Consumer Unit Explained: What Every Switch Does (And What to Do When One Trips)
Open your consumer unit - the box most people still call the fuse box - and you're faced with a row of switches, usually labelled things like "lights," "sockets" and "cooker." Most of the time you only ever look at it when the power's gone off and you're trying to work out which switch has flipped.
This guide explains, in plain English, what each switch and device in a modern consumer unit actually does, what it's protecting you from, and what to do (and what not to do) when one trips. No jargon, no assumptions - just a clear explanation of the box that keeps your home's electrics safe. At MS Electrical Solutions, we get asked these questions all of the time, so any questions you may have regarding your Consumer Unit installation and the many switches, below should answer them.
What Is a Consumer Unit?
The consumer unit is the central hub of your home's electrical system. It's where the electricity supply enters your property and gets divided up into the separate circuits that run your lights, sockets, cooker, shower and everything else.
Its job isn't just distribution, though - it's safety. Every device inside a modern consumer unit exists to cut the power automatically when something goes wrong, before that fault can cause an electric shock or a fire. Different devices protect against different types of fault, which is why there's more than one kind of switch in there.
The Main Switch
The large switch, usually on one side of the unit (often bigger than the rest), is the main switch - sometimes called the main isolator.
Flicking this off cuts all power to your entire property in one action. It's what you'd use to safely isolate the electrics before certain work is carried out, or in an emergency where you need everything off quickly.
You can use the main switch yourself to turn off the whole supply if you ever need to - it's the one switch in the unit that's designed for you to operate normally.
MCBs - Protecting the Circuits from Overload
The row of smaller switches, each usually labelled with a circuit name, are most often MCBs (Miniature Circuit Breakers). You might hear them called trip switches.
An MCB protects an individual circuit from overload and short circuits - in other words, from drawing too much current. If a circuit tries to pull more power than its cabling is designed to handle (too many appliances on one circuit, or a faulty appliance causing a short), the MCB trips and cuts that circuit off. This stops the cabling overheating, which is what could otherwise lead to a fire.
Each MCB protects one circuit, which is why when one trips, you lose the lights or the sockets in one area rather than the whole house. MCBs replaced the old rewirable fuses you'd find in much older fuse boxes - instead of a blown fuse wire you have to replace, you get a switch you can reset.
RCDs — Protecting You from Electric Shock
An RCD (Residual Current Device) does a completely different job to an MCB, and this is the bit most people don't realise: they protect against different things.
While an MCB protects the cabling from overload, an RCD protects you from electric shock. It constantly monitors the electricity flowing out to a circuit and the amount flowing back. If some of that current goes missing - because it's leaking to earth, potentially through a person who's touched something live - the RCD detects the imbalance and cuts the power in a fraction of a second.
In UK homes, RCD protection trips at a leakage of 30 milliamps (30mA), which is the level considered necessary to protect against serious harm.
In older consumer units, you'll often see one or two RCDs each protecting a whole bank of circuits - so if one trips, you lose everything on that side of the board (for example, all your sockets, or all your lighting). That's why sometimes a single fault can seem to take out half the house.
RCBOs — The Best of Both, Per Circuit
A newer arrangement, and now considered best practice, uses RCBOs (Residual Current Breakers with Overcurrent protection).
An RCBO combines both jobs - the overload protection of an MCB and the shock protection of an RCD - into a single device, on a single circuit. The big advantage is that if a fault occurs, only that one circuit trips. Not half the house, not a whole bank of circuits - just the one affected.
Under the current wiring regulations (BS 7671), RCBO-per-circuit boards are increasingly the preferred layout for new and upgraded installations, because losing a single circuit to a fault is far less disruptive than losing several - and it makes finding the fault much easier.
SPDs - Protecting Against Power Surges
Many modern consumer units also include an SPD (Surge Protection Device).
An SPD protects your installation and everything plugged into it from sudden spikes in voltage - power surges - caused by things like lightning strikes on the network or switching events on the grid. It diverts that excess energy safely to earth before it can reach and damage sensitive electronics like TVs, computers and smart home equipment.
SPDs are now a standard requirement to consider in new installations and consumer unit replacements under current regulations.
What to Do When a Switch Trips
If a switch in your consumer unit trips, here's the sensible approach:
Open the consumer unit and look for the switch that's flipped to the off position - it'll be out of line with the others.
Unplug or turn off whatever was being used when the power went - often it's a specific appliance that's caused it.
Try resetting the switch by flicking it firmly back to the on position.
If it stays on - problem likely solved. It may have been a one-off, or a faulty appliance you've now disconnected.
If it trips straight back off, or keeps tripping - stop. A switch that won't reset, or trips repeatedly, is telling you there's a genuine fault on that circuit. That's not something to keep forcing or to investigate yourself.
This is the important bit: repeatedly resetting a switch that keeps tripping, or trying to work around it, is exactly the wrong thing to do - the switch is doing its job by cutting power to a fault. If a circuit keeps tripping, or you're ever unsure, that's the point to call a qualified electrician. It's a routine callout for us, and far cheaper than the alternative.
How Old Is Your Consumer Unit?
If your consumer unit still uses old rewirable fuses (with fuse wire rather than switches), has no RCD protection at all, or is made of plastic rather than metal, it may be worth having it assessed. Older boards can still be working, but they often don't offer the level of shock and fire protection that modern regulations require - and an upgrade is usually a straightforward job.
We carry out consumer unit upgrades across Horley, Crawley, Reigate, Redhill and the wider Surrey and West Sussex area, always to current BS 7671 standards with certification on completion.
Frequently Asked Questions
What's the difference between an MCB and an RCD?
An MCB (Miniature Circuit Breaker) protects a circuit's cabling from overload and short circuits - from drawing too much current. An RCD (Residual Current Device) protects people from electric shock by detecting current leaking to earth and cutting the power. They protect against completely different things, which is why a consumer unit needs both.
What is an RCBO?
An RCBO combines the functions of an MCB and an RCD into a single device protecting one circuit. It provides both overload protection and shock protection, and because each circuit has its own RCBO, a fault trips only that circuit rather than a whole group. It's now considered best practice in modern consumer units.
Why does my switch keep tripping?
A switch that trips repeatedly is detecting a genuine fault on that circuit - often a faulty appliance, but sometimes a wiring issue. Try unplugging whatever was in use and resetting it once. If it trips again, stop resetting it and call a qualified electrician, as repeatedly forcing it doesn't fix the underlying fault.
Can I turn off my own consumer unit?
Yes. The main switch (the large one, usually to one side) is designed for you to operate and turns off all power to your property in one action. The individual circuit switches can also be turned off safely. What you shouldn't do is attempt any work inside the unit itself - that's for a qualified electrician.
What is an SPD in a consumer unit?
An SPD (Surge Protection Device) protects your electrics and connected equipment from voltage spikes, such as those caused by lightning or grid switching, by diverting the excess energy safely to earth. It's now a standard consideration in new consumer units and replacements.
