Q: How Long Should Headlight Fuses Last?

All fuses in your vehicle are technically designed under normal conditions to last the life of the vehicle. A fuses primary job is to open up if the current flowing through the circuit it is protecting exceeds its rated capacity. Fuses are rated in amps or amperage which represents the amount of energy that can pass through them safely. The number one reason a fuse will open up is most often from a condition known as a short to ground. This can occur when the power and ground wire accidentally connect or the power wire is rubbed through and touches the chassis of the vehicle which is ground. Wiring can become loose or out of place due to an accident or broken mounting clips.

If you have ever looked under the hood of your vehicle you will notice the wires are run through a plastic conduit and secured in place to protect them from contact or damage. A short will cause the amperage or current to flow uncontrolled resulting in the fuse heating up and opening up to protect the circuit. If a fuse or in some cases a circuit breaker are not used with the right current rating the circuit can be damaged or even cause a fire.

Fuses can also open if the current in a particular circuit just slightly exceeds the rating. In a headlight circuit this could happen if additional lighting is added to the circuit like a light bar for off-road use. It is also possible when adding a trailer as the additional lights will actually lower the overall resistance of the circuit allowing for additional current flow.

A quick review of electrical theory will help in the understanding here so voltage from the battery is the push force that will get electrons moving in the circuit. The actual movement and amount of energy or electrons moving is called current or amperage. The devices we use to illuminate or operate such as motors or bulbs are referred to as the load and they provide resistance to the flow of current.

When a circuit is shorted or the connection in effect bypasses the load, the amount of current that will flow is at the maximum that the battery can supply. This can be found on the battery label under CCA or Cold Cranking Amps rating. Reducing the resistance will allow more current to flow which can cause heat that can damage the parts. To prevent this, the fuse or circuit breaker is the safety device that will open and stop the excess current flow. If the circuit operates in the proper range and without a short condition fuses can last the life of your vehicle. Always remember to [replace the fuse]((https://www.yourmechanic.com/services/fuse-replacement) with the proper rating as a higher rating will put the circuit at risk of a part or wire failure. A lower rated fuse may actually open during the normal operation of the circuit.