RCD Testing Procedure: How to Test RCDs per BS 7671

RCD (Residual Current Device) testing verifies that every RCD in an electrical installation — whether RCCB (Residual Current Circuit Breaker), RCBO (Residual Current Breaker with Overcurrent protection), or socket-outlet RCD — operates correctly at the specified current levels and within the required trip times. RCDs provide additional protection against electric shock by detecting small leakage currents to earth and disconnecting the supply before a lethal shock can occur.

RCD testing is test number seven in the GN3 testing sequence, performed after all dead tests and the other live tests (earth fault loop impedance and prospective fault current). It is a live test that requires the circuit to be energised and deliberately injects fault current through the earth path.

The test procedure involves applying specific multiples of the rated residual operating current (IΔn) and verifying that the device trips within the required time — or, in the case of the half-rated test, does NOT trip. The tests must be performed on both the positive and negative half-cycles of the supply waveform to verify correct operation under all conditions.

A4:2026 introduced an important change to Reg 643.3: regardless of RCD type (AC, A, F, B etc.), an alternating current test at rated residual operating current (IΔn) shall be used to verify the effectiveness of the RCD. This means the 1x IΔn effectiveness test is always performed as an AC sinusoidal test, even for Type A, F, or B devices. Where your MFT has a type selector, select Type AC for the 1x IΔn effectiveness test; instruments without a type selector can only perform Type AC tests and this remains compliant for the effectiveness verification.

For each RCD in the installation, the following tests must be performed in order. Each test must be carried out on both the positive (0 degree) and negative (180 degree) half-cycles.

The worst-case (longest) trip time from the 0 degree and 180 degree tests at each current level is the value recorded on the schedule of test results. For example, if the 5x test gives 28 ms at 0 degrees and 34 ms at 180 degrees, you record 34 ms.

The half-rated current test (0.5x IΔn) is often overlooked but is an essential part of the RCD test sequence. For a 30 mA RCD, this means applying 15 mA of fault current. The RCD must NOT trip at this current on either half-cycle.

The purpose of this test is to verify that the RCD is not overly sensitive. BS EN 61008/61009 specifies that an RCD must trip between 50% and 100% of its rated residual operating current. A device that trips below 50% (below 15 mA for a 30 mA device) is excessively sensitive and presents a nuisance tripping risk. Nuisance tripping is not just an inconvenience — it can lead occupants to bypass or remove the RCD entirely, leaving circuits unprotected.

If the RCD trips at half-rated current, investigate the cause. There may be existing earth leakage on the downstream circuits (from electronic equipment, long cable runs, or damp conditions) that, combined with the test current, exceeds the trip threshold. Alternatively, the RCD itself may be faulty and should be replaced.

The rated current test (1x IΔn) applies the full rated residual operating current — 30 mA for a 30 mA RCD. The device must trip within 300 milliseconds on both the positive and negative half-cycles.

The 300 ms limit applies to general-type (non-time-delayed) RCDs. This is the maximum acceptable trip time at the rated current. In practice, most healthy RCDs trip significantly faster — typically between 15 ms and 30 ms at rated current. A trip time approaching 300 ms, while technically a pass, may indicate a device that is nearing the end of its useful life.

The test is performed on both half-cycles because the RCD's sensitivity may differ depending on the polarity of the fault current at the instant the test is applied. The worst-case (longest) trip time from either half-cycle is recorded on the certificate.

The five times rated current test (5x IΔn) is the most critical test for shock protection. For a 30 mA RCD, this applies 150 mA of fault current. The device must trip within 40 milliseconds for a general-type RCD.

The 40 ms limit is specifically designed to ensure protection against ventricular fibrillation — the most dangerous consequence of electric shock. Research has shown that currents above 30 mA flowing through the body for more than approximately 40 ms can induce fibrillation. The 5x test simulates a significant fault condition where a large leakage current is flowing to earth, and the rapid 40 ms disconnection prevents the shock duration from reaching dangerous levels.

If an RCD fails the 5x test (trips in more than 40 ms), it must be replaced. This is a C2 (potentially dangerous) observation on an EICR because the delayed disconnection could result in a lethal electric shock.

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The AC supply waveform is a continuous sine wave that oscillates between positive and negative peaks. When a fault occurs, it can happen at any point in this waveform. The magnitude of the fault current at the instant of the fault depends on where in the waveform cycle the fault occurs — at the peak (maximum voltage, maximum current) or at the zero crossing (zero voltage, zero current).

