How to replace fusewire in a rewireable fuse box

Helpful video reference. This UK tutorial demonstrates the correct procedure for replacing fuse wire in a Wylex rewireable fuse box, including isolation at the main switch, identifying the blown carrier, fitting the right gauge of wire and the importance of not overtightening the terminal screws. We have included it because it gets the UK-specific details right. View the original at youtube.com/watch?v=7yywsC8dTp8.

1. Work out which circuit has tripped

Before switching anything off, note which items in the house have gone dead: lights upstairs, sockets downstairs, the cooker, the immersion heater. This tells you roughly which circuit has gone and saves you pulling out every carrier in turn.

In a rewireable fuse box, the carriers are typically colour-coded or labelled by current rating: white for 5A (lighting), yellow for 15A (immersion), blue for 20A (shower or cooker), and red for 30A (ring main). Some older boards are not labelled at all -- in that case you work through them one by one.

2. Switch off the main isolator

The main switch on the fuse box is the large lever or rotary switch at the top of the board. Turn it to the OFF position. You will lose power to the whole house while you work. Switch off the main switch before pulling out any fuse carrier.

3. Remove and inspect each carrier

Pull out the carrier for the circuit you think has gone. The fuse wire bridges the two terminal screws across a small porcelain saddle or bridge in the middle of the carrier. A blown wire is usually visibly broken or missing. Sometimes it melts inside the porcelain and is not obviously broken -- if in doubt, try fitting new wire.

If all the fusewires look intact, the problem may not be a blown fuse. In that case, restore the main switch and investigate further -- a tripped RCD (if there is a separate RCD unit) or a loose connection elsewhere may be the cause.

4. Remove the old fusewire

Loosen the two terminal screws on the carrier -- one at each end of the fusewire. Pull out the old wire and any remnants. Run a finger over the porcelain bridge to check there is no buried piece of wire that could short against the new one.

5. Choose the correct fusewire

The wire must match the rating of the circuit it protects:

• 5A -- lighting circuits
• 15A -- immersion heater circuits
• 20A -- cooker circuits (some older cookers) or electric shower circuits
• 30A -- ring main circuits (power sockets)

Fusewire is sold in small reels at electrical factors and most hardware shops. Buy by ampere rating. Using a higher-rated wire than the circuit needs removes protection -- if the circuit develops a fault, the wire will not blow when it should.

6. Thread and fit the new wire

Cut a length of wire slightly longer than you need. Thread it through the porcelain bridge (or around the saddle, depending on the carrier type), then loop it under one terminal screw and tighten that screw. Pull the wire taut across the bridge, loop it under the second screw and tighten. The wire should be snug but not stretched tight -- over-tensioning reduces the blowing current.

Trim any excess wire so it does not hang loose. If the carrier type has a wrap-around bridge, wrap the wire in the direction the screw tightens, so tightening the screw grips the wire rather than pushing it out.

7. Reinsert the carrier and restore power

Push the carrier firmly back into its slot. It should click or seat positively. Once all carriers are in, restore the main switch. Check that the previously dead circuit has come back on. Test a socket with a plug-in tester or switch on a light.

When to call us

Replacing a blown fuse is the one electrical job most homeowners with a rewireable board end up doing. But if your board has no RCD protection and you are spending money getting it rewired or extended anyway, the honest advice is to budget for a consumer unit upgrade at the same time. A modern unit with full RCBO protection costs around £600 fitted and provides a substantially higher standard of protection than any rewireable board.