How to understand electrical cables, flexes & fuses
Introduction
In your home, the electrical current flows along different types of cable and flex. You have to strip these to expose the cores before you can connect them to new terminals. Fitting new plugs is one of the most straightforward electrical jobs, but you need to make sure you insert the correct fuse for each appliance.
Flex connects your appliances and pendant lights to the fixed wiring. It's usually round in cross-section and contains three cores within a white or coloured PVC outer sheath (although irons can use non-kinking rubber-sheathed flex with a braided outer covering).
All three cores have coloured insulation: brown for live, blue for neutral and green/yellow for earth. Two-core flex (with no earth core) is used for double-insulated appliances and some light fittings that don't have metal components. Special heat-resistant flex is also available for immersion heaters.
Flex sizes
Flex is available in a range of sizes. Check the table to see the size you need for different appliance ratings.
| Appliance rating | Flex size |
|---|---|
| 700W | 0.5mm2 |
| 1.4kW | 0.75mm2 |
| 2.3kW | 1mm2 |
| 3kW | 1.25/1.5mm2 |
Cable is used for all fixed wiring - which is usually hidden behind walls, ceiling and floor. It has an oval cross-section and contains cores that are set side-by-side in a thick white or grey PVC outer sheath.
The live core is separately insulated in brown sheath, the neutral core in blue sheath, and the earth core is bare and runs between them. This is known as two-core-and earth cable. Special three-core-and-earth cable (with brown, black and grey cores, plus a bare earth) is used for the two-way switching of lights.
When you connect cable, you must protect the exposed earth core with a length of green/yellow electrical sleeving. You'll need bigger cable for circuits with a higher current demand. The common sizes of cable are 1, 1.5, 2.5, 4, 6 and 10mm2 - the measurement represents the cross-sectional area of the individual cores.
The current-carrying capacity of cable varies according to a number of installation design factors. If you're in any doubt about your requirements, make sure you ask a qualified electrician. As normal cable sheathing isn't frost-resistant, you'll need to protect it if it runs outdoors with a conduit or steel-wire-armoured cable (SWA).
These days, all appliance plugs contain cartridge fuses that are normally rated at 3, 5 or 13 amp, depending on the wattage of the appliance concerned. You should protect an appliance that's rated up to 700 watts with a 3 amp fuse, and one that's rated above 700 watts with a 13 amp fuse.
Some appliances (such as televisions) might need a 5 amp fuse, but always check the appliance manufacturer's instructions first. Never fit a fuse with the wrong rating - a new plug usually contains a 13 amp fuse, but this could be too high for your appliance.
The colours of the live and neutral cores in cable have changed. The reason for this was to bring the UK more closely in line with the rest of Europe, which is why the new colours are often referred to as 'harmonised' colours.
The live core, which was red, has changed to brown; while the neutral core, which was black, has changed to blue. These are the colours that have been used in flex for many years.
The earth core is still identified by the colour combination green/yellow. Three-core-and-earth cable colours changed from red, yellow and blue to brown, black and grey. These harmonised colours have been in use since 31 March 2004 and became compulsory in all new installations from 31 March 2006.
Working safely with the old and new colours
The core colour changes could cause confusion. For example, black is used for neutral in old two-core and-earth cable, but is one of the live cores in the new three-core-and-earth cable. Also, blue is used for neutral in the new two-core-and-earth cable but is one of the live cores in old three-core-and-earth cable. If you make an alteration or addition to an existing electrical installation, it may well involve working with a mixture of old and new cable colours. Where old and new two-core and-earth cables are joined at socket or junction-box terminals, it's essential to take great care to connect the cores correctly (for example, old red to new brown (for live) and old black to new blue (for neutral). You must also fix a warning notice in a prominent position at the distribution board or consumer unit.
Changing a fuse in a plug
Unplug your appliance first, then unscrew the cover of the plug and lift it off. Prise the fuse from its clips, insert a new one of the correct amperage and refit the cover. Bear in mind that some plugs are moulded in one piece, with the fuse in a lever-out holder in the base. If this is the case, make sure you've pushed this holder fully back into the plug after you've changed the fuse.
To prepare your flex or cable for connection, you'll need to strip back the plastic sheath to expose the copper cores. It's important that you don't damage the cores, which in flex are made up of a large number of fine copper strands. If you cut through any of these, it reduces the core's current capacity - which could lead to overheating.
Stripping flex
Mark the amount of flex outer sheath you need to remove, then bend the flex double and make a shallow cut at the mark. This will open up automatically. Repeat this around the flex until you can pull the piece of outer sheath free. Cut the flex cores to length with side cutters. Next, remove about 10mm of the insulation from the end of each core, using wire strippers. Some wire strippers are adjustable to suit a range of core sizes, while others have jaws with cut-outs for different sizes of core. Twist the exposed fine copper wires of each core together so you can insert them easily in their terminals.
Stripping cable
Mark the amount of outer sheathing you want to remove, and lay the cable on a flat surface. Use a sharp knife to carefully slit the sheathing down the centre (above the earth core). Peel back the sheathing and cut it off using side cutters, then cut the cores to the length you need. Use wire strippers to remove just enough of the coloured cores' insulation so they extend to the full depth of the terminal, with no wire exposed. Add green/yellow sheathing to the bare copper core.