Heat pump installation – 18th Edition RCD update

Air source heat pumps (ASHPs) contain large speed control inverters in comparison to normal domestic appliances. Inverter based speed control is central to the economic performance of HP compressors and fans, reducing compressor starting current peaks and enabling close motor speed variation to match the heating and cooling demands of the system.

The leakage currents and associated residual currents produced by HP inverters exceed the safety performance limits for Type A RCDs – reference BS 7671 Reg 531.3.3 – see explanation below.

Guidance

Currently, unlike PV and EV, there is no specific guidance given in BS 7671 or the IET Guide (Electrified Heating) relating to HP installation. RCDs connected in circuit with HPs are covered by the generic requirements of Reg 531.3.3. Where fault disconnection relies on the operation of an upstream RCD, using the incorrect “Type” of RCD in association with a HP leads to the risk of RCD “blinding”. The RCD cannot reliably detect fault currents as required by BS 7671, increasing the risk of a fatal “accident”.

For those involved in HP design and associated standards committees, see BS EN IEC 62477 H.3: Fault current waveforms produced by speed control inverter topology and the recommended RCD Type.

The large area exposed conductive parts of a ground mounted HP pose an elevated risk in the event of a fault. Take note of the HP manufacturer’s grounding and protection recommendations. Installations containing Type A RCDs in series with inverter controlled HPs, will not meet the existing recommendations in BS 7671. If in doubt, contact your trade association for technical advice.

HP equipment design characteristics

In installations requiring RCD protection, the characteristics of the in-built inverter equipment determine the sensitivity and Type of RCD that can safely be installed up-stream (Ref 133.1.3). Type A RCDs are designed for operation with sinusoidal or pulsed residual current generated at 50 Hz –  Ref 531.3.3 (i). Transients associated with powering up, and or powering down the inverter, can exceed the transient switch-on limit for Type A RCDs resulting in unwanted tripping – a warning sign associated with incorrect RCD selection.

HP inverters produce AC leakage currents with frequency components above and below 50 Hz, resulting in composite (mixed frequency) residual currents – requiring the application of Type F and B RCDs – see 531.3.3 (ii) & (iii).

Type F or Type B for single phase HPs?

Electrical installers do not have access to the HP manufacturer’s inverter design characteristics and consequently cannot make the decision to use Type F in place of Type B RCDs without the agreement of the HP manufacturer. The characteristics of the HP inverter, and the location of the fault within the equipment, determine the characteristics of the residual current.

Example 1: Generic scheme for single phase supplied – 3 phase out inverter, without a power factor correction stage – Ref. source BS EN IEC 62477-1 Fig H.3.

An insulation fault IF1 located on the DC side of the bridge rectifier produces a pulsed residual current at the fundamental (mains) frequency. This pulsed current may be superimposed on a smooth DC component*, the effective value of which is related to the DC link voltage, smoothing capacitance, and fault resistance.

*Limits for smooth DC fault current component: ≤ 10 mA Type F or > 10 mA Type B.

Note: Type F RCDs do not trip on smooth DC. Do not use for smooth DC fault current > 10mA.

A fault IF2 located on the output stage (supply to motor) results in a residual current containing multiple  frequency components. Referred to as a composite residual current for design/test purposes – see table 3.

The composite residual current components have relative values defined at the specific frequency; mains frequency, the motor frequency at 10 Hz, and inverter switching frequency at 1 kHz. The values  10Hz and 1 kHz represent the most severe condition for Type F RCD tripping, providing reliable operation and effective protection when using Type F RCDs.

HP design limits for use of Type F checked through equipment design calculations & testing:     

• Ratio of composite leakages current components compared to RCD sensitivity I∆n.
• Minimum motor speed control setting (frequency) 10 Hz.
• Maximum inverter switching frequency < 1 kHz.
• Maximum smooth DC fault current limit < 10 mA.

If the equipment design characteristics cannot be guaranteed to meet the Type F limits, use Type B.

Note: Type Bs may have an operational limit of 1 kHz – refer to the manufacturer’s documentation.

Type B RCD unexplained tripping

HP inverters can operate at switching frequencies* in the region of 2 < 16 kHz. BS EN 62423 (Type B RCD standard) only requires manufacturers to test < 1k Hz. If these products are used with HP inverters, they may trip unexpectedly when subject to leakage currents > 1 kHz – see diagram below.

Before purchasing Type B RCDs, refer to the manufacturer’s technical data (frequency response characteristics), to verify that they are suitable for use above 1 kHz – see Reg. 133.1.3

*The rate at which the control stage switches the DC voltage to produce the AC output to the motor.

Conclusion

As with any complex equipment, HP 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, in conjunction with the individual site design requirements based on BS 7671. Remember: look, check, ask, think, do.

To provide years of trouble-free service, HPs require a reliable electrical supply. The duty of care is shared between the HP manufacturer/UK supplier and the electrical installer. This can be met with a bit of planning, checking and discussion, if necessary, prior to installation. Big metal boxes mounted outside and supplied with electricity represent a potential hazard. To reduce the risk of an accident, the electrical installation and method of protection (ADS) must meet the specific requirements of BS 7671 to the letter.

Chaz Andrews – Technical Manager, Doepke UK Ltd