Fuse Boxes: Where They Are & How They Work

A mains fuse box (also known as a fuse board or consumer unit) is essentially the brain of your home’s electricity.

It’s designed to operate and safeguard your home’s power. So, it’s helpful to understand:

• What a fuse and a fuse box is
• Where your fuse box is located
• What kind of fuse box you have.

And more!

Understanding your fuse box

A fuse is a safety component in an electric circuit. It’s essentially a small piece of wire designed to break, burn or melt to stop the flow of electricity in a circuit.

A cartridge fuse is the traditional cylindrical design found in plug tops and older fuse boxes. It works as follows.

Current enters a fuse through one of its two end terminals, then travels through the following components:

• The end caps: Metal casings at each end of the fuse. They form the electrical connection between the fuse and its holder, completing the circuit.
• A centre post: A metal pin or rod that carries current from the end cap into the fuse element.
• A bead of solder: A small amount of low-melting-point metal alloy that joins the wire to the centre post.
• The wire: This carries the current through the fuse.
• The body: The outer casing, typically made of glass or ceramic. It houses and protects the internal components but does not carry current.

When the current exceeds a certain level in amps, it generates heat which melts the solder bead. This causes the wire to pull away from the centre post.

When this happens, the fuse is described as having ‘blown’, creating a gap in the circuit and cutting power to that specific circuit.

Note: Many modern homes now use miniature circuit breakers (MCBs) instead. These work on a similar principle but can be reset when circuits trip with a switch rather than needing to be replaced. Nowadays, people most often replace fuses in plugs, not fuse boxes.

You will know if a fuse has blown if your lights suddenly go out or your electrical appliances stop working.

Locating your fuse box

Fuse boxes can be found in different parts of homes. But they are commonly found in:

• Utility rooms
• Porches
• Cupboards under the stairs
• Hallways
• Garages.

Knowing where yours is important. You may need to turn it off in an emergency.

Maintaining easy access

Wherever your fuse box is, you should have easy access to it. Keep the area surrounding your fuse box clear from obstructions and clutter.

Remember: If the lights go out, you may need to find your fuse box with a torch! And if it’s high up on a wall, keeping a stepladder nearby will be useful.

Fuse boxes have changed over time

In the UK, fuse boxes first entered homes in the early 20th century. Thomas Edison had patented the fuse not long before, in 1890.

Over the decades, technology for fuse boxes has improved. Today, they can be broadly categorised into three main types.

Understanding which type you have is useful when considering:

• Risk and safety in your home
• Upgrades to your fuse box
• Upgrades to your home (adding an extra socket or light, building an extension, etc.)
• Home insurance.

Traditional fuse boxes

Traditional fuse boxes (sometimes referred to as rewireable fuse boxes) typically have at least three of the following:

• Several different rewirable fuses
• No RCD switches (see below)
• Fuse carriers (often called bridges). (These may carry asbestos.)
• A wooden backboard.

It’s estimated that 12 – 18% of domestic UK fuse boxes are in this category, which can be further subdivided.

Old wire fuse boxes

Fuse boxes that pre-date the 1960s are generally considered ‘old wire fuse boxes’.

In these, each rewirable fuse burns through or breaks to cut off power to a circuit. So, when a fuse blows, a replacement new piece of wire is needed.

Repairing a fuse used to be a relatively simple DIY task, but it was also time-consuming. This led to some people replacing fuses with incorrect materials nearer to hand (foils, nails, etc.), which caused fires.

Fuses are now no longer widely available in UK electrical hardware stores.

Cartridge fuse boxes

These fuse boxes use sealed glass or ceramic cartridges (or fuse holders). This made replacing fuses, which are stored in the cartridges filled with sand, easier and safer.

Cartridge fuse boxes were introduced in the 1960s but were popular until the 1980s.

Miniature Circuit Breakers (MCB) fuse boxes

MCB technology was invented in Germany in 1924. By the early 1950s, it was being used in industrial settings (and some homes) in the UK. But it wasn’t until the 1970s that they were widespread here.

They were the first fuse boxes to simply require a switch to be flicked rather than a fuse (or a cartridge containing a fuse) replacement.

Dual RCD Boards

Dual RCD Boards (also known as 17th Edition Boards) are typically encased in plastic boxes. They have two large RCD switches (see below) and several smaller MCB switches. 

They are estimated to make up between 35 – 42% of all UK fuse boxes.

Dual RCD Boards are safer than traditional fuse boxes. They cut electricity almost immediately when electricity breaks, reducing the chances of fires or electrocutions.

However, they also blanket cut up to five circuits at a time. This leads to ‘nuisance tripping’ where multiple devices close at once. 

This makes it harder to identify the cause and therefore more dangerous, too – as homeowners may be unaware of which appliance is faulty.

Residual Current Device (RCD)

RCDs are switches that turn off the electricity supply when a dangerous fault is detected.

They save lives by cutting electricity when there’s a household appliance fault or someone touches a live cable.

