RCD Types Explained: Type AC vs Type A vs Type B
RCD Types Explained: Type AC vs Type A vs Type B
Short answer: Type AC detects AC residual current only. Type A detects AC and pulsating DC. Type B detects AC, pulsating DC and smooth DC, plus high frequencies up to 1 kHz. For most domestic circuits a Type A is now the sensible default. For EV charge points and solar PV inverters you need protection against smooth DC fault current — that means either a Type B RCD, or a Type A combined with a charger that has built-in 6 mA DC fault detection (an RDC-DD). Always follow the equipment manufacturer's instructions and I.S. 10101.
Comparison table
| RCD type | Detects | Typical use | Not suitable for |
|---|---|---|---|
| Type AC | Sinusoidal AC residual current only | Legacy installations, purely resistive loads | Any circuit with electronic loads producing DC components. Increasingly considered inadequate for modern installations. |
| Type A | AC + pulsating DC residual current | General domestic and commercial circuits — sockets, lighting, appliances, most electronic equipment | Equipment that can produce smooth DC residual current, such as three-phase inverters and many EV chargers |
| Type B | AC + pulsating DC + smooth DC + high frequency to 1 kHz | EV charge points, PV solar inverters, frequency inverters, variable speed drives, medical and industrial equipment | Nothing functionally — it is the most capable type. Cost is the only reason not to use one. |
Type AC
A Type AC RCD responds only to sinusoidal alternating residual current. It will not reliably detect a fault that contains a DC component.
Because almost every modern installation now contains electronic equipment — LED drivers, switch-mode power supplies, variable speed drives, appliance controllers — a fault can easily produce a residual current that a Type AC device will not see. For that reason Type AC is no longer the appropriate default choice for new work in most cases.
Type A
A Type A RCD detects sinusoidal AC residual current and pulsating DC residual current. This covers the vast majority of real-world faults in a modern domestic or commercial installation, including those involving single-phase electronic equipment.
Type A is the sensible default for general circuits: socket outlets, lighting, kitchen appliances, immersion heaters and so on.
What Type A does not cover is smooth DC residual current — a steady DC fault current with no ripple. A smooth DC fault can magnetically saturate the core of a Type A or Type AC device and prevent it tripping at all, including for a subsequent AC fault. This is the specific risk that Type B exists to address.
Type B
A Type B RCD detects everything a Type A does, plus smooth DC residual current and high-frequency residual current up to 1 kHz.
This matters wherever power electronics can put DC onto the supply side of the installation:
- EV charge points — the vehicle's on-board charger is a rectifier, and a fault can present as smooth DC
- Solar PV inverters, particularly transformerless designs
- Frequency inverters and variable speed drives
- Battery storage systems
- Medical, laboratory and certain industrial equipment
Do I need a Type B RCD for an EV charger?
You need protection against smooth DC fault current. There are two accepted ways to achieve it:
- A Type B RCD upstream of the charge point, or
- A Type A RCD upstream, where the charge point itself incorporates DC fault current detection — a residual direct current detecting device (RDC-DD) rated at 6 mA, to IEC 62955. Many modern chargers include this and state it explicitly in their documentation.
So the honest answer is: it depends on the charger. Check the manufacturer's installation instructions. If the charger does not integrate 6 mA DC detection, a Type B RCD is required. If it does, a Type A is permitted upstream.
Some specifiers fit a Type B regardless, on the basis that it removes any dependency on the charger's internal protection and future-proofs the installation. That is a design decision for the installer.
All installations in Ireland must comply with I.S. 10101, the National Rules for Electrical Installations. This page is general guidance, not a substitute for the rules or for the judgement of a registered electrical contractor.
Do I need a Type B RCD for solar PV?
Where a PV inverter is transformerless — which most modern string inverters are — a DC fault can pass to the AC side. Unless the inverter's documentation confirms it provides integrated DC fault current monitoring that permits a lesser type, Type B protection is the appropriate choice. Again, check the inverter manufacturer's instructions.
RCCB, RCBO or MCB — what's the difference?
- MCB (miniature circuit breaker) — protects against overload and short circuit. It does not provide any earth fault or shock protection.
- RCCB (residual current circuit breaker) — protects against earth fault (residual current). It does not provide overload protection, so it must be used together with an MCB or fuse.
- RCBO — both functions in one device: residual current protection and overload/short-circuit protection, usually in a 2-module width. Using RCBOs per circuit means a fault on one circuit doesn't take out everything downstream of a shared RCCB.
MCB tripping curves: B, C and D
| Curve | Trips at | Use for |
|---|---|---|
| B | 3–5 × rated current | Resistive loads, general domestic circuits — lighting, sockets, heating |
| C | 5–10 × rated current | Moderate inrush — motors, transformers, banks of fluorescent or LED lighting. The most common choice in commercial work. |
| D | 10–20 × rated current | High inrush — large transformers, welding sets, X-ray equipment, capacitor banks, direct-on-line motor starting |
Choosing a curve that is too low causes nuisance tripping on start-up. Choosing one that is too high can compromise disconnection times — the curve must still satisfy the required disconnection time for the circuit's earth fault loop impedance.
What we supply
Shamrock Electrical Supplies is an electrical wholesaler based in Rathcoole, Dublin. We supply the full NOARK circuit protection range — MCBs, RCBOs, RCCBs and isolators — including Type B RCCBs (Ex9LB63) rated 25 A, 40 A and 63 A at 30, 100 and 300 mA, in 2-pole and 4-pole versions. All NOARK devices are CE marked, manufactured to IEC/EN standards and backed by a 5-year warranty.
We also build Equipped Distribution Boards — fully populated consumer units assembled to your circuit schedule, including Type B protection for EV and PV work.
Trade enquiries: sales@shamrockelectrical.ie or (01) 401 9907.
Frequently asked questions
Do I need a Type B RCD for an EV charger?
You need protection against smooth DC fault current. That means either a Type B RCD upstream, or a Type A RCD where the charge point itself includes a 6 mA DC fault detection device (RDC-DD) to IEC 62955. Check the charger manufacturer's installation instructions to determine which applies.
What is the difference between a Type A and a Type B RCD?
A Type A detects AC and pulsating DC residual current. A Type B detects those plus smooth DC residual current and high frequencies up to 1 kHz. Only a Type B is suitable where smooth DC fault current can occur, such as with transformerless PV inverters.
Can I use a Type AC RCD in a new installation?
Type AC only detects sinusoidal AC residual current and will not reliably detect faults with a DC component. Given the amount of electronic equipment in modern installations, Type A or better is the appropriate choice for new work.
What is the difference between an RCCB and an RCBO?
An RCCB provides residual current (earth fault) protection only and must be paired with an MCB or fuse for overload protection. An RCBO combines both functions in a single device, so each circuit gets its own residual current and overload protection.
Which MCB curve should I use for a motor?
Curve C (5–10 × rated current) suits most motors and other moderate-inrush loads. For high-inrush applications such as large transformers or direct-on-line starting, Curve D (10–20 ×) may be required.
What residual current rating do I need — 30 mA, 100 mA or 300 mA?
30 mA is the rating used for protection of people against direct and indirect contact, and is what final circuits serving socket outlets and similar require. 100 mA and 300 mA devices are used for fire protection and for selectivity in upstream positions, not for personal protection.