Most people know where their fuse box is and have a rough idea of what it does. But when an electrician starts talking about consumer units, MCBs, RCDs, and RCBOs, the terminology can quickly become confusing. This guide explains what a consumer unit actually is, what each component inside it does, how modern units differ from older fuse boxes, and when you should consider replacing one.
What is a Consumer Unit?
A consumer unit is the technical name for what most people call a fuse box or fuse board. It is the central distribution point for electricity in your home. Power enters the property from the electricity network through the service cable, passes through the meter, and then reaches the consumer unit. From there, it is distributed to individual circuits — lighting, sockets, kitchen, immersion heater, shower, and so on.
The consumer unit serves two critical functions. First, it divides the incoming supply into separate circuits, which makes the installation more manageable and means that a fault on one circuit does not necessarily affect the others. Second, it contains the protective devices — fuses, circuit breakers, and residual current devices — that disconnect a circuit when something goes wrong. These protective devices are the safety system that prevents electrical faults from causing fires or electrocution.
The word "consumer" in consumer unit simply refers to the consumer side of the electricity supply — as opposed to the distribution network side. The electricity company's equipment ends at the meter; everything from the meter onwards is the consumer installation.
What is Inside a Consumer Unit?
Open the door of a modern consumer unit and you will typically see a row of switches. Each one has a function, and understanding what each type does is helpful when things go wrong — or when an electrician tells you that your unit needs upgrading.
The Main Switch
On the left-hand side of almost every domestic consumer unit you will find the main switch. This is typically a large double-pole switch that disconnects the entire installation from the supply. Turning off the main switch cuts power to everything in the consumer unit — all circuits, all MCBs, all RCDs. This is the switch an electrician will turn off before working on the board, and it is the one you should turn off in an emergency where you need to cut power to the whole property.
Miniature Circuit Breakers (MCBs)
Miniature circuit breakers are the individual switches that protect each circuit. When a circuit is overloaded — drawing more current than it is rated for — or when a short circuit occurs, the MCB trips and disconnects that circuit. MCBs replaced rewirable fuses in domestic installations from the 1970s onwards. They are far more reliable and consistent than fuse wire, and crucially, they can be reset by flicking the switch back up, rather than requiring the wire to be replaced.
MCBs are rated in amperes — you will typically see ratings of 6A (for lighting circuits), 16A, 20A, 32A (for socket ring circuits and kitchen circuits), and 40A or 45A for electric showers or cookers. The rating must match the cable it is protecting: a 32A MCB must be connected to cable that is rated to carry at least 32A, otherwise the MCB will allow more current to flow than the cable can safely handle.
Residual Current Devices (RCDs)
An RCD is a different type of protective device to an MCB. While an MCB protects against overload and short circuit, an RCD protects against earth faults — including the potentially fatal situation where a person becomes part of the circuit by touching a live conductor. An RCD continuously monitors the current flowing on the live and neutral conductors. In a healthy circuit these are equal. If current is leaking — perhaps through damaged insulation, through water, or through a person — the live current will be higher than the neutral current. When the difference reaches 30 milliamps, the RCD trips and disconnects the supply in a fraction of a second.
Modern consumer units must provide RCD protection to final circuits. Older consumer units often provided no RCD protection at all, which is one of the main reasons that old units are still upgraded today even when the MCBs themselves are working correctly.
RCBOs — The Best of Both Worlds
An RCBO combines the functions of an MCB and an RCD in a single device. It protects against overload, short circuit, and earth fault — all in one unit. In a consumer unit fitted with RCBOs, every circuit has its own individual protection. This has a significant practical advantage: if a fault causes one circuit to disconnect, only that circuit trips. Other circuits — and crucially, the rest of the installation — remain live.
Compare this to a split-load consumer unit where one RCD protects multiple circuits. In that arrangement, a fault on the shower circuit might take out all the sockets on one half of the board as well. This is the situation many householders experience as "everything going off at once" — and it is one of the reasons all-RCBO consumer units have become the preferred choice in modern installations.
Old Fuse Boxes vs Modern Consumer Units
If your property was built before the mid-1970s and the electrical installation has never been significantly updated, you may have a fuse box rather than a modern consumer unit. The most common type is the rewirable fuse board, where each circuit is protected by a piece of fuse wire threaded through a ceramic fuse carrier. These boards are recognisable by their individual fuse holders, often coloured differently by circuit type.
