Switch Fuse and Fused Connection Unit (FCU): The Complete Guide

Your electrician mounts a new extractor fan, runs the cable back, and asks where you want the spur. You shrug and say "wherever". Six months later the fan starts buzzing, and to kill the power you have to pull a fuse at the consumer unit and work in the dark. The fan is wired into a buried junction box with no local isolation. A switched fused connection unit on the wall would have solved that on day one.

What it is and what it's for

A switch fuse is a small wall-mounted device that does two jobs for a single fixed appliance: it lets you switch the appliance off locally, and (in its most common form) it protects the appliance with a fuse. The proper trade name for the everyday version is a fused connection unit (FCU), and you will also hear it called a fused spur, a fused spur outlet, a switched fused connection unit (SFCU), or just "a spur". They are all the same family of device.

The reason it matters comes down to one word: isolation. Most domestic switches and fuses only break the live conductor (the wire carrying the supply). A switch fuse breaks both the live and the neutral conductors at once. That is what "double-pole" means: two poles, both interrupted by the same switch. For a portable lamp that is overkill, but for a fixed appliance that someone will one day open up to clean or repair, double-pole isolation is the difference between safe and lethal. Pull the live only and the appliance terminals can still sit at supply potential through the neutral under certain fault conditions.

There is also a regulations reason you cannot simply put a plug socket next to a fixed appliance and call it done. BS 7671, the UK wiring regulations, expects fixed equipment that is unlikely to be moved (an extractor fan, a towel rail, an immersion heater) to have a dedicated permanent connection rather than a plug and socket. A socket outlet is intended for portable appliances you plug in and unplug. The fused connection unit is the approved way to give a fixed appliance its own switched, fused, permanent connection.

Types, sizes, and specifications

There are two devices doing the "switch fuse" job, and the distinction matters enormously. Choosing the wrong one is one of the few electrical mistakes a homeowner can spot in a finished install.

The 13A fused connection unit is the one you will see most often. It carries a replaceable cartridge fuse, exactly like the fuse inside a UK three-pin plug, rated anywhere from 1A up to 13A. You match the fuse to the appliance: a bathroom extractor fan typically takes a 3A fuse, a towel rail 3A to 5A, an electric cooker hood around 3A, a bathroom heat lamp 3A. That cartridge fuse is the appliance's last line of defence. If the appliance develops a fault and starts drawing more current than its flex can safely carry, the fuse blows before the flex overheats.

The 20A double-pole switch looks similar from across the room but has no fuse carrier. It is purely a heavy-duty double-pole isolator. You use it where the appliance is on its own dedicated radial circuit and the overcurrent protection is provided back at the consumer unit by the MCB (miniature circuit breaker) or RCBO (residual current breaker with overcurrent protection) on that circuit. The classic example is an immersion heater, which can draw 13A continuously for hours. A 13A cartridge fuse run flat out for that long tends to weaken and nuisance-blow, so the immersion gets its own circuit and a fuse-free 20A switch instead.

That is the key safety point worth holding onto: for sustained high loads, the 20A double-pole switch on a dedicated circuit is safer than a 13A fused connection unit. A cartridge fuse is designed for intermittent duty, not for sitting at its rated current all day.

Both types come in a few flavours you will be asked to choose between:

• Switched or unswitched. Switched units have a rocker you can flick to isolate the appliance. Almost always get switched: the whole point is local isolation.
• With or without neon indicator. The neon is a small light that glows when the unit is on. Handy on an extractor or immersion so you can see at a glance whether it is powered. A pound or two more.
• Flex outlet or no flex outlet. A flex outlet is a hole in the faceplate where the appliance's flexible cable passes through. Choose this when the appliance has a trailing flex (a towel rail, a fan). Without a flex outlet, the cable enters from behind through the back box.

Faceplate finishes run from standard white moulded plastic up to brushed steel, chrome, and black nickel for a kitchen that has been styled. The internals are identical; you are paying for the look.

How to work with it

A fused connection unit is not heavy or awkward, and the physical fixing is the same as any wall accessory. The unit screws onto a single-gang back box, the same metal or plastic box used for a light switch or single socket. The box needs to be at least 35mm deep so the wiring and the unit's body fit without crushing the conductors. Shallower 25mm boxes that suit a slim light switch are too tight for an FCU; fit a deep box at first fix.

The actual wiring is a job for your electrician, and in England and Wales adding or altering a circuit in a kitchen or bathroom is notifiable work under the building regulations, so it should be done by a registered electrician who can self-certify it. What you should understand is the layout, because you will be asked to decide where these units go before the walls are plastered.

A fused connection unit has two sets of terminals: supply (marked with arrows pointing in, or labelled "mains" / "feed" / "supply") and load (arrows out, or "load" / "out"). The supply side connects to the circuit cable coming from the consumer unit or from a ring final circuit. The load side connects to the appliance. Get those reversed and the fuse and switch sit on the wrong side of the appliance, leaving it live when the unit is switched off. The arrows are there precisely so this does not happen.

