AFDDs Explained: Purpose, Requirements and Differences from RCBOs
Understand arc-fault protection, check which requirements apply, and separate new-work decisions from the assessment of existing wiring.
By EICR Certificates London · Updated 1 November 2026
An AFDD is an arc fault detection device that disconnects a circuit when it detects dangerous electrical arcing, helping reduce the risk of fire.
Its protection differs from an RCBO’s residual-current and overcurrent functions. For London property owners, the practical questions concern the building, its circuits and any proposed work.
How Does an AFDD Detect an Arc Fault?
An AFDD analyses electrical waveforms for patterns associated with dangerous arcing and disconnects the affected circuit when it detects them. An arc occurs when electricity crosses a gap through material that would normally insulate it, such as air. Damaged conductors, loose connections and deteriorated insulation can create these conditions.
The device is designed to distinguish hazardous fault patterns from normal switching activity. As the IET’s AFDD consumer guidance explains, waveform analysis provides protection beyond conventional circuit breakers. However, an AFDD cannot repair damaged wiring or detect every overheating connection. Thermal damage still needs investigation, including where PVC twin and earth cable or accessories appear affected.
| Arc type | Conceptual explanation |
|---|---|
| Series arcing | Current crosses a damaged point along its normal path, such as a break within one conductor or a loose termination. |
| Parallel arcing | Current crosses between conductors through damaged insulation, creating an unintended path alongside the connected load. |
Where Are AFDDs Required?
AFDD requirements depend on the premises, the circuit being protected and the scope of the electrical work. The relevant provision concerns single-phase AC final circuits supplying socket-outlets rated no higher than 32 A. That wording refers to the socket-outlet rating; identifying a circuit only by its breaker rating is insufficient.
| Premises | Circuit scope | Required or recommended for relevant installation work | What this means for an existing installation |
|---|---|---|---|
| Houses in multiple occupation | Single-phase AC finals serving socket-outlets up to 32 A. | Required. | Consider the proposed alterations alongside inspection findings and applicable licence conditions. |
| Purpose-built student accommodation | The socket-supplying circuits described above. | Required. | Establish which circuits serve the accommodation and what work is being undertaken. |
| Care homes | Relevant single-phase socket-outlet final circuits. | Required. | Assess existing protection and the scope of any installation project. |
| High rise residential buildings within the standard’s category | Final circuits meeting the stated supply and socket-rating criteria. | Required. | Confirm the building’s classification before specifying circuit protection. |
| Other premises | Single-phase AC circuits supplying sockets within that rating limit. | Recommended. | Consider the recommendation through a design assessment; absence alone does not establish danger. |
This matrix neither assigns an EICR outcome nor creates a universal instruction to retrofit every existing board. Ring final circuits and radial circuits both need assessment according to what they supply and the work proposed.
When board replacement is being considered, discuss consumer unit upgrade options with the electrician so the specification addresses the relevant protection requirements.
What Do the AFDD Requirements Mean for an HMO?
For an HMO, the electrician must distinguish AFDD provision for relevant installation work from the assessment of wiring already in service. The HMO entry above identifies required protection for the specified socket circuits. It does not establish the condition of a particular property. HMO status and licensing coverage are related but separate: whether a property requires a particular licence is not the sole test for AFDD applicability.
- Proposed work: identify whether the quotation concerns new circuits, alterations, protective-device changes or replacement of the consumer unit.
- Inspection findings: provide the existing report and observation schedule, including the inspector’s reasons for recommended work.
- Licence wording: have the actual conditions available; clarify any uncertainty with the local housing licensing authority.
For a board-change discussion, bring these documents when arranging HMO and property licence EICRs. They help separate the installation specification from inspection findings and licensing obligations.
AFDD Recommendations for Other Homes and Premises
Outside the categories requiring AFDDs, the standard’s recommendations still need to be considered for the relevant circuits. A designer assesses circuit use, the premises and the consequences of arcing. Combustible construction and valuable or irreplaceable contents can inform that discussion. A documented recommendation explains the proposed design; it does not, by itself, establish that an existing home is unsafe.
- Which circuits would benefit from arc-fault protection, and why?
- How do the building’s construction, use and contents affect the assessment?
- What protection is included in the electrical works quotation, and how is the design decision recorded?
BS 7671 Requirements, Legal Duties and Existing Installations
BS 7671 sets technical installation requirements; an existing installation’s condition and a landlord’s legal duties require separate assessment.
- Installation standard: BS 7671:2018+A4:2026 is the current edition. Older AFDD explanations may describe earlier recommendations or premises terminology. What Is BS 7671 explains its wider role and legal status.
- Legal duties: England’s 2020 electrical-safety regulations, extended to social rented housing by the 2025 amendment, impose landlord obligations. BS 7671 is not itself an Act of Parliament.
- Condition assessment: periodic inspection and testing examines whether existing wiring remains suitable for continued service. This differs from designing and certifying new electrical work.
A newer edition does not automatically make an older installation unsafe or require wholesale replacement. Decisions should reflect its actual condition, applicable duties and the proposed changes.
