AFDDs: Arc Fault Detection Devices
An AFDD watches for the electrical signature of an arcing fault — the loose terminal or damaged cable that overheats without ever drawing enough current to trip a breaker — and disconnects before it becomes a fire.
MCBs trip on overcurrent, RCDs trip on earth leakage — but neither reliably catches a series arc at a loose connection, which can smoulder and start a fire while drawing entirely normal current. Arc Fault Detection Devices exist to fill that gap, recognising the high-frequency electrical signature of an arc and disconnecting the circuit.
This guide explains what an AFDD does, the difference between series and parallel arc faults, how the device distinguishes a dangerous arc from harmless everyday switching, and where BS 7671 Regulation 421.1.7 recommends or requires them.
Key takeaways
- An AFDD detects arcing faults — the electrical signature of an arc — that MCBs and RCDs don't catch.
- Series arcs occur in a single conductor (a loose terminal, a broken strand) at normal current; parallel arcs occur between conductors.
- AFDDs use signal analysis to tell a dangerous arc from normal switching and appliance arcing.
- Regulation 421.1.7 recommends AFDDs for single-phase final circuits, and requires them for certain higher-risk premises.
- They're a fire-protection measure — complementary to overcurrent and RCD protection, not a replacement for either.
The gap AFDDs fill
Conventional protection has a blind spot. An MCB responds to overcurrent, an RCD to earth leakage. But a series arc — the tiny, sustained spark at a loose terminal, a partially broken conductor, or a damaged cable — draws no more current than the load normally would and leaks nothing to earth. Neither device sees it, yet the arc generates intense localised heat that can ignite surrounding materials.
Loose connections in accessories, nail or screw damage to cables, degraded insulation and poorly made terminations are exactly the sort of faults that arc without tripping anything. AFDDs were introduced to catch these fire-risk faults that fall between the coverage of overcurrent and residual current protection.
Series and parallel arc faults
A series arc occurs in a single conductor — where the current has to jump a gap in the line it's already flowing along, such as a loose screw terminal or a conductor with most of its strands broken. The current is limited by the load, so it stays at or below normal levels; that's precisely why an MCB never trips on it, and why it's so dangerous.
A parallel arc occurs between conductors — line to neutral, or line to earth — through damaged or degraded insulation. A parallel arc can draw high current and may eventually trip an MCB or RCD, but often not before it has done damage or started a fire. AFDDs are designed to detect both series and parallel arcing by their electrical signature rather than by current magnitude.
Series arcs are the sneaky ones
A series arc at a loose terminal carries normal current, so an MCB and an RCD both ignore it — while it quietly cooks the back of the socket. That's the fault an AFDD is really there for.
How an AFDD tells arc from noise
Arcing produces a characteristic high-frequency, erratic signature in the current and voltage waveform. An AFDD continuously analyses the waveform and looks for that signature — the broadband noise and irregular pattern that a genuine fault arc produces.
The hard part is discrimination. Normal operation is full of harmless arcing and switching: a light switch, a thermostat, a brushed motor, a plug pushed into a socket. A well-designed AFDD uses signal processing to distinguish these benign, expected events from the sustained, dangerous arc of a fault, so it disconnects on the real thing without nuisance-tripping every time someone flicks a switch. This discrimination is what separates a good device from a frustrating one.
What Regulation 421.1.7 says
BS 7671 addresses AFDDs in Regulation 421.1.7. Following Amendment 2, the regulation requires AFDDs for single-phase AC final circuits supplying socket-outlets rated up to 32A in certain higher-risk premises — specifically higher-risk residential buildings (such as buildings with sleeping accommodation like care homes), houses in multiple occupation (HMOs), purpose-built student accommodation and similar. For other installations the regulation recommends AFDDs as a protective measure the designer should consider.
So the position is a tiered one: mandatory in the defined higher-risk premises, recommended and to be considered elsewhere on a risk basis. As always, verify the exact wording and scope against the current edition of BS 7671 for the installation in front of you — the list of premises and the circuit scope are set precisely in the regulation and its amendments.
Check the current wording
The premises where AFDDs are required and the circuits they cover are defined precisely in 421.1.7 and its amendments. Read the current regulation for the specific job rather than relying on a summary.
Fitting AFDDs in practice
AFDDs are commonly available as a combined device — an AFDD/RCBO that provides arc fault detection, overcurrent protection and residual current protection in one module — or as a standalone AFDD used with a separate protective device. The combined unit is the tidy option in a consumer unit, giving one module per circuit that covers arc, overcurrent and earth-leakage faults together.
Treat the AFDD as an addition to your protection strategy, not a substitute for any of it. You still size the overcurrent device and provide RCD protection where required; the AFDD adds fire protection against arcing faults on top. Where the regulation requires them, they're not optional; where it recommends them, they're a genuine safety upgrade worth putting to the client, particularly on circuits in older properties where loose terminations and aged cabling are more likely.
Frequently asked questions
What does an AFDD do?
It detects the electrical signature of an arcing fault — a loose terminal, damaged cable or degraded insulation that arcs — and disconnects the circuit. It catches fire-risk faults that MCBs (which respond to overcurrent) and RCDs (which respond to earth leakage) don't reliably detect.
What's the difference between a series and parallel arc?
A series arc is in a single conductor — a loose terminal or broken strands — and carries normal current, so an MCB never trips on it. A parallel arc is between conductors through damaged insulation and can draw high current. AFDDs detect both by their electrical signature.
Are AFDDs required by BS 7671?
Regulation 421.1.7 requires them for single-phase final circuits supplying socket-outlets up to 32A in certain higher-risk premises, such as higher-risk residential buildings, HMOs and purpose-built student accommodation, and recommends them as a measure to consider elsewhere. Check the current edition for the exact scope.
Do AFDDs replace RCDs or MCBs?
No. An AFDD adds arc-fault fire protection on top of overcurrent and residual current protection. Combined AFDD/RCBO devices provide all three in one module, but the arc detection complements the other protection rather than replacing it.
From guidance to action
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