RCBO vs RCD – What's the Difference?

RCBOs and RCDs are both commonly found inside modern consumer units, but they perform different functions.

Understanding the difference is particularly important when replacing an existing device, upgrading a consumer unit or trying to identify an older electrical protective device.

In this guide, we explain what RCBOs and RCDs do, how they differ and what information you should check when identifying a replacement.

What Is an RCD?

RCD stands for Residual Current Device.

An RCD monitors the electrical current flowing through a circuit or group of circuits. If it detects an imbalance that could indicate current flowing to earth, it can disconnect the supply.

RCD protection is designed to reduce the risk associated with electric shock and certain earth faults.

A common rating found on domestic RCDs is:

30mA

This refers to the rated residual operating current of the device.

You may also see current ratings such as:

• 40A
• 63A
• 80A
• 100A

These ratings should not be confused with the 30mA residual current rating.

For example, a device marked 80A 30mA has an 80A rated current and a 30mA residual operating current.

What Is an RCBO?

RCBO stands for Residual Current Breaker with Overcurrent protection.

An RCBO combines two important forms of protection within one device:

Residual current protection – protection against earth leakage faults.

Overcurrent protection – protection against overload and short-circuit conditions.

This means an RCBO effectively combines residual current and circuit overcurrent protection for an individual final circuit.

RCBO vs RCD – The Main Difference

The simplest distinction is:

RCD = residual current protection

RCBO = residual current protection + overcurrent protection

An RCD will often protect a group of circuits, with separate circuit breakers providing overcurrent protection to each individual circuit.

An RCBO provides both types of protection to the individual circuit connected to it.

What Does an RCBO Look Like?

RCBO designs vary considerably between manufacturers and generations.

Common markings can include:

• Manufacturer
• Product range
• Current rating
• Trip curve
• Residual current rating
• RCD type
• Breaking capacity
• Model or part number

For example, an RCBO might be marked:

B32
30mA
Type A

This provides several important pieces of information about the device.

What Does B32 Mean on an RCBO?

The letter identifies the trip curve and the number identifies the device's rated current.

For example:

B32 = Type B trip characteristic, 32A rated current

Other examples include:

B6 – B curve, 6A
B16 – B curve, 16A
B20 – B curve, 20A
B40 – B curve, 40A
C16 – C curve, 16A
C32 – C curve, 32A

The correct rating and characteristic must be selected for the circuit rather than simply replacing a device with one that physically fits.

For more information, see our B Curve vs C Curve RCBO Guide.

What Does 30mA Mean?

Many domestic RCDs and RCBOs are marked 30mA.

This relates to the residual current operating characteristic of the device.

It is completely different from the circuit's normal current rating.

For example:

32A / 30mA RCBO

means that the device has a rated current of 32A and a rated residual operating current of 30mA.

What Does Type A Mean on an RCD or RCBO?

You may see devices described as:

Type AC

or

Type A

This refers to the type of residual current waveform the device is designed to detect.

It does not refer to the device's amp rating or trip curve.

Type A devices are designed to detect residual sinusoidal alternating currents as well as certain pulsating DC residual currents.

The correct RCD type depends on the installation and connected equipment.

We'll cover this in more detail in our Type A vs Type AC RCD & RCBO Guide.

Does an RCBO Replace an RCD?

Not in the sense of simply removing an RCD and fitting one RCBO in its place.

The protective arrangement of the consumer unit needs to be considered as a complete system.

A traditional arrangement may have one RCD protecting several circuits, with individual MCBs providing overcurrent protection.

An RCBO-based arrangement can instead provide individual residual current and overcurrent protection for each circuit.

Changing between these arrangements is an electrical design decision and isn't simply a matter of swapping devices.

What Is the Advantage of Individual RCBO Protection?

One practical advantage of using RCBOs is that residual current protection can be provided individually to each circuit.

With a shared RCD arrangement, a residual current fault on one circuit may disconnect other circuits protected by the same RCD.

With individual RCBO protection, a fault can generally be isolated to the affected circuit rather than disconnecting an entire group of circuits.

This can reduce inconvenience and make fault identification easier.

Can I Fit Any Brand of RCBO Into My Consumer Unit?

No.

An RCBO should not be selected simply because it appears to physically fit the DIN rail or consumer unit.

Different manufacturers and product ranges can have differences in:

• Busbar arrangements
• Terminal positions
• Physical dimensions
• Connection systems
• Technical characteristics
• Manufacturer-approved combinations

Even products from the same manufacturer can belong to different generations or ranges.

For example, identifying a device as Wylex, Hager, Crabtree, Eaton or another manufacturer is only the first step.

You should also identify the specific product range and part number.

We'll cover this in detail in our RCBO Compatibility Guide.

How Do I Identify My Existing RCBO?

Before looking for a replacement, record as much information from the existing device as possible.

Look for:

• Manufacturer
• Product range
• Manufacturer part number
• Current rating
• Trip curve
• RCD type
• Residual current rating
• Breaking capacity
• Number of poles
• Consumer unit model or range

A clear photograph of the front and side markings can also be useful when identifying older or discontinued devices.

Why Does the Exact RCBO Model Matter?

Electrical manufacturers regularly update their consumer unit and protective device ranges.

An older RCBO may therefore be discontinued even though the consumer unit containing it remains in service.

A newer RCBO from the same manufacturer should not automatically be assumed to be a direct replacement.

This is why the manufacturer part number (MPN) can be particularly useful when searching for an existing device.

Buying Replacement RCBOs & RCDs

ReCircuit Resale stocks a changing selection of new, new-other and used electrical protective devices from manufacturers including Wylex, Hager, Crabtree, Eaton and other established brands.

Our stock includes current and discontinued RCBOs, RCDs and related consumer unit equipment.

Where possible, our listings provide important identification information including:

• Manufacturer
• Product range
• MPN
• Current rating
• Trip curve
• RCD type
• Condition

If you are trying to identify an existing RCBO or RCD, send us clear photographs of the device and its markings and we can help identify the product and point you towards relevant stock we currently have available.

Important: RCBOs and RCDs are electrical safety devices. Selection and compatibility should be determined for the specific installation and in accordance with the manufacturer's instructions and applicable wiring regulations. Consumer unit work should be carried out by a suitably competent person.