Max Zs Values 18th Edition BS 7671

BS 7671 18th Edition Maximum Zs

BS7671 2018 + Amendment 2 2022

The Max Zs is the maximum permissible values of earth fault loop impedance (Ω). Some maximum Zs values are listed in BS 7671.

The maximum earth loop impedance values listed in the Zs tables are used to compare against the actual measured earth loop impedance values to ensure compliance with BS 7671.

The Max Zs values can also be found in our popular Max Zs Values App which also includes a handy max Zs calculator and MCCB tables.

We have received a number of emails regarding the 18th edition max Zs values table 41.3, so we have created this post for the 18th edition maximum Zs values tables.

The maximum Zs values for BS 7671 18th edition have not changed from the 17th edition wiring regulations to the 18th edition 2018, Zs values can be found in table 41.3.

The 17th edition of the IET wiring regulations amendment 3 introduced the Cmin (0.95) factor which reduced the old maximum zs values to allow for the fluctuation of the voltage.

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

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

Max Zs Values App

Save yourself the time and headache of calculating maximum Zs values with the Max Zs Values App.

Max Zs Calculation Formula

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 calculation would be:

1. ​5×32(In) = 160 Amps (la)
2. 218.5(Uo) / 160 (la) = 1.36525 (rounded to 1.37) Maximum Zs Value

​Step 2 above shows the voltage to earth 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 must be applied to this value when comparing it against the actual readings obtained on site when carrying out an earth loop impedance test.

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

​When carrying out an earth loop impedance test you are carrying out the testing under normal conditions, so to take into account of the rise in cable temperature under fault conditions we need to apply the 0.8 correction factor to the maximum Zs value, therefore 1.37 * 0.8 = 1.096 (rounded to 1.1), which you then compare against your actual measured zs value.

​If your Zs test results exceeds the temperature adjusted (80%) max Zs value and if it’s for an EICR you would need to record this as a C2 because the maximum disconnection time can not be achieved due to the high Zs readings.

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

BS 7671 (big blue book) shows the 100% maximum Zs values. The On Site Guide (little blue book) shows the temperature adjusted Zs values having applied the 0.8 correction factor.

​The 100% values should be recorded as the maximum permitted Zs value on the electrical test certificate and the temperature adjusted 80% values are used to compare against the actual readings obtained when testing the circuit.

60947-2 Max Zs Values

The max Zs for 60947-2 devices vary from manufacturer to manufacture, hence why BS EN 60947-2 MCCB max Zs value tables are not in BS 7671 or the On-Site Guide.

Many manufactures have ZS tables for their 60947-2 MCCB’s on their website.

Our Max Zs Values App has a number of Zs tables for manufacture specific BS EN 60947-2 MCCB’s.

18th Edition Max Zs Values

The 18th edition max Zs values are the same as the 17th edition maximum Zs values. ​The last time the maximum Zs values changed was with the release of the 17th edition amendment 3.