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.
- You need provision for more circuits
- The existing CU is damaged in some way
- You need to better integrate (or provide for the first time) RCD protection for circuits.
- There is a risk that uninformed people may attempt to re-wire a fuse with the incorrect rating wire.
- You have older PVC T&E power cabling with undersized earth wires and re-wireable fuses.
- You need to separate out circuits to allow independent control - say for time switched electric heating, or for a power feed to an outbuilding.
- 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.
- 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
- 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.
- 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.
Location Ways RCDs Split Load Vs Multiple Henley block External main switch PSSC PFC Breaking Capacity
Main eq bonds Existing Circuit tests
Disconnecting the power
Removing Old CU
Installing new CU
Testing CU main wiring
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.
Pre flight checks
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.
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 RCD tripping when a CU is replaced are:
- a circuit miswired during the works (eg when fixing another fault)
- wrong rating of MCB fitted