Do I need an RCD?

What is an RCD? 

An RCD, or residual current device, is a life-saving device which is designed to prevent you from getting a fatal electric shock if you come into contact with a live wire or current. RCDs offer a level of personal protection that ordinary fuses and circuit-breakers cannot provide. The majority of our leads have an option to add in an RCD, to provide this invaluable level of protection.

How does it work? 

An RCD is designed to protect against the risks of electrocution, for example if you cut through a cable and accidentally touched the exposed live wires. An RCD constantly monitors the electric current flowing through the live and neutral wire. If a fault occurs and the live and neutral current become unbalanced (e.g. someone has come into contact with a live wire), the current will flow to earth and the RCD will detect this fault and cut the power immediately.

This useful YouTube video helps explain how an RCD works.

What is the difference between an RCD and an MCB?

RCDs are often confused with Miniature Circuit Breakers (MCBs) or fuses. Circuit breakers protect property, RCDs protect people. Circuit breakers are found in consumer units. They are specifically designed to trip and cut off the power if too much current flows through a circuit or there is a short circuit. Unlike RCDs they do not protect against the risk of electrocution, as the current required for them to trip would not be generated in such a situation. The fuse in a 13A plug operates in the similar manner, providing protection from an overload or short circuit.

RCDs in contrast, do not protect against overload or short circuit. It is therefore important that any circuit also has overload protection from an MCB. Some houses may have RCBOs in the consumer unit - they are basically an MCB and RCD combined into one unit. 

       MCB              RCD            RCBO

Do I need an RCD?

We are often asked the question of whether an extension should include an RCD and the simple answer is yes if ANY of the following apply:

  • The lead if 20m or longer.
  • The socket that you will use is not RCD protected.
  • You may use the extension in various locations.


How can I tell if my socket is RCD protected?

Check whether the socket is an RCD socket - these have a TEST and RESET button. If it is, plug something in and then press the TEST to check that it trips, followed by the RESET button to restore the power. If it functions correctly then you have a functioning RCD socket. 

If it is not an RCD socket then continue with the steps below. Since 1992 any newly installed socket that may reasonably be expected to supply equipment outdoors should be RCD protected. This changed to a requirement for all 13A sockets to be protected from 2008. To see if a socket is protected by an RCD, follow these steps:

1. Look at your consumer unit using the pictures below as a guide, to find out whether it has any RCDs or RCBOs. If it looks like the top 4, there is no RCD, and one would be required as part of the extension lead. If it has any RCDs or RCBOs, move to step 2.

Rewireable Fuse

Original consumer units relied on fuse carriers which had a piece of fuse wire fixed between two terminals.

No RCD protection.

Cartridge Fuse

A slightly later variation used cartridge fuses - essentially a larger version of what is used in a 13A plug.

No RCD protection.

MCB (plug in)

With the first introduction of MCBs, these plugged into the old style consumer units instead of fuses. Whilst they have a small red test button, they are circuit breakers not RCDs.

No RCD protection.


As MCBs became standard, consumer units were designed to contain just these and a main switch. The MCBs do not have test buttons.

No RCD protection.


More modern consumer units have one or more RCDs (circled). They are generally twice the width of the MCBs for each circuit and have a test button (yellow in the picture). 

Sometimes all circuits are protected by an RCD, whilst in other cases some are not. All of the MCBs for the circuits to the left of an RCD should be protected.

RCD protection of some or all circuits.


The latest consumer units may have RCBOs. They are a combined MCB and RCD. Each one has a test button (yellow in the picture).

RCD protection of all circuits with an RCBO.


2. Check that it states '30mA' on the RCD/RCBO - if it does, move to step 3. Some RCDs in older installations may state 100mA and are not sensitive enough for modern standards - a 30mA RCD would be required as part of the extension lead.

3. Plug a lamp or something which you can easily tell is on/off into the socket:

  • Press the test button on the RCD which is to the right of the circuit breaker (MCB) labelled for the socket circuit you are using. RCBOs protect single circuits, so will be labelled for each socket circuit.
  • If operating correctly, the switch on the unit will flick downwards.
  • Check that the item plugged into the socket has turned off - if it has, the socket is protected by the RCD.
  • To turn the power back on, push the switch on the unit upwards. On some older units it is necessary to first push the switch all the way down, before pushing it up.

If the test button turns off the circuit, then it is RCD protected. Consider whether you also need an RCD with the extension lead. If the test button doesn't work the unit may be faulty and should be checked by an electrician.

The above is provided as a general guide only - if in any doubt, please refer to an Electrician.

Choosing the Right RCD

If an RCD is required, there are two types of unit:

Plug RCD

Instead of a normal 13A plug, the lead has a larger plug RCD. It provides a more cost effective option when the extension will only be connected to indoor sockets.

Although it is IP55 rated and has a unique design with the plug pins at the top to enable it to fit into most outdoor sockets, the unit will stop the socket cover from closing. Where the extension will regularly be connected to an outdoor socket, the in-line RCD is therefore recommended.

Inline RCD

The unit is weatherproof with an IP66 rating. As the extension has a standard 13A plug fitted before the RCD, it fits all outdoor sockets and enables the socket cover to fully close.