Martin Plumbridge, Doepke Technical Manager, discusses heat pumps and RCD selection
Electrical characteristics of inverter-driven heat pumps, residual current waveforms, and practical selection of RCDs/RCBOs to support safe and reliable operation.
Introduction
Modern air source and ground source heat pumps (ASHPs/GSHPs) are electrically powered refrigeration systems. Unlike legacy electric heaters, most heat pumps use inverter drives (power electronics) to vary compressor and fan speed for efficiency and low starting current. Power electronics change what the electrical installation “sees”: the equipment can generate leakage currents that are not purely 50Hz sinusoidal AC.
This matters because RCDs/RCBOs are waveform-dependent protective devices. Selecting an unsuitable RCD type can lead to nuisance tripping (loss of heating), or in the worst case “blinding” (reduced ability to detect a dangerous earth fault). This article summarises the electrical behaviour of inverter-driven heat pumps and provides a practical approach to upstream RCD selection in line with BS 7671 and manufacturers’ instructions.
Why RCD type matters
An RCD measures imbalance between current carrying conductors. If current is leaking to earth (through insulation failure, moisture ingress, damaged cable, or a person), the imbalance rises and the device trips.
However, not all RCDs respond to all residual current waveforms. Smooth DC and some high frequency components can saturate the sensing core of certain RCD types. This is commonly described as RCD blinding. Separately, normal inverter-related leakage and switch-on transients can cause unwanted tripping if the RCD sensitivity/type is not suited to the equipment.
Standards context
RCD selection for inverter driven equipment is primarily governed by the general requirements of BS 7671 rather than a single dedicated heat pump section. Key themes are:
- Follow manufacturers’ instructions (BS 7671 Reg 134.1.1): if the heat pump specifies an RCD type or special characteristics, design accordingly.
- RCD type selection by waveform (Reg 531.3.3): choose an RCD type suitable for the residual current that may occur.
- RCD rating/sensitivity and cumulative leakage (Reg 531.3.2): avoid unwanted tripping by considering protective conductor currents of connected equipment.
- Additional protection (typically 30 mA): often required in domestic circuits depending on cable installation method and location (e.g., cables in walls, outdoor equipment feeds).
RCD/RCBO types in practice
For single phase equipment, the use of Type F or B RCDs is governed by the level of smooth dc residual current produced under certain fault conditions. Some examples of basic configurations for different inverter topologies (Power Electronic Control System – PECS), and the associated Types of RCD are listed in BS 7671 Figure A53.1. The level of smooth DC fault current is related to the value of the “DC link voltage” i.e. it is function of the original equipment design and can only be defined by the inverter manufacturer.
The AC operational leakage current (PE current), must be compatible with the installation design and the RCD sensitivity. Consider the possible addition of leakage currents, when adding new equipment to an existing installation, as this may require sub-division of existing circuits.
DC link voltage determines the Type of RCD
The use of Type F or Type B RCDs for inverters connected to a single phase supply, is governed by the value of smooth dc residual current produced under fault conditions i.e. dc fault = link voltage/fault resistance.
Figure 1 gives a generic example of a single phase combined compressor and fan unit. The DC voltage output from the rectifier will be a minimum of 1.41 x the AC rms voltage. There may be a step-up voltage section, to increase the DC voltage before it is fed into the control section, based on the characteristics of the motors connected to the inverter output.
A capacitor connected across the DC bus provides smoothing and a power reservoir, for the control section during the non-conducting period of the AC supply. These characteristics of the inverter design determine the maximum value of smooth DC residual current.
A fault to earth on the DC link section, results in a smooth DC residual current flowing in the PE conductor, back to the source earth and returning via the line conductors through any upstream RCDs.
Operational leakage currents
Heat pumps can have high leakage currents due to EMC filtering and inverter operation. When multiple circuits share one 30mA RCD, the cumulative leakage can approach the trip threshold, especially during transients (power-up/defrost).
Relatively small inverter setups designed for permanent connection to the supply, may require a minimum of 300 mA RCDs. Check with the manufacturer first, do not leave it until you get to site.
BS7671 543.7.1 places limits on the operational leakage current, based on the method of connection: Check the HP manufactures installation instructions for the correct method of connection and earthing.
High frequency residual currents
Inverter switching frequencies are typically in the kHz range. While Type F devices are intended for composite residual currents and Type B devices for smooth DC, real world performance at higher frequencies can vary by product. If the heat pump manufacturer specifies enhanced frequency performance (or if persistent nuisance tripping/compatibility concerns exist), verify the chosen RCD/RCBO’s frequency characteristics with the protective device manufacturer.
Conclusion
Heat pumps are high efficiency electrical loads that rely on inverter technology, and that technology can introduce composite and DC residual currents that traditional RCD assumptions do not cover. For safe, robust installations, select the protective device type based on the equipment’s residual-current characteristics (starting with the manufacturer’s instructions), manage leakage current to minimise nuisance tripping, and verify performance through appropriate testing and certification.
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