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Fuses, breakers and fusible links
Circuit protection exists to protect the wire, not the component. Once you internalise that, every question about what fuse to fit answers itself — and it becomes obvious why fitting a bigger one is never a repair.
The fuse protects the wire
A fuse is sized so it opens before the wire it feeds gets hot enough to melt insulation. It is not sized to protect the motor or lamp on the end.
So when someone fits a 30 A fuse where a 15 A belongs, the device may well survive. The harness becomes the fuse instead, and it fails somewhere inside a loom where you cannot see it.
Breakers where nuisance trips are dangerous
Circuit breakers are used where losing the circuit permanently is worse than a momentary overload — wipers, power windows, and on apparatus, warning lights and pump controls.
Type I auto-resets cycle on and off while the fault is present, which shows up as a light that flashes at a steady beat. Type III manual-reset breakers stay open until someone pushes the button.
Fusible links
A fusible link is a deliberately undersized piece of special wire, usually four gauge sizes smaller than what it protects, sitting close to the battery on high-current feeds.
They fail rarely and usually indicate a serious fault upstream. Replace with the correct link wire, never with ordinary wire of the same look.
How to test it
- Confirm it is really openCheck across the fuse in place, with the circuit powered — you should read near zero volts across a good fuse and full battery voltage across a blown one.
- Find out whyA fuse that blows instantly points to a hard short. One that blows after a while points to an overload or a dragging motor.
- Measure the loadCurrent draw on the circuit against spec. This is what tells you whether the fuse or the load is the problem.
- Replace like for likeSame rating, same type. If the correct fuse will not hold, the fault is still there.
Numbers to work to
| Blade fuse colours | 5 A tan, 10 A red, 15 A blue, 20 A yellow, 25 A clear, 30 A green | standard ATO/ATC colour code |
| Sizing rule of thumb | about 125–150% of normal load | and never above the wire's rating |
| Fusible link | about 4 gauge sizes smaller than the wire | must be link wire, not ordinary wire |
| Good fuse, powered | under 0.1 V across it | in-circuit check, no need to pull it |
Common mistakes
- Fitting a bigger fuse because the right one keeps blowing.
- Testing a fuse by eye. Cracked elements can look intact, especially in high-amp cartridge fuses.
- Replacing a fusible link with plain wire of the same gauge, removing the protection entirely.
- Missing an auto-reset breaker cycling — a rhythmic flash is the clue, not a bad lamp.
Drill this until it's automatic
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