What Is Inside a Consumer Unit?
A consumer unit is the main electrical control point in most homes. It is sometimes still called a fuse box, although modern consumer units use circuit breakers and other safety devices rather than old-style fuses.
Inside a consumer unit you will usually find a number of different components, each with an important job. These parts help control the electrical installation, protect circuits from overloads and faults, and improve safety throughout the property.
Understanding what is inside a consumer unit can help homeowners and landlords better understand their electrical system and any recommendations made by an electrician.
Quick Summary
A consumer unit contains several key components including the main switch, circuit breakers (MCBs), RCDs or RCBOs, and sometimes surge protection (SPD) or arc fault detection (AFDD).
It also includes internal parts such as the main tails, neutral bar, earth bar and busbar, along with circuit labels and important safety notices.
Older Fuse Boxes and Modern Consumer Units
Not every consumer unit looks the same inside.
Older fuse boxes may contain rewirable fuses, cartridge fuses or other older protective devices instead of modern breakers. Newer consumer units are more likely to contain MCBs, RCDs, RCBOs and surge protection devices.
The exact layout depends on the age of the installation, the type of board fitted, and whether the unit has been upgraded over time.
Rewirable Fuses
Some older fuse boxes contain rewirable fuses instead of modern circuit breakers.
These fuses use a length of fuse wire designed to melt if too much current flows on the circuit. If a fault or overload occurs, the fuse wire blows and disconnects the supply to help protect the circuit.
Rewirable fuses were common in older installations, but they provide less protection and are generally less convenient than modern MCBs and RCBOs.
If a rewirable fuse blows, the fuse wire has to be replaced with the correct size and type. Incorrect replacement can create a safety risk.
Older fuse boxes with rewirable fuses also usually lack the additional safety features found in modern consumer units, such as RCD or RCBO protection.
Main Switch
The main switch allows the power to the installation to be turned off.
This isolates the outgoing circuits so they can be worked on more safely. In most homes, switching off the main switch cuts power to the electrical circuits in the property, but it does not usually make the whole consumer unit dead.
Image of a Main Switch:
Main Tails
The main tails are the large insulated cables that bring the electrical supply into the consumer unit.
They usually connect from the electricity meter or isolator to the main switch and are typically 16 mm² or 25 mm² in size, depending on the installation.
These cables remain live on the supply side of the main switch unless the supply has been isolated upstream, so they should only ever be worked on by a qualified electrician.
Image of Main Tails:
Main Earthing Conductor and Bonding Conductors
In addition to the internal earth bar, there will also be a main earthing conductor and main protective bonding conductors connected to the installation.
The main earthing conductor connects the electrical installation to the supply earth. It is commonly 16 mm² in size in many domestic installations, although this can vary.
The main protective bonding conductors connect metal services such as gas, water or oil pipework to the earthing system. These are typically 10 mm² in domestic properties, depending on the installation.
Earthing is one of the most important parts of an electrical installation, as it allows protective devices such as MCBs, RCDs and RCBOs to operate correctly and disconnect the supply quickly in the event of a fault.
During an electrical inspection such as an EICR, it is not uncommon to find older bonding conductors that are 6 mm². Where there are no signs of thermal damage and the installation is otherwise satisfactory, this is typically recorded as a C3 (improvement recommended).
These conductors are an important part of the overall safety system, helping ensure that exposed metalwork remains at a safe potential in the event of a fault.
They are usually connected near the consumer unit or at the main earthing terminal and should only be installed or modified by a qualified electrician.
Image of Earthing and Bonding Conductors:
Circuit Breakers (MCBs)
MCBs, or miniature circuit breakers, protect individual circuits from overloads and short circuits. They are often referred to as trip switches.
Each MCB is usually assigned to a specific circuit, such as lighting, sockets, cooker, shower, garage or smoke alarms.
If too much current flows on a circuit, or if a fault occurs, the MCB will trip and switch that circuit off. In this way, it performs a similar function to a fuse, but can be reset rather than replaced.
Image of an MCB:
RCDs
An RCD, or residual current device, provides protection against electric shock by detecting earth leakage faults.
If it detects electricity flowing where it should not — such as through faulty appliances, damaged wiring, or potentially through people or pets — it can quickly disconnect the supply, typically in less than half a second.
In some consumer units, one RCD may protect a group of circuits. This is common in split-load consumer units. When an RCD trips, all of the circuits it protects will lose power at the same time, which can make fault finding more difficult, as it is not always obvious which circuit is causing the tripping.
Modern installations use Type A RCDs, which are designed to detect a wider range of fault currents. Older Type AC RCDs may not detect certain fault conditions caused by modern electronic equipment and are now generally replaced with Type A devices.
Image of an RCD:
RCBOs
An RCBO combines the job of an MCB and an RCD into one device.
This means it protects against both overcurrent and earth leakage faults on an individual circuit.
