When the current carried exceeds the rated amount for an extended period of time, the fuse "blows." When a circuit is overloaded, or exceeds the preset amount of amps stated on the fuse, the fuse blows (or the circuit opens) to keep the wires from breaking down and burning. Fuse failure is sometimes caused by electrical shorts. If you expose wiring to air through holes in its casing, it will oxidize and break open these circuits even though there is no load attached to them. This can also happen if you use wire that has been chemically treated to prevent corrosion.
How do you know if a fuse is about to blow? There are several signs that a fuse is about to fail: if any one of them is present, replace the fuse immediately before other parts of the system fail due to lack of electricity.
Using overcurrent protection ensures that your facilities are safe and doesn't cause unnecessary damage to wiring or components. Fuses serve this purpose by blowing when they reach their limit, which prevents further current flow and associated damage.
The initial symptom is the same as with an overload: the metal ribbon within the fuse burns through, and all lights and fixtures on the circuit go dark. In the case of a short circuit, however, just replacing the fuse would almost certainly result in the new one blowing immediately—unless the short circuit has been repaired. For example, if a wire has come into contact with another piece of metal, such as when a dog bites into an extension cord, the entire section of cord containing the bit of metal should be replaced.
If you are lucky enough to have power back after repairing a short circuit, it may be that nothing else needs fixing on the circuit. If not, then follow these steps again to locate the problem: check all wiring under the floorboards or inside walls for damage; check all wiring connections for breaks or corrosion; check all wiring terminals for corrosion or damage; check all switch contacts for corrosion or dirt; replace any burned out light bulbs.
Finally, check the owner's manual or call the manufacturer for information on how to test fuses and what symptoms indicate they need to be replaced. Fuses are very important for your safety so don't try to fix them yourself. A professional electrician should always do the work on your house wiring.
A common fuse is constructed of materials that melt at high temperatures; hence, when a short circuit develops, the large quantity of current causes the temperature to rise extremely quickly, melting the fuse and so breaking the circuit, blocking further current flow. As soon as the heat dissipated by the fuse no longer increases its temperature, it refreezes and reforms again.
The most common way for a short to develop is if there is an open wire or metal part within the radius of the arc between two wires. This creates a second path for current to take through the open structure, causing additional heat and eventually leading to destruction of those wires as well. If enough current flows through an open wire, it will actually vaporize into gas which will fill the space inside the casing and cause an explosion. This is why electrical wiring should never be done without protection against electricity, such as using ground cables or fuses.
If you short-circuit a diode, it will stay hot for some time after the power source is removed, but it will not burn up. Diodes are very efficient at turning off electricity from a circuit, so even though both ends of the diode are connected together, very little current flows through it.
Batteries also consist of two connections in parallel, so if you were to connect both terminals together, it would cause the battery to discharge itself completely.
When an overload develops, a fuse or circuit breaker prevents this by "blowing," or severing the circuit. Fuse wires are threaded into threaded sockets within the fuse box. The higher-rated fuse will enable more current to flow than the wires can safely manage, perhaps resulting in overheating and fire. A fuse is simply a device designed to break under excess current conditions. Fuses are available in various sizes for different applications. The two main types of fuses are thermal fuses and magnetic fuses.
Thermal fuses open when enough heat is applied to them to cause a deformation or melting point. This allows the current to continue through the fuse, but no longer able to flow into other parts of the circuit. Thermal fuses are usually made from thin strips of metal or glass coated with organic material that melts at a low temperature. When heated to its threshold value, the fuse blows with little or no damage to surrounding components. Modern fuses are also made from semiconductors that fail by tunneling rather than melting, but these are less common.
Magnetic fuses consist of a thin wire wound around a core material. A strong magnet placed close to the fuse will hold it in place, while another closer to the power source will keep it in circuit. If the windings on the fuse are damaged or the core material is fractured, then the fuse will not function properly.
The fuse element is normally at a temperature below its melting point. However, if there is a short circuit or an overload, an excessive current will flow through the fuse element. The temperature rises as a result, and the fuse element melts extremely fast. An electric fuse, in this manner, disconnects the power supply and safeguards electric circuits or equipment.
Fuses are available in many different sizes and ratings to meet the needs of various applications. The type of fuse used in a circuit depends on the current it must carry and the voltage it must withstand. A regular household fuse is designed to interrupt current at normal levels (about 15 amps) with voltages up to about 120 volts. More powerful fuses for use in power lines or large appliances may be approved by electrical codes for use with higher currents and/or voltage limits. Fuse boxes, which are enclosures containing several fuses, are available for special-purpose applications such as supplying alternative current (AC) to motors from battery backup systems or solar panels.
The choice of fuse for any given application should take into account its physical size, its melting point, and its tolerance to overcurrent and voltage. For example, a fuse that is too small will cause more problems than it solves because it will fail before it has time to melt. On the other hand, a fuse that is too large can cause damage to circuit components due to overheating.