Max Zs Values 18th Edition BS 7671

We have received a number of emails regarding the 18th edition max Zs values so we have created this post for the 18th edition maximum Zs values chart. The maximum Zs values for BS7671 18th edition have not changed from the 17th edition wiring regulations to the 18th edition 2018.

The 17th edition 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 the calculation would be 230×0.95 = 218.5 volts.

Max Zs Calculation Formula

So the Zs calculation formula for a 0.1s to 5s disconnection time for 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 test 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 to your test results.

​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 | BS 7671 & 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 of 60947-2 devices vary from manufacturer to manufacture hence why BS EN 60947-2 MCCB max Zs values chart is not in BS 7671 or the On Site Guide. Many manufactures have charts for their 60947-2 MCCB’s on their website. Our Max Zs Values app has a number of Zs charts 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 max Zs values. ​The last time the maximum Zs values changed was with the release of the 17th edition amendment 3.

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