RCD testing and functional verification
What is being proved
The RCD must operate within the required time at its rated residual current, must not operate at half that current, must operate quickly at five times, and must operate on both half-cycles. Types AC, A, F and B respond to different residual current waveforms; type A is the usual choice for socket outlets and circuits with electronic equipment, and type B is needed where smooth DC residual current can occur, such as some EV chargers and PV inverters.
The tests
½ IΔn: no trip. IΔn: trip within 300 ms for a general RCD, or between 130 ms and 500 ms for an S-type time-delayed device. 5 IΔn: trip within 40 ms for a general RCD, or within 150 ms for an S-type. Perform the IΔn and 5 IΔn tests at both 0° and 180°. Operate the integral test button as the functional check, and record the times.
Where to test
Test at the device, and for 30 mA additional protection also test at the furthest point of the circuit, because that includes the circuit conductors. Test with the load disconnected.
Trap: relying on the test button alone - it proves the mechanism, not the sensitivity or the time.
Trap: only testing at the RCD and never at the extremity of the circuit.
Trap: forgetting that on a TT system the RCD is the fault protection and RA must be verified separately.
Rule of thumb: 30 mA RCD - no trip at 15 mA, 300 ms at 30 mA, 40 ms at 150 mA, both polarities.