What Is an RCD and What Does It Do?
An RCD (Residual Current Device) is an electrical safety device designed to quickly disconnect the electricity supply when it detects current flowing somewhere it shouldn't.
RCD protection is extremely common in modern UK electrical installations and you'll usually find RCDs inside the consumer unit or distribution board.
They provide an important additional level of protection against electric shock and certain electrical faults.
What Does an RCD Protect Against?
Under normal conditions, the current flowing through the live conductor should correspond with the current returning through the neutral conductor.
An RCD monitors this balance.
If some current begins flowing through an unintended path – potentially through a person or to earth because of an electrical fault – the RCD detects the imbalance.
If the residual current reaches the device's operating threshold, the RCD disconnects the supply.
This can happen extremely quickly.
What Does 30mA Mean on an RCD?
Many RCDs found in domestic consumer units are marked:
30mA
The mA figure refers to the RCD's rated residual operating current, rather than the amount of normal electrical load the RCD can carry.
A 30mA RCD is commonly used to provide additional protection against electric shock.
This should not be confused with ratings such as:
63A
80A
100A
These figures relate to the rated current of the RCD.
For example, an RCD could be marked:
80A 30mA
These are two separate specifications and both are important when identifying or replacing an RCD.
What Is the Difference Between an RCD and an MCB?
An RCD and an MCB provide different types of protection.
An MCB (Miniature Circuit Breaker) primarily provides protection against overcurrent conditions such as overloads and short circuits.
An RCD monitors residual current and provides protection by disconnecting the supply when an earth leakage/residual current fault reaches its operating threshold.
This is why traditional consumer units may have one RCD protecting a group of individual MCBs.
What Is the Difference Between an RCD and an RCBO?
An RCBO (Residual Current Breaker with Overcurrent protection) combines residual current protection and overcurrent protection within an individual device.
A traditional arrangement may therefore look like:
RCD → Multiple MCBs
Whereas an RCBO consumer unit can have:
Individual RCBO → Individual Circuit
Using individual RCBOs can reduce the number of circuits affected by a single fault compared with an arrangement where several circuits share one RCD.
What Are Type AC and Type A RCDs?
The RCD's type describes the forms of residual current that the device is designed to detect.
Two types you're particularly likely to encounter in UK consumer units are Type AC and Type A.
Type AC
Type AC RCDs are designed to operate with alternating sinusoidal residual currents.
They were extremely common in older installations and many older consumer units still contain Type AC devices.
Type A
Type A RCDs can detect alternating sinusoidal residual current as well as pulsating DC residual current.
Modern electrical equipment frequently contains electronic components, which means selecting the appropriate RCD type has become increasingly important.
You'll commonly find Type A RCDs in newer consumer units and electrical installations.
There are also Type F and Type B RCDs for applications involving other residual-current characteristics.
We cover the differences between these RCD types in more detail in our separate Type AC vs Type A RCD Guide.
How Can I Identify My RCD?
Most of the information required to identify an RCD is printed directly on the front of the device.
Look for:
- Manufacturer – for example Wylex, Hager, Crabtree or Schneider
- Product/model number
- Current rating – such as 63A, 80A or 100A
- Residual operating current – commonly 30mA
- RCD type – such as Type AC or Type A
- Number of poles
- Test button
- Any additional symbols or markings
The model number is particularly important when trying to find an exact replacement.
Two RCDs that both say 80A 30mA are not necessarily interchangeable.
Can I Replace an RCD With Another Brand?
You should not assume that an RCD from one manufacturer can simply be installed into another manufacturer's consumer unit.
Consumer units are assemblies, and compatibility with the specific consumer unit or distribution board needs to be established.
Even when two devices appear physically similar and have the same electrical ratings, this does not automatically make them suitable replacements for each other.
When looking for a replacement RCD, identify the:
Manufacturer → Consumer unit range → Existing model → RCD type → Current rating → Residual current rating
This information makes it much easier to identify the correct device.
Why Does My RCD Keep Tripping?
An RCD that repeatedly trips may be responding to residual current somewhere within the installation.
Possible causes can include faulty appliances, damaged equipment, wiring faults, moisture or the cumulative leakage current from multiple connected devices.
Repeated RCD tripping should not simply be ignored or solved by repeatedly resetting the device.
If the cause isn't obvious, the installation should be investigated and tested by a suitably competent electrician.
What Does the Test Button Do?
RCDs have a test button, normally marked T or Test.
The test function is designed to confirm that the RCD's operating mechanism functions when the test circuit is activated.
Follow the manufacturer's instructions regarding testing and recommended test intervals.
If an RCD does not operate correctly when tested, it should be investigated rather than assumed to be providing the required protection.
Buying a Replacement RCD
If you're replacing an existing RCD, don't select a replacement based solely on its current rating.
Check the complete specification, including:
Manufacturer
Model number
Consumer unit range
Current rating
Residual current rating
RCD type
Number of poles
Physical and manufacturer compatibility
At ReCircuit Resale, we stock RCDs from manufacturers including Wylex, Hager, Crabtree and other major electrical brands, including devices from older and discontinued ranges.
If you're struggling to identify a replacement, the easiest option is to send us the model number or a clear photograph of your existing RCD and consumer unit so the correct product can be identified.
This guide provides general information about RCDs and should not be used as a substitute for electrical design, inspection or testing. Work on consumer units and fixed electrical installations should be undertaken by a suitably competent person and in accordance with the applicable requirements.