A guest fitted a new plug to a desk lamp using the only fuse he had lying around, a 13A pulled from an old kettle plug, and couldn't understand why the lamp's cable got warm to the touch within a week.

Why Does a UK Plug Have a Fuse in It at All?

The UK's BS 1363 plug system is unusual internationally in requiring a fuse inside every plug, not just at the consumer unit. The fuse protects the flexible cable running from the plug to the appliance, which is thinner and more vulnerable than the fixed wiring in the wall. If a fault develops in the appliance or its cable, the plug fuse is designed to blow before the cable overheats, well before the circuit's main protection at the consumer unit would ever trip.

What Actually Happens If the Fuse Rating Is Wrong?

A fuse rated too high for the appliance is the more dangerous mistake. If a 13A fuse is fitted to a lamp that only draws 1A under normal use, a fault developing in the lamp's thin flexible cable could push the current up toward 5A or 6A, warming the cable significantly, without ever approaching the 13A threshold needed to blow the fuse. The cable can overheat well before the fuse reacts, which is exactly the scenario in the story above.

A fuse rated too low causes a different, more immediately noticeable problem: it blows under normal operation, because the appliance's genuine current draw exceeds what the fuse is designed to carry. This is inconvenient but safe, the fuse is doing its job correctly, just at a rating that doesn't match the appliance.

How Do You Work Out Which Fuse an Appliance Actually Needs?

The appliance's power rating in watts, printed on a label somewhere on the unit or in its manual, divided by the UK mains voltage of 230V, gives the current draw in amps. A 700W appliance draws roughly 3A, a 1000W appliance draws roughly 4.3A, and a 3000W kettle draws roughly 13A. Round up to the next standard fuse rating above the calculated figure, rather than down, since a fuse rated below the appliance's normal draw will blow immediately under regular use.

What Should You Check Before Assuming a Plug's Fuse Is Correct?

• Check the fuse rating printed on the fuse itself, visible through the small window on most fuse carriers, rather than assuming it matches the appliance.
• Calculate the appliance's actual current draw from its wattage rating, and compare it against the fitted fuse rather than guessing.
• Never reuse a fuse from a different appliance just because it happens to fit, as the story at the top of this guide shows why.
• Buy fuses in the correct BS 1362 standard, since generic fuses not built to this specification may not blow reliably at the stated rating.

For fixed wiring rather than plug-in appliances, the same principle applies through a fused connection unit sized to the specific appliance, rather than assuming every FCU on a property takes the same rating.

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