Consumer unit (Fuse box)

What’s in it?:

A modern consumer unit will contain a main switch, MCB’s, RCD’s and/or RCBO’s. Older Consumer units can contain Fuses instead of MCB’s either replaceable or rewireable, like the modern consumer unit they will have a main switch but they will not have RCD’s or RCBO’s.

What does everything do:

Main switch: This is a double poll switch, (It cuts both live and neutral). It will always have a rating shown on it, this will show the Amps that are safe to pass through it.

MCB’s: They are the modern fuse, they cut off power on an individual circuit if the current rises too high, they can cut power within 5 seconds.

RCD’s: These are extra protection, in modern consumer units you would typically see two of them. Both the Live and Neutral pass through it before the MCB’s, they detect an imbalance between live and neutral , they can cut the power within 0.4 seconds, (usually allot faster).

You may also find RCBO’s or RCCB’s , RCCB’s perform the same operation as an RCD, RCBO are Residual current device with build in over protection, put simply they are RCD with MCB’s in the same unit.

What is the best configuration?

Ideally you could have an RCBO on every circuit, the benefit of this is, for example if one of you have an imbalance on one circuit and the RCBO trips it only cut the power on one circuit, it will not affect your kitchen circuit with your cooker or fridge, computer or smoke alarms (if they’re mains wired) or house alarm as long as they are not the affected circuit.

The latest standard recommends that you have a two RCD’s protecting your circuits, the idea is that your lighting and power circuits are split between the two. E.G on RCD1 you would have downstairs lights and upstairs power and RCD2 would have upstairs lights and downstairs power,(all other circuits split evenly between the two). The result is that if one RCD trips the other is still on, so if the lights go out in one area the power sockets are still on.

There are many different possible arrangements, There are a few rules to follow.

Meter tails into main switch, main switch into RCD, RCD into MCB’s.

Arranging the MBC’s: They should always be in order from highest amps to lowest, the reason is that in the case of an earth fault only the circuit effected trips.(more information)

Safety:

Remember Always cut the power before removing the front of a consumer unit as there will be live components inside!

A modern consumer unit will have the following warning signs:

Warning 230V, to warn you that there are live electrical parts inside: