abstract： Concerned Synonyms of fuses are: circuit breaker, load switch, isolating switch, earth leakage circuit breaker, high voltage fuse, fuse, air switch, voltage transformer, thermal relay and lightning arrester.

Fuse refers to an electrical appliance that fuses the melt and breaks the circuit with its own heat when the current exceeds the specified value. The fuse melts the melt with its own heat after the current exceeds the specified value for a period of time, thus breaking the circuit; A current protector made of this principle. Fuses are widely used in high and low voltage distribution systems, control systems and electrical equipment. As short-circuit and overcurrent protectors, fuses are one of the most widely used protection devices. A metal conductor is used as a melt connected in series. When overload or current passes through the melt, it will fuse due to its own heat, thus breaking the circuit. Fuses are simple in structure and convenient to use, and are widely used as protection devices in various electrical equipment and household appliances. Spiral fuse: the upper end of the melt is covered with a fuse indicator. Once the melt is fused, the indicator will pop up immediately, which can be observed through the glass hole in the porcelain cap. It is often used in the electrical control equipment of machine tools. Spiral fuse. The breaking current is large, which can be used for short-circuit protection in circuits with voltage level below 500V and current level below 200A A. Enclosed fuse: Enclosed fuse is divided into two types: packed fuse and unfilled fuse, as shown in Figure 3 and Figure 4. Filled fuses are generally made of square porcelain tubes with quartz sand and melt inside, which have strong breaking ability, and are used in circuits with voltage level below 500V and current level below 1KA. Packing-free closed fuse puts the melt into a closed cylinder, and its breaking capacity is slightly smaller. It is used in power network or power distribution equipment below 500V and below 600A A. Fast fuse: Fast fuse is mainly used for short-circuit protection of semiconductor rectifying elements or rectifying devices. Because the overload capacity of semiconductor elements is very low. It can only withstand a large overload current in a very short time, so short-circuit protection is required to have the ability to fuse quickly. The structure of fast fuse is basically the same as that of closed fuse with filler, but the material and shape of melt are different. It is a variable cross-section melt with V-shaped deep groove punched by silver sheet. Self-fuse: Sodium metal is used as melt, which has high conductivity at room temperature. When a short-circuit fault occurs in the circuit, the short-circuit current generates high temperature, which makes sodium vaporize rapidly, and the vapor sodium presents high impedance state, thus limiting the short-circuit current. When the short-circuit current disappears, the temperature drops, and the metal sodium restores its original good conductivity. Self-fuse can only limit short-circuit current, but can't really break the circuit. Its advantage is that it does not need to change the melt and can be reused. The fuse is not equal to the rated current of the fuse. The rated current of the fuse should be greater than the rated current of the fuse according to the choice of the protected equipment, and it should be determined in coordination with the main electrical appliance. Fuse is mainly composed of melt, shell and support, in which melt is the key element to control the fusing characteristics. The melting characteristics are determined by the material, size and shape of the melt. Melts are divided into low melting point and high melting point. Low melting point materials, such as lead and lead alloy, are easy to fuse because of their low melting point. Because of their high resistivity, the cross-section size of the melt made is large, and more metal vapor is generated when fusing, so it is only suitable for fuses with low breaking capacity. High melting point materials, such as copper and silver, have high melting points and are not easy to fuse. However, due to their low resistivity, they can be made into smaller cross-sectional dimensions than low melting point melts, and less metal vapor is generated when fusing, so they are suitable for fuses with high breaking capacity. The shape of the melt is divided into two types: filiform and ribbon. Changing the shape of the variable cross-section can significantly change the fusing characteristics of the fuse. Fuses have different fusing characteristic curves, which can meet the needs of different types of protection objects. For the melt, its operating current and characteristics are ampere-second characteristics of the fuse, which is also called 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. Understanding the ampere-second characteristic, we can see Q=I*R*T from Joule's law. In series circuits, the R value of the fuse is basically constant, and the calorific value is proportional to the square of the current I and the heating time T. That is to say, when the current is larger, the time required for melting the melt is shorter. When the current is small, it takes a long time for the melt to fuse. Even if the heat accumulation rate is less than the heat diffusion rate, the fuse temperature will not rise to the melting point, and the fuse will not even fuse. Therefore, in a certain overload current range, when the current returns to normal, the fuse will not blow and can be used continuously. Therefore, each melt has a minimum melting current. The minimum melting current varies with different temperatures. Although the current is affected by the external environment, it can be ignored in practical application. Generally, the ratio of the minimum fusing current of the melt to the rated current of the melt is defined as the minimum fusing coefficient. The fusing coefficient of commonly used melts is greater than 1.25, which means that the melt with the rated current of 10A will not fuse 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 overload current of the line with the rated current of the fuse. For example, the melt of 8A is used in the circuit of 10A 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 load and the size of short-circuit current. For motors and lighting branches 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 smaller. RQA series fuses with lead-tin alloy melt are usually selected. For large-capacity motors and lighting mains, short-circuit protection and breaking capacity should be considered emphatically. Generally, RM10 and RL1 series fuses with high breaking capacity are selected; When the short-circuit current is large, RT0 and RTl2 series fuses with current limiting function should be adopted. Type IRN-- melt rated current; IN-- 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. Fuses can be divided into high-voltage fuses and low-voltage fuses according to the applied voltage. According to the protection objects, it can be divided into fuses for protecting transformers and general electrical equipment, fuses for protecting voltage transformers, fuses for protecting power capacitors, fuses for protecting semiconductor components, fuses for protecting