An RCD (residual current device) disconnects a circuit when the current leaving on the line conductor differs from the current returning on the neutral by more than its rated residual operating current, IΔn, because the difference is leaking to earth, possibly through a person. Testing an RCD means injecting a known residual current with a multifunction tester and timing the trip. Under BS 7671:2018+A4:2026 a general-type RCD must trip within 300 ms at IΔn and must not trip at half IΔn; a 30 mA RCD providing additional protection under Regulation 415.1.1 must also trip within 40 ms at five times IΔn. The result in milliseconds goes on the schedule of test results.
- Three instrument tests: ½ IΔn (must not trip), 1 × IΔn (≤300 ms general, 130 to 500 ms selective) and 5 × IΔn (≤40 ms) for 30 mA additional protection. Test at 0° and 180° and record the longer time.
- RCD type (AC, A, F, B) must match the load; Type A is the usual minimum for circuits with electronic equipment.
- The test button proves the mechanism, not the sensitivity. It does not replace the instrument test.
- RCBOs are tested exactly like RCDs, one circuit at a time.
- Ramp tests are diagnostic: useful for nuisance tripping and standing leakage, not a certification requirement.
What types of RCD are there and why does it matter?
An RCD's type describes the waveform of residual current it can detect. Modern loads with switched-mode power supplies, inverters, LED drivers and motors can produce residual currents that a basic RCD will not see, so the type has to be chosen for the equipment on the circuit, and the type is recorded on the schedule of test results.
| Type | Detects | Typical use | Marking |
|---|---|---|---|
| AC | Sinusoidal a.c. only | Legacy installations, purely resistive loads | Sine wave symbol |
| A | a.c. plus pulsating d.c. | General circuits with electronic loads; the default for new domestic work | Sine wave over a half-wave symbol |
| F | As A plus mixed-frequency residual currents | Single-phase variable speed drives, some heat pumps and washing machines | A symbol plus a frequency symbol |
| B | As F plus smooth d.c. | Three-phase drives, some EV charging points (Section 722), some PV inverters (Section 712) | A symbol plus a smooth d.c. line |
The IET has been clear since the 18th Edition that Type AC RCDs are not appropriate where d.c. components are likely, which in practice is most circuits in a house. On an EICR, a Type AC device protecting circuits with electronic loads is usually recorded as a C3 rather than a fail, unless specific equipment requires a Type A or B by its section of BS 7671. Section 722 for EV charging, for example, requires either a Type B RCD or a Type A with 6 mA d.c. detection built into the charger.
Separately from the waveform type, RCDs are either general (no intentional delay) or selective, S-type, with a built-in delay so that a 100 or 300 mA device at the origin does not trip before a 30 mA device downstream. The trip time limits differ.
What tests are required and what are the pass values?
The tester connects between line and earth on the load side of the RCD, at a socket-outlet or the furthest point of the circuit, and draws a residual current for a set time. Each test is run on both half-cycles (0° and 180°) and the longer time is recorded.
| Test current | Purpose | General RCD | Selective (S-type) RCD |
|---|---|---|---|
| ½ × IΔn (15 mA for a 30 mA device) | Check the device does not trip below its rating | No trip | No trip |
| 1 × IΔn (30 mA) | Rated operation | ≤300 ms | 130 to 500 ms |
| 5 × IΔn (150 mA) | Additional protection, 30 mA devices only | ≤40 ms | Not applicable |
The acceptance value at 1 × IΔn follows the product standards BS EN 61008 and BS EN 61009: 300 ms for a general device and 130 to 500 ms for a selective one. Guidance Note 3 now uses these product-standard figures; the older 200 ms test limit applied to BS 4293 devices and should not be confused with the 0.2 s disconnection time for TT systems in Table 41.1. The 40 ms at 5 × IΔn requirement comes from Regulation 415.1.1, which defines additional protection as a 30 mA RCD operating within 40 ms at 150 mA; it applies to the 30 mA devices required for socket-outlets up to 32 A (Regulation 411.3.3), domestic lighting (411.3.4), cables concealed in walls (522.6.203) and bathrooms (Section 701).
Typical healthy results: a good 30 mA RCD or RCBO trips in roughly 15 to 30 ms at IΔn and under 15 ms at 5 × IΔn.
Where one RCD protects several circuits, test it once at the furthest point of one circuit and enter the same result against every circuit it covers, with a remark. For a board of RCBOs, test each from its own circuit.