Testing at 0 degrees applies the test current as the waveform passes through zero going positive. Testing at 180 degrees applies it as the waveform passes through zero going negative. These represent the two worst-case starting conditions for the RCD — the points where the fault current starts from zero and must build up before the RCD can detect it.

Your multifunction tester has a setting to select 0 degree or 180 degree phase angle for RCD testing. Both tests must be performed at each current level, and the longer trip time is the one recorded. Some electricians abbreviate this by only testing on one half-cycle — this is not compliant with BS 7671 and could miss a device that fails on the other half-cycle.

The ramp test is a diagnostic test that gradually increases the leakage current from zero until the RCD trips. Unlike the standard tests which apply a fixed current and measure time, the ramp test measures the actual trip current — the precise current level at which the device operates.

BS EN 61008/61009 requires that a general-type RCD must trip between 50% and 100% of its rated residual operating current. For a 30 mA device, this means it must trip between 15 mA and 30 mA during a ramp test. A device that trips below 15 mA is overly sensitive and a nuisance tripping risk. A device that does not trip until above 30 mA is not providing adequate protection and must be replaced.

The ramp test is not always required on the schedule of test results, but it is a valuable diagnostic tool. If you are investigating nuisance tripping, the ramp test can confirm whether the RCD itself is overly sensitive or whether the issue is background leakage current on the circuit.

Every RCD has a test button on its front panel marked "T" or "Test." Pressing this button creates an artificial imbalance in the sensing toroid by passing a small current through an internal resistor, simulating a fault. The RCD should trip immediately.

The push-button test confirms three things: (1) the trip mechanism is free and operates correctly, (2) the contacts separate cleanly, and (3) the mechanical reset mechanism works. It does NOT confirm that the RCD will trip at the correct current level or within the required time — for that, you need the full instrument test sequence.

The push-button test should be performed first, before any instrument testing. If the button test fails (the RCD does not trip when the button is pressed), the device is faulty and must be replaced — there is no point proceeding with instrument tests. Occupants should be advised to press the test button six-monthly as a routine maintenance check between periodic inspections — this is the interval specified in the mandatory RCD instruction notice required by BS 7671 Regs 514.12.1 and 514.12.2.

Time-delayed RCDs, designated Type S (selective), have an intentional time delay built into their operation. They are designed to achieve discrimination with downstream general-type RCDs — when a fault occurs, the downstream device should trip first, without the upstream Type S device also tripping.

Note the critical difference: for a Type S device, there is both a maximum AND a minimum trip time. If the device trips faster than the minimum (for example, tripping at 1x in less than 130 ms), it is not providing the intended time delay and discrimination will fail. If it trips slower than the maximum (for example, tripping at 5x in more than 200 ms), it is not providing adequate protection. Both conditions are failures.

RCD discrimination ensures that when a fault occurs, only the RCD nearest to the fault trips — the upstream RCD remains closed, maintaining supply to other circuits. This is essential in modern consumer units where a main switch RCCB protects multiple circuits that also have individual RCBOs.

To achieve discrimination, the upstream device must be a Type S (time-delayed) RCD and the downstream device must be a general-type RCD. The time delay in the Type S device gives the downstream device time to operate first.

Discrimination between two general-type (non-time-delayed) RCDs cannot be guaranteed because both devices will attempt to trip at the same speed. One will trip first, but which one trips is essentially random and depends on the manufacturing tolerances of each device.

RCD test results are recorded on the schedule of test results attached to the EICR or EIC. For each RCD, you must record the type (RCCB, RCBO, socket-outlet), the rated residual operating current (typically 30 mA), and the trip times at each test level.

The worst-case (longest) trip time from the 0 degree and 180 degree half-cycle tests is the value recorded. For example, if the 5x test gives 25 ms at 0 degrees and 32 ms at 180 degrees, you record 32 ms. For the half-rated test, record "Did not trip" or the appropriate pass indication.

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If any test fails, record the failure and classify the observation appropriately. An RCD that fails the 5x test (does not trip within 40 ms) is typically C2 (potentially dangerous). An RCD that trips at half-rated current may be C3 (improvement recommended) if it is causing nuisance tripping, or C2 if the overly sensitive operation is likely to cause occupants to bypass the device.

Several common issues can affect RCD test results or cause confusion during testing.

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