The Fuse Protects the Wire, Not the Thing You Plugged In

Start from the wire. Find the current that wire can safely carry, pick a fuse below it, and then confirm that the load's normal current — including its startup surge — sits comfortably under the fuse. If the load needs more than the cable can carry, the answer is a thicker cable, not a bigger fuse. Fitting a larger fuse to stop nuisance blowing is the single most dangerous thing in this rung, and it is extremely common. Put the fuse at the SUPPLY end of the cable, as close to the power source as possible. A fuse at the far end protects nothing: the whole cable between the battery and the fuse is unprotected, and that is the part most likely to chafe against a chassis.

A FAST-BLOW fuse opens almost immediately at its rating. It protects semiconductors, and it will also trip on the inrush of anything with a motor or a large capacitor. A SLOW-BLOW (time-delay, T-rated) fuse tolerates a brief overload and opens on a sustained one, which is what a motor, a transformer or a supply with big input capacitors needs. Fitting fast where slow is needed produces a fuse that blows every time you switch on, and the usual response is to fit a bigger one — which is the wrong fix for the right symptom. A POLYFUSE or PTC resets itself when it cools. Convenient for a USB port or something inaccessible, but it is slower and less precise than a real fuse, and it does not protect against everything a fuse does.

Test a fuse with the meter on continuity, out of its holder. Judging by eye is unreliable: a fuse can open without any visible break, especially a slow-blow one. A fuse that has blown is reporting a fault. Replacing it with the same rating and seeing whether it goes again is a reasonable test exactly once; fitting a larger one, or wrapping it in foil, or pushing a nail into the holder, converts a contained failure into a fire. Check the HOLDER too. A corroded or loose fuse holder adds resistance, heats up, and is a common fault in its own right — and it will make a correctly sized fuse blow for no apparent reason.

Everything in this batch is written for low-voltage DC work. Mains wiring is governed by law in most countries, requires specific cable types, earthing, enclosures and protective devices, and in many places must be done or inspected by a licensed electrician. The principles carry over — the fuse or breaker still protects the cable, strain relief still matters, a loose terminal still heats up — but the consequences of an error change completely, and so do the rules. Earthing and RCD/GFCI protection have no equivalent at 12 V and are not optional. So: learn the craft on low voltage, where a mistake costs a component. For anything connected to a wall socket, find out what your jurisdiction actually requires before you start, and get it inspected. This is one of the few places in making where 'try it and see' is the wrong method.