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.

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?

It is essential to ensure that any extension lead used outside is protected by an RCD. We would always recommend having an RCD as part of the extension lead unless it is 15m or shorter and you are absolutely certain that any sockets to which it will be connected are already RCD protected. If you are only using the lead at home, then this can easily be checked. However if you plan to use your lead at multiple locations, this is unlikely to be possible. All leads over 15m will only be supplied fitted with an RCD in order to comply with UK requirements.

How can I tell if my socket is RCD protected?

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.

Re-wireable 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.

MCB 

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.

RCD + MCB

More modern consumer units have 1 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 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.

RCBO

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

RCD Plug

Instead of a normal 13A plug, the lead has a larger RCD plug, which looks similar to the RCD adaptors you can buy. Whilst being the most cost effective option, it does have a number of limitations:

  • As the unit is not weatherproof, the plug cannot be used outside.
  • The size of the plug means that it will not fit into outside sockets.
  • Due to it's size, the plug cannot be used in a socket which has an obstruction above it e.g. a shelf.
  • In-line RCD

    The unit is weatherproof with an IP66 rating. As the unit has a standard 13A plug, it does not have any of the limitations of an RCD plug.