RCD vs RCBO – What's the Difference and Which Do I Have?
RCDs and RCBOs are both commonly found inside UK consumer units, but they are not the same device.
The main difference is the protection they provide.
An RCD provides residual-current protection, while an RCBO combines residual-current protection with overcurrent protection for an individual circuit.
Understanding the difference can help when identifying an existing device or searching for a replacement.
What Is an RCD?
RCD stands for:
Residual Current Device
An RCD monitors the current flowing through the conductors passing through it.
If it detects residual current meeting its operating criteria, it disconnects the supply.
In many older and split-load consumer units, a single RCD provides residual-current protection for several circuits.
The individual circuits then have separate MCBs providing overcurrent protection.
A typical arrangement might therefore be:
RCD → Multiple MCBs → Multiple Circuits
If the RCD operates, several circuits can lose power simultaneously.
What Is an RCBO?
RCBO stands for:
Residual Current Breaker with Overcurrent Protection
An RCBO combines two important functions within one device:
Residual-current protection
and
Overcurrent protection
Instead of one RCD protecting several MCB-controlled circuits, individual circuits can each have their own RCBO.
A typical arrangement is:
RCBO → Individual Circuit
Each RCBO therefore provides protection for its respective circuit.
RCD vs RCBO – Simple Comparison
| Feature | RCD | RCBO |
|---|---|---|
| Residual-current protection | Yes | Yes |
| Overload protection | No | Yes |
| Short-circuit protection | No | Yes |
| Can protect several circuits | Yes | Normally protects an individual circuit |
| Common 30mA versions | Yes | Yes |
| Common in UK consumer units | Yes | Yes |
The key difference is that an RCBO incorporates overcurrent protection while an RCD does not provide that function on its own.
How Can I Tell Whether I Have an RCD or RCBO?
Look at the markings on the front of the device.
Typical RCD Markings
An RCD may show ratings such as:
63A 30mA
80A 30mA
100A 30mA
It will normally also have a test button and markings identifying its RCD type.
Typical RCBO Markings
An RCBO will normally include a circuit-breaker rating and characteristic such as:
B6
B16
B20
B32
B40
alongside its residual-current information.
For example:
B32 30mA Type A
This indicates an RCBO providing overcurrent protection for the circuit as well as residual-current protection.
Why Do Some Consumer Units Have RCDs and MCBs?
Many existing UK consumer units use an arrangement where one or two RCDs provide residual-current protection to groups of circuits.
Each individual circuit then has its own MCB.
For example:
80A 30mA RCD
may protect several circuits fitted with:
B6 MCB – Lighting
B16 MCB – Individual circuit
B32 MCB – Socket circuit
The MCBs provide the individual circuits with overcurrent protection while the shared RCD provides residual-current protection.
Why Do Newer Consumer Units Often Use RCBOs?
Using individual RCBOs allows residual-current protection to be provided separately to individual circuits.
One major advantage is improved circuit separation.
With a shared RCD arrangement, a fault on one circuit can cause the RCD to operate and disconnect several other circuits at the same time.
With individual RCBOs, a fault affecting one circuit is less likely to disconnect unrelated circuits.
For example, a fault on an outdoor circuit doesn't necessarily have to disconnect other circuits protected by separate RCBOs.
Is an RCBO Better Than an RCD?
It isn't simply a case of one device being "better".
They perform different functions and form part of different consumer unit arrangements.
An RCD can provide residual-current protection to multiple circuits when used as part of an appropriately designed assembly.
An RCBO combines residual-current and overcurrent protection for an individual circuit.
The appropriate arrangement depends on the design of the electrical installation and applicable requirements.
Can I Replace an RCD With an RCBO?
This isn't normally a direct one-for-one component swap.
Changing from a shared RCD and MCB arrangement to individual RCBO protection can involve alterations to the configuration of the consumer unit.
Compatibility with the existing consumer unit also needs to be established.
Not every RCBO is suitable for every consumer unit, even when it is made by the same manufacturer.
An electrician should assess the existing board and determine what equipment is appropriate.
Can I Replace an MCB With an RCBO?
Again, don't assume that an RCBO can simply replace an existing MCB because it appears to occupy the same space.
The consumer unit, wiring arrangement, device compatibility and circuit requirements all need to be considered.
If you're looking to upgrade an older consumer unit from MCBs to RCBOs, identify the exact consumer unit manufacturer and range first.
RCD vs RCBO Tripping
The arrangement can also affect what happens when a fault occurs.
Shared RCD
If one RCD protects several MCB-controlled circuits and the RCD operates, all circuits supplied through that RCD may lose power.
This can make identifying the affected circuit more difficult.
Individual RCBO
If each circuit has its own RCBO, a residual-current fault on one circuit will generally cause the protective device associated with that circuit to operate rather than a shared RCD disconnecting a whole group.
This can improve fault separation and reduce disruption.
Does an RCBO Still Have a Test Button?
Yes.
RCBOs incorporating residual-current protection normally have a test function.
The appearance and position of the test button varies between manufacturers and product ranges.
Always follow the manufacturer's instructions regarding testing.
Do RCDs and RCBOs Come in Type A and Type AC?
Yes.
RCD characteristics such as Type AC and Type A are relevant to residual-current protection.
You may therefore encounter both RCDs and RCBOs with different RCD classifications.
When ordering a replacement, don't identify a device solely by its amp rating.
Check its complete specification.
See our Type A vs Type AC RCD Guide for more information.
Does the Manufacturer Matter?
Yes.
Protective devices should not be treated as universally interchangeable between consumer units.
A Wylex RCBO should not automatically be replaced with a Hager or Crabtree RCBO simply because the electrical ratings appear similar.
The same applies to RCDs.
Even products from different generations produced by the same manufacturer may have compatibility differences.
Always identify:
Manufacturer → Consumer unit range → Existing device → Required replacement
Which Device Do I Need?
If you're replacing an existing device, start by identifying what is already installed.
If You Have an RCD
Check:
- Manufacturer
- Model number
- Current rating
- Residual-current rating
- RCD type
- Number of poles
- Consumer unit range
If You Have an RCBO
Check:
- Manufacturer
- Model number
- Current rating
- Trip characteristic
- Residual-current rating
- RCD type
- Consumer unit range
The model number is particularly useful when dealing with older or discontinued equipment.
Need Help Identifying an RCD or RCBO?
At ReCircuit Resale, we stock a range of new, new-other and used RCDs and RCBOs, including current and discontinued electrical equipment from manufacturers such as Wylex, Hager and Crabtree.
If you're unsure which device is installed in your consumer unit, send us:
A clear photograph of the complete consumer unit
and
A close-up photograph of the RCD or RCBO showing its model number and markings.
We can help identify the device and check whether we have the product you're looking for.
You can also visit our RCBO Guides for more information on RCBO types, ratings, identification and replacement compatibility.
This guide is intended for general product identification and information. Protective-device selection and work within consumer units should be undertaken by a suitably competent person in accordance with BS 7671 and the relevant manufacturer's instructions.