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This article is about different types of Consumer Unit (CU) and their features. For details on changing and upgrading consumer units see here
Types of CU
The term "consumer unit" is generic name for the box of fuses or circuit breakers that normally connects each of the electrical circuits in a property to the electricity supply after it comes out of the electricity meter.
Older CUs usually have a main switch and some form of fuse based protection. Different manufacturers opting for different incompatible designs.
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" to. 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, and the "live" connections made to the individual circuit breakers.
Rewireable fuse CU
A simple CU with a single main switch and a number of outgoing ways. Small designed having one or two ways, and the larger ones up to 10 or so ways. In most designs the fuse carrier for each circuit can be removed to facilitate replacement of the fusewire. The fuse carriers are often also marked with the current rating. Better designs include a mechanism that stopped carriers of the worng rating being inserted into fuse ways that did not match the rating, although note that none of the carriers include any way of preventing the wrong rating of fuse wire from being installed.
Cartridge fuse CU
Popular for a short while between the transition between rewireable fuses and MCBs was the cartridge fuse based CU. Typically a single main switch design, plus a number of outgoing ways protected by a cartridge fuse. Fuses can only usually be inserted into a fuse way of matching current, by virtue of the different ratings of fuse being different physical sizes. Note that cartridge fuses can still be used in many modern CUs via DIN rail mounting fuse carriers.
Standard "Main Switch" CU with MCBs
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. Note that in addition to carrying MCBs, these CUs are also able to house a number of other DIN rail mounted devices such as fuse carriers, RCBOs. Timers, Relays, and Contactors etc.
Split Load CU
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 multiple sections. The main switch will usually control power to all the sections, but each section can be independently switched off via its RCD. This allows RCD protection to be applied to some circuits but not others.
With the 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 become more popular. Originally split load units featured only one split, whereas current models not often allow three or four split sections.
- Cheap and cheerful with a good range of MCBs, single module RCBOs, RCDs etc.
- Plenty of space
- Custom split load arrangements possible by cutting the bus bar at a user selected position
- Plastic models are easy to cut and drill for cable entry and exits
- Beware the flimsy busbar cover
- Note recent design change has made MCB geometry subtly different from previous model. This makes interchangeability harder since the height of the protruding bit of the MCB/RCD is slightly different. The plastic models are however usually flimsy enough to make fitting with a little brute force possible!
- Wider than usual range of MCBs and RCBOs, including double pole.
- Snappable busbar with good quality slip on sleeve makes live working safer and allows custom split load configurations to be made
- MCBs and RCBOs comptable across the domestic plastic and industrial metal enclosures.
- Double pole (SPSN) and single pole devices may be mixed freely on a standard single phase busbar (insulated busbar way on the neutral side and flying lead for the neutral). Double pole devices are general double width.
- Expensive, but RCD "pods" are available for some of the MCBs allowing many custom RCBOs to be constructed.
- Probably the only manufacturer listing a true double pole busbar carrying both phase and neutral on a split comb arrangement, avoiding the need for "flying neutrals".
- Innovative enclosed and hidden busbar which allows devices to be clipped in without removing the busbar from all devices.
Three types of Wylex enclosure are commonly met: The classic rewireable fuse type, the NN type - early bus bar design using MCBs, and current DIN rail MCB designs.
Rewireable fuse boards
- Basic Wylex fuseboxes are very common in existing installs
- No RCD provision in the fusebox
- Fuses expose live pins if pulled with power on
- Overheating occasionally happens on high current circuit fuses (greater than 30A)
- Older designs have intumescent oak back boxes - this is not a safety issue.
- Can be upgraded with Plug in MCBs (3kA breaking capacity only)
Similar in concept to modern CUs however the bus bar design is different having U shaped notches cut into the top of it in place of the more common "fingers" of todays CUs. The notches enable a screw to pass through the busbar into the NN MCBs to make electrical contact.
- MCBs tend to have Fuse equivalent trip ratings (i.e. 5A, 15A, 30A).
- MCBs have the older type II trip response rather than the more modern type B, C, or D, and the breaking capacity is often only 3kA
- MCBs commonly thermal trip when loaded close to their marker rating.
- Finding spares for the CUs is now difficult. eBay will often source additional MCBs although high trip currents (anything over 30A are hard to find).
- It is possible to get a modern style MCB into one of these CUs - but only at the extreme left of the busbar, The busbar will need to be trimmed shorter su that it does not iminge on the new MCB. The lower metal flange on the base of its din rail needs to be bend down with a pair of pliers. This will enable the new MCB to hang on the rail. The feed to the MCB can now be hard wired using a bridge wire. Attache a loop terminal to one end and place it under one of the existing MCB screw connections on the basbar, and connect the other end to the base of the MCB in place of the modern busbar finger.
Many devices will work in other brands of CU, although often there is a slight misalignment of entry and exit terminals or protruding section of the device. If possible, try and stick to all one brand within the CU. It should also be noted that mixing devices from other manufacturers will be outside the manufacturer's type testing, so the assembly will have to be specially tested and certified to show compliance with British Standards, which is required by the Wiring Regulations.