RCDs Explained By   a Professional Electrician

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 touch something live, such as a bare wire. It can also provide some protection against electrical fires. RCDs offer a level of personal protection that ordinary fuses and circuit-breakers cannot provide.
Below are the questions most commonly asked about RCDs (click to expand answers).
What does an RCD do?
An RCD is a sensitive safety device that switches off electricity automatically if there is a fault.
An RCD is designed to protect against the risks of electrocution and fire caused by earth faults.  For example, if you cut through the cable when mowing the lawn and accidentally touched the exposed live wires or a faulty appliance overheats causing electric current to flow to earth.
How does it work?
An RCD constantly monitors the electric current flowing through one or more circuits it is used to protect. If it detects electricity flowing down an unintended path, such as through a person who has touched a live part, the RCD will switch the circuit off very quickly, significantly reducing the risk of death or serious injury.
What are the main types of RCD?
Fixed RCDs
These are installed in the consumer unit (fusebox) and can provide protection to individual or groups of circuits. A fixed RCD provides the highest level of protection as it protects all the wiring and the sockets on a circuit, and any connected appliances.
Fixed RCDs

Socket-Outlet RCDs
These are special socket-outlets with an RCD built into them which can be used in place of a standard socket-outlet. This type of RCD provides protection only to the person in contact with equipment, including its lead, plugged into the special socket-outlet.
Socket-Outlet RCDs

Portable RCDs
These plug into any standard socket-outlet. An appliance can then be plugged into the RCD. They are useful when neither fixed nor socket-outlet RCDs are available but, as with socket-outlet RCDs, they provide protection only to the person in contact with the equipment, including its lead, plugged into the portable RCD.
Portable RCDs
  • What are the main types of RCD?
  • Why is RCD protection important?
    RCD protection can save lives by protecting you and your family from fatal electric shock, and can provide some protection against fire.
    Every year in the UK around 70 people die and 350,000 are injured as a result of electrical accidents at home.  A Government report also indicated that, each year, about 4,000 fires caused by electricity in homes might have been prevented if RCD protection had been fitted in the consumer unit.  Despite this, more than half of UK homes – that’s 13 million - don’t yet have any, or an adequate level of, such additional protection.
    RCD protection is particularly important whilst using mains-powered electrical equipment outdoors, where there is an increased risk of electric shock.
    (To provide additional protection against electric shock, an RCD must have a rated tripping current of no more than 30 mA. The rating will be marked on the RCD.)
How much will RCD protection cost?
A plug-in RCD can cost as little as £10. A fixed RCD will cost more, but will provide a greater degree of protection to help keep your family safe. Installation costs will vary, so we recommend getting several quotes before proceeding.
How do I check whether I already have fixed RCD protection?
To check if you have fixed RCD protection, go to your consumer unit and have a look to see if there is a device with a pushbutton marked ‘T’ or ‘Test’. This ‘test’ button is part of an RCD. If an RCD is fitted, there should also be a label on or near the consumer unit stating ‘test quarterly’.
If you have an RCD, you should check that it is functioning properly by pushing the test button every three months. When tested, the RCD should switch off the power to the areas of the home it protects. 
The RCD (or RCDs) in your consumer unit may not cover everything in your home, such as the lighting circuits, so it’s a good idea to check - while the RCD is off - which sockets and lights are no longer working, showing that they are protected by that RCD.
Switch the RCD back on to restore the supply.
consumer unit RCDs
If your RCD does not switch off the electricity supply to the protected circuits when the test button is pressed, or if it does not reset, get advice from a registered electrician.
If you don’t have RCD protection fitted in your consumer unit, the best way to protect yourself in the short term is by using a portable RCD, which you can find in most DIY or gardening stores.  All you have to do is plug it into a socket and plug the appliance into the RCD.  The appliance will then be safer to use.
However if, for example, you damage a hidden cable when fixing something to a wall, you won’t be protected against electric shock unless you have an RCD in your consumer unit protecting that circuit.
Is it a requirement to install RCDs?
In 2008 a new edition of the UK standard for the safety of electrical installations, BS 7671, came into effect. This standard calls for virtually all electrical circuits installed in homes since then to be provided with additional protection by means of an RCD.
The Electrical Safety Council has electrical safety advice on a range of issues regarding home electrics and other areas.
if in doubt contact an electrician westend glasgow


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