Protection is the least glamorous part of a project. You don't think about it until things go wrong — and by then, it's too late. A fuse is not a concession to caution. It is part of good design.
What exactly does a fuse do?
A fuse has one job: to interrupt the circuit as soon as more current flows through it than intended. The fuse is the weakest link in the chain — and that is exactly the point. Better a broken fuse than a broken component, a burnt PCB, or worse.
Choosing fuses: the most important parameters
Current rating
The most common mistake: choosing a fuse that is just high enough for the maximum current draw. A rule of thumb: choose a fuse at 125–150% of the expected operating current. If your circuit normally consumes 1 A, a 1.25 A or 1.5 A fuse is a better choice.
Voltage
The voltage rating of a fuse indicates at which voltage it can safely interrupt the circuit. You can use a 250 V fuse in a 12 V system, but never the other way around.
Fast or slow-blow?
| Type | Behavior | Application |
|---|---|---|
| Fast (F) | Reacts instantly to overcurrent | Resistive loads, no inrush current |
| Slow (T) | Tolerates short current spikes | Motors, transformers, capacitors |
Size
The most commonly used sizes are 5×20 mm (Europe) and 6.3×32 mm (international). Always ensure a matching fuse holder in your design — a loose fuse in a clip is not a solution for a final product.
More than one fuse?
In complex systems, you protect different sections separately. A main fuse at the input protects the power supply and cabling. Sub-fuses per module protect individual components and make it easier to isolate a fault.
When do you need an RCD?
A fuse reacts to too much current. An earth leakage circuit breaker — officially an RCD (Residual Current Device) — reacts to the difference between the current entering the circuit via the phase and the current returning via the neutral. An RCD detects that difference and cuts the power within milliseconds.
When is an RCD mandatory or highly recommended?
- If your project is connected to 230 V mains power
- If you are working in a damp environment (basement, garden, garage)
- If anyone other than yourself uses or touches your project
- If your project is used outdoors
