TESTING GUIDE

RCD Testing Guide: RCD Test Procedures to BS 7671

The complete UK electrician's guide to RCD testing to BS 7671:2018+A4:2026 — half-rated current test, rated current trip time (300ms maximum for 30mA general type, Reg 643.8), 5 times current test, ramp test, correct instrument connection, recording results, and diagnosing nuisance tripping. Updated for A4:2026 including Reg 411.3.4 (30mA RCD on domestic lighting circuits).

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13 min readUpdated 2026-06-10Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

  • 1BS 7671 Regulation 643.3 and 643.8 (Chapter 64) require RCDs to be tested at the time of installation and as part of every periodic inspection and test. The single A4:2026 verification criterion is an AC test at rated residual operating current (IΔn): the RCD must disconnect within 300ms (general non-delay type). Note: Appendix 3 Table 3A (the previous time/current criteria framework) has been deleted by A4:2026 — half-rated and 5× values now derive from BS EN 61008/61009 product standards, not BS 7671 Chapter 64.
  • 2A 30mA RCD tested at its rated residual operating current (30mA) must trip within 300ms (0.3 seconds) per BS EN 61008/61009 and Reg 643.8. The AC test at IΔn applies regardless of RCD type (AC, A, F, B) per Reg 643.3.
  • 3S-type (time-delayed) RCDs have different limits to general type RCDs and are used in series to provide discrimination. They must not operate in less than 130ms at rated current to allow a downstream standard RCD to trip first.
  • 4The test must be performed with the instrument connected between the line terminal (downstream of the RCD) and the main earth terminal. Never connect to the neutral — the instrument must drive current through the RCD sensing coil.
  • 5All RCDs must be tested at 0 degrees (positive half-cycle) and 180 degrees (negative half-cycle) phase angles. Prove the test instrument on a proving unit before and after use — an unproved instrument invalidates all results.
  • 6A4:2026 Reg 411.3.4 — new requirement: within domestic (household) premises, additional protection by a 30mA RCD shall be provided for all AC final circuits supplying luminaires. Existing lighting circuits without RCD protection now require a C2 or C3 code on an EICR.
01 · Testing Guide

What Is RCD Testing and Why Is It Required?

A residual current device (RCD) protects against electric shock by detecting imbalance between the current in the line and neutral conductors — caused by current flowing to earth through a person or fault path. When the residual current (the imbalance) exceeds the rated sensitivity, the RCD must operate and disconnect the supply within the required time.

RCDs are mechanical and electronic devices that can fail or degrade over time without any obvious external sign. A mechanically stuck or electrically degraded RCD may fail to trip when required, leaving connected equipment and persons unprotected. BS 7671 Regulation 643.3 and 643.8 (Chapter 64) therefore require RCDs to be verified at the time of installation and as part of every periodic inspection and test.

RCDs covered by this guide: This guide covers testing of 30mA general type RCDs (the most common type in UK domestic and commercial installations, required for socket outlets under Regulation 411.3.3), 100mA and 300mA RCDs used for fire protection, S-type (time-delayed) RCDs, and Type A RCDs/RCBOs (which detect pulsating DC residual currents and are required for circuits supplying EV chargers, washing machines, and PV inverters). The same AC test at IΔn applies to all types per Reg 643.3. The same test principles apply to RCBOs (combined MCB and RCD in a single device).
A4:2026 — Reg 411.3.4: 30mA RCD now mandatory for all domestic lighting circuits: BS 7671:2018+A4:2026 introduced Regulation 411.3.4, which requires that, within domestic (household) premises, additional protection by a 30mA RCD shall be provided for all AC final circuits supplying luminaires. This is new in A4:2026. For EICRs, existing domestic lighting circuits that are not RCD-protected now require a C2 or C3 observation code — this is one of the most actionable A4:2026 changes for periodic inspection work. Note: Reg 411.3.4 is explicitly scoped to domestic (household) premises — it does not, in its own wording, extend to commercial or industrial premises. Commercial inspectors should apply the general additional protection rules under Reg 411.3.3 and assess lighting circuits against other applicable regulations rather than applying 411.3.4 directly.
A4:2026 — Reg 421.1.7: AFDDs recommended for AC final circuits: Alongside Reg 411.3.4, BS 7671:2018+A4:2026 introduced Regulation 421.1.7, recommending the installation of arc fault detection devices (AFDDs) in AC final circuits of a fixed installation to mitigate the risk of fire due to arc fault currents. Unlike Reg 411.3.4 (which uses "shall" and is mandatory), Reg 421.1.7 uses recommendatory language. AFDDs are board-level devices installed alongside or in place of MCBs or RCBOs. On an EICR for a new or rewired domestic installation, the absence of AFDDs where Reg 421.1.7 would apply may attract an observation code.
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02 · Testing Guide

Half-Rated Current Test — Must NOT Trip in 2 Seconds

The half-rated current test verifies that the RCD does not have an excessively low operating threshold that would cause it to trip on normal earth leakage currents. At half the rated tripping current, the RCD must remain closed for at least 2 seconds.

