Its Ohms law
V=IR therefore R=V/I
V= 50 as metalwork cannot rise above this figure
I= 30mA RCD trip current
therefore R=50/0.03 which comes to 1666.66666666
Yes and fine if it's a TT system, but, when designing a circuit for a TN system you should really have the maximum earth fault loop impedance figures for the primary overcurrent device in mind (MCB, BS88 fuse etc) and not the maximum permissible figures for a 30mA RCD (1666ohms).
If I ever see a Zs of 1666 ohms for any circuit I'd be looking for a problem!
Even with TT the Ra should not exceed 200 ohms for reliability. Do you have a copy of GN3? Take a look at page 48 (17th ed).
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