ElectricalOM Support > Calculations & Checks

Max Earth Fault Loop Impedance figures

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RCDGuy:
I have been going through the EICR that i am carrying out and have noticed some discrepancies from the BS7671 and your list . For example 61009 C 32 30ma - 7283.33 Zs in the regs 0.68 Zs on 0.4 secs, also 60898 D 63 - 0.65 in the regs 0.35 Zs on 5 secs. Can you clarify that they are correct please.

Mert:
This is actually correct, considering the RCD will be responsible to protect during the earth fault conditions. The maths are:

MaxZs =( Cmin x Uo ) / Ia

For a 30mA RCD Ia = 30mA ==> MaxZs = (0.95x230)/0.03 = 7283.3 Ohms

Now in case, you want the overcurrent device to be responsible for the protection (recommended for TN systems) you need to check the relevant option in the circuit editor. See the attached image 1.

You can have this as the default behaviour for your circuits by setting it at the project option defaults (main menu-->Edit-->Project info and options). See the attached image 2.

In your current project, you can multi-select and apply this setting from the circuit details page. See the attached image 3.

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