Replacing a Consumer Unit

Published: 19th January 2012
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Updating an older consumer unit to one including two residual current systems, RCDs is a project that many many people will currently have to think about. The 17th edition of the cabling legislation demands most circuits in a domestic home to be safe by an RCD. It is usually a better and less expensive choice to update the consumer unit rather than modify the active set up in order to conform with the new rules.

A consumer unit applying rewireable BS 3036 fuse carriers was improved to a Seventeenth model split load board making use of circuit breakers sometimes referred to as MCBs, miniture circuit breakers. The circuits on the new consumer units are protected by two residual current devices. Power and lighting circuits for the same floor are split between the RCDs on different sides of the board. This is so that in the event of a fault all the circuits on a particular floor are not cut-off from the supply.

Consumer unit to be improved with fuse carriers removed

17th edition cabling laws relating to consumer units

The new wiring regulations presented in 2008 require that all circuits are secured by RCDs. Generally the best way to comply, especially if the existing fuse box is dated, is to upgrade.

Replacing the consumer unit may also require some supplemental electrical work, to allow the installation to satisfy the expectations set by the new wiring polices. This may possibly include splitting circuits or running in new cable. An older property might have all the lighting and sockets supplied by just two circuits. It would be better practice to split these so that the lighting and sockets for each floor are on separate circuits with their own circuit breaker. Other concerns that require consideration include assimilated neutrals. This is where a neutral return path was not accessible and a link to a further circuit has been produced. Solving this error would, in many scenarios, will need a new cable to be laid back to the consumer unit.

Old consumer unit removed and removing out of date cables

Checking earthing and bonding arrangements

Previous to the consumer unit is changed, the electrician will have examined the distributor's apparatus at the origin of the installation and the earthing and bonding arrangements. This is to make sure that they are safe and that they conform with the regulations. A number of older properties do not have a double pole isolation switch between the meter and the consumer unit which means that the electrical supplier will have to be called in before work on the consumer unit can start. The supplier may assist by installing this primary switch.

A lot of domestic properties have the following arrangement at the source: the service cable going into a sealed unit with a fused line (live) conductor with a 100amp main fuse or service cut-out, 25mm meter tails, a electronic or analogue meter, 25mm tails to consumer unit. It is here that the principal switch would be located. The ‘automatic disconnection of supply' would be provided for by 16mm earthing conductor and 10mm main bonding conductors to gas, normal water and additional services. These arrangements vary with larger or smaller installations; your electrical contractor will suggest you on what is required.

Installing wall support
Making first connections to 17th edition RCD board

Installing the new consumer unit

The characteristics of this work, disconnecting the overload safeguard and attaching the cable to a new means of basic and fault safety signifies that the electrical installer will have worked on all circuits. He will consequently have to perform an electric experiment on all circuits to make sure that they are safe and will then have to issue an electrical installment document before the job is passed over. Any defects identified on a circuit will have to be remedied before the circuit is energised.

Setting up circuit connections
New consumer unit in location in a position for labelling

Safe practices Note
Before any work is carried out on an electrical circuit, the circuit must be separated and locked off or the fuse carrier removed. A sign must be placed at the isolation point declaring that work is being carried out and that electric power must not be returned to the circuit.

If you are not 100% positive that you understand precisely what you are doing call in a certified electrical installer. Building rules are becoming stricter, demanding that competent persons only carry out electrical work, with most works having to be advised to the local authority. Modifying the electrical installation in your residence could be at variance with the new rules and could invalidate your home insurance policy, if in doubt check first!

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