What is the difference between a Type A and Type B RCD?
A residual current device (RCD) disconnects a circuit when it detects certain leakage currents. Type A detects alternating and pulsating direct residual currents; Type B also detects smooth direct current [S2]. IET guidance describes individual protection rated no higher than 30 mA for the charging points it covers [S1]. Your designer must specify the arrangement. Checked October 2026.
What is a Type A or Type B RCD?
A residual current device (RCD) monitors current imbalance and disconnects the supply when its operating conditions are met. Type A and Type B describe the residual-current waveforms it can detect. Alternating current (AC) changes direction; direct current (DC) flows in one direction [S2].
| RCD type | Main distinction relevant here |
|---|---|
| Type A | Detects sinusoidal AC and pulsating DC residual currents |
| Type B | Also detects smooth DC residual current |
The distinction matters for equipment containing power electronics, including electric vehicle charging equipment [S2]. Ask the designer to record the required protection in the proposal rather than accepting an unspecified protective device.
Does every EV charger need a Type B RCD?
IET guidance describes Type B protection where the charging equipment lacks DC residual-current protection. It also describes Type A or Type F arrangements with an appropriate residual direct current detecting device (RDC-DD). Follow the equipment instructions and the applicable installation standard [S1].
An illustration, not a quote: for an assumed installation with two charging points, request a schedule identifying protection for each point: two points need two individually protected connecting points. The IET article describes RCD protection rated no higher than 30 milliamps (mA), subject to its stated electrical-separation exception [S1]. This is a design check, not a shopping list.
What does built-in protection mean for the installation?
The IET’s 2026 guidance includes a residual direct current protective device (RDC-PD) option combining AC and pulsating DC protection with 6 mA DC detection [S3]. Its edition checker says the previous edition remained valid until 15 October 2026 [S4].
A claim of integrated protection needs supporting product documentation. Ask the electrical designer to confirm device standards, DC detection, disconnection and the effect on upstream protection [S1].
Do not replace a device yourself or assume that DC detection alone fulfils every protection requirement. Request the design basis and commissioning records.
Use the wiring requirements guide, earthing explained and installation certificates when preparing questions for your installer.
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Common questions
Is a Type AC RCD the same as Type A?
No. Type AC responds to sinusoidal alternating residual current. Type A also covers pulsating direct residual current; the labels describe different detection characteristics [S2].
Does three-phase supply automatically decide the RCD type?
No. IET describes Type B applications on both single-phase and three-phase equipment. Selection depends on the connected equipment and residual-current characteristics [S2].
Does a Type B circuit-breaker label identify a Type B RCD?
No. Check the residual-current specification separately from overcurrent protection. IET distinguishes RCD types by the fault-current waveforms they detect [S2]. Have the designer identify the exact device.
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Sources
- RCDs for electric vehicle supply equipment, Institution of Engineering and Technology,Last checked 16 October 2026
- Which RCD Type?, Institution of Engineering and Technology,Last checked 16 October 2026
- Protective, isolation and switching devices explained, Institution of Engineering and Technology,Last checked 16 October 2026
- Stay compliant with the current edition of BS 7671, Institution of Engineering and Technology,Last checked 16 October 2026