Functional Testing: What to Check After Installation

Functional testing is the final stage of the verification process defined in BS 7671. It comes after the dead tests (continuity, insulation resistance, polarity) and the live tests (earth fault loop impedance, prospective fault current, RCD trip times). Where dead and live tests measure electrical values with instruments, functional testing involves physically operating equipment to confirm it works correctly under normal service conditions.

Regulation 643.10 of BS 7671:2018+A2:2022 states that assemblies such as switchgear, controlgear, interlocks, and similar equipment shall be subjected to a functional test to verify that they are properly mounted, adjusted, and installed in accordance with the relevant requirements of the Regulations. This means every switch, every isolator, every RCD test button, every interlock, and every circuit label must be checked.

Functional testing is often treated as a tick-box exercise, but it is genuinely important. An installation can pass every dead and live test and still be unsafe if a switching device controls the wrong circuit, if an RCD test button does not work, or if circuit labelling is missing or incorrect. These are the issues that functional testing catches.

Functional testing is carried out in two main situations: during initial verification of a new installation or alteration, and during periodic inspection and testing (EICR). In both cases, functional testing is performed after all dead and live tests have been completed and passed.

For initial verification, functional testing confirms that everything has been installed correctly and operates as intended before the installation is energised for normal use. For an EICR, functional testing assesses whether the existing installation continues to operate safely and whether any deterioration or changes have affected the function of switchgear, controls, and protective devices.

The testing sequence defined in IET Guidance Note 3 (GN3) places functional testing after all instrument-based tests. This is deliberate — there is no point checking whether a switch operates correctly if the circuit behind it has not passed its dead and live tests.

Every RCD and RCBO has a built-in test button. Pressing this button passes a small current through the device's trip coil, simulating an earth leakage fault. The device should disconnect immediately. This test confirms that the mechanical tripping mechanism inside the RCD is functioning — it does not measure the trip time or the trip current, which are measured separately during live testing with an RCD tester.

During functional testing, you must press the test button on every RCD and RCBO in the installation. The device must trip. After confirming the trip, reset the device and confirm it re-engages correctly. If the test button does not cause the RCD to trip, the device is faulty and must be replaced — this is a serious safety defect.

Note that the RCD test button check is in addition to the instrument-based RCD trip time test carried out during live testing. The instrument test measures the actual trip time and verifies it is within the BS 7671 limits. The test button check simply confirms the mechanical function of the device. Both are required.

It is good practice to advise the client that they should press the RCD test button themselves at regular intervals (typically quarterly) as an ongoing safety check. Many domestic clients are unaware that the test button exists or what it does.

Every switching device in the installation must be functionally tested. This includes light switches, fused connection units, timer switches, dimmer switches, fan isolators, cooker switches, contactors, and any other device that makes or breaks a circuit. For each device, you must confirm the following:

For circuit breakers at the distribution board, confirm each MCB and RCBO switches on and trips off smoothly. Any device that is stiff, difficult to operate, or does not latch in the on position may have a mechanical fault and should be investigated or replaced.

Board scanner reads your MCB/RCBO ratings automatically

Point your phone at the distribution board and Elec-Mate's AI reads the device ratings, types, and circuit references.

Try it free for 7 days

Isolators are devices designed to disconnect a circuit from the supply so that work can be carried out safely. Unlike switches, isolators are not intended for making or breaking load current — they are used for isolation only. During functional testing, you must confirm that every isolator fully disconnects all live conductors from the downstream circuit when operated.

Common isolators that require functional testing include: the main switch on the consumer unit or distribution board, local isolators for fixed equipment (cooker isolators, boiler isolators, immersion heater switches), and isolators for motors or other plant equipment.

Interlocking is a mechanism that prevents equipment from being operated in an unsafe sequence. The most common domestic example is the consumer unit cover interlock — the cover should not be removable while the main switch is in the on position. This prevents accidental contact with live busbars. During functional testing, you must confirm that the interlock mechanism works correctly. Try to remove the cover with the main switch on — it should be physically prevented. For commercial installations, interlocks on changeover switches, motor starters, and access panels must all be tested.

If an interlock mechanism has been defeated or bypassed (for example, a consumer unit cover that can be removed with the main switch on because the locking tab has been broken), this is a deficiency that must be recorded. On an EICR, a defeated interlock would typically attract a C2 or C3 observation code.

Regulation 514.9.1 of BS 7671 requires that a durable label or arrangement of labels shall be provided at each distribution board or consumer unit, indicating the purpose of each circuit and the area it serves. Every MCB, RCBO, or fuse must be identified with a unique circuit designation, and the designation must be recorded on a circuit chart.

During functional testing, you must verify the following for every circuit:

Incorrect or missing labelling is one of the most common observations on EICRs. It may seem like a minor issue, but incorrect labelling can have serious consequences — if an electrician isolates the wrong circuit because the labels are wrong, they could be working on a live circuit they believe is dead. Labelling accuracy is a safety issue, not just a paperwork issue.

Beyond switching devices, isolators, interlocks, and labelling, functional testing may also include the following checks depending on the type of installation:

Functional test results are recorded on the inspection schedule of the EIC or EICR. The inspection schedule includes a section for functional testing where each item is recorded as satisfactory, unsatisfactory, or not applicable.

For an EICR, any unsatisfactory functional test results are recorded as observations with the appropriate observation code. For example, a non-functional RCD test button might be C1 (danger present), missing circuit labels might be C3 (improvement recommended), and a defeated interlock might be C2 (potentially dangerous).

For an EIC or Minor Works, unsatisfactory functional test results indicate that the installation does not comply with BS 7671 and the certificate should not be issued until the deficiency is rectified.

Elec-Mate includes a comprehensive functional testing checklist integrated into every EIC, EICR, and Minor Works certificate. The checklist covers RCD test button verification, switching device operation, isolator checks, interlocking, circuit identification, labelling, and all other functional test items required by BS 7671.

Never miss a functional test item

Elec-Mate's functional testing checklist is built into every certificate. Each item has a pass/fail/N/A selector.

Try it free for 7 days

The app also auto-generates the circuit schedule from the board scanner, so your functional test checklist is already populated with the correct circuit references, device types, and ratings. The 8 Elec-AI agents can answer questions about functional testing requirements in real time while you are on site.