  • Test current: For a 30mA RCD, the test current is 15mA (half of 30mA). For a 100mA RCD, the test current is 50mA. For a 300mA RCD, the test current is 150mA.
  • Required result: The RCD must NOT trip during the 2-second test period. If the RCD trips at half rated current, it is operating below its rated threshold. This is a fail — the RCD is defective and must be replaced. Record the actual trip time if the RCD does trip during the half-rated test.
  • Test at both phase angles: Test at 0 degrees and 180 degrees. An RCD that passes at 0 degrees but trips at 180 degrees (or vice versa) has an asymmetric fault and must be replaced.
03 · Testing Guide

Rated Current Test — Maximum 300ms for 30mA General Type

The rated current test applies the full rated residual operating current and verifies that the RCD trips within the required time. This is the primary safety verification test.

  • Standard general type RCD (30mA): Must trip within 300ms (0.3 seconds) at 30mA. This is the maximum disconnection time at rated current per BS EN 61008-1 and BS EN 61009-1. A 30mA RCD that takes longer than 300ms to trip at rated current is defective and must be replaced.
  • 100mA and 300mA RCDs (fire protection type): Same maximum disconnection time of 300ms at rated current. These higher-sensitivity RCDs protect against fire risk from sustained low-level earth faults rather than personal protection.
  • S-type (time-delayed) RCD: Must NOT trip before 130ms and must trip within 500ms at rated current. The intentional delay allows downstream standard RCDs to operate first. If an S-type RCD trips in less than 130ms, it is defective.
  • Test at both phase angles (0 and 180 degrees): The trip time at both phase angles must be within the required limit. Record both results on the schedule of test results.
04 · Testing Guide

5 Times Rated Current Test — Rapid Operation at 5×IΔn

The 5 times rated current test verifies that the RCD operates very rapidly under conditions simulating a severe direct contact fault. At 5 times the rated operating current, virtually all the current is flowing through a person in direct contact with a live part — the RCD must disconnect quickly to limit the energy delivered to the body.

A4:2026 update — Appendix 3 Table 3A deleted: BS 7671:2018+A4:2026 deleted Table 3A (Time/current performance criteria for RCDs) from Appendix 3. The half-rated and 5× trip-time values that test instruments display derive from BS EN 61008/61009 product standards, not from a BS 7671 Chapter 64 pass/fail table. The single BS 7671 A4:2026 verification criterion is the AC test at IΔn, with disconnection within 300ms for a general non-delay type (Reg 643.8 NOTE).
  • Test current: For a 30mA RCD, the 5 times test current is 150mA. For a 100mA RCD, it is 500mA.
  • General type — product standard reference: BS EN 61008 and BS EN 61009 specify 40ms as the maximum disconnection time at 5 times rated current for a standard (non-time-delayed) RCD. Test instruments use this as their pass/fail threshold. An RCD that trips in more than 40ms at 5×IΔn is likely defective.
  • S-type — product standard reference: BS EN product standards specify that an S-type RCD at 5 times rated current must NOT trip before 50ms and must trip within 200ms. The intentional delay is reduced at higher currents to prevent energy accumulation, but remains longer than the standard type to maintain discrimination.
Test sequencing: Perform the half-rated test first, then the rated current test, then the 5 times test. Running the 5 times test first may trip the RCD before the other tests can be performed. Always reset the RCD between each test and allow a brief settling period before the next test.
05 · Testing Guide

Ramp Test — Finding the Actual Trip Threshold

The ramp test gradually increases the residual current from zero until the RCD trips, recording the actual trip current. Per GN3 Ch4 Reg 4.6 (9th Ed A4), the ramp starts from below 50% of the RCD rating (i.e. below 15mA for a 30mA device) and increases to 110% of the rating (i.e. to 33mA). The RCD must trip somewhere within this range.

  • Acceptable range for a 30mA RCD: The ramp test must show the RCD tripping at a current between 15mA and 33mA (50%–110% of rated current per GN3 Ch4 Reg 4.6). A trip below 15mA indicates an excessively sensitive device prone to nuisance tripping. A trip above 33mA (110% of rating) indicates a device that may not provide adequate protection and should be investigated.
  • When to use the ramp test: The ramp test is most useful when investigating nuisance tripping complaints — to verify the RCD's actual threshold. It is not always mandated by BS 7671 as a standalone test, but many competent inspectors include it as good practice. Check whether your certification scheme requires it.
  • Record the ramp trip current: Note the actual trip current on the schedule of test results alongside the standard test results.

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06 · Testing Guide

Instrument Connection Method

Correct instrument connection is essential for valid RCD testing. The instrument must be connected so that the test current flows through the RCD sensing coil but not through the neutral conductor.

