What Is a Consumer Unit? What's Inside Your Fuse Box and How It Works
Updated: Oct 9
A consumer unit is the box that takes your home's electricity supply from the meter, splits it into separate circuits for lights, sockets, cooker and shower, and switches each circuit off automatically if something goes wrong. It is the modern version of what most people still call the fuse box or fuse board. Inside are a main switch and a row of protective devices, each guarding its own circuit against overload, short circuits and electric shock.
What does a consumer unit do?
A consumer unit does two jobs: distribution and protection. Distribution means feeding each circuit from one incoming supply. Protection means disconnecting a circuit quickly when it draws too much current, develops a short circuit, or leaks current to earth.
Because each circuit has its own device, a fault on the upstairs sockets trips only that circuit (or the group it shares an RCD with) and leaves the rest of the house running. That is also what makes faults easier to trace.
The supply reaches it in a set order. The network operator's cable ends at the service head, which holds the main fuse and is sealed. From there it passes through your meter, then along two thick cables called meter tails into the consumer unit. The service head and main fuse belong to the network operator and the meter to your supplier; the meter tails, the consumer unit and everything after it are normally the owner's responsibility.
Consumer unit vs fuse box: what is the difference?
In everyday speech they mean the same thing: the box where your circuits start. The technical difference is what sits inside.
An old fuse box uses fuses, either rewireable (a length of fuse wire in a ceramic carrier) or cartridge fuses. When a fault occurs the fuse melts and has to be replaced. Most older fuse boxes also have no RCD, so they lack the extra protection against electric shock that an RCD gives when current leaks to earth.
A modern consumer unit uses circuit breakers that switch off and can be reset, plus RCD protection. Electrical Safety First recommends upgrading a fusebox that does not have RCD protection so it meets the latest standard (it says this is not urgent), and that is the most common reason we carry out a consumer unit replacement. Replacing an old fuse box does not always mean rewiring the house, and we explain when it does in can you replace a fuse box without rewiring.
What is inside a consumer unit?
Open the front cover of a modern board and you will see some or all of these:
Main switch: isolates the whole installation at once.
MCBs (miniature circuit breakers): one per circuit, tripping on overload or short circuit. Lighting circuits are commonly 6 A, and a socket ring is commonly 32 A.
RCDs (residual current devices): monitor the balance between line and neutral and trip if current leaks to earth. Those protecting sockets are rated at 30 mA, which is fast enough to reduce the risk of a fatal shock.
RCBOs: an MCB and an RCD in one device, protecting a single circuit against both kinds of fault.
Surge protection device (SPD): diverts voltage spikes from lightning or switching on the network, now common in new boards.
AFDDs (arc fault detection devices): fitted in some newer boards to detect the dangerous arcing caused by damaged cables or loose connections.
Neutral and earth bars: terminal strips where the neutral and earth conductors of each circuit connect.
A circuit chart: the labels that tell you which breaker controls what.
The difference between RCD and RCBO boards matters when something trips, because one faulty appliance on a shared RCD can take out half the house; we compare the two in RCBO vs RCD.
What does the main switch do?
The main switch disconnects every circuit in the consumer unit at once. On a modern board it is double pole, meaning it breaks both the line and the neutral. It is the switch to use in an emergency, such as water coming through a ceiling onto a light fitting, or before any work on the installation.
On most boards the main switch is a simple isolator and does not trip for a fault. Some older split boards use an RCD as the main switch, in which case a single earth fault anywhere cuts power to the whole house. Turning the main switch off does not make the meter tails or the service head safe; they stay live.
Types of consumer unit
Split load (dual RCD): the circuits are shared between two RCDs, with MCBs for each circuit. Common and economical, but one fault can switch off every circuit on that side.
RCBO board: every circuit has its own RCBO, so a fault only affects the circuit it is on.
High integrity: a mix, with some circuits on RCDs and others on RCBOs.
For domestic premises, BS 7671 requires consumer units to have an enclosure made of non-combustible material, or to sit inside a non-combustible cabinet, which in practice means a metal (usually steel) housing. Plastic units fitted before that rule came in are not automatically unsafe, but new and replacement boards must meet it.
Can you replace a consumer unit yourself?
No. Replacing a consumer unit is notifiable work under Part P of the Building Regulations in England. It must be carried out by a registered electrician who can self-certify the work under Part P, or notified to building control before work starts. The electrician tests every circuit before and after connecting the new board and issues an Electrical Installation Certificate.
Those tests matter. A new board with RCD protection will expose existing earth leakage and wiring faults that an old fuse box ignored, so some circuits need repairs before they can be connected. Our upgrade service starts by checking the existing wiring and planning the circuit layout, and finishes with every circuit tested and full certification.
Signs your fuse box needs attention
rewireable or cartridge fuses instead of circuit breakers
no RCD protection on sockets
a wooden back board, cracked casing or missing covers
scorch marks, a burning smell, buzzing or a board that feels warm
regular tripping with no obvious cause
plans for an EV charger, a new kitchen or an extension that adds circuits
Any of these is a good reason to have the installation inspected. An EICR tells you whether the board and the circuits behind it are in a satisfactory condition before you spend money on either.
Consumer unit questions, answered
Where is my consumer unit?
Usually near where the supply enters the building: under the stairs, in a hallway cupboard, in the garage, or close to the meter. In flats it is often in a cupboard by the front door.
Can I reset a tripped breaker myself?
Yes. Switching a tripped MCB or RCD back on is exactly what they are designed for. If it trips again straight away, leave it off and find the cause rather than forcing it.
Should I test my RCD?
Electrical Safety First suggests checking RCDs regularly with the test button marked "T". If pressing it does not switch the RCD off, have a registered electrician investigate.
Upgrading your consumer unit
Whether you need an electrician in Stafford, Telford or Wolverhampton, we have over 20 years of experience replacing old fuse boxes and consumer units in homes, rentals and businesses across Shropshire, Staffordshire and the West Midlands from our base in Shifnal. As a registered electrician, we self-certify the work under Part P and issue the certification. If your board is old or you are not sure what you have, get in touch and we will take a look.
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