Smashing the myths about arc fault detection devices

Technical

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Exploding the common AFDD myths

Since being introduced to the IET Wiring Regulations in 2018, AFDDs have been the subject of much debate. Unfortunately, these discussions have also led to some misunderstanding about the technology, so our Associate Member is here to shatter five frequent misconceptions

DAVID ENEFER

Product Manager, Circuit Protection Systems

MYTH NO.1: AFDDS AREN’T REALLY REQUIRED

Regulation 421.1.7 of BS 7671:2018 +A2:2022 now states that AFDDs conforming to BS EN 62606 SHALL be provided for single phase AC final circuits serving socket outlets not exceeding 32A in: • Higher risk residential buildings (HRRB) • Houses in multiple occupation (HMO) • Purpose-built student accommodation • Care homes.

For all other premises, the use of AFDDs conforming to BS EN 62606 is recommended for single phase AC final circuits supplying socket outlets not exceeding 32A. Amendment 2 also provides further guidance, on the terms ‘shall’ and

“AFDDS DETECT AND DISCONNECT ARC FAULTS ON RING CIRCUITS, RADIAL CIRCUITS, SPURS AND LEADS, WHATEVER THE MODE OF CONNECTION”

‘recommended’ to help users understand the verbal forms used within BS 7671. ‘Shall’ is a requirement, ‘recommended’ means should. This clarification was necessary because the term ‘recommended’ in BS 7671 was either misunderstood or not being taken seriously by some organisations.

MYTH NO.2: AFDDS WON’T WORK ON RING CIRCUITS

I wish I had a fiver for every time I’ve heard this. It’s simply not true! AFDDs detect and disconnect arc faults on ring circuits, radial circuits, spurs and leads, whatever the mode of connection. Even if a ring becomes broken it becomes two radial circuits – and as we know, AFDDs work on radial circuits! The confusion may have come from the belief that arc faults can’t occur on a ring circuit because a broken ring is two radials and there is no potential difference at the break in the ring. This means that there can be no arc fault at that precise point, but it doesn’t mean that arc faults can’t occur at other points on that same circuit. Ring or radial, AFDDs will detect arc faults wherever they can occur in such final circuits.

MYTH NO.3: AFDDS ARE SUSCEPTIBLE TO NUISANCE TRIPPING

Once again, this is simply not the case. The microprocessor in an AFDD is looking for a specific arc fault signature on the wave form and uses a sophisticated algorithm. In addition, the product standard, BS EN 62606, includes a range of tests to ensure AFDDs won’t respond to expected conventional

‘operational arcing’ conditions when loads such as vacuum cleaners, washing machines, switches, etc are used.

MYTH NO. 4: AFDDS ARE COMPLICATED TO INSTALL

Early versions of AFDDs were often odd modular sizes and had unusual termination layouts that weren’t compatible with standard busbars in consumer units (CUs) and distribution boards (DBs). Consequently, installers were concerned about complicated assemblies and wiring arrangements. However, that’s changed and there is now a wide choice of AFDDs to suit standard CUs. One or two AFDDs fit standard DBs, which is ideal for landlords’ socketoutlets in care homes, while there are also some off-theshelf boards/enclosures designed specifically for accommodating AFDD circuits. Although it depends which brand is chosen, AFDDs are now more regular in shape and size and are as easy to integrate into installations as miniature circuit breakers (MCBs) and residual current breakers with overcurrent (RCBOs).

If you have a question about how AFDDs work and which consumer units to choose, it’s best to contact the manufacturer of those particular circuit protection systems. For installation requirements and processes, refer to BS 7671 and the manufacturer’s instructions.

MYTH NO.5: AFDDS ARE TRICKY TO TEST

In an effort to become familiar with AFDD technology, some early adopters made DIY test rigs and did ‘tests’ by touching wires together to create an arc. However, all this did was replicate the switching arcs which AFDDs are designed to ignore. It is therefore essential that valid tests are performed in an accurate and repeatable way in accordance with the relevant product standard, i.e. BS EN 62606. Such tests must be conducted by qualified competent persons in suitably equipped and approved test facilities. DIY tests will produce inconsistent and erroneous results.

For AFDDs installed in an electrical installation, there is no requirement in the IET Wiring Regulations to test them with an external test instrument. However, AFDDs include a self-test function which activates when the device is first energised and repeats this process at regular intervals. Self-testing intervals are timed so the self-test is done at least daily without tripping the device, unless a malfunction is detected.

Where installations include AFDDs with a manual test facility, i.e. a test button, these should be pressed every six months and when an EICR is carried out. In all cases, the manufacturer’s instructions and the requirements of BS 7671 shall be followed with respect to all testing activity and test button operation.

There are still some misconceptions surrounding AFDDs

SELECT Member on the benefits of using AFDDs: Pages 40 & 41