Fused Connection Units, Switched Fused Spurs, and FCU Wiring — A UK Electrician's Guide

A fused connection unit (FCU), often called a fused spur, provides a fused connection for fixed equipment. This guide explains the main choices and the checks a competent electrician should make. It is not a substitute for the current edition of BS 7671, the accessory instructions or the connected equipment manufacturer's instructions.

What an FCU does

An FCU contains a replaceable cartridge fuse and terminals for the supply and load conductors. The fuse limits current to the downstream connection; it does not replace the protective device or other protective measures required for the final circuit.

A switched FCU also offers local switching. Do not assume that every switch is a suitable means of electrical isolation: confirm the poles switched, contact separation, rating and intended use from the manufacturer's data and the installation design.

Choosing the right type

• Switched or unswitched: decide whether local switching is required and whether another suitable means of isolation is provided.
• Single-pole or double-pole: select according to the circuit, equipment instructions and isolation requirements.
• Neon indicator: useful as a status indication, but never use an indicator alone to prove that conductors are dead.
• Flex outlet: use where the accessory is designed to grip and route a flexible cord through the faceplate. Follow the stated cord range and strain-relief instructions.
• Metal clad: useful where greater mechanical robustness is wanted. Use the matching enclosure and earthing arrangement specified by the manufacturer; “metal clad” does not by itself mean weatherproof.

Fuse selection

Use only the fuse type and rating specified for the FCU and connected equipment. Select the rating from the equipment instructions and the circuit design, taking account of the flexible cord or downstream cable. Do not choose a fuse from appliance type or wattage alone.

Many 13A FCUs accept BS 1362 fuses, but the fitted fuse and permitted replacements must be checked against the individual product instructions. A lower-rated fuse is appropriate only where the manufacturer and design call for it.

Connection and circuit-design checks

Terminal markings and capacities vary. Identify the supply and load terminals on the actual accessory, observe conductor preparation and tightening requirements, and make provision for the circuit protective conductor and any metal back box as required.

Before adding an FCU to a ring final or radial circuit, verify the circuit arrangement, protective device, conductor capacity, existing spurs, load, RCD requirements and test results. A fused spur can support downstream wiring or fixed equipment within its design limits, but it is not a general remedy for an unsuitable or overloaded circuit.

Boilers, fans and immersion heaters

• Boilers: follow the boiler manufacturer's instructions for supply, fuse rating, isolation and location. Do not apply a universal cable size or fuse rating.
• Extractor fans: timer and humidity models may need permanent and switched supplies. Follow the fan wiring diagram and its stated fuse and isolation requirements.
• Immersion heaters: confirm element rating, circuit design, switching duty and the heater manufacturer's instructions. Do not treat a standard 13A FCU as interchangeable with a suitably rated double-pole control switch.

Bathrooms and other special locations

Equipment in a room containing a bath or shower must be selected and located under the current requirements for that special location, including zoning, external influences, IP rating and RCD protection. A product is not suitable merely because it is double-pole or metal clad. Check Section 701 of the current BS 7671 and the manufacturer's declared installation conditions.

Safe isolation, inspection and testing

Before work, identify the correct circuit, isolate it securely and prove dead with suitable equipment. The HSE warns that supplies should be made dead and proved dead before work begins; see HSE: maintaining portable and transportable electrical equipment.

After installation, carry out the inspection, testing and certification appropriate to the work before energising. In dwellings, also assess whether the work is notifiable under the applicable building regulations. See GOV.UK: Approved Document P — electrical safety in dwellings.

For an IET discussion of isolation and switching for mechanical maintenance, including an extractor-fan manufacturer example, see IET Wiring Matters: isolation and switching for mechanical maintenance.

Final checklist

• Confirm the FCU rating, switch type, fuse and enclosure from current manufacturer data.
• Follow the connected equipment instructions rather than generic appliance assumptions.
• Verify supply/load identification, conductor capacity, earthing and terminal torque.
• Assess the complete circuit and any special-location requirements.
• Safely isolate, prove dead, inspect, test and document the work.