Type A vs Type AC RCDs & RCBOs – What's the Difference?

When looking at an RCD or RCBO, you may see it described as Type A or Type AC.

These classifications refer to the types of residual current the device is designed to detect. They are separate from ratings such as 30mA, B32 or 80A.

Understanding the difference is particularly important when identifying or replacing an existing RCD or RCBO.

This guide explains what Type A and Type AC mean, how to identify them and what information you should check before choosing a replacement.

What Does the RCD Type Mean?

RCDs monitor for residual current that may indicate current flowing through an unintended path to earth.

However, residual current does not always have the same waveform.

Modern electrical and electronic equipment can produce different forms of residual current, which is why different RCD types exist.

Two commonly encountered types in domestic electrical installations are:

Type AC
Type A

There are other RCD classifications for more specialised applications, but this guide focuses on Type A and Type AC.

What Is a Type AC RCD?

A Type AC RCD is designed to respond to residual sinusoidal alternating current.

Type AC devices have historically been widely installed in domestic consumer units.

You may therefore encounter Type AC RCDs and RCBOs when maintaining or identifying older installations.

The AC classification should not be confused with the normal AC mains supply. It specifically describes the residual current detection characteristics of the device.

What Is a Type A RCD?

A Type A RCD is designed to detect residual sinusoidal alternating current as well as residual pulsating DC current.

This additional capability is important because modern electronic equipment can produce residual currents that are different from a simple sinusoidal AC waveform.

Electronic equipment is now commonplace throughout homes and other buildings, including equipment containing electronic power supplies and control circuits.

Type A vs Type AC – The Main Difference

The simplest comparison is:

Type AC
Designed for residual sinusoidal alternating current.

Type A
Designed for residual sinusoidal alternating current and residual pulsating DC.

This means Type A has additional residual-current detection capability compared with Type AC.

However, this does not mean you should simply replace every Type AC device with any Type A device.

The complete specification and compatibility of the replacement still need to be correct for the installation.

Why Are Type A Devices Common on Modern Installations?

The electrical loads found in properties have changed considerably.

Modern homes can contain equipment such as:

LED lighting and drivers
Computers
Televisions
Washing machines
Electronic heating controls
Induction cooking equipment
Chargers and power supplies
Other electronically controlled appliances

Electronic circuits can affect the characteristics of residual current under fault conditions.

For this reason, Type A residual current protection is now commonly encountered in modern consumer units and protective devices.

How Can I Identify a Type A RCD or RCBO?

Do not identify the RCD type based solely on the appearance or colour of the device.

Look at the markings printed on the front or side of the RCD or RCBO.

Manufacturers use standardised waveform symbols to identify the residual-current characteristics of the device.

Product descriptions and manufacturer specifications may also explicitly state:

RCD Type: A

RCD Type: AC

When purchasing a replacement device, check the manufacturer's part number as well as the markings.

Is 30mA the Same as Type A?

No.

These are two completely different specifications.

For example, an RCBO might be described as:

B32 30mA Type A

Each part tells you something different.

B = trip characteristic

32A = rated current

30mA = rated residual operating current

Type A = residual-current detection classification

You therefore need to consider all of these specifications when identifying a device.

Is B Curve the Same as Type A?

No.

This is another common source of confusion.

B Curve or C Curve refers to the overcurrent tripping characteristic of an MCB or RCBO.

Type A or Type AC refers to the residual-current detection characteristics.

An RCBO can therefore be both:

B Curve + Type A

or, depending on the product:

C Curve + Type A

These specifications describe different functions of the same protective device.

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

Can I Replace a Type AC RCBO With a Type A RCBO?

A Type A device has broader residual-current detection capability than a Type AC device, but that alone does not establish whether a particular RCBO is a suitable replacement.

You must also consider:

Manufacturer
Product range
Consumer unit compatibility
Current rating
Trip curve
Residual current rating
Breaking capacity
Number of poles
Terminal and busbar arrangement
Manufacturer instructions
Requirements of the circuit

An RCBO should therefore never be selected based solely on its RCD type.

Can I Fit a Different Brand of Type A RCBO?

The fact that two RCBOs are both Type A does not mean that they are interchangeable.

For example, a Type A RCBO manufactured by Wylex should not automatically be treated as interchangeable with a Type A RCBO manufactured by Hager, Crabtree, Eaton or another manufacturer.

Physical appearance and DIN rail mounting are not sufficient to establish compatibility.

The consumer unit and manufacturer's approved equipment need to be considered.

We'll cover this in detail in our RCBO Compatibility Guide – Will an RCBO Fit My Consumer Unit?

What Other RCD Types Are There?

Type A and Type AC are not the only residual-current device classifications.

Other types include Type F and Type B, which are designed for applications involving additional residual-current characteristics.

These should not be confused with B Curve circuit breakers.

For example:

Type B RCD = residual-current device classification.

B Curve RCBO = overcurrent trip characteristic.

The letter B is being used to describe two different electrical characteristics.

This is why reading the complete specification of a protective device is important.

What Should I Check When Replacing an RCD?

If you need to identify an existing RCD, record:

Manufacturer
Product range
Manufacturer part number
Rated current
Residual current rating
RCD type
Number of poles
Breaking or conditional ratings where applicable
Consumer unit model or range

For example, an RCD could be marked:

80A
30mA
Type A

All three pieces of information are relevant.

What Should I Check When Replacing an RCBO?

An RCBO contains both residual-current and overcurrent protection, so there are additional specifications to identify.

Check:

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

Taking a clear photograph of the existing device and its markings can make identification considerably easier.

Buying Type A & Type AC RCDs and RCBOs

ReCircuit Resale stocks a changing selection of new, new-other and used RCDs and RCBOs from established manufacturers including Wylex, Hager, Crabtree and Eaton.

Our stock includes both current and discontinued electrical protective devices, which can be useful when a specific existing model is required.

Where possible, our listings provide specifications including:

Manufacturer
Product range
MPN
Current rating
Trip curve
Residual current rating
RCD type
Condition

If you are unsure which device you currently have, send us clear photographs of the front and side markings and we can help identify it and point you towards relevant products currently available.

Important: RCDs and RCBOs are electrical safety devices. The correct type and specification must be selected for the individual installation in accordance with the manufacturer's instructions and applicable wiring regulations. Consumer unit work should be carried out by a suitably competent person.