Modern RCBOs are typically Type A, allowing them to detect a wider range of fault currents produced by modern electronic equipment.
RCBO consumer units are now very common because they provide better circuit separation. If one RCBO trips, it usually affects only that one circuit rather than several circuits at once.
Image of an RCBO
Surge Protection Device (SPD)
Modern consumer units will usually include a surge protection device, often called an SPD.
An SPD helps protect the electrical installation and connected equipment from sudden increases in voltage, known as power surges. These can be caused by events such as lightning strikes, switching of large electrical loads, or disturbances on the electricity network.
These surges can damage sensitive electronic equipment such as TVs, computers, broadband routers and appliances with circuit boards.
An SPD works by diverting excess voltage safely to earth, helping to protect both the electrical installation and the equipment connected to it.
SPDs are typically fitted within the consumer unit and may include an indicator window to show whether they are still operational.
Image of an SPD:
Arc Fault Detection Device (AFDD)
Some consumer units may also contain an arc fault detection device, usually called an AFDD.
An AFDD is designed to detect certain types of dangerous arcing faults in a circuit. These faults can happen if a cable or connection becomes damaged, loose or deteriorated and starts to arc.
If this type of fault is detected, the AFDD disconnects the circuit to help reduce the risk of fire.
Under current wiring regulations, AFDDs are required in some higher-risk types of building, such as HMOs, care homes and certain multi-occupancy buildings, and are recommended in many other situations.
AFDDs are not found in every consumer unit and are not usually installed in standard family homes at present. As costs reduce, they may become more widely used in domestic installations.
Image of an AFDD:
Circuit Cabling
Each protective device in the consumer unit is connected to a circuit cable that supplies power to different parts of the property.
These cables run from the consumer unit to points such as lighting, socket outlets, cookers, showers and other equipment.
They are sized and installed to suit the type of circuit and the expected electrical load. For example:
• lighting circuits are typically 1.0 mm² or 1.5 mm²
• socket circuits are usually 2.5 mm²
• cooker circuits are often 6 mm² or 10 mm²
• shower circuits are typically 6 mm² or 10 mm²
All of these circuits are protected by the devices within the consumer unit, such as MCBs, RCDs, RCBOs or AFDDs, which disconnect the supply if a fault or overload occurs.
Neutral Bar
The neutral bar is the terminal bar where the circuit neutral conductors are connected.
It forms part of the return path for the electrical circuits in the installation and is an essential part of the consumer unit.
Depending on the type of consumer unit, there may be more than one neutral bar inside.
Earth Bar
The earth bar is the terminal bar where the circuit protective conductors, often called earth wires, are connected.
It provides the earthing connection for the circuits in the installation and plays an important role in electrical safety.
As with other parts inside the consumer unit, this should only be accessed by a competent electrician.
Busbar
Inside the consumer unit there is usually a busbar, although this is not normally visible once the front cover is in place.
The busbar is a metal strip that distributes power to the protective devices, such as MCBs and RCBOs.
This allows the consumer unit to safely supply multiple circuits from a single incoming supply.
Blanks and Spare Ways
You may notice blanking plates on the front of a consumer unit.
These cover unused ways where no protective device has been fitted. They are important because they prevent access to live parts inside the unit.
Some consumer units have spare ways left for possible future additions, such as an EV charger, shower circuit or garage supply.
Circuit Identification Labels
A consumer unit should be clearly labelled to show what each device controls.
This may be written on a circuit chart or identification label inside or next to the unit.
Clear labelling makes it easier to identify the correct circuit for maintenance, testing or fault finding.
Safety Labels and Warning Notices
You may also find a number of warning stickers or notices on or near the consumer unit.
These can include RCD test notices, mixed cable colour warning labels, periodic inspection labels, main switch identification, or surge protection and dual supply warning notices.
These labels help users and electricians understand important safety information about the installation.
Important Safety Note
Although the main switch turns off the outgoing circuits, it does not usually make the whole consumer unit dead.
The incoming supply side of the main switch, including the supply tails feeding it, will usually remain live unless isolated elsewhere.
That means dangerous live parts can still be present inside the consumer unit even when the main switch is off.
For this reason, consumer units should only be opened or worked on by a qualified electrician working for a Part P compliant company. Any new circuits or significant electrical work must be properly notified to local building control and certified in accordance with current regulations.
Need Advice on a Consumer Unit?
If you are unsure what is inside your consumer unit, or you have an older fuse box that may need upgrading, RW Currion Electrical can help.
We carry out consumer unit upgrades, EICRs, fault finding and general electrical work for homeowners, landlords and businesses in Banbury and the surrounding towns and villages.
Why Choose RW Currion for Your Electrical Work?
• Fully qualified and insured
• Clear explanations — no jargon
• Work completed to the latest BS 7671 standards
• Safety-first approach to protect your home, family, and electronics
• All installations come with full certification and are emailed for your records.
Honest advice on consumer units
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