 A with socket-outlets, and up to 32 A supplying fixed equipment.
5 s: distribution circuits, and final circuits above those ratings (Regulation 411.3.2.3).

Why measured readings are checked against 80%

The tabulated figures assume the conductors are at their maximum operating temperature (70 °C for thermoplastic cable). When you test, the cable is cold, so its resistance is lower. The rule of thumb in the IET On-Site Guide and Guidance Note 3 is to compare a reading taken at about 10–20 °C with roughly 80% of the tabulated value. If the reading is above 80% but below the tabulated limit, correct it properly using the temperature method in Appendix 3 before deciding.

Where the numbers come from

For circuit-breakers, max Zs = (U₀ × Cmin) ÷ Ia, where Ia is the current that trips the breaker instantly: 5 × In for Type B, 10 × In for Type C and 20 × In for Type D. For a 32 A Type B: 230 × 0.95 ÷ 160 = 1.37 Ω. Fuse values are read from the time–current curves in Appendix 3.

RCDs and TT systems

On a TT system fault protection normally comes from an RCD, and the limit is very different: Table 41.5 allows up to 1667 Ω for a 30 mA RCD at 50 V, although an earth electrode resistance above 200 Ω may not be stable. The values on this page are for TN systems (TN-S and TN-C-S).

Questions electricians ask

What is the max Zs for a 32 A Type B MCB?

Under BS 7671:2018+A4:2026 Table 41.3 the maximum Zs for a 32 A Type B circuit-breaker or RCBO is 1.37 Ω (230 V, Cmin 0.95). For a reading taken at room temperature, compare it with about 80% of that figure, 1.10 Ω.

Why do electricians use 80% of the max Zs?

The tables assume conductors at their full operating temperature (70 °C for PVC cable). Test readings are taken cold, when resistance is lower, so a measured value is compared with about 80% of the tabulated limit to allow for the rise in resistance when the circuit is loaded.

Did Amendment 4 change the max Zs tables?

No. BS 7671:2018+A4:2026 (published April 2026) kept Tables 41.2, 41.3 and 41.4 and the Cmin 0.95 factor that were introduced in earlier amendments, so the max Zs values are the same as in A2 and A3.

Where is BS 1361 in the 18th Edition?

BS 1361 has been withdrawn and replaced by BS 88-3, so the 18th Edition tables list BS 88-3 fuses. Use the BS 88-3 values for older BS 1361 cartridge fuses of the same rating, and check the fuse marking.

More free tools for electricians

Voltage drop calculator — mV/A/m × Ib × L, 3% / 5% limits› Cable size calculator — Twin & earth, Table 4D5, Ca · Cg · Ci›

Test sheets on the phone

EIC, EICR and Minor Works with live Zs, insulation and RCD checks, printed on the official IET forms.

See Graftum for electricians