RCD testing: verification and extra diagnostics

Identify the RCD type, rated residual operating current and any intentional delay before selecting the tester settings. Take account of the device and instrument instructions.

For the normal AC test at rated residual operating current, a general non-delay RCD has a maximum operating time of 300 ms. Do not apply that limit indiscriminately to a selective time-delayed device.

Extra tests need a stated purpose

Since the 2022 changes, half-current, five-times-current and tests at both phase angles are not all mandatory parts of BS 7671 RCD verification. They may still be useful diagnostics or be specified by the manufacturer. Interpret them against the appropriate waveform, current and criterion.

If a device trips during an extra half-current check, first investigate the setup and existing leakage. The tester's injected current may not be the only residual current present.

Functional checks still matter

Operate the test button as required and verify the associated equipment functions correctly. A working button does not measure the disconnection time, and a good RCD result does not cover the other protective measures.

Decision check: a technician selects a saved five-times-current programme because it was used on every job years ago. Check the current verification requirements, and the reason for any additional test, first.

More on RCDs and functional checks

• Additional shock protection uses an RCD with a residual-current rating no greater than 30 mA. Typical duties include general-use socket-outlets up to 32 A and shallow concealed cables using the prescribed-zone-and-RCD arrangement. Other permitted cable-protection methods have their own conditions. An RCBO also provides overcurrent protection.
• Type A identifies an RCD’s ability to respond to sinusoidal AC and pulsating DC residual current. Type AC covers sinusoidal AC only. Electronic loads may need another type, so check their instructions: Type A does not cover every waveform.
• Type S identifies an intentional time delay, not a waveform category; an RCD can carry both S and A markings. The delay supports discrimination with a downstream RCD. For the usual selective arrangement, the upstream residual-current rating is at least three times the downstream rating; also check timing and manufacturer coordination guidance. A Type S device cannot be the sole provision for 30 mA additional protection, which requires prompt operation.
• A ramp programme increases its injected current and records the value when the RCD opens. Use it as diagnostic evidence for the chosen waveform and conditions: existing leakage can alter the reading. A timed programme measures operating time, while the integral button gives a routine operation check without a calibrated timing result.
• For an extra AC half-current check, exclude other residual current before expecting non-operation. A supplied five-times-current criterion for a general non-delay device may be 40 ms; apply the actual device and test instructions. If a diagnostic procedure asks for tests at 0° and 180°, record the longer of those two results, without treating it as every possible worst case.
• Unexpected tripping during a no-trip loop measurement calls for checks of load leakage, wiring, tester mode and compatibility, and the RCD itself. Changing to a higher residual-current rating is not a diagnosis or a substitute for required protection.
• Keep the results and test details for verification and future comparison. A later longer operating time needs assessment under comparable conditions; it does not always mean that the RCD is approaching failure.
• Plan interruptions with the occupants and follow connected-equipment guidance. Restore normal service only where safe. If a defect remains, leave the affected equipment made safe, record it and tell the responsible person.
• Functional testing operates switches, controls and interlocks, such as two-way lighting, to confirm the assembled installation works as intended.