A Guide to Selecting and Applying Residual Current Devices (RCDs)

Introduction

Residual Current Devices (RCDs) are a fundamental safety component in modern electrical installations, protecting people from electric shock and guarding against fire caused by persistent earth fault current. But selecting the right one has gotten a lot more complicated. The rise of inverter-driven equipment — heat pumps, solar PV, EV chargers — introduces residual current characteristics that traditional RCDs simply weren't designed to handle. BEAMA's updated Guide to the Selection and Application of Residual Current Devices addresses exactly this, and this article summarises its key points on choosing the right RCD type for modern installations.

Why RCD Type Matters

An RCD works by detecting an imbalance between the current flowing in the line and neutral conductors, and disconnecting the supply once that residual current exceeds a preset limit. The complication is that different equipment produces residual currents with very different characteristics: a simple resistive load produces a clean AC residual current, while modern inverter-driven electronics — like heat pumps — generate more complex currents that can include pulsating DC components and mixed frequencies. Using the wrong RCD type risks two failure modes: unwanted nuisance tripping, or worse, "blinding" — a failure to trip at all during a genuine fault. Getting the type right isn't a formality; it's central to a safe, reliable installation.

RCD Types: AC, A, F, and B

Since Amendment 2 of BS 7671, Type AC RCDs are restricted to fixed equipment known to carry no DC component in its load current — meaning they're generally unsuitable for circuits feeding modern electronics.

• Type AC detects AC sinusoidal residual currents only, and its use is limited to loads with no DC component. Typical applications: standard lighting circuits and resistive heating with no DC present.
• Type A detects AC and pulsating DC residual currents, but isn't suitable for the complex composite currents inverters produce. Typical applications: socket outlets, general electronics, and single-phase equipment without inverters.
• Type F detects AC, pulsating DC, and composite residual currents, and is built to handle mixed frequencies plus up to 10mA of smooth DC without desensitisation. Typical applications: single-phase inverter-driven equipment such as some heat pumps, washing machines, and lighting controls.
• Type B detects AC, pulsating DC, and smooth DC residual currents, giving the broadest protection available for complex, multi-inverter systems. Typical applications: three-phase inverter equipment, large heat pumps, PV systems, EV chargers, and elevator drives.

Case Study: Heat Pumps

Heat pumps illustrate exactly how modern technology has complicated RCD selection. The inverters inside them can produce AC leakage currents with frequency components both above and below 50Hz, combined with DC components — a mixed-frequency composite residual current that a standard Type A RCD can't reliably detect.

BS 7671 doesn't contain heat-pump-specific RCD requirements, but the product safety standard BS EN IEC 60335-2-40 recommends a 30mA RCD. Choosing the type comes down to the equipment itself: if the manufacturer specifies an RCD type, Regulation 134.1.1 requires the installer to follow it; if not, the installer needs to judge between Type F and Type B based on the equipment's likely characteristics, with Type B offering the more comprehensive (and more expensive) protection.

A real-world example reported by Professional Electrician shows what happens when this goes wrong: a former BEAMA CEO experienced two years of persistent, unwanted tripping on his heat pump. The cause was the interaction between his heat pump, solar PV, and battery storage — all inverter-based — sharing a consumer unit with a Type A RCBO, which simply couldn't cope with the complex leakage currents the combined system produced. The fix came only after reconfiguring the circuit with a specialist Type B RCCB designed for exactly this environment.

RCD Selection for EV Charging

BEAMA has also published a dedicated guide, updated in 2025, on RCD selection for EV charging installations, covering how to comply with BS 7671 Amendment 2 in this specific context. It addresses how DC fault currents affect RCDs and the role of Residual Direct Current Detecting Devices (RDC-DDs), which can pair with standard RCDs or Type A devices to deliver the protection an EV charging circuit needs.

E Marking and What's Changing

The guide also clarifies the "E marking" found on some RCDs, which currently categorises devices by their method of operation and protection capability. That marking system is being phased out — the next revision of the relevant product standards, expected around 2027, will remove E marking classification entirely, with device suitability defined directly by conformity to performance requirements instead. That should simplify product marking for installers and specifiers going forward.

Conclusion

Choosing the right RCD is a genuine design decision with real safety and reliability consequences, and "one size fits all" no longer holds up against the spread of inverter-based technology in modern buildings. Understanding the practical differences between Type AC, A, F, and B — and where each one actually belongs — matters more than ever. For anything involving heat pumps, solar PV, battery storage, or EV charging, working from a detailed resource like the BEAMA RCD guide is the difference between a trouble-free installation and years of chasing nuisance trips.

This article summarises key concepts from the "BEAMA Guide to the Selection and Application of Residual Current Devices (RCDs)." The full guide is available as a free download from BEAMA.