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Choosing the RCD: RA × I∆n ≤ 50 V300 milliamps or 30. Which, and why?
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Guidance Note 3 · pp.83–84. The formula that decides it:
RA × I∆n ≤ 50 V
RA = 82 Ω, trying a 300 mA device
Put the milliamps into amps first: 300 mA = 0.3 A 82 × 0.3 = 24.6 V ≤ 50 V, so it satisfies the requirement
And a 30 mA device, for comparison
82 × 0.03 = 2.46 V also well inside 50 V
So both pass the formula. The choice between them comes from what else the circuit needs:
✋Both satisfy RA × IΔn ≤ 50 V. A 30 mA device goes wherever additional protection is required. A 300 mA device upstream is chosen for selectivity with the 30 mA devices below it, and to avoid nuisance tripping. And a TT electrode reading can drift upward — 82 Ω today could be 150 Ω in a dry spell.
Check the headroom too. If a circuit's R1 + R2 takes Zs to 82.5 Ω, ask whether that still leaves room for the ground to dry out. Choosing the device is a judgement, not just a calculation that passes.
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