Rewirable fuse boards have no RCD protection whatsoever. They provide a much slower and less reliable response to faults than modern MCBs. The fuse wire can be incorrectly rated — a deliberate or accidental over-fusing removes the protection entirely. And the boards themselves can be physically deteriorated: the wooden backboards found in very old installations are a fire risk in themselves, and some boards from the 1960s and 1970s used insulation materials that are now known to be hazardous.
Between the rewirable boards of the 1960s and the fully RCD-protected units of today, there were several intermediate generations: early MCB-only boards with no RCD protection; split-load boards with a single RCD protecting half the circuits; and various proprietary systems from manufacturers that are now discontinued and for which replacement parts are unavailable. Each generation represents an improvement on the last, but older examples of each are still commonly found in Devon's housing stock.
Why RCD Protection Matters So Much
The importance of RCD protection cannot be overstated. Electrical Safety First, the electrical safety charity, estimates that around 70% of all electrical fires and a significant proportion of electrocution deaths could be prevented with RCD protection. The 30mA trip threshold is specifically chosen because currents above that level, flowing through the human body, can cause cardiac arrest. The speed of disconnection — typically within 25 to 40 milliseconds — is fast enough to significantly reduce the risk of a fatal outcome even where contact with a live conductor does occur.
RCDs also protect against fire caused by leakage currents that are too small to operate an MCB but large enough to generate heat at a fault point. A current of a few hundred milliamps flowing through degraded insulation will not trip a 32A MCB, but it will trip an RCD and may prevent a fire.
Types of Consumer Unit
There are three main types of consumer unit you will encounter in domestic properties today:
Main switch units: A single main switch and individual MCBs with no RCD protection. These are obsolete by current standards for most residential applications and would attract a C2 observation on an EICR.
Split-load units: A main switch, one or two RCDs protecting groups of circuits, and MCBs for each circuit. This type was the standard for many years and is still found in a large proportion of UK homes. It provides RCD protection but has the disadvantage that a single RCD fault takes out all circuits protected by that device.
All-RCBO units: A main switch and individual RCBOs for every circuit. This is the current best practice for new installations. Every circuit has its own RCD protection, meaning a fault on one circuit disconnects only that circuit. This type offers the highest level of protection and the greatest convenience.
When Should a Consumer Unit Be Replaced?
A consumer unit should be replaced when it no longer provides adequate protection, when it is physically deteriorated, when it cannot accommodate the circuits the property requires, or when an EICR has identified it as a source of concern. The specific triggers include: the presence of rewirable fuses; no RCD protection on circuits supplying socket outlets; physical damage to the enclosure; the unit having reached the end of its expected service life; or the circuits requiring more capacity than the current board can provide.
There is no fixed service life for a consumer unit — a well-made unit from a reputable manufacturer can last thirty years or more if the underlying installation remains sound. But many consumer units in domestic properties are considerably older than this, and some contain components that have been discontinued and can no longer be serviced or replaced.
If you are unsure about the condition or adequacy of your current consumer unit, the right starting point is an EICR inspection. The inspector will assess the unit as part of the overall installation and will make clear whether it needs replacing and on what timescale. If replacement is recommended, our consumer unit replacement service in Exeter can arrange for a NICEIC-registered electrician to carry out the work, issue the necessary Part P certification, and leave you with a fully documented installation.
Who Can Replace a Consumer Unit?
Consumer unit replacement is notifiable work under Part P of the Building Regulations in England. This means it must either be carried out by a qualified electrician who is registered with an approved competent person scheme — such as NICEIC or NAPIT — or it must be notified to the local authority before work begins.
In practice, virtually all consumer unit replacements in domestic properties are self-certified by the installing electrician through their competent person scheme registration. On completion of the work, the electrician issues an Electrical Installation Certificate (EIC) and notifies the work to the relevant authority. You should receive a copy of this certificate, and you should keep it. It is a legal document confirming that the work was carried out by a qualified person and has been tested and certified as safe.
This certification matters when you come to sell your property. A buyer's solicitor will ask for evidence of building regulations compliance for any notifiable work carried out since 2005. Without an Electrical Installation Certificate for a consumer unit replacement, you may face delays or complications at the point of sale.
Never allow unlicensed or unregistered persons to replace a consumer unit. Apart from the safety risk, uncertified work will not be compliant with building regulations and may create problems with your insurance and with any future sale of the property.