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. Type B is for equipment that can put smooth DC onto the supply, such as transformerless PV inverters and EV charge points. All installation work in Ireland must comply with I.S. 10101 and be certified by a registered electrical contractor.
What is an RCD?
An RCD is a safety switch that cuts the supply when it detects current leaking to earth. A 30mA device operates in well under a second, fast enough to protect a person.
It works by comparing the current flowing out on the line conductor with the current returning on the neutral. In a healthy circuit the two match. If some of that current is going to earth instead, through faulty insulation, a damaged cable or a person, the two no longer balance and the device trips.
An RCD does not protect against overload or short circuit. That is the job of a fuse or an MCB, which is why the two functions are either combined in one device or fitted alongside each other.
It also cannot protect against a line to neutral shock, where somebody bridges both conductors. The current still balances, so the device sees nothing wrong. This is one reason isolation before work matters and an RCD is never treated as a substitute for it.
What is the difference between Type AC, Type A and Type B?
Type AC sees sinusoidal AC leakage only. Type A adds pulsating DC. Type B adds smooth DC and high frequencies to 1 kHz, which is what modern power electronics can produce.
| 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 and 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 |
| Type B | AC, pulsating DC, smooth DC and high frequency to 1 kHz | PV solar inverters, frequency inverters, variable speed drives, battery storage, medical and industrial equipment | Nothing functionally. It is the most capable type and cost is the only reason not to use one. |
Type AC responds only to sinusoidal alternating residual current and will not reliably detect a fault containing a DC component. Because almost every modern installation contains electronic equipment, LED drivers, switch mode power supplies, variable speed drives and appliance controllers, a fault can easily produce a residual current a Type AC device never sees. It is no longer the appropriate default for new work.
Type A covers the vast majority of real world faults in a modern installation, including those involving single phase electronic equipment. It is the sensible default for general circuits: socket outlets, lighting, kitchen appliances and immersion heaters.
What Type A does not cover is smooth DC residual current, a steady DC fault with no ripple. Smooth DC 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. That specific failure is the reason Type B exists.
Type B detects everything a Type A does plus smooth DC and high frequency residual current. It matters wherever power electronics can put DC onto the installation: transformerless solar PV inverters, frequency inverters and variable speed drives, battery storage systems, EV charge points, and certain medical, laboratory and industrial equipment.
Do I need a Type B RCD for an EV charge point?
You need protection against smooth DC fault current. That is either a Type B upstream, or a Type A where the charge point itself has 6mA DC detection built in.
There are two accepted ways to achieve it. The first is a Type B RCD upstream of the charge point. The second is a Type A RCD upstream, where the charge point incorporates a residual direct current detecting device (RDC DD) rated at 6mA to IEC 62955. Many modern units include this and state it explicitly in their documentation.
So the honest answer is that it depends on the equipment. Check the manufacturer's installation instructions. If the unit does not integrate 6mA 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 equipment's internal protection and future proofs the installation. That is a design decision for the installer.
Do I need a Type B RCD for solar PV?
Where a PV inverter is transformerless, and most modern string inverters are, a DC fault can pass to the AC side. Type B is the appropriate choice unless the inverter documentation says otherwise.
Transformerless designs are used because they are lighter, cheaper and more efficient, but removing the transformer also removes the galvanic separation that would otherwise block a DC fault from reaching the AC installation.
Some inverters provide integrated DC fault current monitoring that permits a lesser type upstream. That has to be confirmed from the inverter manufacturer's instructions for the specific model, not assumed from the category.
What is the difference between an RCCB, an RCBO and an MCB?
An MCB protects against overload and short circuit. An RCCB protects against earth fault. An RCBO does both jobs in a single device.
- MCB, miniature circuit breaker. Protects the cable against overload and short circuit. It provides no earth fault or shock protection whatsoever.
- RCCB, residual current circuit breaker. Provides residual current protection only, so it must be used together with an MCB or a fuse for overload protection.
- RCBO. Both functions in one device, residual current protection and overload protection, usually in a 2 module width.
The practical argument for RCBOs is discrimination. On a board using one shared RCCB across several circuits, a single earth fault anywhere takes out everything downstream of it. With an RCBO per circuit, a fault on the kitchen sockets leaves the lighting and the freezer alone.
The trade off is width and cost. RCBOs take more space on the board and cost more per way, which is why mixed arrangements are common, with RCBOs on the circuits that matter most.
