The rated current of the melt is not equal to the rated current of the fuse. The rated current of the melt is selected according to the load current of the protected equipment. The rated current of the fuse shall be greater than the rated current of the melt and determined in cooperation with the main electrical appliance.
The fuse is mainly composed of melt, shell and support, in which the melt is the key element to control the fusing characteristics. The material, size and shape of the melt determine the fusing characteristics. Melt materials can be divided into low melting point and high melting point. Low melting point materials such as lead and lead alloys have low melting point and are easy to fuse. Due to their high resistivity, the cross-section size of the melt is large, and more metal vapor is generated during fusing. It is only suitable for fuses with low breaking capacity. High melting point materials such as copper and silver have high melting point and are not easy to fuse. However, due to their low resistivity, they can be made into smaller cross-section size than low melting point melts and produce less metal vapor when fusing. They are suitable for fuses with high breaking capacity. The shape of melt can be divided into filamentous and banded. Changing the shape of variable section can significantly change the fusing characteristics of fuse. Fuse has various fusing characteristic curves, which can be applied to the needs of different types of protection objects.
Ampere second characteristic:
The action of the fuse is realized by the fusing of the melt. The fuse has a very obvious characteristic, which is the ampere second characteristic.
For the melt, its action current and action time characteristics are the ampere second characteristics of the fuse, also known as the anti time 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 ampere second characteristics, 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 directly proportional to the square of current I and the heating time t, that is, when the current is large, the time required for melt fusing is short. When the current is small, the melt fusing takes a long time. Even if the heat accumulation speed is less than the heat diffusion speed, the fuse temperature will not rise to the melting point, and the fuse will not even fuse. Therefore, within a certain overload current range, when the current returns to normal, the fuse will not fuse and can continue to be used.
Therefore, each melt has a minimum melting current. Corresponding to different temperatures, the minimum melting current is also different. Although the current is affected by the external environment, it can not be considered in practical application. It is generally defined that the ratio of the minimum melting current of the melt to the rated current of the melt is the minimum melting coefficient. The melting coefficient of the common melt is greater than 1.25, that is, the melt with rated current of 10A will not fuse when the current is less than 12.5A.
It can be seen from here that the fuse has excellent short-circuit protection performance and general overload protection performance. If it really needs to be used in overload protection, it is necessary to carefully match the overload current of the line with the rated current of the fuse. For example, 8A melt is used in 10A circuit for short circuit protection and overload protection, but the overload protection characteristic is not ideal at this time.
The selection of fuse is mainly based on the protection characteristics of load and the size of short-circuit current. For motors and lighting branch lines with small capacity, fuses are often used as overload and short-circuit protection, so it is hoped that the melting coefficient of the melt will be appropriately smaller. RQA series fuse with lead tin alloy melt is usually selected. For motors and lighting trunk lines with large capacity, short-circuit protection and breaking capacity shall be emphatically considered. RM10 and RL1 series fuses with high breaking capacity are usually selected; When the short-circuit current is large, RT0 and rtl2 series fuses with current limiting effect should be used
The rated current of the melt can be selected according to the following methods:
- When protecting the stable load without starting process, such as lighting circuit, resistance, electric furnace, etc., the melt rated current is slightly greater than or equal to the rated current in the load circuit.
- The melt current for protecting a single motor working for a long time can be selected according to the maximum starting current or the following formula:
IRN ≥ (1.5～2.5)IN
Where IRN — rated current of melt; In — rated current of motor. If the motor starts frequently, the coefficient in the formula can be appropriately increased to 3 ~ 3.5, which shall be determined according to the actual situation.
- Protect multiple long-term motors (power supply trunk line)
IRN ≥ (1.5～2.5)IN max+ Σ IN
Max rated current of a single motor. Σ In rest Sum of rated current of motor.