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1. What is a fuse?
　　The fuse is an electric element protecting electric devices. It is commonly connected with a circuit in series, and when fault current increases to a certain value, the fuse blows out to cut off the circuit so as to fulfill an aim of protecting other equipment in the circuit. The fuse is the most commonly used component in a over-current protection circuit. The traditional fuse mainly consists of two parts of a tube body, of which both ends have metal connecting terminals, and metal melt in the tube body. The majority of fuses are cylinder-shaped, namely cartridge construction. But nowadays, with the fast development of advanced technology, some special materials are well applied, the fuse industry has started to move forward in the direction of sub-miniature products and a series of new products such as Surface Mounted Devices type fuse are successively developed.
2. Fuse's function
　　a. The fuse can bear overload in a certain range when current fluctuation occurs because of power supply or external interference ;
　　b. When bigger overload current appears in the circuit, the fuse should cut off the circuit in the set time so as to protect other electric parts in the circuit ;
　　c. When very big current appears because of short circuit, the fuse can safely cut off the circuit so as to avoid damage by overload current .
3. Technical parameter term
　　a. Rated current: Normally the rated current value is: 100mA, 160mA,200mA, 315mA, 400mA, 500mA, 630mA, 800mA, 1A, 1.6A, 2A, 2.5A, 3.15A, 4A, 5A, 6.3A, etc. ;
　　b. Rated voltage: Normally the rated voltage value is: 24V, 32V, 63V, 125V, 250V, etc. The fuse can be used at the voltage less than or equal to its rated voltage, but normally can’t be used at the voltage is more than its rated voltage ;
　　c. Voltage drop: The fuse is operated at the rated current, when the fuse reaches heat balance, namely the temperature is stabilized, the voltage difference measured at both ends of the fuse is voltage drop. Because the voltage drop has certainly influence on the circuit, the voltage drop is clearly regulated in the European standard, but not defined in the American standard ;
　　d. Overload current: The overload current refers to the current which is higher than that when the circuit normally works. If the overload can’t be cut off in time, it is quite possible that electric parts in the circuit are damaged. The short circuit current refers to the current generated because of partial or whole short circuit, and normally is very high, much higher than the overload current ;
　　e. Fusing characteristic: Namely time-current characteristic. Normally, the fusing characteristic is expressed in two ways: I-T diagram and test report. The I-T diagram is a curve formed by connecting average operating time point of the fuse under different current loads in a coordinate system in which the overload current is X coordinate and the operating time is Y coordinate. Each fuse model has a corresponding curve representing its fusing character. While selecting fuses, you can refer to the curve. The test report is the test data record obtained by finishing the test items required by the standard ;
　　f. Interrupting rating: Also called as rated short circuit capacity, namely, at the rated voltage, the maximum current value (effective value for AC) at which the fuse can safely cut off the circuit. The interrupting rating is an important safety parameter to the fuse ;
　　g. Melting heat energy value: Namely the heat energy value needed for melting the fuse, its code is I² t. The melting heat energy value is the product of the square of corresponding current when the fuse blows out in 8ms or shorter time and opeating time, the limited time is in 8ms in order to make all heat generated by the fuse-wire used to melt the fuse other than be dissipated. It is a constant for each kind of different fuse-wire, and also a parameter of fuse-wire and determined when the fuse-wire is designed ;
　　h. Temperature rise: The temperature rise is the difference between the temperature which is stabilized when the fuse is charged with regulated current and room temperature .
4. Construction of fuse
　　The fuse normally consists of three part: The fuse-element part, it is the core of the fuse and has the function of cutting off the circuit when melted,resistance as far as possible small and be consistent, and more importantly, the fusing characteristic must be consistent; The electrode part, it has two parts and is connected with the melt and the circuit, the electrode part must have high electrical conductivity and doesn't generate obvious installation contact resistance; The support part, fuse-element is commonly slim and soft, the fuse support part is used to fix the fuse-element and make the three parts form a complete rigid body, so that the fuse can be easily installed and used, the fuse support part must have high mechanical strength, insulating property, heat resistance and flame resistance, and the phenomena of breakage, deformation, burning and short circuit don't occur while in use .
5. Working principle of fuse
　　When the fuse is electrified, because of the fuse's resistance, the electric energy is converted into heat energy to heat the melt, simultaneously the heat energy generated by the current radiates toward ambient environment by the melt and shell and is dissipated by means of convection and conduction.When the fuse is charged with the allowed working current, the dissipated heat energy and the generated heat energy are balanced, the heat energy will not be accumulated on the melt, so that the fuse will not heated to its melting point and operation.When the current passing the fuse reaches a certain value, the heat converted by electric energy increases, the heat dissipation speed can't catch up with the heating speed, the heat energy will be gradually accumulated on the melt, the temperature of the melt increases, when the melting point of the melt is reached, the melting wire starts to be melted and continues to absorb heat energy for further and to be converted into liquid, later, the temperature of the melt further increases to the evaporation point, the electric arc is formed. The electric arc is a gas discharge phenomenon, the strength of the electric arc is related to the voltage of the circuit, the higher the voltage, the stronger the electric arc,  the fuse can't be used in the circuit, of which the voltage is higher than the fuse' s rated voltage, that' s because the electric arc is difficultly extinguished when the circuit voltage is higher than the fuse's rated voltage. In addition, the strength of the electric arc is also related to the circuit current, the bigger the current, the stronger the electric arc. If the electric arc can't be extinguished in time, not only the circuit can't be cut off, but also the other components in the circuit are possibly burned, and fire accidents are easily caused. For the high anti-explosion type fuse, the electric arc is extinguished by adding explosion-proof sand, the fuse can really cut of the circuit after the electric arc is extinguished so as to achieve the aim of protecting other equipment .
6. Type of fuse
　　According to protection type: The fuse can be divided into over current protection and overheat protection. The fuse used for over current protection is the commonly used fuse (also called current-limiting fuse). The fuse used for overheat protection is commonly called "thermal cutoff fuse". the thermal cutoff fuse is used to protect heating electric devices, for example, electric drier, electric iron, electric cooker, electric furnace, transformer, motor, etc.; and the fuse responds to the temperature rise of the electric devices and has nothing to do with the working current of the circuit. The working principle of the thermal cutoff fuse is different from the current-limiting fuse
　　According to range of use: The fuse is divided into electric device fuse, electric power fuse, vehicle fuse, etc. ;
　　According to volume: The fuse is divided into large type, medium type, small type and micro type ;
　　According to rated voltage: The fuse is divided into high voltage fuse, low voltage fuse and safe voltage fuse ;
　　According to shape: The fuse is divided into flat-head tube-shaped fuse, sharp-head tube-shaped fuse, cutting knife type fuse, screw type fuse, blade  type fuse, flat plate type fuse and SMD type fuse ;
　　According to fusing speed: The fuse is commonly divided into Time-Lag fuse normally expressed by T and Fast-Acting fuse normally expressed by F ;
　　According to standard: The fuse is divided into European standard fuse (IEC60127 series), American standard fuse (UL248-1/14 series), Japanese standard fuse (METI Ordinance Article 1), etc. ;
　　According to protection type: The fuse is divided into current fuse and thermal cutoff fuse ;
　　According to size: The fuse is divided into surface mounted devices type: 0603, 0805, 1206 and 1812; plug-in type: Φ2.4*7, Φ3.6*10, Φ4.5*15, Φ5.2*20, Φ6.35*32, Φ8.5*8, 8.5*8*4, etc.
 

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