The fuse carrier is the other thing you will interact with over the appliance's life. It is a small slot, usually below the switch, that pops out with a flat screwdriver or a coin to reveal the cartridge fuse. When an extractor fan dies, the first thing to check is whether the 3A fuse in its spur has blown, not the appliance itself. Keep a few spare cartridge fuses (3A and 13A cover most cases) in the same drawer as your spare lamps.

These units are unaffected by weather indoors, but the standard white plastic versions are not rated for outdoor or wet locations. An outside tap heater or a garden room appliance needs an IP-rated weatherproof unit, which is a different product in a sealed enclosure.

How much do you need

Quantities are simple: one switch fuse per fixed appliance that needs local isolation. There is no per-square-metre calculation here, just a checklist of the fixed appliances in your extension and what each one wants.

Walk the rooms and count. A typical single-storey kitchen extension with a utility area and a shower room ends up wanting something like:

• Kitchen extractor / cooker hood: one 13A FCU, 3A fuse (above and to the side of the hob)
• Utility room extractor fan: one 13A FCU, 3A fuse
• Shower or bathroom extractor fan: one 13A FCU, 3A fuse (switch usually outside the room or on a ceiling pull)
• Heated towel rail: one 13A FCU, 3A to 5A fuse
• Underfloor heating wiring centre or manifold: one 13A FCU (check the manufacturer's stated fuse rating)
• Immersion heater or thermal store: one 20A double-pole switch on its own circuit

So a realistic extension uses four to six 13A fused connection units and, if you have hot water from an immersion or a heat battery, one 20A double-pole switch. Buy one or two spares; they cost little and save a second trip to the merchant when you find an appliance you forgot. There is no meaningful wastage allowance because these are not cut or trimmed; you either fit a unit or you do not.

Cost and where to buy

These are cheap components, and the price gap between budget and premium is about brand and faceplate finish, not function. Expect to pay £5 – £20 for a 13A fused connection unit and £10 – £25 for a 20A double-pole switch, with the higher end reflecting switched versions with a neon indicator and metal faceplates. White plastic basic units sit at the bottom of those ranges.

The dependable brands are MK (often regarded as the trade default for switches and sockets), Crabtree, Hamilton, BG, Schneider, and Hager. Every one of these fits a standard UK single-gang back box, so you are not locked into a system; you can mix a budget BG fused spur in the utility room with styled Hamilton plates in the kitchen if you want. For a hidden unit behind an appliance, there is no reason to spend more than the basic white MK or BG. For a visible kitchen wall, the metal-finish ranges look the part.

You will find these at every electrical wholesaler and DIY merchant: Screwfix, Toolstation, Wickes, and Travis Perkins all stock them off the shelf, and your electrician will often supply them as part of the job. Because they are small and light, buying online or picking up in store is straightforward; there are no delivery considerations like there are with bulk building materials.

External resource

IET Wiring Regulations (BS 7671)

The UK standard governing fixed electrical installations, including the requirement for fixed appliances to have a dedicated connection rather than a plug and socket.

electrical.theiet.org

Alternatives

The most common alternative homeowners reach for is a plug and socket, and for a genuinely portable appliance that is correct. If the appliance gets unplugged and moved (a kettle, a toaster, a freestanding fan heater), a socket is the right answer. The fused connection unit is specifically for appliances that stay put.

For appliances that must never be casually switched off, an unswitched fused connection unit is the alternative to the switched version. A fridge or freezer is sometimes fed this way so nobody knocks the switch and silently defrosts the contents. The trade-off is that isolating it for service means pulling the fuse carrier rather than flicking a switch.

Where an appliance needs both local isolation and a clear visual warning that it is live, the neon-indicator version is worth the small premium over the plain switched unit, particularly for an immersion heater you might otherwise leave on by accident.

Where you'll need this

• First fix electrics - fused spurs and switch positions are set out during first fix, before plastering
• Kitchen electrical provisions - the extractor and cooker hood spur is part of the kitchen first-fix layout
• Electrical layout planning - decide where each fixed appliance and its isolator go before any cable is run

These units appear during the first-fix electrical stage of any extension or renovation project, anywhere a fixed appliance needs its own switched, fused connection.

Common mistakes

The mistake that bites hardest is wiring a fixed appliance straight into a junction box with no local isolator at all. It passes inspection if done correctly, but it leaves you with nothing to switch off when the appliance needs servicing. Always specify a switched fused connection unit for fixed appliances; the few pounds it costs is trivial against the inconvenience later.

The second common error is reversing the supply and load terminals, which leaves the appliance live even when the unit is switched off. This is your electrician's job to get right, but it is worth knowing the arrows on the unit point from supply to load so you can sanity-check a self-install if you ever do simple like-for-like work.

Finally, do not under-spec the back box. Fitting a shallow 25mm box at first fix to save depth means the FCU body and its wiring will not seat properly, and you end up forcing the faceplate against crushed conductors. Use a 35mm-deep box as standard for any fused connection unit.