Consumer Units

Consumer unitMCB

What is a Consumer Unit?

The modern consumer unit is the centre, or heart, of the wiring system in the home. The unit distributes the electricity, via fuses of one kind or another, to the different circuits in the house. The older fuse wires are being replaced gradually by their modern equivalent, the MCB or miniature circuit breaker.

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What you need to know

We will deal with the two main types of consumer unit found in the home today . The first one is a single load fuse board where the power coming in is taken through a double pole switch to a live buss bar. Each fuse, or MCB, is clipped onto a DIN bar and the "teeth" of the buss bar are inserted into the MCB's. The cables to the house circuits are connected to the other side of the MCB's. All of this is explained more thoroughly when we deal with the second type of unit, which is the split load unit. The photographs are of a split load fuse board. The ordinary consumer unit is exactly the same in principle without the RCD.

Just as a matter of interest, DIN stands for Deutsche Industrie Norm and, originating in Germany, is any of a series of technical standards, used Internationally, to designate electrical connections, film speeds and paper sizes. Shown in the consumer unit casing above, it is a metal, pressed bar, to which the MCB's clip. They simply push on via a spring loaded lock at their back.

A split load board is designed for total safety and incorporates an RCD (Residual Current Device, shown as E in the picture above) as well as the double pole switch. The split board shown here has one RCD and is a simple version to show the user how the board works.

Consumer unit for installationThe RCD is not just a manually operated isolating switch, but a very sensitive safety device which cuts off in fractions of a second if it senses an earth fault. RCDs can be bought in different current ratings and various sensitivities to current leakage. It also detects any imbalance between Live and Neutral conductors which is essential for Class 2 double insulated equipment. You will need to check which ratings are appropriate for your home.

As they are very sensitive, it is not practical to fit one RCD to protect the whole house. If a fault develops on one circuit, all circuits would be switched off immediately...This would, most often, leave you in the unnecessary position of having no lights or power. Because of this it actually contravenes the most recent (16th Edition) wiring regulations. This is easily prevented by using an RCD in conjunction with a main, double pole isolating switch so that it protects only some of the more vulnerable circuits. This then becomes a split load consumer unit.

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