If a failure in an appliance causes too much current to flow, the fuse trips the circuit. If the current flowing through the fuse is too high, the wire warms up and melts, breaking the circuit. The amount of current that will cause this to happen depends on the size of the fuse.
The usual purpose for using fuses is to protect other equipment or people from damage if an electrical component fails. For example, if a light bulb burns out, there is no need to worry about someone walking into the room while you are trying to fix the problem. But if someone were to come into the room while a power tool was operating, they could be seriously injured or even killed by being hit by a falling blade block. Fuses prevent overcurrent conditions from reaching dangerous levels by "fusing" (that is, destroying) the element which creates electricity. A fused power supply does not mean that your power is out; it means that an overload condition has been detected and acted upon. The next time electricity is needed from this source, it will be automatically restored.
Fuses can be divided into two main categories: thermal-type and magnetic-type. Magnetic fuses use a magnet to break the current path inside the fuse, while thermal fuses use the heat created by the current itself to destroy the fuse element.
Because the current in series stays constant, the fuse wire melts and breaks the circuit, sparing the appliance from harm. As a result, an electric fuse is necessary in all electrical appliances for the purpose of protecting the equipment. Fuses are available in sizes ranging from small for use in circuits requiring less current to large for high-current applications.
If a fuse blows quickly when removed from the appliance or circuit, it has been damaged by overcurrent. The entire fuse assembly should be replaced before further use in case some parts were not destroyed when the fuse blew. Checking other fuses in the vicinity as well as other components in the circuit may also help identify the cause of the problem.
Fuses are simple devices that break the connection between their terminals when a certain level of current flows through them. They perform this function by melting or vaporizing a material that connects each terminal to its corresponding conductor. When the current stops, the fuse link re-establishes the connection, thereby preventing any further damage to the appliance or circuit.
There are two main types of failures: open circuit and short circuit.
Because of the massive current flow, the wire becomes heated (heating effect of electric current) and may catch fire. A fuse wire is built of a low melting point tin and lead alloy. The strong current's heat immediately melts the fuse wire and destroys the circuit.
Heated wires should never be touched. A bare hand will do, but better use a metal glove or handle plug/outlet with proper protection from electrical shock.
If you are sure that no live current is flowing through the wire, then you can try to touch it first with your insulated hand or tool. If there is no reaction, then it is safe to work on. Be careful not to let any water drip onto the wire because it will cause an short circuit if the droplets freeze.
Fuses are designed to break at exactly this temperature for just this reason. If they failed at any lower temperature, then everyone would be killed by their own car engines!
The actual damage to the wire occurs at the melting point, which for tin and lead is about 100 degrees C (212 degrees F). Above this temperature the atoms of the wire's material move around too quickly, causing them to collide with other atoms often enough to make new bonds with them. This process leaves behind many new particles which continue to move around too quickly to stay together, so they fall back into original shape.
This piece of wire must also carry the current to the appliances. If a short circuit develops, or if too many appliances are connected to one wire, causing too much current to flow, the wire in the fuse quickly warms up and melts, breaking the circuit and preventing a fire from beginning. The metal inside the fuse box is very conductive, so it doesn't heat up when current passes through it.
The length of time that a fuse will stay intact before melting depends on the type of fuse used. A dry-type fuse stays hot for several minutes after the current stops flowing through it. This allows time for someone to locate the fuse and replace it before it burns out. A wet-type fuse evaporates an acid compound that causes the metal within the fuse to melt instead of heating up like the dry-type fuse. These fuses should be replaced by a qualified technician as soon as they have been opened or exposed parts of the fuse become corroded. Corrosion can cause a fuse to fail even though it appears to be working properly.
Fuses are available in sizes ranging from 1 amp up, depending on the expected current load of the circuit. Size matters here: The more current that flows through a circuit, the larger the wire needed to provide enough voltage drop across the circuit.