Changing a consumer unit

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Note that most of the work described here would be classed as a "notifiable work"
under Part P of the building regulations.

Changing a consumer unit

This article discusses the reasons why you may need to change or upgrade a Consumer Unit (CU) aka "Fuse Box", and covers the procedures to follow. Note that this represent major electrical work, and should not be attempted unless you are confident that you understand the technicalities involved, and can produce an adequate standard of workmanship.

Reasons for a change

There is often an implicit assumption that a modern CU with resettable Miniture CIrcuit Breakers (MCBs) will be "better" than an existing one that has cartridge or re-wireable fuses. It is important to understand that both types of fused circuit protection are still permissible in the current wiring regulations, and can offer the required levels of protection. There are also disadvantages to changing from fuses to MCBs in some cases.

Pros

1. You need provision for more circuits
2. The existing CU is damaged in some way
3. You need to better integrate (or provide for the first time) RCD protection for circuits.
4. There is a risk that uninformed people may attempt to re-wire a fuse with the incorrect rating wire.
5. You have older PVC T&E power cabling with undersized earth wires and re-wireable fuses.
6. You need to separate out circuits to allow independent control - say for time switched electric heating, or for a power feed to an outbuilding.

Cons

1. Nuisance trips. Modern MCBs react more quickly to very short term overloads, and may result in loss of power to a whole lighting circuit when a bulb blows.
2. Discrimination: it can be harder to ensure that the circuit protective device nearest to a fault will be the only one to open when you have cascaded MCBs - sometimes upstream fuses interoperate better with downstream MCBs
3. Expense: Changing a CU can be expensive, and may not bring significant benefits in overall safety. There may be other more serious problems with an electrical installation that are better addressed first.
4. Extra work: Fitting a CU with RCD can often result in the installation not working initially due to a borrowed neutral or leakage.

Types of CU

Modern CUs for domestic properties tend to be based round similar designs regardless of the manufacturer. Typically they use a DIN mounting rail system onto which the protective devices "clip on". Incoming power is fed via one or more switches and RCDs to a "busbar", which feeds power to each protective device. The circuit wires are connected to Neutral and Earth busbars, plus the individual circuit breakers.

The simplest CUs have just a single main switch, and space for a number of protective devices. The number of spaces (or "ways") dictates how many circuits can be connected. The smallest CUs have 1 or 2 ways and are suitable for some applications like a garage or shed supply. Larger ones can have 16 or more ways.

A popular type of CU is the "split load" type. This has the busbar divided at some point and allows the CU to be powered in two sections. Typically one section under the control of a switch, and the other section fed via a RCD. This allows RCD protection to be applied to some circuits but not others.

With forthcoming changes in the 17th Edition of the Wiring Regulations requiring 30mA RCD protection for many more types of cable location, it is likely that the use of RCBOs, which combine an MCB and an RCD in one device, on individual circuits will be more popular. There are also starting to appear a new style of split load unit with more than the typical two section split.

Planning

Location

Consumer units typically need to be within 2m of the electricity meter unless an additional switch fuse is fitted to protect the tails. They ought not be mounted on the electricity suppliers meter board (although this is quite often seen). An ideal location should also make access to the CU easy without need to resort to ladders or climbing past obstructions - since you may be doing it in the dark.

Ways

When selecting a new CU, it is wise to choose one with enough ways to accommodate all of the proposed circuits, but to leave a few spare ones for future applications. Note also that some devices that can be mounted in a CU such as contactors, time clocks, bell transformers etc, may take more than one modules width.

RCDs

Any modern CU will typically feature at least one RCD, and quite probably more in new designs.

Split Load Vs Multiple

When selecting a CU, one option that is often overlooked is the use of more than one CU. This can make it far simpler to position a CU with sufficient ways in a limited space. In the case of houses with TT earthing, it can also save the cost of a time delayed RCD since the circuits do not need protection via a chain of cascaded RCD.

Henley block

Multiple CUs can each be fed from their own set of tails. A Service Connector Block (aka "Henley Block"), can be used to split the tails from the meter or main switch enclosure, to allow the feeding of multiple CUs.

External main switch

When more that one CU is fitted, it is often worthwhile fitting an additional single "main switch" so that the capability to kill all power to all circuits with one action is retained.

PSSC

PFC

Breaking Capacity

Other work

Main eq bonds Existing Circuit tests

Starting work

Disconnecting the power

Prior to replacement of the CU it is necessary to isolate the power feed to it. Some installations have an isolation switch for this purpose, however many don't and the other most readily available method is to disconnect the main service fuse. Removal of the electricity distributor's main fuse is not legal, however it is common practice. Unofficially it is reported that many of the supply companies would prefer people to do this, rather than take the risk of working live.

Great care must be taken when removing the fuse. It must only ever be pulled when there is *no* load on any of the circuits (i.e. all existing CU(s) turned off). These fuseholders may be of very old design, and may lack even fairly basic safety guarding. Touchable metalwork inside the fuse enclosure can be assumed to be live. Please see the additional notes in this section.

Working Live

It is possible to remove the tails and tape the ends over, but this is not recommended, as it does constitute a risk of injury, loss of sight and to life. Live working should not be attempted unless one has the correct equipment and knowledge to allow this to be done safely.

Removing the Old CU

All wires should be labelled as they're removed, as one mistake later can cause a fair bit of trouble.

Take care if there is any old [Historic Mains Cables#VIR rubbber cable] present, since these will have fragile insulation easily damaged. If rubber wiring has insulation damage at the CU end, but is otherwise in fit state to be reused, sleeving may be used to restore effective insulation. However a high percentage of rubber wired circuits are not fit for re-use.

Installing new CU

Labelling

The MCBs or fuses should be labelled to show what circuits they supply. Installations using both wiring colour codes are required to have a notice fixed by the CU stating that both colour codes are in use.

Resolving Problems

Nuisance RCD trips

It is not uncommon for a new CU to trip the RCD as soon as power is turned on. There are a number of common causes of this, which are addressed in the RCD article.

MCB trips

MCBs trip when too much current flows in the circuit they supply. Since the odds of an appliance fault developing while you replace a CU are small, the likely causes of MCB tripping when a CU is replaced are:

• a circuit miswired during the works (eg when fixing another fault)
• wrong rating of MCB fitted

See Also

Rewiring Tips