AFDD vs RCD
Different Faults
An AFDD plus an RCD detect entirely different faults. They are not really vs each other but layered together. What each catches, where each fits plus how the BS 7671 protection layers stack up in 2026.
An RCD detects residual current, the small leakage that flows when a person touches a live conductor or when an appliance has an earth fault. An AFDD detects arc faults, the high-frequency current signatures of damaged or loose wiring before they cause fire. They look for completely different faults at completely different frequencies. They do not replace each other. On modern UK installs, especially HMOs plus higher-risk residential buildings, BS 7671 Section 421.1.7 calls for both to be present together.
Three figures
worth knowing
RCD Threshold
Residual current at which an RCD must trip for shock protection. Detection at 50Hz mains frequency. Disconnection within 40ms.
AFDD Detection
AFDDs analyse current signatures up to several kHz to spot the high-frequency arc patterns that earth fault detection cannot see.
BS 7671 Section
Section 421.1.7 mandates AFDDs on socket circuits in HMOs, HRRBs, care homes plus PBSA. Other premises remain risk-assessed.
What each device
actually detects
An RCD plus an AFDD are looking for completely different things at completely different frequencies. Understanding what each detects is the key to understanding why both exist.
Residual Current Device
UK domestic standard since 2008What it detects
Imbalance between the line plus neutral currents at the device. If 10A flows in the line plus only 9.97A returns through the neutral, the missing 30mA must be flowing somewhere else. Typically through a person to earth or through a faulty appliance casing to earth.
What it catches
Direct contact with a live conductor (someone poking a metal object into a socket). Earth faults on appliance casings. Damaged flex insulation that puts the line into contact with the case. Wet equipment leaking current to earth.
What it cannot catch
Faults that do not produce a residual imbalance: series arcing in a damaged cable behind plasterboard, parallel arcing between two intact conductors, loose terminations. None of these create a leakage to earth so an RCD sees nothing.
Arc Fault Detection Device
UK mandate from 2018 onwards in HMOs etcWhat it detects
The high-frequency electrical signature that an arc fault produces in the current waveform. An arc fault generates rapid plus chaotic current changes at frequencies far above the 50Hz mains. The AFDD's onboard processor analyses the live conductor current looking for this arc-fault waveform pattern.
What it catches
Series arcing in a damaged cable (a partly cut cable behind plasterboard slowly burning). Parallel arcing between two intact conductors with degraded insulation. Loose connections at terminations. Damaged appliance flex with internal arcing. All the fire-starting fault modes that pre-date a complete circuit failure.
What it cannot catch
Earth faults that are not yet arcing. Direct contact between a person plus a live conductor (no arc waveform produced). Slow earth leakage. The earth-fault failure modes are exactly what an RCD is designed to handle, which is why both devices belong together.
Four layers of protection
on a modern UK board
Each protective device is designed to catch one specific kind of fault. Together they form a layered defence rather than a competing set of options.
Earth faults plus arc faults are different things
The reason an RCD does not replace an AFDD (plus vice versa) is that the two faults look completely different from the device's point of view. To see why, picture the two underlying physical scenarios.
The earth fault scenario (RCD territory)
A person touches a live conductor. The current that flows through that person to earth has to come from somewhere. It comes from the line conductor of the circuit. But the neutral conductor never sees that current return. The current went through the person to earth instead. The line current plus the neutral current are now imbalanced. The RCD measures both currents simultaneously plus trips when the imbalance exceeds 30mA. Operation time is within 40ms which is fast enough to prevent ventricular fibrillation in a healthy adult.
The same scenario plays out when an appliance casing develops an earth fault: current flows through the casing to earth via the protective conductor instead of returning through the neutral. Imbalance detected, trip activated, danger averted.
The arc fault scenario (AFDD territory)
A nail driven into a wall has nicked a buried cable. The line plus neutral conductors are still intact but the insulation between them has been compromised. Tiny arcs jump across the damaged section every few mains cycles. The arc current is small, intermittent plus varies wildly in frequency. From an MCB's point of view nothing is wrong because the mean current is normal. From an RCD's point of view nothing is wrong because the line plus neutral currents still balance (all the arc current returns through the neutral conductor).
What is wrong is that the arc generates intense localised heat at the damage point. Over weeks or months that heat can ignite the surrounding plaster, dust or insulation. Arc faults are the leading single cause of UK electrical fires. The AFDD is the only protective device on a domestic board that can detect the arc waveform plus disconnect the circuit before the fault becomes a fire.
Why both are needed
Each device is blind to the fault type the other is designed to catch:
- An RCD does not see arc faults because parallel arcing between intact line plus neutral conductors does not produce a residual imbalance.
- An AFDD does not see classic earth faults because direct contact between a person plus a live conductor does not produce an arc waveform.
The 18th Edition of BS 7671 plus its amendments have made both devices increasingly common together. Section 421.1.7 mandates AFDDs on socket circuits in HMOs, higher-risk residential buildings, care homes plus purpose-built student accommodation. Section 411.3.3 plus 411.3.4 mandate 30mA RCDs on most circuits in dwellings. The two requirements layer together rather than competing.
Where the AFDD is going next
The current 2022 Amendment 2 position is that AFDDs are required only in higher-risk premises. A risk-assessment approach permits them in ordinary dwellings. As AFDD module costs continue to fall most installers expect the next BS 7671 amendment to widen the mandate. Several manufacturers now produce combined AFDD-RCBO modules that fit a single way on the busbar plus deliver all three functions (overload, earth fault plus arc fault) in one device. These are likely to become the default on new installs over the next few years.
Four things every homeowner
should know
Different faults, different devices
RCDs handle earth faults plus electric shock. AFDDs handle arc faults plus fire prevention. Neither replaces the other.
AFDD is mandatory in some premises
HMOs, higher-risk residential buildings, care homes plus PBSA require AFDDs on socket circuits under Section 421.1.7.
Combined modules are arriving
AFDD-RCBO combined devices fit one way on the busbar plus deliver all three functions. Expect these to dominate new installs in the next few years.
Layered protection is the goal
MCB, RCD plus AFDD plus SPD form four layers of defence. Each catches a different fault type. Modern boards stack them all.
RCBO plus AFDD Boards in Milton Keynes
C-Lec Electrical fits BS 7671 compliant consumer units with full RCBO protection plus AFDD modules where required across Milton Keynes, Bedford plus the surrounding Bedfordshire area. EIC plus Building Control notification handled in full.
For the wider context on consumer units, RCBOs, AFDDs plus the regulations behind UK distribution boards, head back to our full guide to consumer units where every common question is answered in one place.
Back to the Consumer Units Guide
This article sits inside our complete Consumer Units knowledge base. The hub covers everything from board types plus RCBOs through to landlord requirements plus BS 7671 wiring regulations.
More on protection
devices
To dig deeper into the AFDD device alone, head to what is an AFDD board. To understand how the RCD legal framework works in UK domestic installs, see are RCDs legally required. To understand the SPD layer that completes the four-layer modern protection stack, see what is an SPD. If you need a new consumer unit fitted in Milton Keynes or Bedford, our consumer unit upgrades service page is the fastest route to a quote.