Installing a Consumer Unit

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ABB MCB, S200M SeriesType C, 16A, Single

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Installing A Consumer Unit http://www.diydoctor.org.uk/projects/consumer_unit.htm

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WELCOME TO OUR CONSUMER UNIT PROJECTThis project is presented by www.neweysonline.co.uk - Quick Click: JOB DONE!

Summary: Consumer Unit. Installing a consumer unit. Consumer unit explained

Please also see our project on the New Wiring and Cable Colours.

Click on books to buy and scroll down for tools and equipment

All cables and connections to the consumer unit must be checked and tested by a qualified electrician. Do not

attempt removal or replacement unless you are you are qualified to Part P regulation standards.. It is also an offence

to interfere with the seals on the electricity meter.

Please see our project on PART P BUILDING REGULATIONS for information on DIY Electrical Installation.

The modern 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.

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 bus bar. Each fuse, or MCB, is

clipped onto a DIN bar and the "teeth" of the bus 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 (D). The split board shown here has one RCD and is a simple version

to show the user how the board works.

The 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

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2 of 6 04/11/2011 03:55

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 and is shown

above. The key to this photo is..

A: Neutral terminal block for the main isolating switch side of the unit.

B: Neutral terminal block for RCD side of the unit

C: Earth terminal block (takes both sides of unit)

D: Main double pole isolating switch.

E: Generally 80 - 100 Amp RCD with 30mA (milliamp) sensitivity.

F: Neutral link cable from Main switch to terminal block

G: Live feed from main switch to RCD.

H: Neutral link cable from main terminal block to RCD

I: Neutral link cable from RCD to RCD neutral terminal block.

The use of RCDs is necessary, on two occasions, to meet the wiring regulations. The first of these is to protect any

socket which may be reasonably expected to supply equipment outside the house (Strimmers, lawn mowers, hedge

trimmers, pond pumps, sheds, lights etc etc). This rule pretty much covers every single socket in the house so the

ring main should be placed on the RCD protected side of the consumer unit.

RCDs have a test button which creates an earth leak with a resistor, and it is usually recommended that they are

tested once a month. An information label near the consumer unit explains this The resistor passes more current

than that required for tripping, and the duration of the test is not limited, so testing in this way does not provide

proof that the unit is working to specification. RCDs can also be switched off manually and can take the place of the

isolator switch in the consumer unit, if they break live and neutral.

The second condition applies to any circuits where an earth fault current is not sufficient to blow the fuse, or trip the

switch, in the designated time allowance made in the regulations. This generally applies to higher Amp rated

appliances (showers, cookers etc) but it should always be checked with an electrician as to which circuits need to be

placed on the RCD side of the consumer unit.

The Current ratings which MCBs are designated to deal with are now conforming to the international equivalent of our

old imperial ratings. This standard is called the Renard current rating and supercedes the imperial ratings as far as

MCBs are concerned. A 5 amp fuse can be replaced with a 6 amp MCB. A 15 amp fuse becomes a 16 amp MCB, a 20

amp fuse can be a 20 amp MCB, 30 amp fuse = 32 amp MCB and a 45 amp fuse must now be a 40 amp MCB.

Top left shows the mains double pole isolating switch and an MCB clipped into place on the DIN bar. Top right shows

two MCBs and the bus bar. When the MCBs are clipped onto the DIN bar, the bus bar is inserted into the live

terminal at the bottom of the MCB. When screwed tight this gives live feed to all the MCBs.


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Top left shows the live (K) and neutral (J) coming into the main double pole switch from the meter. These cables are

called tails, and in this instance, with this type of unit, are 16mm cables. This will vary and you should always check

with a qualified electrician. Top left shows the earth cable from the meter (L). This screws into the earth terminal

block.


4 of 6 04/11/2011 03:55

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Electrical Safety Requirements In Dwellings

Electrical Conductivity To Elevated Floor

Which Electric Cables To Use Around The Home

How To Extend A Ring Main From The Consumer

Unit

How To Wire A Lighting Circuit From The

Consumer Unit

Other Useful Projects

Top right of the above image shows how a circuit cable is introduced to the unit. In this case it is a 2.5mm cable for

the ring main. The live (O) is fixed into the top of an MCB on the RCD side of the unit, while the neutral (M) goes to

the RCD neutral terminal block. The earth (N*) (which has been left unsheathed for clarity ). Each circuit cable is fed

into the consumer unit and connected to an MCB of the appropriate rating.

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For regulations governing heights of sockets etc, please click here

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Installing a Consumer Unit