An FCU or fused connection unit is essentially an inline fuse and in the UK they are mainly used where a new electrical supply is taken from an existing electrical supply that is going to feature more than 1 single additional socket.
This is often the case when a new radial circuit (more than 1 new socket) is taken from an existing radial circuit or ring main.
If any issues occur on the new circuit due to an appliance that has been plugged in creating too much load, the fuse in the FCU blows and isolates the new branch to reduce the chances of any damage or shock injury.
Installing a fused connection unit is quite an easy job but one that should never be taken lightly, electricity is dangerous and can easily kill! Read on below to find out more.
NOTE: Before starting any form of work, always isolate and turn off the circuit you are working on at your consumer unit!
When are Fused Connection Units Used?
As we have touched on above, the fused connection unit is an electrical unit that features a fuse. The incoming electrical supply runs through the fuse and the outgoing supply then goes to the load or appliance.
If the load being drawn by the appliance becomes too great for the circuit, the fuse blows and cuts the supply preventing the chances of overheating and fire.
With this in mind a fused connection unit is generally used where there is a chance that the appliance or object drawing power could pull too much.
It is for this reason that they are required when adding additional radial circuits. If you add an additional 4 sockets to an existing circuit and a washing machine, tumble dryer, vacuum and kettle could all be plugged in and turned on at the same time.
As each of these items can draw a large amount of current (some more than others) if they were all running at the same time they could easily pull too much current, cause massive overheating and then ultimately a fire.
The above is a bit of an extreme example but paints a good picture. As a summary an FCU is generally used where there is the risk of either too much current being pulled from a circuit or the circuit itself is in a potentially dangerous location such as a bathroom.
Some common uses for fused connection units are as follows:
- Creating new radial circuit from an existing radial or ring main
- To power an appliance that pulls a lot of current e.g. washing machine, cooker, dishwasher, boiler etc
- For electric power showers
- For appliances in bathrooms e.g. shaver sockets
Different Types of Fused Connection Unit
In the UK there are 3 different types of fused connection unit; switched with a flex outlet, switched without a flex outlet, single and double pole and unswitched.
Each different type of FCU is generally used for a specific scenario, here’s a brief run down of each type and when it’s generally used:
Switched With Flex Outlet
A switched fused connection unit with a flex outlet features an integrated internal fuse that cuts power to the connected device in the event that large amounts of current are drawn.
The unit also features a manual switch to the allow the connected appliance to be manually turned on and off.
With other FCU’s the electrical supply cable normally runs out of the unit in either trunking or is chased into the surface behind, but with this unit a flex wire runs out of the unit itself to the connected device.
Switched Without Flex Outlet
As you might guess form the name, a switched FCU features not only a fused unit but also an isolation switch so the power supply can be manually turned off.
These are generally used for appliances where being able to turn them off manually can be a great benefit. A good example of this is an electric heater.
As an electric heater draws a large amount of current it needs a fused connection unit but at the same time it may not be left on all the time so a means of turning it off is required.
Again as the name suggests, unlike the example above that has a manual switch, an unswitched FCU only features a fuse and no manual switch.
These units are generally used for appliances that require a constant supply of power. A good example is a freezer.
Single and Double Pole FCU’s
A single pole FCU is generally regarded as being less comprehensive then a double pole unit. With a single pole, the supply is only cut to the live wire and the neutral is left still connected.
With double pole units the supply is cut to both the live and neutral making it a more complete safety device. Double pole units are generally used for devices the use large amounts of current such as cookers and boilers.
How to Fit a Fused Connection Unit
Fitting a fused connection unit is a fairly easy task if you have some basic electrical knowledge and the correct tools for the job.
To do the job properly you will need a drill/driver, small flat blade screwdriver, Philips screwdriver, utility knife, wire strippers, plastic/metal back box, pliers, 8x 30mm screws or similar and if fixing to a solid wall, 6mm masonry bit and wall plugs.
In this example we are going to spur off of an existing socket to add 2 new sockets by adding a fused connection unit between our existing socket and new sockets.
as we have mentioned several times in this guide already, and we will do again here, before you start anything, first turn off the power to the circuit you are working on at your consumer unit!
Fit Back Box
The first job is to fix a back box in place where your FCU is to be installed. You can use either a metal back box (find out how to install metal back boxes here) or a plastic back box (find out how to install plastic back boxes here).
In this example we are using a plastic back box as our FCU is being installed in a shed. If it was being installed in a house a better more professional finish would be to use a metal back box and recess it into the wall.
As the supply and feed cables are being run in a length of trunking we notched out the trunking, removed the blanks from the back box and screwed it to the wall ensuring it was straight and level.
Run Electrical Cables
The next job is to run our feed and supply cables from our spur point (existing socket) and from the FCU to the new sockets.
Open up the existing socket and remove some blanks from the back box where the cable is being run.
Using the correct sized cable, in this case as we are wiring sockets we will need to use 2.5mm twin and earth, cut the cable to the required length and strip the wires at both ends. Find out how to strip wires in our project here.
Once stripped run the cable from the existing socket, through the trunking to the FCU back box.
Connect up the wire at the existing socket to the correct terminals on the rear of the socket, e.g. brown live to live, neutral blue to neutral and earth to earth.
Depending on how old the property is the wire colours may differ slightly. To find out about old and new wire colours see our project here.
Next cut another length of cable and run this from the fused connection unit back box out to where the new socket will be placed. Strip both ends fo the cable before installing.
Connect up Fused Connection Unit
Once the feed and supply cables have been run you can now connect up the fused connection unit.
If you look at the rear of the FCU you will see 6 separate terminals, 3 are for the supply in to the unit from the supply and the other 3 are for the load going out of the unit.
First connect up the incoming supply wires. The brown live should be connected to “L IN”, the blue neutral to “N IN” and the earth to “EARTH IN” (ensuring to add earth sheath to the bare earth wire).
Next connect up the load wires. Connect the brown live wire to “L LOAD”, the blue neutral to “N LOAD” and the bare earth wire to “EARTH LOAD”
Screw FCU Face Plate to Back Box
Once all the wires have been connected to the correct terminals on the back of the FCU face plate you can now go ahead and screw the face plate to the back box.
Pop each screw supplied with the unit through each hole on the front of the unit and then offer it up to the back box, gently moving it until each screw sites over a screw hole on either side of the back box.
Using a small screw driver, screw each screw into its respective hole and tighten it up. Don’t tighten it too much as you can easily crack the face plate or back box
Connect up Load
With the FCU now successfully connected up you can now connect up the load cable to the new socket.
Fit the back box in place and then wire the supply cable to the reverse of the plug socket into the correct terminals, brown live to “L”, blue neutral to “N” and earth to earth terminal.
How to Replace a Fuse in a Fused Connection Unit
In the event that the internal fuse in your FCU blows due to excessive current being drawn you will need to replace it.
This is a fairly simple job to do but before you do anything make sure you isolate the circuit and turn off the power at your consumer unit first!
Once isolated it’s now just a question of removing the fuse and replacing it. Depending on the type of FCU you have, the fuse panel will either prise out using a small flat blade screwdriver or you may need to unscrew a small screw first.
Once the fuse panel has been opened, remove the old fuse and replace it with a new one. You must replace the old fuse with a new fuse of exactly the same amperage!
Once replaced, push the fuse panel back in and turn the power back on and ensure any connected devices now have power.
One final important point is that you will need to find out what caused the fuse to blow in the first place so that the issue doesn’t occur again. If it was an appliance that caused the issue it may be faulty and will need testing.