What is the difference between an RCCB and an RCBO?
An RCCB detects earth leakage only and needs an MCB or fuse for overload. An RCBO does both jobs, in one device, per circuit.
In practice an RCCB usually sits upstream of a group of circuits, each with its own MCB, so an earth fault on any one of them trips the RCCB and every circuit in that group goes off together. An RCBO gives each circuit its own residual current protection, so the fault stays on the circuit where it happened. Both are available in the residual current types described above, and the same 30mA rating applies where personal protection is needed. The trade off is space and cost per way on the board. Which arrangement suits an installation is a design decision for a registered electrical contractor working to I.S. 10101.
Why use an RCBO instead of an MCB?
Because an MCB gives no protection against earth leakage or electric shock. An RCBO adds residual current protection to that circuit in one device.
An MCB only protects the cable against overload and short circuit. Where a circuit needs 30mA protection, such as final circuits serving socket outlets, an MCB on its own is not enough: it has to sit downstream of an RCCB, or be replaced by an RCBO. Using RCBOs removes the shared RCCB from those circuits, so a fault on one circuit no longer takes out its neighbours. An MCB still makes sense where residual current protection is already provided upstream. RCBOs, RCCBs and MCBs are counter items at Shamrock Electrical and are not sold online. Call 01 401 9907 or email sales@shamrockelectrical.ie with the board, the ways and the ratings you need.
Which MCB tripping curve should I use?
Curve B for general domestic circuits, Curve C for motors and lighting banks with moderate inrush, Curve D for high inrush loads such as large transformers and welding sets.
| Curve | Trips at | Use for |
|---|---|---|
| B | 3 to 5 times rated current | Resistive loads and general domestic circuits: lighting, sockets, heating |
| C | 5 to 10 times rated current | Moderate inrush: motors, transformers, banks of fluorescent or LED lighting. The most common choice in commercial work. |
| D | 10 to 20 times 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 every time the load starts up. Choosing one that is too high can compromise disconnection times, because the curve must still satisfy the required disconnection time for that circuit's earth fault loop impedance.
That last point is the one most often missed. A Curve D device on a long circuit with a high loop impedance may not disconnect fast enough under fault conditions, which is a design calculation rather than a preference.
What residual current rating do I need, 30mA, 100mA or 300mA?
30mA is the rating that protects people and is what final circuits serving socket outlets require. 100mA and 300mA are for fire protection and upstream selectivity, not personal protection.
The distinction is worth being clear about, because the numbers look like a simple sensitivity scale and they are not. A 300mA device will let a lethal current flow for far too long to protect a person. It is there to catch a developing earth fault before it becomes an ignition source.
Where devices are used in series, the upstream one is given a higher rating and often a time delay, so that a fault on a final circuit trips the local device and not the main switch. That is selectivity, and getting it wrong is why a whole building sometimes goes dark for a fault on one socket.
Where can I buy RCDs, RCBOs and MCBs in Dublin?
At the Shamrock Electrical trade counter in Greenogue Business Park, Rathcoole, just off the N7 and well positioned for the M50. Circuit protection is a counter item.
We supply the NOARK circuit protection range: MCBs, RCBOs, RCCBs and isolators, including Type B RCCBs (Ex9LB63) rated 25A, 40A and 63A at 30, 100 and 300mA, in 2 pole and 4 pole versions. NOARK devices are CE marked, manufactured to IEC and EN standards and backed by a 5 year warranty.
Circuit protection is not sold through the website. Getting the right device depends on the board, the curve, the rating and the type, and that is a two minute conversation rather than a guess at a basket. We also build equipped distribution boards assembled to your circuit schedule.
Website prices are retail. Trade pricing is set by account and by quantity, so ring the counter on 01 401 9907 or email sales@shamrockelectrical.ie with the board, the ways and the ratings you need.
Trade counter details
Shamrock Electrical, Unit 22, Block 613, Jordanstown Road, Greenogue Business Park, Rathcoole, Co. Dublin, D24 TX98.
Phone 01 401 9907. Email sales@shamrockelectrical.ie.
Opening hours: Monday to Friday 7am to 5pm, Saturday 9am to 1pm, closed Sunday.
This page is general guidance. It is not a substitute for I.S. 10101, the National Rules for Electrical Installations, or for the judgement of a registered electrical contractor.