11.2 Functional Testing

A Different Kind of Question

Every test so far has asked is this value within its limit? Functional testing asks does this actually work?

Regulation 643.10 requires that assemblies — such as switchgear and controlgear assemblies, drives, controls and interlocks — are subjected to a functional test to show that they are properly mounted, adjusted and installed in accordance with the relevant requirements of BS 7671. Protective devices are functionally tested where necessary to confirm they are properly installed and adjusted.

It is a live activity, carried out after the other live tests, with the installation energised and in its normal operating configuration. It is also the point at which many installations reveal problems that no meter would ever find: a perfectly compliant circuit wired to the wrong switch, or an isolator labelled for a machine it does not control.

What Is Functionally Tested

The labelling problem

The single most valuable outcome of functional testing is confirming that labels tell the truth. An isolator marked "Compressor 2" that actually feeds Compressor 3 is not a cosmetic error: a fitter will lock it off, prove dead at the wrong machine, and work on a live one. The same applies to circuit charts inside a distribution board.

Operate each device and confirm what actually goes off. Where a label is wrong, correct the label — and where the circuit chart is wrong, correct the chart, because that is what the next person will read.

The RCD Test Facility

It is worth being precise about why the RCD button check appears here and not in Chapter 9.

• Chapter 9 (Regs 643.7.1 and 643.8) covers verifying that the RCD operates within its time limit — an instrument measurement of the operating time at IΔn.
• Regulation 643.10 covers operating the device’s own test facility — the "T" button — to confirm the facility itself works and the mechanism is not seized.

The two prove different things and neither replaces the other. A device with a functioning button can be far outside its time limits; a device that passes the instrument test but whose button does nothing has a defective test facility, which matters because the user is the only person operating the device between inspections.

The notice required by Regulation 514.12.2, instructing the user to operate the test button, is itself an inspection item — confirm it is present and legible, and check the interval it prints. Earlier editions of BS 7671 required a quarterly user test; BS 7671:2018 changed the instruction to six months. A legacy notice giving the old interval is a recordable observation, not a compliant label.

The same logic applies to the test facilities on other devices. An arc fault detection device (AFDD) has an integral test facility that should be operated to prove the device responds, and a surge protective device (SPD) carries a status indicator that must be checked to confirm the module has not reached end of life. Neither check measures performance; both confirm that the device is present, alive and able to be maintained by the user.

"Properly Mounted, Adjusted and Installed"

The three words in Regulation 643.10 are worth taking one at a time, because each generates a different kind of defect:

• Mounted — the assembly is secure, the right way up, and fixed to a surface that will support it. A distribution board that flexes when the main switch is operated is not properly mounted.
• Adjusted — any device with a setting is set to the value the design requires. Adjustable overcurrent releases, motor overloads, adjustable RCDs and time-delay relays all leave the factory at a default that is rarely the designed figure. The setting actually applied is recorded, because a later inspector has no other way of knowing what it should be.
• Installed — the device is wired the way its manufacturer requires, including load-side connections, auxiliary supplies and any specific terminal arrangement.

This is why functional testing is not simply "switch it on and see". A motor starter whose overload is left at its default setting will either trip nuisance-fast or fail to protect the motor at all, and no instrument test elsewhere in the sequence would reveal it.

Procedure and Recording

1. Complete the other live tests first, so the installation is known to be electrically sound before you start operating plant.
2. Warn anybody affected. Functional testing means things start, stop and move. On an occupied site, agree in advance what may be operated and when.
3. Operate each device and observe the result — not the indicator lamp, the actual load.
4. Restore every device to its intended operating state afterwards. Leaving a circuit switched off after a functional test is a common and embarrassing handover failure.
5. Record the outcome on the schedule of test results, and raise any defect as an observation on the report where the work is a periodic inspection.

Two-way and intermediate switching deserves a specific mention because it is the most commonly under-tested arrangement in domestic work. A two-way circuit must be proved from both positions, and an intermediate circuit from every position — operating one switch and seeing the lamp change state proves only one of several possible combinations, and a miswired intermediate switch will still work from two of its four states.

At periodic inspection

Functional testing is often restricted on an occupied installation — you cannot stop a production line or a server room to prove an isolator. Whatever cannot be operated is recorded as a limitation, agreed with the client in advance where possible. A device that cannot be functionally tested has not been verified, and saying so on the report is part of the job.