AFDD vs RCBO: Which Protection Does Each Provide?
An AFDD detects dangerous arcing, while an RCBO combines residual-current and overcurrent protection; a combined AFDD/RCBO provides both sets of functions.
| Device or function | Overload and short-circuit protection | Residual-current protection | Arc-fault detection |
|---|---|---|---|
| MCB | Yes | No | No dedicated detection |
| RCCB | No | Yes | No dedicated detection |
| RCBO | Yes | Yes | No dedicated detection |
| AFDD function alone | No | No | Yes |
| Combined AFDD/RCBO | Yes | Yes | Yes |
An AFDD label alone does not establish which other functions the product incorporates. Check its documentation. Residual-current protection, including 30mA additional protection where specified, remains necessary. RCD, RCBO, MCB and RCCB Explained covers the broader differences between these devices and the RCD family.
For example, a series arc at a damaged connection may carry ordinary load current without leakage to earth. Neither excessive current nor a sufficient residual-current imbalance may occur to trip an RCBO. Conversely, some faults involving arcing can operate conventional protection. Dedicated arc detection addresses a different fault characteristic; it does not remove the need for the other protective functions.
Can an AFDD Be Retrofitted to an Existing Consumer Unit?
An AFDD can be retrofitted only where the consumer unit, selected device and existing circuits are suitable for the proposed arrangement.
- Assembly compatibility: check documented manufacturer support for the board and device combination, considering the consumer-unit assembly standard BS EN 61439-3.
- Space and connections: assess available ways, spare capacity, terminals and connection arrangements. Physical fit or a shared brand name does not establish compatibility.
- Circuit condition: undertake appropriate pre-upgrade checks, including insulation resistance testing, with precautions for connected electronic equipment.
- Protective functions: select suitable arc-fault, residual-current and overcurrent protection for each affected circuit.
- Verification and records: include inspection, testing and the appropriate completion certificate in the work scope.
These checks determine whether adding protection to an existing consumer unit is practical. Replacement may be appropriate when no supported arrangement is available, but it is not an automatic consequence of considering AFDDs.
An EICR records condition. Completed installation work requires its own documentation: a consumer unit replacement receives an Electrical Installation Certificate, while some smaller alterations may qualify for a Minor Electrical Installation Works Certificate.
Discuss Your EICR and Protection Options in London
EICR Certificates London can help you arrange an EICR and discuss electrical protection work arising from its findings. Your EICR is carried out by a NICEIC-registered engineer.
Open 8am to 10pm, seven days a week. Free, no-obligation quotes for remedial and repair work are available; the inspection and any subsequent work have separate scopes.
Have your property type, postcode, existing report and proposed work ready so the discussion can address your installation and the decisions you need to make.
AFDD Questions About Circuits, EICRs and Maintenance
The remaining questions concern circuit coverage, inspection findings, user checks, costs and device indications.
Do I Need an AFDD on Every Circuit, Including Lighting?
AFDD requirements do not amount to a blanket instruction to fit one on every circuit, including every lighting circuit.
The applicability table concerns final circuits feeding socket-outlets within the stated rating. A lighting-only circuit has a different use, although a design assessment may specify additional protection. Circuit labels should accurately identify what is supplied: calling something a radial or ring circuit does not settle the question.
Does Having No AFDD Automatically Mean an EICR Fails?
The absence of an AFDD does not, by itself, establish an automatic unsatisfactory EICR outcome. A qualified and competent inspector evaluates the installation and records observations in context. A design requirement and a classification code answer different questions.
Read EICR Codes Explained when interpreting an actual observation. Neither C2 nor C3 should be assumed simply from knowing that no AFDD is fitted; the recorded finding and its reasoning matter.
How Should an Occupier Test an AFDD?
Follow the instructions for the fitted AFDD, including use of its accessible test button where a manual test is specified. Manual checks and automatic self-testing depend on the product; a combined device may also have instructions for its RCD function.
A user check does not replace periodic inspection or the electrician’s recorded test results. If the prescribed check fails, seek an electrician’s assessment. Do not remove covers, simulate arcing or improvise test equipment.
What Affects the Cost of Adding AFDD Protection?
The cost depends on the circuits involved, compatible equipment, the condition of the existing installation and whether the consumer unit needs replacement. Spare ways and the required protective functions also affect the scope. A retail device price is not an installed quotation; this work is quoted.
Quotation checklist: devices and quantities; labour; inspection and testing; certification appropriate to the work; VAT; and any separately quoted remedial repairs. Confirm whether board replacement is included.
What Do the Lights or Fault Codes on a Wylex AFDD Mean?
The meaning of an AFDD indicator depends on the exact model and the manufacturer’s instructions. A combined AFDD/RCBO contains several protective functions, so an indication alone does not establish the underlying fault.
Record the identification: note the visible model reference and circuit label without removing covers.
Record the indication: describe the displayed light or pattern. Consult the matching Wylex instructions or give those details to the electrician. Avoid repeated resetting to investigate the cause.