What Is a Type A RCD? Full Installer Guide
A Type A RCD is designed for circuits where residual current may include ordinary sinusoidal AC and pulsating DC components. That makes it a common choice for modern equipment containing rectifiers, electronic controls and inverter-driven loads.
Quick answer: A Type A RCD detects sinusoidal AC residual current and pulsating DC residual current. It is commonly selected for modern circuits, but it is not a universal answer for every inverter or DC-producing load.
Choose the RCD type from the expected residual-current waveform and the equipment manufacturer's instructions — not from the appliance name alone.
Compare Type A RCDs or browse the wider RCD range.
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What Does a Type A RCD Detect?
- Sinusoidal AC residual current
- Pulsating DC residual current
- Residual-current waveforms within the limits of the relevant Type A product standard
The symbol and markings on the device identify its type. The rated current, residual operating current, pole arrangement and time characteristic are separate parts of the specification.
Type A vs Type AC RCD
| Feature | Type AC | Type A |
|---|---|---|
| Residual current detected | Sinusoidal AC | Sinusoidal AC and pulsating DC |
| Typical selection constraint | Only where the load is known not to contain DC components | Commonly suited to modern electronic loads |
| Smooth DC capability | No | No — move to a suitable higher RCD type or another manufacturer-approved arrangement |
Type AC is not simply “old and illegal”. Its use is limited to fixed equipment where it is known that the load current contains no DC components. Because that condition is increasingly uncommon, Type A is often the practical starting point.
When Did Type A RCDs Become Mandatory?
There is no single date on which Type A became mandatory for every circuit. BS 7671 requires the RCD type to be selected for the residual currents expected from the connected load. The result is that Type A is widely used on new work, while Type AC has a much narrower suitable application.
Selection reminder: “Type A as a sensible default” is not the same as “Type A is suitable for every load”. Always check the appliance or equipment manufacturer's instructions.
Where Is a Type A RCD Commonly Used?
Type A may be appropriate for circuits supplying equipment such as washing machines, induction hobs, electronic power supplies and other fixed loads using single-phase rectification. Heat pumps, EV chargers, PV inverters and variable-speed drives need closer attention because smooth DC or higher-frequency residual currents may be possible.
For EV charging, a Type A or Type F RCD may be used where suitable 6 mA DC residual-current detection is provided by the charging equipment or an RDC-DD. Otherwise, a Type B arrangement may be required. See Type A vs Type B RCDs for EV Chargers.
What a Type A RCD Does Not Do
- It does not provide overcurrent protection unless it is part of an RCBO.
- It is not a surge protective device.
- It is not generally intended to detect smooth DC residual current.
- It does not guarantee freedom from unwanted tripping.
The often-quoted “6 mA DC” figure relates to avoiding DC blinding in specific arrangements; it should not be presented as a general smooth-DC detection capability of a Type A RCD.
How to Select a Type A RCD
- Rated current: coordinated with upstream protection and load.
- Residual-current rating: selected for the protective measure.
- Poles: suitable for the circuit and required disconnection.
- Time characteristic: instantaneous, selective or another specified function.
- Compatibility: confirmed with the distribution-board and equipment manufacturers.
Options include 63A 2-pole Type A high-immunity RCDs and 80A 4-pole Type A high-immunity RCDs.
Type A RCD FAQs
Is a Type A RCD suitable for a heat pump?
It may be, but the heat-pump manufacturer's instructions determine the minimum suitable RCD type. Some equipment requires Type B or a specifically defined alternative.
Does Type A detect smooth DC?
No. Type A is intended for AC and pulsating DC residual current, not general smooth DC detection.
Is Type A the same as a Type A RCBO?
No. “Type A” describes residual-current sensitivity. An RCBO also includes overcurrent protection.
Can I replace Type AC with Type A?
Often, but replacement still requires correct ratings, board compatibility, testing and verification.
Final Word
Type A is the normal choice for many modern circuits because it recognises pulsating DC as well as AC residual current. Its limits still matter: select from the load characteristics and manufacturer data rather than treating it as a universal upgrade.
Ready to specify? Browse Type A RCDs by current rating and pole configuration.
For combined overcurrent and residual-current protection, compare the RCBO range.