Why do I need a fuse?

Okay... lesson one in our fuseology series.

Most fundamental, why do I need a fuse? The short answer is to keep your car from burning to the ground. The more detailed answer can be summarized in the favorite saying of my former mentor at Littelfuse: "Fuses protect wires from fires". This phrase was ground into us as we were being indoctrinated into the engineering group. Even worse was the fact that the phrase was always spoken with an exaggerated, overly-fake southern accent.

This leads to an important point and the second most asked question related to fuses. Fuses don't protect the end load. They protect the wire carrying the current to the load. For example, you don't need to fuse the wire going to your headlights to protect the filament in the bulb. That can take care of its self. Rather, the fuse is intended to be the weakest electrical link in the circuit. If more current flows through the wire than intended, the fuse should open in a controlled way, instead of the wire getting hot and causing damage.

All wires have a maximum current carrying capability. In most cases, the limiting factor is not the copper conductor, it is the insulation. When the current flowing through a wire exceeds its safe rating, the conductor heats up. The safe current carrying capacity for a wire is ultimately determined by the temperature rise of the insulation.

Different types of insulation react differently to this increase in temperature. Some materials get soft and melt off of the conductor. Other materials get brittle and crack. Some materials smoke and burst into flames. An overheating wire can cause significant damage to other wires or even ignite materials in the car.

As I said above, fuses are designed to be the weakest electrical link in the system. They are made of special conductive alloys that melt at specific temperatures. They are encased in housings made of high-temperature materials. The goal is to have the fuse open gracefully, without damaging anything in the electrical system, long before the wire gets hot enough for something bad to happen.

So in closing for this post, check out this video. This is an extreme example of what can happen to an un-protected wire. Click here to see it. Make sure that you turn up your volume. Do you want this to happen in your car?

Enjoy!

Jay