I know you need fuses for cars, electrical equipments, etc. I don't get what they are really used for?
A fuse is a circuit breaker. It is placed in the line of an electrical circuit to protect against power surges that could potentially damage the electrical component. If a power surge happens, the fuse will blow, cutting off the flow of current and protecting the component. Without the fuse in the line any spike or surge of current in the circuit could be sent into the electrical component and cause serious damage.
Fuses are placed in the path of an electrical current as a fail-safe to prevent your equipment from frying. It is essentially a piece of metal that breaks when too much current is applied to it. What this does is stop the electrical current from getting to your radio/ECU and doing damage to it. Usually if a fuse has blown in your car it is a sign that something has failed and you should check the related system before simply replacing the fuse and repeating the process.
If there occurs a short circuit or an overload beyond the current rating of the fuse, the thing melts inside, breaking the contact, stopping current flow, and sending someone off in search of a new fuse rather than repairing the situation which blew out the last one. Nothing in the fuse breaks, it melts. A fuse is not a mechanical circuit breaker, which is an actual switch actuated either magnetically (in AC circuits) or a bimetallic strip which has the current going through it and gets hot enough on excessive current that it self-bends and breaks the connection temporarily, and can be reset. Once a fuse blows, it's over for that fuse.
Fuses and circuit breakers are used to prevent damage to an electrical circuit due to a short or overload. All circuits have at least a positive side and a ground (negative) side. If the positive and ground sides of the circuit touch each other the result is intense heat generated by the electrical current. The job of the fuse or circuit breaker is to stop the flow of electricity when an overload occurs in the circuit so excessive damage to the components of the circuit does not take place (wiring,computers,relays,etc.). All circuits are designed to carry a certain load (amps), DO NOT EVER replace a fuse with one of a higher rating(example: do not replace a 10amp fuse with a 15 or 20amp one) if the normal fuse blows there is a problem that needs to be located and repaired! I have seen MANY vehicles severly damaged because an owner or backyard mechanic replaced a blown fuse with one that was not the same rating as the one removed. I have seen burnt up vehicles from this action. If the fuse blows there is a problem, putting a higher rated fuse in WILL NOT fix the problem, it will more than likely cause major damage to the circuit or possibly cause a fire that will destroy the vehicle. (retired mechanic, from an electrical repair garage)
a fuse is the last defense for an electrical device. given that electricity needs a complete path to flow in order for the circuit to be active a fuse acts as a switch to an extent. that same fuse is amp sensitive. if the circuit draws more than the fuse allows the fuse will over heat and open the circuit. typically a fuse is set up to be about 20 % more than the circuits typical amp draw.
