Helpful video reference. We use Electrium's official overview "Arc Fault Detection Devices from Crabtree" as the video reference here. Electrium manufactures Crabtree, Wylex and Volex products in the UK and their AFDD guidance is grounded in BS 7671 Amendment 2 requirements. It covers why the devices are required, how they detect arc faults, and how the single-module Crabtree version clips into a Starbreaker board in the same position as an MCB.
1. Understand what an AFDD does
A conventional MCB trips on overcurrent or short circuit. An RCD trips on earth leakage. Neither detects the arcing that occurs in damaged cable insulation, a loose connection in a back box, or a frayed flex in a hidden cavity. That arcing can ignite surrounding material long before enough current flows to trip anything else.
An AFDD monitors the waveform on its circuit continuously. It recognises the irregular current pattern that arcing produces and disconnects the circuit within milliseconds. BS 7671 Amendment 2 (October 2022) recommends fitting them on AC final circuits in high-risk locations in dwellings: bedrooms, living rooms, and similar areas where fires starting from concealed wiring could be catastrophic.
They are not retrospectively mandatory in existing homes unless major work is being done, but they are cheap insurance on a vulnerable circuit. A single-module Crabtree AFDD occupies one way in a Starbreaker board, exactly where the old MCB was.
2. Check your consumer unit is compatible
The first thing to establish is whether your board can accept an AFDD at all. Since 2016, new consumer units in the UK have been required to have a steel enclosure under Amendment 3 of the 17th Edition. These metal boards accept AFDDs. Old plastic-bodied consumer units do not, and the answer in those cases is usually a full board replacement rather than trying to retrofit.
AFDDs are also brand-specific. Crabtree units fit Crabtree Starbreaker boards. Wylex AFDDs fit Wylex boards. Hager, Schneider and others have their own ranges. Check the manufacturer's compatibility chart before buying.
3. Choose your moment: isolate fully
Turn off the main switch on the consumer unit. If the board has a hasp, lock it out. Tape a warning notice to the board if anyone else is in the building. Use an approved voltage indicator to confirm that the busbars on the load side of the main switch are dead before touching anything. The supply side of the main switch and the meter tails are still live: do not go near them.
4. Remove the existing MCB
Note which position you are working on and which circuit cable is connected. Take a clear photo. Loosen the load-side terminal screw and withdraw the cable conductor. On a clip-in busbar type, insert a flat-bladed screwdriver into the slot at the base of the MCB and lever the device off the busbar. It will require a firm push. Keep the MCB if it is in good condition: it can go back in if needed.
5. Clip the AFDD onto the busbar
Align the AFDD with the busbar rail and press it firmly into the vacated position. You should feel it seat with a positive click. Connect the load-side cable to the AFDD load terminal and tighten to the manufacturer's specified torque. The neutral return from the circuit goes to the neutral bar, exactly as it did from the MCB.
If your existing circuit used 2.5mm twin and earth for sockets or 1.5mm for lighting, the conductor terminations are the same as they were. Just transfer them carefully one at a time.
6. Restore power and test
Close the consumer unit cover, turn the main switch on, and switch the new AFDD to the on position. The circuit should operate normally. Now press the test button on the face of the AFDD: it should trip and disconnect the circuit. Reset it by switching it back to on. Check that the sockets or lights on that circuit are working correctly.
The test button simulates an arc fault signal. Press it regularly, at least once every six months, just as you would for an RCD test.
7. Label the circuit and notify
Update the consumer unit label to show that the circuit now has AFDD protection. Fitting an AFDD is notifiable electrical work under Part P. If you are a homeowner doing this yourself, you need to notify your local Building Control before starting (or use a scheme member who self-certifies). A qualified electrician registered with NICEIC, NAPIT or a similar scheme can self-certify and issue an electrical installation certificate, which is the right route for most people.
When to call us
If your board is plastic, old, or does not have AFDD-compatible positions, the sensible answer is usually a full consumer unit upgrade rather than a patch. Richard can supply and fit a steel 12-way board with individual RCBOs and AFDDs on bedrooms and living rooms, complete with EICR and Part P certification. Call or message for a quote.
Want AFDDs fitted in Sandwich?
Richard can replace or upgrade a consumer unit with AFDD protection on bedroom and living room circuits, fully certified under Part P.
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