RCD Testing: Types, Trip Times and Common Faults

Why RCD Testing Matters

RCDs are the single most important protective device for preventing electric shock fatalities. Every Electrical Installation Condition Report (EICR) and every new installation certificate requires RCD testing, and getting it wrong — or skipping it — puts lives at risk and your certification on the line.

BS 7671 (the IET Wiring Regulations) and the associated Guidance Note 3 lay out exactly what's expected. This guide covers the practical side: which RCD types you'll encounter, the trip times you need to hit, how to run the tests properly, and the faults that cause most failures.

RCD Types Explained

Not all RCDs are created equal. The type determines what kind of fault current the device can detect. Getting the wrong type installed is a surprisingly common issue, especially in older installations that have had new equipment added.

Type AC

Detects sinusoidal AC residual currents only. This is the most basic type and was standard for decades. You'll still find them in older installations, but they're increasingly being replaced because modern electronics produce fault currents that Type AC simply cannot see.

Type A

Detects sinusoidal AC residual currents and pulsating DC residual currents. This is now the minimum standard for most circuits in new installations under BS 7671. Pulsating DC faults are common with devices containing rectifiers — think EV chargers, LED drivers, washing machines and IT equipment.

Type F

Everything Type A does, plus detection of composite residual currents generated by single-phase variable frequency drives. Required where inverter-driven equipment is connected — most commonly seen with air conditioning units, heat pumps and certain motor-driven appliances.

Type B

The most comprehensive. Detects AC, pulsating DC, and smooth DC residual currents. Required for three-phase variable speed drives, some EV chargers (particularly three-phase units), and certain medical equipment. These are significantly more expensive, so you need to know when they're genuinely required.

Required Trip Times

BS EN 61008 and BS EN 61009 define the maximum trip times for RCDs. These are the figures you need to know when interpreting your test results.

For a 30mA RCD (general type, non-delayed)

1. At 50% rated residual current (15mA): The RCD must not trip. If it does, it's oversensitive and needs replacing.
2. At 100% rated residual current (30mA): Must trip within 300ms.
3. At 500% rated residual current (150mA): Must trip within 40ms.

For time-delayed (Type S / selective) RCDs

1. At 100% rated residual current: Must trip between 130ms and 500ms.
2. At 500% rated residual current: Must trip between 60ms and 200ms.

The minimum time delay on Type S RCDs exists to allow downstream non-delayed RCDs to trip first, providing discrimination. If a Type S device trips too quickly, you lose discrimination and the whole board goes down on a single circuit fault.

Test button response

Every RCD test must also include pressing the integral test button. This tests the mechanical trip mechanism. If the test button doesn't trip the device, the RCD must be replaced regardless of what your instrument says — the mechanical linkage has failed.

Test Procedure Step by Step

This assumes you're using a calibrated multifunction installation tester (MFT) such as a Megger, Metrel or Fluke unit.

1. Identify the RCD type and rating — check the label on the device for type (AC, A, F, B), rated residual current (IΔn), and whether it's time-delayed (S).
2. Warn the occupant — RCD testing will disconnect circuits. Make sure nobody is relying on life-support equipment or has unsaved work on computers.
3. Connect your tester — line to phase, neutral to neutral (or earth depending on your instrument and the test). Refer to your instrument manual for the correct connection method.
4. Run the ½× IΔn (no-trip) test — the RCD must NOT trip. If it does, it's oversensitive.
5. Run the 1× IΔn test at 0° — record the trip time.
6. Reset the RCD and run 1× IΔn test at 180° — record the trip time.
7. Run the 5× IΔn test at 0° and 180° — record trip times.
8. Press the integral test button — confirm it trips mechanically.
9. Record the worst-case trip time for each test on your schedule of results.

For EICRs, Guidance Note 3 (Inspection and Testing) from the IET provides the definitive reference on how to record and interpret these results.

Common Reasons RCDs Fail Testing

After years of testing, these are the faults that come up again and again.

1. Accumulated leakage current

This is the number one cause of nuisance tripping and borderline test results. Every circuit has some natural leakage to earth — long cable runs, damp environments, and lots of electronic equipment all add up. When the standing leakage on a circuit gets close to the RCD threshold, it only takes a small additional fault to trip it. Regulation 531.3.2 of BS 7671 notes that the cumulative earth leakage current should not exceed 30% of the rated residual operating current in normal service.

2. Neutral-earth faults downstream

A neutral conductor touching earth anywhere downstream of the RCD creates a parallel return path. Some of the load current returns via earth instead of the neutral, and the RCD sees this as an imbalance. These faults can be intermittent and frustrating to find — often caused by damaged cables, poor terminations, or borrowed neutrals between circuits.

3. Wrong RCD type for the load

As discussed above, a Type AC RCD protecting a circuit with equipment that produces pulsating DC fault currents won't detect those faults. It will appear to pass testing with your MFT (which applies a sinusoidal test current) but won't protect against real-world faults from that equipment.

4. Mechanical failure

RCDs are mechanical devices. The trip mechanism can seize or become sluggish, especially if the test button hasn't been pressed regularly. BS 7671 Regulation 514.12.2 requires a notice at the distribution board advising the user to test RCDs quarterly. If they haven't been tested for years, don't be surprised when the mechanism is stiff or slow.

5. Damaged or contaminated RCDs

Moisture ingress, insect contamination (particularly in outdoor or garage boards), and physical damage can all affect RCD performance. If an RCD is giving inconsistent readings across multiple tests, inspect it physically before assuming the fault is elsewhere.

6. Shared neutrals (borrowed neutrals)

Where circuits share a neutral conductor downstream of an RCD — whether by design or by a wiring error — the RCD will see a current imbalance because the return current from one circuit is flowing through another circuit's neutral path. This is a common finding in older domestic installations that have been modified over the years.

Recording Results on Certificates

When completing your schedule of test results — whether on paper or in an app like CertBox — record the following for each RCD:

• RCD type (AC, A, F, B)
• Rated residual current (IΔn) — typically 30mA or 100mA
• Operating time at IΔn (worst case of 0° and 180° results)
• Operating time at 5× IΔn (worst case of 0° and 180° results)
• Test button operation — confirm functional

Accurate recording is essential. If your figures are ever questioned — whether by a client, a building control officer, or in a legal dispute — your test results are your evidence that the installation was safe at the time of inspection.

Key Regulations Reference

• BS 7671:2018+A2:2022 — IET Wiring Regulations, 18th Edition (Regulations 411.3.3, 531.3, 514.12.2)
• BS EN 61008-1 — RCCBs without integral overcurrent protection
• BS EN 61009-1 — RCBOs with integral overcurrent protection
• IET Guidance Note 3 — Inspection and Testing (the definitive test procedure reference)
• ECA / NICEIC inspection guides — for coding guidance on observations

Published 2026-03-06. This article is for general guidance only and does not constitute legal or professional advice. Always refer to the relevant standards and consult qualified professionals for definitive requirements.