RCD protection is the most frequently deficient safety measure found in HMOs, and AFDDs are the requirement most often missed. Three regulations do the work: 411.3.3 for socket-outlets, 411.3.4 for lighting, and 421.1.7 for arc fault detection.
- Reg 411.3.3 — 30 mA RCD on socket-outlets up to 32 A
- Additional protection by a 30 mA RCD is required for socket-outlets with a rated current not exceeding 32 A, and for mobile equipment up to 32 A used outdoors. The documented risk assessment exception applies only to indent (b) — it cannot be used for socket-outlets liable to be used by ordinary persons or children, which is the normal case throughout an HMO. Age of the property is not a defence.
- Reg 411.3.4 — 30 mA RCD on lighting circuits
- Within domestic (household) premises, AC final circuits supplying luminaires require additional protection by a 30 mA RCD. HMOs are domestic premises, so every lighting circuit is caught. This regulation was introduced with BS 7671:2018 and is not new at A4:2026 — installations designed since 2019 should already comply.
- Reg 701.411.3.3 — bath and shower rooms
- RCD protection is required for low voltage circuits serving a location containing a bath or shower, and for circuits passing through zones 1 and 2 that do not serve the location.
- An RCBO per circuit, not a split-load board
- An RCBO on each circuit gives per-circuit discrimination, so a fault in one bedroom does not put the whole house in darkness. In a shared property that difference matters more than it does in a single-family home.
AFDDs are required in HMOs, not recommended
Regulation 421.1.7 of BS 7671:2018+A4:2026 states that arc fault detection devices conforming to BS EN 62606 shall be provided for single-phase AC final circuits supplying socket-outlets with a rated current not exceeding 32 A in four named premises types: high rise residential buildings, houses in multiple occupation, purpose-built student accommodation, and care homes. For every other type of premises the same regulation only recommends them. Where AFDDs are used, they shall be placed at the origin of the circuit to be protected.
BS 7671 does not assign EICR observation codes to individual non-compliances, so how a missing AFDD in an existing HMO is coded is a matter of the inspector’s judgement against the installation in front of them. Our AFDD guide for HMOs and care homes works through the coding decision in detail.