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In this technical article, Chaz Andrews, Technical Manager at Doepke UK, looks at Type B RCDs for use with heat pumps and explores the requirements of BS 7671 Amendment 2.
Heat pumps (HPs) producing leakage currents above 1kHz will not be compatible with Type B RCDs that are only manufactured to meet the lowest requirement of BS EN 62423, ie. operation up to 1kHz.
Type B RCDs rated for use up to 1kHz may trip unexpectedly when subjected to leakage currents above 1kHz, or worse, the detection circuit may be saturated (blinded). This will result in the device not detecting residual currents under fault conditions.
BS 7671 offers essential guidance for the design of the installation for those who have the experience to ask the right questions. If unsure, the ‘skilled person’ will seek the advice of the heat pump and RCD manufacturers to verify that the characteristics of the equipment they intend to use are compatible.
Compatibility of characteristics – ref BS 7671 331.1
BS 7671 states, “An assessment shall be made of any characteristics of equipment likely to have harmful effects upon other electrical equipment.”
Leakage current in AC circuits is an inherent function of circuit capacitance and increases as a function of frequency. Modern heat pumps, to improve efficiency, apply inverter technology with switching frequencies in the kHz range.
Heat pump leakage current and the associated harmonic currents must not have a detrimental effect on the protection offered by the RCD, and the RCD should not trip as a result of normal circuit leakage current.
Compared to normal domestic equipment designed to be plugged into a 13A socket, heat pumps contain larger inverters, the characteristics of which must be considered when designing the installation. Installers who have not been involved in commercial or industrial applications may not have had experience with this type of equipment or the application of Type B RCDs.
If you have no experience with heat pump installations, do not make assumptions about the installation requirements. Before quoting and buying products, check with the heat pump and RCD manufacturers to verify the RCD characteristics for use with the heat pump model.
Type B for use with heat pumps – understanding the standards
At the time of writing, BS 7671 Appendix 1 refers to BS EN 62423 2012 for Type B: to claim compliance, RCDs do not have to work above 1kHz. The VDE standard 0664-400 (B+) sets an upper limit of 20 kHz*.
This is a recognised issue for larger inverter applications, usually associated with non-domestic installations. Doepke and several European manufacturers’ Type B RCDs exceed the requirements of the existing BS EN. Consequently, for applications exceeding the 1kHz value (see clause 133.1.1), reference must be made to the individual manufacturer’s Type B RCD characteristics to check their suitability for heat pump applications.
The diagram below gives an example of two 30mA Doepke Type B characteristics for use with heat pumps (red dashed line shows the existing 1kHz requirement for BS EN 624232).
*The performance limit for an RCD with regard to frequency is a function of the quality and properties of the magnetic materials and the design of detection circuits used in the product:
Low < 1kHz, Medium < 20kHz, High < 150kHz.
Characteristics above 1kHz are based on extrapolation of the tripping requirements in 62423 and the advice/limits given in IEC 60479-2 thresholds < 150 kHz. 30 mA BSK characteristic is within the threshold for ventricular fibrillation < 150kHz / IEC 60947-2.
Heat pump installation
Greener homes require more innovative solutions (equipment) than those traditionally associated with the simplicity of our old domestic installations. This, in turn, requires more knowledge of the characteristics of the equipment we intend to supply with electricity. Planning, checking and verification before attending the site are prerequisites for quick, safe and reliable installation.
The requirements for skilled electrical installers are given in BS 7671. Manufacturers such as Doepke publish characteristics for their Type B RCCBs; these can be used to verify that they are compatible with the heat pump model. Heat pump manufacturers who do not provide a clear statement on RCD compatibility requirements risk costly rework when things go wrong.
In addition to safety, using inappropriate products in the heat pump installation will result in system availability problems. This, in turn, affects the reputation of the heat pump brand, the companies involved in the installation, and the final customer.
As with any complex equipment, heat pump characteristics vary from one manufacturer to another. Consequently, the design of the electrical supply requirements and associated protection devices may differ. This must be considered with the individual site design requirements based on BS 7671.
Remember, look, check, ask, think, do.
RCCB suitable for HP application < 20kHz – marking the RCCB with HP symbol aids identification on site
This article appeared in Electrotechnical News Mar/Apr 2025 edition – read it here
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