Whilst this is a basic guide to RCD consumer units, you should not attempt to install one yourself unless you are an electrician qualified to part P. Any work carried out on your consumer unit must be carried out by an electrician qualified to part P. Also note that it is a criminal offence to interfere with the seals on an electricity meter, and any such work that needs the seals removing may need your electricity company to unseal and reseal the electricity meter.
Consumer units come in two different formats - single load fuse boards and split load fuse boards. A single load fuse board consists of a double pole switch connected to a single bus bar that has the individual fuses, now known as MCB's attached to it. However the most commonly installed consumer units are the split load fuse boards. They are designed with the ultimate safety in mind and will incorporate a master RCD as well as the double pole switch. The rest of the board will also be fitted with RCD switches, which replace the older style fuses that need new fuse wire every time they blow.
However RCD switches can not only be manually turned on and off, but their RCD mechanism trips out the mains power in milliseconds in the event of an Earth fault, such as a hedge trimmer cutting through its own power cable.
RCD's are a key factor in preventing household electrocutions, however as they are very sensitive it would not be sensible to use the same one for the whole of the house. So you can buy them much the same as fuse wire in different ampage ratings, as well as different sensitivities. So you will need to select the right RCD's for your household applications.
A split load consumer unit is often installed as if just one RCD covered the whole house then in the event of an electrical fault tripping out the RCD breaker, then the whole consumer unit would be dead. This would mean that all of your light and power circuits would be dead at once. This is now actually against the latest (16th edition) regulations.
As an example if all of the power went out at night, then you would be faced with having to wait until morning to investigate any fault. Whereas if just one circuit has tripped out on the consumer unit then you should have power to put on a light to investigate the fault.
This is why you should install a new RCD consumer unit if you do not currently have one.
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