Max Zs Values 18th Edition BS 7671 A4:2026

Max Zs Values – BS 7671:2018+A4:2026 (Amendment 4)

BS7671 2018 Amendment 4 2026.

Max Zs is the maximum permissible value of earth fault loop impedance (Ω) as set out in BS 7671 and manufacturer data sheets.

Amendment 4 left the Max Zs tables (41.2 to 41.4) and disconnection times unchanged. A4 was published on 15 April 2026, and the previous edition is withdrawn on 15 October 2026.

If you have an old Zs table from a previous version of BS 7671, then see our new Zs Converter to convert old Zs values to the 18th edition values.

Max Zs Values Table – MCBs and RCBOs (Table 41.3)

0.4 s and 5 s disconnection for BS EN 60898 and BS EN 61009 protective devices, 230 V. Record the 100% value on the certificate, compare your measured reading against the 80% value.

Rating (A)	Type B 0.4 & 5 s 100% (Ω)	Type B 0.4 & 5 s 80% (Ω)	Type C 0.4 & 5 s 100% (Ω)	Type C 0.4 & 5 s 80% (Ω)	Type D 0.4 s 100% (Ω)	Type D 0.4 s 80% (Ω)	Type D 5 s 100% (Ω)	Type D 5 s 80% (Ω)
6	7.28	5.82	3.64	2.91	1.82	1.46	3.64	2.91
10	4.37	3.50	2.19	1.75	1.09	0.87	2.19	1.75
16	2.73	2.18	1.37	1.10	0.68	0.54	1.37	1.10
20	2.19	1.75	1.09	0.87	0.55	0.44	1.09	0.87
25	1.75	1.40	0.87	0.70	0.44	0.35	0.87	0.70
32	1.37	1.10	0.68	0.54	0.34	0.27	0.68	0.54
40	1.09	0.87	0.55	0.44	0.27	0.22	0.55	0.44
50	0.87	0.70	0.44	0.35	0.22	0.18	0.44	0.35
63	0.69	0.55	0.35	0.28	0.17	0.14	0.35	0.28
80	0.55	0.44	0.27	0.22	0.14	0.11	0.27	0.22
100	0.44	0.35	0.22	0.18	0.11	0.09	0.22	0.18
125	0.35	0.28	0.17	0.14	0.09	0.07	0.17	0.14

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Max Zs and Cmin

The 17th edition of the IET wiring regulations (BS 7671:2008+A3:2015) introduced a Cmin factor of 0.95, which reduced the maximum Zs values to allow for voltage fluctuations.

The Cmin factor is applied to the voltage to earth value (Uo), so, if the voltage is 230v, the calculation would be: 230 (v) × 0.95 (Cmin) = 218.5 volts.

What is Zs?

Zs is the total impedance of the earth fault path: the supply (Ze) plus the line and protective conductors of the circuit (R1+R2).

Zs = Ze + (R1+R2)

If Zs is too high, not enough fault current flows to trip the protective device quickly enough. The circuit could then stay live during a fault.

How Max Zs is Calculated

The tabulated values use the nominal voltage to earth (230 V) with the minimum voltage factor Cmin (0.95), divided by the current that trips the device instantly (Ia).

Worked Example:

The maximum Zs calculation for a 0.1s to 5s disconnection time and a BS EN 60898 MCB or BS 3871 to calculate the maximum Zs would be:

la = The amount of current required to trip the MCB within the given time.
In = The rating of the MCB in Amps.
Uo = Voltage to earth​

• Type B: la = 5 x In
• Type C: la = 10 x In
• Type D: la = 20 x In

So, for a Type B 32 Amp MCB to BS EN 60898 the max zs calculation is,

• la = 5 x 32 (In) = 160 Amps
• Max Zs = 218.5 (Uo) ÷ 160 (A) = 1.365625 (rounded to 1.37) Ω.

​Note: The calculation above shows the voltage to earth value AFTER applying the 0.95 Cmin factor.

​The 1.37 ohms value is the 100% max Zs value which should be recorded on the test certificate. A correction factor of 0.8 must be applied to this value when comparing it against the actual measured values.

The 0.8 correction factor is to allow for the rise in temperature under fault conditions.

For the 32 A Type B example: 1.37 × 0.8 = 1.096 Ω. A measured Zs of 1.05 Ω passes, 1.20 Ω fails.

​EICR codes and Zs Values: If your measured Zs exceeds the temperature adjusted (80%) maximum Zs value this should be recorded as a C2 observation because the maximum disconnection time can not be achieved due to the high Zs readings – An example of the EICR observation could be something like, “DB.2 circuit 3 exceeds the maximum permissible Zs (80%) = C2“

TT systems with a 30 mA RCD

In TT earthing systems with a 30 mA RCD the max Zs is 1667 Ω (Table 41.5), provided the RCD trips within the required time, note, a reading above 200 Ω may not be stable.

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Max Zs Values for BS 7671 and the On-Site Guide

100% vs 80% values

BS 7671 provides the 100% maximum Zs values. The On-Site Guide provides the temperature adjusted (80%) Zs values having applied the 0.8 correction factor (Zs x 0.8).

The tables in BS 7671 assume the conductors are at their maximum operating temperature (70 °C for PVC cable). When you test, the cables are cold, so their resistance is lower. To allow for this, multiply the table value by 0.8 before comparing it with your measured reading.

• 100% value: record this as the maximum permitted Zs on the certificate.
• 80% value: your measured Zs must not exceed this.

Calculation Rounding: You may find that different sources listed 80% values may vary slightly due to calculation rounding where some sources have rounded it up, but other sources have rounded it down, i.e. 1.096 Ω has been rounded up to 1.10 Ω, or rounded down to 1.09 Ω, if in doubt, it might be a good idea to go on the side of caution and use the lower (rounded down) value.

BS EN 60947-2 Max Zs Values

The max Zs for 60947-2 devices varies between manufacturer to manufacturer (and specific device settings), hence why BS EN 60947-2 MCCB max Zs values are not in BS 7671 or the On-Site Guide, you will need to refer to the manufacturers’ data sheet time current curve tables to get the la to calculate the maximum Zs.

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