Type A vs Type AC RCD – What's the Difference?

If you're replacing an RCD in a consumer unit, one of the most important markings to identify is the RCD type.

Two of the most common types you'll encounter in UK domestic electrical installations are:

Type AC RCD

and

Type A RCD

Although they can look almost identical, they are designed to detect different forms of residual current.

This guide explains the difference and what to look for when identifying a replacement.

What Is a Type AC RCD?

A Type AC RCD is designed to operate when it detects a residual sinusoidal alternating current.

Type AC RCDs were widely installed in older UK consumer units, so they are still commonly found when maintaining or upgrading existing electrical installations.

Older Wylex, Hager, Crabtree, MK and other consumer units may therefore contain Type AC devices.

However, electrical equipment has changed considerably since many of these installations were originally designed.

Modern appliances increasingly contain electronic components capable of producing residual currents with characteristics that need to be considered when selecting the appropriate RCD.

What Is a Type A RCD?

A Type A RCD can detect sinusoidal alternating residual currents as well as pulsating DC residual currents.

This makes Type A suitable for a wider range of modern electrical equipment.

Electronic equipment is now extremely common within homes and commercial properties.

Examples can include equipment containing:

  • Electronic power supplies
  • LED drivers
  • Electronic controls
  • Variable-speed electronics
  • Modern household appliances
  • IT equipment
  • Chargers and power supplies

The presence of electronic equipment is one reason Type A RCDs have become increasingly common in modern consumer units.

Type A vs Type AC – Simple Comparison

RCD Type AC Residual Current Pulsating DC Residual Current
Type AC Yes No
Type A Yes Yes

This is the fundamental difference between the two.

There are also other RCD classifications, including Type F and Type B, which are designed for applications involving additional residual-current characteristics.

How Can I Tell If My RCD Is Type A or Type AC?

Don't rely solely on the appearance of the RCD.

The RCD type can normally be identified from the markings or symbols printed on the front of the device.

Type AC

A Type AC device is identified by the appropriate sinusoidal waveform symbol.

Type A

A Type A device uses a symbol showing its ability to respond to both sinusoidal AC and pulsating DC residual currents.

The exact presentation can vary between manufacturers, so if you're uncertain, use the manufacturer and product number to check the technical specification.

Is an 80A Type A RCD the Same as an 80A Type AC RCD?

No.

The 80A rating and the RCD type describe different characteristics.

For example, you might encounter:

80A 30mA Type AC

and

80A 30mA Type A

Both devices may have the same rated current and rated residual operating current, but they are different RCD types.

When identifying a replacement, all of the relevant specifications need to be considered rather than simply matching the largest number printed on the front.

What Does 30mA Mean?

Many domestic Type A and Type AC RCDs are rated at 30mA.

The 30mA figure refers to the device's rated residual operating current.

It is separate from the RCD's normal current rating.

For example:

80A 30mA Type A

means the device has an 80A rated current, a 30mA rated residual operating current and is a Type A RCD.

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

You should not select a replacement solely because the new device has a more recent RCD classification.

The complete installation needs to be considered.

This includes:

  • RCD type
  • Rated current
  • Rated residual operating current
  • Number of poles
  • Manufacturer
  • Consumer unit range
  • Physical compatibility
  • Manufacturer's instructions
  • Characteristics of the circuit and connected equipment

A Type A device may be appropriate when an installation is being altered or upgraded, but the replacement should be selected by someone competent to assess the installation rather than simply swapping devices based on appearance.

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

A Type AC RCD should not automatically be used as a replacement for an existing Type A RCD.

Current requirements place restrictions on where Type AC RCDs are appropriate because of the characteristics of modern electrical equipment.

If an existing installation has been designed with Type A protection, fitting a Type AC device simply because it physically fits the consumer unit is not an appropriate method of selecting a replacement.

Are Type AC RCDs Obsolete?

You'll still encounter large numbers of Type AC RCDs in existing UK electrical installations.

The fact that an older installation contains a Type AC device does not by itself tell you whether the installation is unsafe or requires immediate replacement.

However, current electrical installations and alterations need to take account of modern RCD selection requirements.

This is why Type A devices are now much more commonly encountered in newer consumer units.

Does the Brand Matter?

Yes.

An RCD isn't a universal component simply because the ratings match.

For example, a:

Wylex 80A 30mA Type A RCD

and a:

Hager 80A 30mA Type A RCD

should not be assumed to be interchangeable.

The RCD must be suitable for the particular consumer unit or distribution board in which it is being installed.

Different manufacturers and even different ranges from the same manufacturer can have different designs, busbar arrangements and compatibility requirements.

Replacing a Discontinued Type AC RCD

This is a common problem with older consumer units.

You may have an older Type AC RCD that has been discontinued and discover that the manufacturer's current range has moved to Type A devices.

Don't simply purchase an RCD that looks similar.

Start by identifying:

Manufacturer → Consumer unit range → Existing RCD model number → Rating → RCD type

You can then establish what compatible replacement options are available for that particular consumer unit.

In some situations an appropriate replacement device may be available, while in others the age or design of the consumer unit may mean a different solution needs to be considered.

Need Help Identifying Your RCD?

At ReCircuit Resale, we stock new, new-other and used electrical protection equipment from major manufacturers including Wylex, Hager and Crabtree, with both current and discontinued products regularly available.

If you're trying to replace an older RCD, send us a clear photograph of the existing RCD and its model number.

Where possible, also provide a photograph of the consumer unit so the range can be identified.

This can make finding the correct replacement considerably easier.

This guide is intended for general product identification and compatibility information. RCD selection and work within consumer units should be carried out by a suitably competent person in accordance with BS 7671, the manufacturer's instructions and the requirements of the particular installation.