Arc Fault Detection Devices (AFDDs)

Fire versus shock: two different problems

Residual Current Devices (RCDs) (and RCBOs that include RCD functionality) are primarily aimed at electric shock protection and certain earth fault conditions. They detect imbalance between live and neutral conductors and disconnect when leakage current exceeds a threshold (typically 30 mA for final circuits in domestic settings).

Arc Fault Detection Devices (AFDDs) are aimed at a different failure mode: series and parallel arcing on final circuits — for example damaged flex, loose terminations, or crushed cable that produces sustained arcing. That arcing may not draw enough fault current to operate an MCB quickly, and may not involve earth leakage that an RCD will see. The result can be localised heating and ignition of nearby combustible material — a fire risk rather than a shock risk.

In short: RCD ≈ shock and certain earth faults. AFDD ≈ arcing fire risk on the circuit it protects. They complement one another; neither replaces the other.

Amendment 4 (2026) and high-risk buildings

BS 7671:2018+A4:2026 continues the evolution of protection against fire and degradation in modern installations. For high-risk scenarios — including certain sleeping accommodation, structures where evacuation is difficult, or where fire development would be rapid — the Regulations and supporting guidance increasingly expect designers to consider arc fault protection on specified final circuits.

Exact requirements depend on circuit use, building type, and risk assessment. Your electrician must apply the current BS 7671, any applicable Building Regulations in England, and manufacturer instructions. Where AFDDs are mandated or selected, they are normally installed at origin of the circuit to be protected (for example RCBO forms combining overcurrent, earth-fault and arc-fault functions).

Tags: Safety · Regulations · Amendment 4 · Fire prevention

Why this matters in Thanet: timber frame and older stock

Timber-framed and lightweight modern dwellings are common in new build and extension projects around Margate, Ramsgate and Broadstairs. Timber structures can allow fire to spread quickly if an electrical ignition source is present; AFDDs form part of a layered strategy alongside correct cable support, zoning, and detection.

Older Thanet homes — Victorian terraces, inter-war semis, and 1960s–1980s estates — often combine original cabling, mixed DIY alterations, and high humidity from coastal air. Accessories and terminations age faster in damp atmospheres; loose screws and damaged rubber or PVC insulation are realistic arc-fault scenarios. An upgrade that includes RCBO protection and, where required or agreed, AFDD protection can materially reduce fire risk on final circuits.

We routinely assess consumer units in the Isle of Thanet against the current edition of BS 7671 and advise proportionately — including whether AFDDs are mandatory for your use-case or a worthwhile voluntary upgrade.

Nuisance tripping and product selection

AFDDs may nuisance-trip on some legitimate loads (certain motor drives, dimmed lighting, or appliance characteristics). Competent selection, segregation of “noisy” loads, and follow-up testing reduce false operations. If an AFDD trips, treat it as a potential fault until a qualified electrician has investigated.