The Cmin factor is applied to the voltage to earth (Uo), so if the voltage is 230 the calculation would be 230x0.95 = 218.5 volts.
Max Zs Calculation Formula
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:
- 5x32(In) = 160 Amps (la)
- 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
60947-2 Max Zs Values
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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