When you see "Type A" or "Type B" on an RCBO, it refers to the residual current detection characteristic of the RCD element — specifically, what waveforms of leakage current the device can detect and respond to. This classification matters increasingly as modern electrical loads — EV chargers, solar inverters, variable speed drives — generate DC and mixed-frequency leakage currents that older Type AC devices cannot detect.

This guide covers all four main type classifications used in UK installations, when each is required, and how to match the correct device to each application.

Type AC devices detect only sinusoidal AC residual currents — the kind produced by a simple live-to-earth fault in a purely resistive circuit. They were the standard for many years in domestic installations but are no longer recommended for new installations. Modern electronic equipment — even basic items like phone chargers and LED drivers — can produce pulsating DC leakage currents that a Type AC device will not reliably detect. BS 7671 now requires at minimum Type A for all new domestic installations.

Type A devices detect AC residual currents and pulsating DC residual currents up to 6 mA. Pulsating DC is the type of leakage produced by single-phase rectifier circuits — the kind found in most domestic electronic equipment, single-phase EV charger power supplies, and LED lighting drivers.

Type A is the minimum classification required for all new UK domestic installations and covers the vast majority of circuits in residential and light commercial buildings. The WCED WRB, WER, WRN, and WFD (AFDD) series are all Type A.

Type F adds detection of composite currents — the mixed-frequency leakage produced by variable speed drives (VSDs) and inverter-driven equipment. Where circuits supply VSD-controlled motors or frequency inverters, Type F provides appropriate protection without the nuisance tripping that Type A devices can experience with these loads. Less commonly specified in domestic settings; more relevant in light industrial and commercial HVAC applications.

Type B devices detect all of the above plus smooth DC residual currents. Smooth DC leakage is generated by 3-phase bridge rectifier circuits — found in 3-phase EV chargers, industrial motor drives, and some solar inverter configurations. A Type A device exposed to smooth DC leakage may become desensitised and fail to trip at the rated threshold — in extreme cases it may fail to trip at all. Type B devices are specified to prevent this.

In UK EV charging installations, Type B RCDs are specifically required when the charger does not include its own integrated DC leakage monitoring. Many modern chargers — including Simpson & Partners units — incorporate internal RDC-DD protection, which removes the need for an external Type B RCD. Always check the charger manufacturer's documentation. See our full guide: What is a Type B RCD?

An AFDD (Arc Fault Detection Device) RCBO is not a separate Type classification — it is an RCBO (typically Type A) with an additional integrated microprocessor that continuously analyses the circuit waveform for arc fault signatures. Arc faults — caused by loose terminals, damaged cable insulation, or rodent damage — generate high-frequency current spikes that MCBs and RCDs cannot detect, but which can cause electrical fires.

AFDD RCBOs are mandatory under BS 7671 Amendment 2 (2022) for socket circuits ≤32A in HMOs, care homes, higher-risk residential buildings, and purpose-built student accommodation. They are strongly recommended for all domestic and commercial socket circuits. See: Arc fault detection (AFDD) explained

Further reading:
What is an RCBO? MCB vs RCD vs RCBO explained
What is a Type B RCD? Why EV charging needs it
Arc fault detection (AFDD) explained
← Back to Electrical Safety Knowledge Base