The Mains Man - home

Replacing a Consumer Unit...

The low down...

Many people across the country still have old consumer units or fuse boards which do not meet current recommendations. Just because it is old doesn't necessarily mean it needs to be replaced, however we would recommend that all domestic properties have a consumer unit with RCD protection.

Only qualified electricians are permitted to replace a consumer unit. A certificate must be issued when a consumer unit is replaced and that requires every circuit connected to that consumer unit to be inspected and tested.


To change a consumer unit we need to be able to disconnect the incoming supply so that it is safe to work on.

If your property already has an isolator between the main fuse and the consumer unit, you do not need to do anything. We can isolate the supply when we need to change the consumer unit.

However, if your property does not have an isolator there are 4 options to get one installed. You will need to ask for quotes to find out which suits you best;

1) Contact your electicity supplier to install an isolator. Some electricity suppliers will install an isolator for free, some charge (some more than others). I have seen Ovo and British Gas install isolators for free, EDF refused to install one at all!
2) Contact your DNO to install an isolator - in the Bristol area that is Western Power Distribution.
3) Contact your electicity supplier to disconnect & reconnect the supply - We would have to be on site to install an isolator while they have disconnected the supply.
4) Contact your DNO to disconnect & reconnect the supply - We would have to be on site to install an isolator while they have disconnected the supply.

Options 3 & 4 can be harder to arrange as they require us to be present at the same time as the supplier/DNO, however it sometimes works out to be cheaper. A visit from the supplier/DNO is usually 20 minutes and is plenty of time for us to install an isolator - you should not need 2 visits.

Once the isolator is installed we can safely turn the supply on and off as we need when it comes to installing your shiney new consumer unit!

Choosing your consumer unit configuration

There are two main options when it comes to RCD protection. You will need to choose the option that best suits you.

a) Split load - this is where there are two RCDs protecting half of the circuits each. This means that if there is a fault on one circuit, the rest of that half will also be turned off by the RDC which can be inconvenient. It is good practice to have upstairs lights and downstairs sockets on one side and vice-versa on the other so that you have either sockets (to plug in a lamp) or lights when there is a fault. This doesn't always work though (in a bungalow for example)

This option is cheaper, but means you could be left in the dark if a fault occurs.

b) RCBOs - An RCBO replaces the MCB and protects again over-current (like an MCB would) and is also an RCD rolled into one and protects each circuit individually. This means that if a circuit has a fault, it is only that circuit which will trip out (be turned off)

This option will cost around £15-£20 per circuit more, but provides you with more specific protection.

What is an RCD?

RCD stands for Residual Current Device and (in basic terms) it disconnects the electricity supply to the circuit(s) it is protecting if electric current "goes out", but doesn't "come back". This is fault current which flows to earth (either via the earth cables or directly possibly via a person or object) instead of flowing back via the neutral as it normally would.

Fuses or MCBs only protect from 'over-current' - too much current flowing. This is important to stop cables from overheating, but isn't necessarily good for protecting again fault currents.

Glossary of terms on this page

DNO - Distribution Network Operator - the people in charge of maintaining

MCB - Miniture Circuit Breaker - these are the switches in a consumer unit. They replaced the old fuses. They provide over-current protection

Over-current protection - The MCB/Fuse/RCBO will disconnect the supply if too much current flows. Too much current flowing down a cable can cause it to overheat and catch fire. The MCB/Fuse/RCBO rating should be appropriate to the size cable that it is protecting.

RCD - Residual Current Device - measures the difference beween the current flowing out throught he Live and the flowing back through the Neutral. This is the term for the group of devices that carry out this task. That includes;

RCCB - Residual Current Circuit Breaker - The type of RCD used in a consumer unit to protect a group of circuits.

RCBO - Specific type of RCD that also provides over-current protection