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 determined in coordination with the main electrical equipment.
The fuse is mainly composed of three parts: the melt, the shell and the support. 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 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 easy to fuse. Due to their large resistivity, the cross-sectional size of the melt is large, and more metal vapor is generated during fusing. It is only suitable for fusing with low breaking capacity. device. High melting point materials such as copper and silver have a high melting point and are not easy to fuse, but due to their low resistivity, they can be made into smaller cross-sectional dimensions than low melting point melts, and generate less metal vapor when fused, which is suitable for high breaking points. capable fuse. The shape of the melt is divided into two types: filament and ribbon. Changing the shape of the variable section can significantly change the fusing characteristics of the fuse. Fuses have various fusing characteristic curves, which can be applied to the needs of different types of protection objects.
Ampere-second characteristics:
The action of the fuse is realized by the melting of the melt. The fuse has a very obvious characteristic, that is, the ampere-second characteristic.
For the melt, its operating current and operating time characteristics are the ampere-second characteristics of the fuse, which are also called inverse 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.
To understand the 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 proportional to the square of the current I, which is proportional to the heating time T. It is proportional, that is to say: when the current is larger, the time required for the melt to blow is shorter. When the current is small, the time required for the melt to fuse is longer, and even if the rate of heat accumulation is less than the rate of thermal diffusion, the temperature of the fuse will not rise to the melting point, and the fuse will not even blow. Therefore, 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 is also different. Although the 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 the minimum melting coefficient. The melting coefficient of the commonly used melt is greater than 1.25, that is to say, the melt with a rated current of 10A will not melt when the current is below 12.5A.
It can be seen from this that the short-circuit protection performance of the fuse is excellent, and the overload protection performance is average. If it is really 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 a 10A circuit for short-circuit protection and overload protection, but the overload protection characteristics at this time are not ideal.
The choice of fuse is mainly based on the protection characteristics of the load and the size of the 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. 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. Generally, fuses of RM10 and RL1 series with higher breaking capacity are selected; when the short-circuit current is large, RT0 and RTl2 series of 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 motor that works for a long time can be selected according to the maximum starting current, or can be selected as follows:
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 to 3.5, which should be determined according to the actual situation.
3. Protect multiple long-term working motors (mains power supply)
IRN ≥ (1.5～2.5)IN max+ΣIN
IN max - the rated current of the single motor with the largest capacity. ΣIN rest. The sum of the rated current of the motor.
