Fuse structure

Fuse structure
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 should be greater than the rated current of the melt and is determined in conjunction with the main electrical appliance.
 A fuse is mainly composed of three parts: melt, shell and support. The melt is the 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 low melting points and are easy to fuse. Due to their large resistivity, the cross-section size of the melt is larger, and more metal vapor is generated during fusing. They are only suitable for fusing with low breaking capacity. device. High melting point materials such as copper and silver have high melting points and are not easy to fuse. However, due to their lower resistivity, they can be made into smaller cross-section sizes than low melting point melts. Less metal vapor is generated when fusing, so they are suitable for high breaking Capability fuse. The shape of the melt is divided into two types: filament and ribbon. Changing the shape of the variable cross-section can significantly change the fusing characteristics of the fuse. Fuses have various fusing characteristic curves, which can be suitable for the needs of different types of protection objects.
 Ampere-second characteristics:
 The action of the fuse is achieved by the melting of the melt. The fuse 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, also called 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.
To understand the ampere-second characteristics, we can see from Joule's law that Q=I2*R*T. In a series circuit, the R value of the fuse is basically unchanged, and the heat generation is proportional to the square of the current I and the heating time T Proportional, that is to say: when the current is larger, the time required for the melt to melt is shorter. When the current is small, the time required for the melt to melt 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 melt. Therefore, within a certain overload current range, when the current returns to normal, the fuse will not blow and can continue to be used.
 Hence, 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. 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. The melting coefficient of commonly used melts is greater than 1.25, which means that a melt with a rated current of 10A will not melt when the current is below 12.5A.
 It can be seen from here that the short-circuit protection performance of the fuse is excellent, and the overload protection performance is average. If it really needs to be used in overload protection, it is necessary to carefully match the line overload current and the rated current of the fuse. 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 selection of fuses mainly depends on the protection characteristics of the load and the size of the short-circuit current. For small-capacity motors and lighting branch lines, fuses are often used as overload and short-circuit protection, so the melting coefficient of the melt is expected to be appropriately small. RQA series fuses with lead-tin alloy melt are usually used. For larger-capacity motors and lighting trunks, short-circuit protection and breaking capacity should be considered. Usually, RM10 and RL1 series fuses with higher breaking capacity are used; when the short-circuit current is large, RT0 and RTl2 series fuses with current limiting function should be used.
The rated current of the melt can be selected as follows:
 1. When protecting stable loads without starting process such as lighting lines, resistors, electric furnaces, etc., the rated current of the melt is slightly greater than or equal to the rated current in the load circuit.
  2. The melt current to protect a single long-term working motor can be selected according to the maximum starting current, or according to the following formula:
  IRN ≥ (1.5~2.5)IN
In the formula, IRN--melt rated current; IN--motor rated current. If the motor starts frequently, the coefficient in the formula can be appropriately increased to 3~3.5, depending on the actual situation.
 3. Protect multiple long-term working motors (power supply trunk lines)
  IRN ≥ (1.5~2.5)IN max+ΣIN
IN max - the rated current of a single motor with the largest capacity. ΣIN the rest. The sum of the rated current of the motor.