RCD selection for heat pump installations

As more customers choose heat pumps, it's vital that installers stay up-to-date on the requirements for safe operation. This includes specifying the correct residual current device (RCD).

Understanding the challenge

Heat pumps, unlike traditional resistive heating appliances, typically use inverter-driven compressors to convert alternating current (AC) to direct current (DC) and back again. This conversion process can introduce complex residual current waveforms — including smooth DC and high-frequency components — that standard RCDs may not detect or handle effectively.

If an RCD is exposed to smooth DC leakage currents beyond its design capability, it can become magnetised, impairing its ability to trip during a genuine earth fault. This situation, commonly referred to as RCD blinding, can compromise electrical safety and leave the installation non-compliant with BS 7671 (Requirements for Electrical Installations).

Types of RCDs and their applications

The industry currently recognises several RCD types — AC, A, F and B — each suited to different current waveforms:

• Type AC: Detects only alternating residual currents.
• Type A: Detects AC and pulsating DC residual currents.
• Type F: Handles mixed-frequency residual currents up to around 1 kHz.
• Type B: Detects AC, pulsating DC, and smooth DC residual currents, and may provide sensitivity to frequencies above 1 kHz.

For inverter-driven equipment such as heat pumps, Type B RCDs are generally the most appropriate, as they provide the broadest detection range and prevent magnetisation from smooth DC components. However, it is worth noting that not all Type B devices are tested to the same performance envelope. Many are certified up to 1 kHz, whereas some inverter systems — particularly heat pumps and EV chargers — can generate residual currents at higher frequencies. This is where enhanced Type B devices designed for high-frequency operation can play an important role.

Compliance and best practice

BS 7671:2018+A2:2022 Regulation 531.3.3 highlights the importance of selecting RCDs that remain functional in the presence of DC components. Meanwhile, Regulation 134.1.1 requires that installers follow both the manufacturer’s guidance and recognised standards when selecting protective devices. In practice, this means:

• verifying that the RCD type is suitable for the specific inverter topology used in the heat pump
• consulting the heat pump manufacturer’s documentation to determine any explicit RCD requirements
• ensuring that the RCD complies with BS EN 61008 or BS EN 61009, as appropriate, and that its frequency and current characteristics align with the installation conditions.

It’s also essential to recognise that the so-called ‘Type B+’ devices — often referenced in European literature — are a VDE-specific classification and not formally recognised within BS 7671. Installers should therefore take care to select products that conform to UK-recognised standards.

The role of manufacturers and industry collaboration

The CIPHE Industrial Associate member Hager has produced an RCD suitable for heat pump applications, with an extended frequency sensitivity that will ultimately lead to the improved performance of a heat pump, protecting consumers against issues such as nuisance tripping. As the UK continues its path in decarbonising its heating systems, the CIPHE advocates industry collaboration between all stakeholders to ensure a smoother transition whilst protecting the consumer.

As heat pump technology evolves, so too must the approach to electrical protection. Installers, specifiers, and designers need to remain aware of emerging guidance, new device classifications, and future updates to BS 7671 that reflect the increasing presence of DC and high-frequency components in domestic and commercial installations.

The CIPHE continues to advocate for competence-led design and installation, where decisions are based on verified technical evidence rather than assumption or marketing claims. Ultimately, ensuring that RCDs perform as intended under all operating conditions is a cornerstone of electrical and user safety within the low-carbon future.

• Read our archive of articles on low-carbon heating systems at pandhengineering.co.uk/advice