Electricity is transmitted to all corners of the city. At present, the supply of electricity in big cities is very huge. Since more and more products need to use this kind of circuit device, the supply of electricity is indispensable for the city itself. Speaking of the current power transmission, it is actually provided by power stations and urban circuits. In the circuit, in addition to the equipment that provides power transmission, the detection device of the circuit itself and the protection equipment of the circuit equipment are also required. These are the main components of the current circuit, and as the fuse cutout, it is actually a relatively common overcurrent circuit protector. The use of this circuit device brings a relatively stable effect to the internal equipment of the circuit to ensure the circuit Stability of safe operation.
The working principle of fuse cutout is:
After the current exceeds the specified value for a period of time, the fuse melts the melt with its own heat, thereby breaking the circuit. In the power distribution system, control system and electrical equipment, the fuse cutout as a short circuit and serious overcurrent protector is one of the most commonly used protection devices. The fuse cutout has reverse delay characteristics, that is, when the overload current is small, the fusing time is long; when the overload current is large, the fusing time is short. Therefore, within a certain range of overload current, when the current returns to normal, the fuse cutout will not be blown and can be used continuously. The fuse cutout has a variety of different fusing characteristic curves, which can adapt to the needs of different types of protection objects. The fuse cutout is connected in series in the circuit. When the circuit or electrical equipment is overloaded and short-circuited, the melt of the fuse will fuse first, cut off the power supply, and protect the circuit or electrical equipment. It is a short-circuit protection electrical appliance.
The action of the fuse cutout is realized by the fusing of the melt. The fuse cutout has a very obvious characteristic, which is the ampere-second characteristic.
For the melt, its operating current and operating time characteristics are the ampere-second characteristics of the fuse cutout, also called the inverse delay characteristics, that is: when the overload current is small, the fusing time is long; when the overload current is large, the fusing time is short.
For the understanding of the ampere-second characteristic, we can see from Joule's law that Q=I2*R*T. In the series circuit, the R value of the fuse is basically unchanged, and the calorific value is proportional to the square of the current I, and it is proportional to the heating time T, It is proportional, that is to say: when the current is large, the time required for the melt to fuse is shorter. When the current is small, the time required for the melt to fuse is longer. Even if the rate of heat accumulation is less than the rate of heat diffusion, the temperature of the fuse will not rise to the melting point, and the fuse will not even be blown. Therefore, within a certain range of overload current, when the current returns to normal, the fuse will not be blown and can be used continuously.
Therefore, each melt has a minimum melting current. Corresponding to different temperatures, the minimum melting current is also different. Although this current is affected by the external environment, it can be ignored in practical applications. Generally, the ratio of the minimum melting current of the melt to the rated current of the melt is defined as the minimum melting coefficient. The melting coefficient of commonly used melts is greater than 1.25, which means that the melt with a rated current of 10A will not fuse when the current is below 12.5A.
It can be seen from this that the short-circuit protection performance of the fuse cutout is excellent, and the overload protection performance is average. If you really need to use it in overload protection, you need to carefully match the line overload current with the rated current of the fuse cutout. For example: 8A melt is used in a 10A circuit for short-circuit protection and overload protection, but the overload protection characteristics at this time are not ideal.
The above is an introduction about the working principle and structural characteristics of fuse cutout. More knowledge of fuses will be sorted and shared in later articles:
The main classification of fuses
Use maintenance and precautions of fuse
Fuse inspection, maintenance and main advantages and characteristics