The fuse is mainly composed of three parts: the melt, the shell, and the support, among which the melt is a key component that controls the fusing characteristics. The material, size, and shape of the melt determine the fusing characteristics. Melt materials are divided into two categories: low melting point and high melting point. Low melting point materials such as lead and lead alloys have a low melting point and are prone to melting. Due to their high electrical resistivity, the cross-sectional size of the melt produced is larger, and the metal vapor generated during melting is more. They are only suitable for fuses with low breaking capacity. High melting point materials such as copper and silver have a high melting point and are not easy to fuse. However, due to their low electrical resistivity, they can be made into smaller cross-sectional sizes than low melting point melts. They produce less metal vapor during melting and are suitable for fuses with high breaking capacity. The shape of the melt can be divided into two types: filamentous and banded. Changing the shape of the variable cross-section can significantly change the fusing characteristics of the fuse. Fuses have various different fusing characteristic curves, which can be suitable for the needs of different types of protection objects.



Our understanding of the ampere second characteristic can be seen from Joule's law that Q=I2 * R * T. In a series circuit, the R value of the fuse remains basically unchanged, and the heat generated is proportional to the square of the current I and the heating time T. This means that when the current is high, the time required for the melt to fuse is shorter. When the current is low, the melting time required for the melt to melt is longer, and 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 blow. So, within a certain overload current range, when the current returns to normal, the fuse will not blow and can continue to be used.
Therefore, each melt has a minimum melting current. Corresponding to different temperatures, the minimum melting current also varies. Although this current is affected by the external environment, it can be disregarded in practical applications. The ratio of the minimum melting current of the melt to the rated current of the melt is generally defined as the minimum melting coefficient. Commonly used melts have a melting coefficient greater than 1.25, which means that a melt with a rated current of 10A will not fuse when the current is below 12.5A.
From this, it can be seen that the short circuit protection performance of the fuse is excellent, while the overload protection performance is average. If it is necessary to use it in overload protection, it is necessary to carefully match the line overload current with the rated current of the fuse. For example, 8A melt is used in 10A circuits for both short circuit protection and overload protection, but the overload protection characteristics at this time are not ideal.

The selection of fuses is mainly based on the protection characteristics of the load and the size of short-circuit current to select the type of fuse. For small capacity motors and lighting branch lines, fuses are often used as overload and short circuit protection, so it is hoped that the melting coefficient of the melt is appropriately small. Usually, RQA series fuses made of lead-tin alloy melt are selected. For larger capacity motors and lighting mainlines, emphasis should be placed on short-circuit protection and breaking capacity. Usually, RM10 and RL1 series fuses with high breaking capacity are selected; When the short-circuit current is high, RT0 and RTl2 series fuses with current limiting effect should be used