Type A RCDs & RCBOs — Why Your London Property Now Needs Them

Anything with a switched-mode power supply, IGBT or thyristor controller, or rectifier feeding a DC load: induction hobs, EV chargers, modern LED drivers, variable-speed washing machines and tumble dryers, heat pump inverters, home battery bidirectional inverters, certain inverter air-conditioning, PV inverters. In a 2026 London home, every kitchen and most lighting circuits will feed at least one.

Type AC detection uses a current transformer optimised for sinusoidal AC waveforms. Pulsating DC saturates the core. Above a threshold of background DC residual current the core no longer responds to additional AC fault current — known as DC blinding. The device passes a button test (which uses an internal AC source) but will not actually trip on a real earth fault. This is why physical inspection or downstream testing is required, not just press-to-test verification.

Type B is required where smooth (continuous) DC fault current can occur — primarily three-phase variable-frequency drives, some industrial inverters, and EV chargers using certain rectification topologies (rare on domestic 7 kW chargers, more common on commercial fast chargers). Domestic single-phase EV chargers typically integrate Type B detection within the charge unit and require only a Type A or Type AC RCD upstream — confirmed against the manufacturer datasheet.

Three tests beyond press-to-test: ramp test (apply increasing residual current and measure trip threshold), trip-time test at rated and 5x rated current, and verification of no-trip below the no-trip threshold. Where the existing board feeds DC-producing loads we additionally measure standing DC residual current — a non-zero reading is itself an indication that Type A is required regardless of test pass.

It depends on what the circuit feeds. Lighting on AC-only Type AC with no known DC load: typically C3 (improvement recommended). Kitchen ring feeding induction hob on Type AC: C2 (potentially dangerous) under most inspector guidance from late 2024 onwards. EV charger on Type AC upstream: C2. Unknown loads where the inspector cannot verify: FI (further investigation required).

On a current-manufacturer board (Hager, Wylex Amendment 3 metal, MK Sentry, Crabtree) with an existing Type AC RCBO range, the physical swap is direct — same slot, same busbar. On an older split-load board where the RCD is a separate device upstream of MCBs, the Type A retrofit may require replacing the RCD module and verifying that downstream MCB ratings remain compliant. Older boards may not accept any compatible Type A device — replacement is then cheaper than retrofit.

Yes, where the per-circuit specification supports it. We commonly install Type A on kitchen ring, EV charger circuit, and any lighting circuit feeding switched-mode LED drivers; Type AC may remain on circuits that demonstrably do not feed DC-producing loads (some bathroom circuits, some pure-resistive heating circuits). Under A4:2026 the default is Type A across the board, with the burden of justification on retaining Type AC.

AFDD modules can be specified as RCBO + AFDD combined devices, or as a separate AFDD module upstream of a standard RCBO. Combined devices save consumer-unit space; separate modules allow easier AFDD addition later. Both options work with Type A — the AFDD adds series-arc detection on top of the existing overcurrent and earth-leakage functions.

For a single 7 kW domestic EV charger, no — the charger circuit is fitted with a dedicated Type A RCBO inside the main consumer unit. Where multiple EV chargers, a home battery, and a heat pump are co-located, a separate sub-consumer-unit with Type A protection and SPD is often the cleanest solution. We quote either approach against your specific load profile.

Yes — a Type A consumer unit upgrade resolves the most common C2 and C3 codes found on landlord EICRs in 2025/26: missing or inadequate RCD protection, plastic-bodied enclosure, and AC-only protection on DC-capable circuits. The installation certificate alongside a fresh EICR forms a complete landlord compliance pack accepted by every London letting agent we work with.