What does the test button do?
Every RCD and RCBO has a test button that connects a resistor from the line on the load side to the neutral on the supply side, creating an imbalance through the sensing toroid and tripping the device. It proves the mechanical parts move. It does not measure trip time, it does not confirm that the device trips at its rated 30 mA (the button current is typically higher), and it says nothing about whether the device is the right type.
On the schedule of test results the test button has its own tick column. Press it after the instrument tests, confirm the device trips, reset it and tick. The label on most RCDs asks the user to press the button periodically; point that out to the customer or landlord.
How do I test an RCBO?
An RCBO (residual current circuit-breaker with overcurrent protection, BS EN 61009) is an RCD and an MCB in one device protecting a single circuit. The RCD element is tested exactly as above: at a socket or the furthest point of that circuit, run ½, 1 × and 5 × IΔn at both polarities and record the longest time. The overcurrent element is not instrument-tested; its type and rating go in the OCPD columns and are verified by the Zs test (see Maximum Zs Values BS 7671: Tables, 80% Rule and Ze).
One practical point: many RCBOs have a functional earth flying lead that must be connected for the electronics to work. An RCBO with the flying lead not terminated may pass the test button and fail the instrument test, or not trip at all.
What is a ramp test and when is it useful?
A ramp test, available on most multifunction testers, increases the residual current slowly from around 10 per cent of IΔn until the device trips and reports the trip current in milliamps. It is not required by BS 7671 and does not go in the operating time column, but it is the quickest way to diagnose nuisance tripping.
A 30 mA RCD is permitted by its product standard to trip anywhere between 50 and 100 per cent of IΔn, so a healthy device will ramp-trip between about 15 and 30 mA. If it trips at 12 mA, the device is probably fine but the circuit already carries 10 mA or more of standing earth leakage from connected equipment, so any additional leakage takes it over the edge. That points you at the load (a heater element, an outdoor socket, an accumulation of switched-mode supplies) rather than the RCD. A ramp trip at 29 mA on a device that also passes the timed tests is a healthy device; if the customer still reports tripping, look for intermittent faults or a Type AC device on a Type A load.
What are the common reasons an RCD fails the test?
| Symptom | Likely cause | Action |
|---|---|---|
| Trips at ½ IΔn | Standing earth leakage on the circuit, or an over-sensitive device | Ramp test; measure leakage with a clamp; split loads across RCDs |
| Does not trip at 1 × IΔn | Seized mechanism, contaminated contacts, failed electronics, RCBO functional earth not connected | Check wiring first; replace the device |
| Trips but slower than 300 ms | Worn or aged device | Replace; C2 if it fails, note if borderline |
| Passes at 1 × but fails 40 ms at 5 × | Device not suitable for additional protection, or degraded | Replace if 30 mA additional protection is required |
| Test button trips it, instrument does not | Mechanism works but sensitivity lost | Replace |
| Trips on connection of tester | Very high standing leakage or a line-earth fault | Insulation resistance test the circuit |
Age is the main factor: RCDs installed in the 1990s and 2000s are now 20 to 30 years old, and a failed or slow RCD on an EICR is a common finding. A 30 mA RCD that does not trip within the required time on a circuit requiring additional protection is generally coded C2. Where no RCD is fitted at all on a socket circuit, Electrical Safety First Best Practice Guide 4 and most inspectors treat it as C3 in an older installation unless there are aggravating factors, such as sockets likely to supply outdoor equipment, which raise it to C2.
How does RCD testing fit into the rest of the live tests?
RCD testing comes after the earth fault loop impedance test, because a loop test on an RCD-protected circuit needs a no-trip setting and you want to know Zs is acceptable before deliberately tripping the device. Functional testing of switchgear follows. See Earth Fault Loop Impedance Testing: Ze, Zs and Ipf Explained for the loop tests and Schedule of Test Results: How to Fill In Every Column for recording the results.
How Certio helps
Certio produces the EICR, EIC, Minor Works certificate and PAT records on the BS 7671 model forms and turns them into branded PDFs. Photograph the consumer unit and the circuit schedule drafts itself; point the phone camera at a Megger MFT-X1 and the reading goes straight into the schedule of test results, or dictate it while your hands stay on the probes. Suggested observation wording with the regulation reference is there for you to check and confirm. The judgement stays yours; the typing goes. Starter is free for 7 days, then £5 a month.