  1. Prove the instrument before use: Before starting any RCD tests, prove the RCD tester on a proving unit and inspect the leads for damage. Prove again after testing. An unproved instrument cannot be relied upon and may invalidate the schedule of test results.
  2. Identify the test point: Connect the instrument at a socket outlet, accessory, or terminal box that is downstream (protected side) of the RCD being tested. The connection must be on the load side of the RCD — not at the source side.
  3. Connect line to line terminal: The instrument's line test lead connects to the line terminal at the test point.
  4. Connect earth to the main earthing terminal: The instrument's earth test lead connects to the main protective earth conductor — ideally at the main earthing terminal at the consumer unit, not at the local earth terminal of the socket outlet. This ensures the test current path is correct.
  5. Do NOT connect to neutral: The neutral terminal must NOT be used for the test connection. The instrument drives test current from line to earth — this creates the residual current imbalance that the RCD detects. Connecting via the neutral would bypass the RCD's current transformer and invalidate the test.
  6. Set the phase angle: Select 0 degrees for the first test, then 180 degrees for the second. Record both results separately.
Testing RCBOs: RCBOs (combined MCB and RCD) are tested using exactly the same method as RCDs. Connect downstream of the RCBO at the first protected outlet. Confirm the trip time at rated current and at 5 times rated current. The RCBO must be reset after each test.
07 · Testing Guide

Recording RCD Test Results

RCD test results are recorded on the Schedule of Test Results as part of the EICR or Electrical Installation Certificate. For each RCD, the following must be recorded:

  • RCD type and rated operating current: General type or S-type, and the rated operating current (e.g., 30mA, 100mA, 300mA).
  • Half-rated test result: Pass (did not trip in 2 seconds) or Fail (tripped). If fail, record the actual trip time.
  • Rated current trip time at 0 and 180 degrees: Record actual trip times in milliseconds for both phase angles.
  • 5 times current trip time at 0 and 180 degrees: Record actual trip times in milliseconds for both phase angles.
  • Ramp test result (if performed): Record the actual trip current in milliamps.
  • Reg 411.3.4 compliance (domestic premises): When certifying a domestic installation against BS 7671:2018+A4:2026 Reg 411.3.4, the schedule of test results or EICR should record: the identification of each AC final circuit supplying luminaires (e.g. "Lighting Circuit 1"), the reference or position of the protecting RCD or RCBO in the consumer unit, and the device's rated residual operating current (confirming it does not exceed 30mA). Where a lighting circuit is not RCD-protected, record the deficiency and the appropriate observation code.

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08 · Testing Guide

Nuisance Tripping — Causes and Solutions

Nuisance tripping is when an RCD operates in the absence of a genuine dangerous fault, causing unnecessary loss of supply. It is a common complaint and one of the most frequent reasons for electricians to be called to domestic properties.

  • Cumulative earth leakage: Every piece of equipment connected to an RCD-protected circuit has some degree of earth leakage — typically due to Y-class filter capacitors in switch-mode power supplies, cookers, and washing machines. Individually these are below the trip threshold, but collectively they may approach or exceed 30mA. Solution: split the load across multiple RCDs, or upgrade to RCBOs on individual circuits.
  • Transient overvoltages (lightning and switching): Voltage transients can cause a momentary current imbalance that trips a sensitive RCD. Surge protective devices (SPDs) reduce this risk. BS 7671:2018+A4:2026 Reg 443.4.1 requires protection against transient overvoltages where the consequence caused by the overvoltage could result in: (a) serious injury to, or loss of, human life; or (c) significant financial or data loss. Limb (b) was deleted by the BS 7671:2018+A2:2022 Corrigendum (May 2023). For all other cases, protection shall be provided unless the owner of the installation declares it is not required because any loss or damage is tolerable and they accept the risk of damage to equipment and any consequential loss.
  • Long cable runs with high capacitance: Underground cables, armoured cables, and long conduit runs have significant capacitance between line and earth. This capacitance causes a leakage current even on healthy circuits. Particularly relevant for outdoor and outbuilding circuits.
  • RCD below specified sensitivity: An RCD with an actual trip threshold below 15mA (confirmed by the ramp test) will cause nuisance trips. Replace the RCD.
09 · Testing Guide

For Electricians: RCD Testing in Practice

RCD testing is a straightforward but time-critical process on site. Testing every RCD and RCBO individually, at both phase angles, and for all three test conditions generates a significant number of data points per board.

Enter RCD Results On Site

The Elec-Mate testing app lets you enter RCD trip times for each phase angle and test level as you go. Auto-checks each result against BS EN 61008/61009 limits and flags any failures before you reset the board and lose track.

Warn Clients Before Testing

RCD testing will trip circuits and interrupt supply. Warn occupants before starting. Check for medical equipment (dialysis machines, stairlifts, alarms) that must not lose power, and plan the test sequence to isolate sensitive circuits last or work with the client to temporarily relocate equipment.

Frequently Asked Questions About RCD Testing

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