So, it’s a good idea to test if your RCDs are working correctly every three months.

All you need to do is press the ‘test’ button located next to your RCD switches. It should throw a test fault to trip an RCD switch.

If the RCDs are working, the switch should move into the down position i.e. ‘off’. You can reset it simply by flicking it back up to the ‘on’ position.

If the test button does not trip an RCD switch, there could be a fault on your fuse box. Contact a qualified electrician to take a look for you.

Do not attempt to fix your electricity board yourself, unless you are a qualified electrician. Electricity can be very dangerous. If you don’t know what you’re doing, you could put yourself and those living in your household at serious risk.

Modern Fuse Boxes

Modern fuse boxes are technically referred to as metal consumer units. 

They became a legal requirement for all new domestic installations in 2016, when Amendment 3 of the 17th Edition Wiring Regulations mandated non-combustible casing. This rule carried forward into the current 18th Edition.

They are the safest kind of fuse boxes. They have more circuits and switches (see below), making isolation easier in case of failures. And they have metal casing, which reduces fire risk.

Contrary to what many expect, modern fuse boxes are usually larger than older ones. This enables them to be upgraded easier (see below section, ‘Upgrading your fuse box’).

Modern electric fuse boxes contain 3 key components, and additional options, each of which serves a different purpose.

1. Main switch

The main switch in your modern fuse box is designed to turn off the electricity supply in your property. 

It’s important to know which switch this is, so you can turn it off in an emergency.

If there is more than one fuse board installed on a property, there will also be more than one main switch.

For example, if you heat your home with electric storage heaters, you’ll likely have a separate electricity board for them.

2. Residual Current Breaker with Overcurrent protection (RCBO)

Modern fuse boxes often contain several small RCBO switches.

RCBOs combine the functions of RCDs with more advanced features.

So, like RCDs, they can also detect leakages. But unlike RCDs, RCBOs can also detect overloads.

They also isolate specific circuits if (for example) one appliance is faulty. By contrast, an RCD would close off several circuits.

Surge Protection Devices (SPDs)

Modern fuse boxes often contain – or can be fitted with –  Surge Protection Devices (SPDs). 

These devices protect domestic appliances from power surges, which may be caused by voltage fluctuations or even lightning strikes.

Arc Fault Detection Device (AFDD)

AFDDs are advanced detectors of ‘arc faults’ (high powered electrical discharges), damaged wiring, and other issues. 

They complement RCBOs. But they aren’t a legal requirement and not all modern fuse boxes have been upgraded to include them.

3. Circuit breakers

Circuit breakers, or MCBs (Miniature Circuit Breakers), are the final type of fuse switch inside your modern fuse box.

They are designed to switch off circuits when they detect problems. They work in a similar way to a traditional wire fuse.

Your electricity board will contain several circuit breakers. Each is sized according to the type of circuit and load it can handle. 

They enable modern fuse boxes to provide more precise protection than older models.If a fault is detected in a circuit, the fuse box switch will trip and move into the downwards ‘off’ position.

To turn the power back onto the circuit, you simply have to flick the switch up to the ‘on’ position. It’s much quicker and more convenient than replacing a fuse wire (like in traditional fuse boxes). 

Just make sure you find and correct the fault first or the switch will keep tripping.

What to do if your fuse box keeps tripping

If your modern fuse box keeps tripping, something may be wrong with the wiring in your home or an appliance you are using.

Avoid continuously resetting your circuit breakers – it will damage wiring. 

If you can’t work out the problem, or you are dealing with faulty electrics, contact a qualified electrician. They have the skills, tools and knowledge to repair fuse boxes safely.

What kind of fuse box should you have?

It’s not illegal for regular homeowners to have older fuseboxes. 

However, landlords’ responsibilities – especially those with HMOs – are different, including:

• Electrical Installation Condition Report (EICR) compliance: In short, this means fuseboxes in rental properties must be inspected by a qualified professional at least once every five years.
• Fuse boxes must be encased in non-combustible units
• Fuse boxes must have RCD protection
• AFDDs in House in Multiple Occupancy (HMOs): HMOs are properties with at least three people living in them who are not one household (i.e., not family). The likelihood of increased electricity usage in these means landlords that own them must follow stricter regulations.

And more. 

Upgrading your fuse box

Upgrading your fuse box is a great way to improve your home’s electrical safety and efficiency. 

If you have an older, traditional fuse box, you are likely relying on outdated rewireable fuses. These offer less protection against fires, electrocution, and power cuts in your home.

Another reason to upgrade is if you are planning home improvements. Whether you are building an extension, adding smoke alarms, or installing a hot tub, etc., the new circuits legally must be RCD protected. 

Old boards generally cannot accommodate this. Modern ones can.

Finally, the upgrade process acts as a comprehensive electrical health check. It can lead electricians to uncovering and fixing hidden wiring faults left by previous owners

Insurance

Whatever type of fuse box you have, you can save money on repairs of it by taking out home cover with Hometree.