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RCD Regulations 2024

The introduction of the 2nd amendment to the 18th edition of the wiring regulations in 2022 has brought some serious changes to the RCD Regulations.

Quite frankly, the choice of RCDs and their application has become nothing short of confusing. We have more choice of types, all for different installations, yet little guidance other than the big brown book

And let’s be fair, BS7671 doesn’t give much in the way of guidance, it just lays out what the RCD Regulations 2024 are and expects you to know where to go.

In this FULL and frank guide, we’re going to look at what the different types of RCDs are, what they are for and how you should make sure you are selecting the right RCD for the right situation.

Before we get started, it’s worth mentioning that the different types of RCD protection discussed in this guide can apply to either standalone RCDs (ie: BS EN 61008) or else combined into overcurrent devices as is the case with RCBOs (ie: BS EN 61009). We will be doing a post on ‘RCD or RCBO’ very shortly and will link back to that as soon as it is ready!

RCD Types – What Are They?

You might have heard the different types of RCD mentioned and wondered what they mean.

The most general types can be summarised as below:

• Type AC
• Type A
• Type F
• Type B
• **Type S**

**It’s worth noting that the “Type S RCD” means something totally different to the rest of the “types”, however no run down of the RCD Regulations 2023 would be complete without looking at these.

Ok, let’s have a look at each type below and what separates them. We’ll also include the typical scenarios where each RCD type should be used.

Type AC RCDs & Their Uses

Type AC RCDs are the “standard” RCD which has now been in use for quite some time!

A type AC RCD will respond to any earth fault current which is of an AC current (which the name might suggest!), however they can easily be “blinded” by DC fault currents.

This means that any appliance which may produce a DC current under fault, could mean that a Type AC RCD could fail to respond in the required time, if at all.

Whilst this wasn’t such a problem in years gone by, over recent times with the introduction of so many electronical items this has meant that the RCD regulations 2024 have had to address this.

The 18th edition AMD2 have introduced some requirements surrounding the use of type AC RCDs. These are to be used ONLY:

• For fixed equipment and NOT socket outlets of any kind
• Where any type of DC fault currents are not to be anticipated

This is a serious departure from the previous RCD Regulations and is the first time that any particular type of RCD has been specified based on what type of fault currents are to be expected.

Type A RCDs & Their Uses

A much newer addition to the different types of RCDs is a Type A RCD.

Whilst also reacting to AC fault currents as per the original type AC units, Type A RCDs are also able to react to pulsating DC fault currents superimposed onto the AC fault current upto 6mA.

This protects against the “blinding” of a typical AC RCD from any possible pulsating DC fault currents caused by electronical equipment or semiconductors.

BS7671:2018(AMD2:2022) has introduced a set of RCD Regulations which basically mean that type A RCDs are now the minimum standard.

Although Type AC RCDs can still be used for fixed equipment with no possibility of DC fault current, Type A RCDs are now the absolute minimum for socket circuits and any fixed equipment which may produce DC fault currents.

Some of the fixed equipment which can cause DC fault currents includes modern dishwashers & washing machines, LED lighting & EV car chargers. We’ll have a look at this further down in another chapter, as the list goes much further than than would imagine.

Type A RCDs were quite expensive until the last couple of years where the cost has come down to pretty much comparable with type AC units as they gradually replace the older RCDs as the standard fitment.

Type F RCDs & Their Uses

Type F RCDs are a fairly new introduction to the world of RCDs and are designed to provide further protection than that offered by Type A RCDs.

These are particularly for use where single phase variable speed drives are to be used. The potential waveform of fault current encountered in this instance could be a mixture of different frequencies including convertor switching frequency, line frequency and motor frequency.

As more and more appliances begin to use variable speed drives, fitting type F RCDs will be a nessecity to ensure that the differing fault current characteristics do no blind the RCD.

The following modern appliances are examples of items which use variable speed drives:

• Washing Machines
• Dishwashers
• Air conditioning compressors
• Some class 1 power tools

Type F RCDs are designed to withstand enhanced disturbance characteristics such that they do not trip on surge current. They are capable of operating and safely tripping even with 10mA of pure DC superimposed onto a sinusoidal or pulsed DC differential current.

Although currently quite rare, it is expected that type F RCDs will become more frequently used in the near future. Hager are one of the early manufacturers to have brought Type F BSEN61008 RCDs out, they are currently double module and quite expensive (as is any new technology in this area!). Expect to see single module RCBOs using this standard as the technology progresses

Type B RCDs & Their Uses

Type B RCDs have been about for a bit longer than type F, however cover AC, pulsating DC, multi frequency DC and smooth DC fault currents.

Most people will be familiar with these from EV charging points, they provide ‘cover all’ protection against a wide variety of different fault currents which may be encountered with modern electronic devices (typically using 3 phase rectifiers).

Examples of instances where a type B RCD may be required are:

• EV charging points (this protection is now often built into the EV charger itself)
• Certain Solar PV installations
• Certain medical equipment

The use of type B RCDs is stipulated in certain instances within the section 7 (special locations) part of the wiring regulations.

A type B RCD will provide coverage of AC, pulsating DC, multi frequency DC and also smooth DC fault currents.

Type S RCDs & Their Uses

The Type S RCD is totally different in that the ‘type’ of the RCD does not dictate the different fault current types which can be detected by that particular RCD.

This is not a new introduction into the RCD regulations, indeed type S time delayed devices have been about for nearly as long as original type AC devices.

Indeed, you can have a Type S – Type AC RCD or a Type S – Type A RCD (confusing, I know!). Although I cannot think of any particular RCDs on the market that are Type S and cover either Type F or Type B fault current, it is not to say that these do not exist, or will be brought to market soon.

The inclusion of Type S into the naming of the RCD denotes that the RCD in question is of a time delayed tripping type.

These RCDs are mainly used in TT earth installations to provide fault protection to circuits which have a particularly high impedance, without interfering with selectivity (previously called discrimination before the 18th edition) with RCDs which may be used for additional protection.

An example of this would be an up-front S Type 100mA RCD on a TT installation. This has a time delay built in (sometimes adjustable) which means that downstream 30mA RCDs/RCBOs have chance to trip before the up front device does (which would remove power for the whole board/installation). This ensures that selectivity is maintained with RCDs fitted in series.

You can easily identify a time delayed RCD by the logo on the device. This is adjacent to the logo displaying the ‘type’ of the RCD and is a letter S in a box (see pic below)

RCD Regulations 2024 – Other Resources

If you are looking for some further reading or other resources relating to the latest RCD regulations then please take a look at the following posts or videos:

A really important factor to consider when considering which RCD to use, is whether or not the existing consumer unit supports these devices. This can mean that an upgrade of the entire unit may be required. As such, please also read our full 18th Edition Consumer Unit Regulations Guide which can help you navigate the best way forward (ie: whether to use RCDs or RCBOs)

• Hager UK – Selection Of RCD Types
• Electrium – RCDs & the 18th Edition
• Schneider Electric – Pick The Right Type Of Residual Current Device (YouTube Video)