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A 30 mA RCD is personal protection and must clear a fault within 300 ms at five times its rating — but only if the earth loop is sound enough to let the fault current flow.
Two residual-current ratings dominate practice, and they are not interchangeable. The 30 mA device protects people and belongs on socket-outlets; the 300 mA device is a whole-board backstop against fire risk and offers no personal protection. Treating a 300 mA unit as socket protection is a category error, not a compromise.
The performance requirement for the 30 mA device is a trip within 300 ms when the residual current reaches five times the rated value. That speed is what makes the device meaningful in a shock scenario, and it is a property of the device only when the circuit around it cooperates.
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The circuit's side of the bargain is loop impedance. A healthy 5 m loop of 1.5 mm² twin-and-earth measures roughly 0.5–1.2 Ω end to end, but one corroded joint can add more than 1 Ω — more resistance than the cable itself — and push the loop past the tripping threshold even though the RCD is rated correctly.
TT earthing is what turns these numbers into obligations. IEC 60364-4-41 requires disconnection within 3 s for circuits up to 32 A and within 0.3 s above 32 A, and on a high-impedance local earth an overcurrent device alone often cannot meet those times. That gap is precisely what forces the RCD rating choice.
Measurement conditions matter as much as the device. Copper resistance rises about 0.4% per °C from its 20 °C baseline, so a loop measured at 60 °C reads roughly 16% higher; a result that passes marginally on a cool morning can fail at working temperature, and the honest reading is the hot one.
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