motors and fuses for protecting household appliances. According to the structure, it can be divided into open fuse, semi-closed fuse, tubular fuse and jet fuse. The open fuse has a simple structure, the melt is completely exposed to the air, it is supported by a porcelain column, and there is no support, so it is suitable for low-voltage outdoor use. When the current is broken, large sound and light are generated in the atmosphere. The melt of the semi-enclosed fuse is mounted on the porcelain frame and inserted into the porcelain box with sockets at both ends, which is suitable for low-voltage indoor use. When breaking the current, the generated sound and light are blocked by the porcelain box. The melt of the tubular fuse is installed in the fuse body. Then insert it into the support or directly connect it to the circuit for use. Fuse is a completely sealed insulating tube with metal caps or contact knives at both ends. If the insulating tube of this fuse is filled with quartz sand, it can limit the current when breaking the current, which can greatly improve the breaking capacity, so it is also called high breaking capacity fuse. If the tube is vacuumized, it is called vacuum fuse. If the tube is filled with SF6 gas, it is called SF6 fuse, and its purpose is to improve the arc extinguishing performance. As quartz sand, vacuum and SF6 gas have good insulation properties, this fuse is suitable for both low voltage and high voltage. Jet fuse is to put the melt in an insulating tube made of solid gas-producing material. Solid gas-producing materials can be electrical white cardboard or plexiglass materials. When the short-circuit current passes through the melt, the melt melts immediately to generate an arc, and the high-temperature arc rapidly decomposes the solid gas-producing material to generate a large amount of high-pressure gas, so that the ionized gas is ejected from both ends of the pipe with an arc, giving off great sound and light, and the arc is extinguished when the alternating current crosses zero, thus breaking the current. Insulating tube is usually installed on an insulating bracket to form the whole fuse. Sometimes, the upper end of the insulating tube is made movable, and then falls off after breaking the current. This kind of jet fuse is commonly called drop fuse. Generally, it is suitable for outdoor occasions with voltage higher than 6 kV. In addition, fuses can be divided into general-purpose fuses, backup fuses and full-range fuses according to the breaking current range. The breaking current range of general-purpose fuses refers to the range from the overload current to the maximum breaking current when it is 1.6 ~ 2 times higher than the rated current. This fuse is mainly used to protect power transformers and general electrical equipment. The breaking current range of the backup fuse refers to the range from the overload current 4 ~ 7 times higher than the rated current to the maximum breaking current. This kind of fuse is often used in series with the contactor, and when the overload current is less than 4 ~ 7 times of the rated current, the contactor will realize the breaking protection. Mainly used to protect motors. In order to prevent leapfrog fuse and expand the accident scope, the fuses of the upper and lower levels (i.e. power supply trunk and branch lines) should have good cooperation. When selecting, the rated current of the fuse of the superior (main power supply line) should be 1 ~ 2 steps higher than that of the inferior (branch power supply line). Commonly used fuses include R1 series, RLl series, RT0 series, RSO and RS3 series, etc. Medium fuse is a kind of electrical appliance that plays a role in safety protection. Fuses are widely used in protection and protection of electrical equipment. When a short-circuit fault or overload occurs in the power grid or electrical equipment, it can automatically cut off the circuit to avoid the damage of electrical equipment and prevent the spread of accidents. Fuse is composed of insulating base (or support), contacts, melt, etc. Melt is the main working part of fuse, which is equivalent to a special section connected in series in the circuit. When the circuit is short-circuited or overloaded, the current is too large, and the melt melts due to overheating, thus cutting off the circuit. Melts are often made into filaments, grids or sheets. The melt has the characteristics of low melting point, stable characteristics and easy melting. Generally, lead-tin alloy, silver-plated copper sheet, zinc, silver and other metals are used. In the process of melting and cutting off the circuit, the electric arc will be generated. In order to extinguish the electric arc safely and effectively, the melt is generally installed in the fuse housing, and measures are taken to extinguish the electric arc quickly. In view of its excellent short-circuit protection performance, fuse is widely used in high and low voltage distribution systems, control systems and electrical equipment. As a short-circuit and overcurrent protector, it is one of the most widely used and important protection devices. 1. The protection characteristics of the fuse should be compatible with the overload characteristics of the protected object. Considering the possible short-circuit current, the fuse with corresponding breaking capacity should be selected; 3) When replacing the new melt, check the internal burns of the fuse tube. If there are serious burns, the fuse tube should be replaced at the same time. When the porcelain fused tube is damaged, it is not allowed to replace it with other materials. When replacing the melt with filler fuse, pay attention to the filler. The same point is that short-circuit protection can be realized. The principle of fuse is that the current flowing through the conductor will make the conductor heat up, and the conductor will melt after reaching the melting point of the conductor, so the circuit will be disconnected for protection and the circuit will not be burnt out. It is an accumulation of heat, so overload protection can also be realized. Once the melt burns out, the melt should be replaced. It can also realize the short circuit and overload protection of the line, but the principle is different. It is the through-bottom magnetic effect (tripper) to realize the open circuit protection, and overload protection (not fusing, so it is unnecessary to replace the device). In practice, when the circuit is close to the fuse used for a long time, it will gradually heat up until it is blown. As mentioned above, the fusing of the fuse is the result of the interaction with time, which plays a role in protecting the circuit. It is one-off. Instead, the current in the circuit suddenly increases, and when it exceeds the load of the circuit breaker, it will automatically disconnect. It is the protection of the circuit with an instantaneous current increase, such as when the leakage is large, or when the instantaneous current is large. When the reason is found out, it can be switched on and used continuously. As mentioned above, the fusing of the fuse is the result of the joint action of current and time, while the circuit breaker will trip as soon as the current exceeds its set value, and the time effect can hardly be considered. Circuit breakers are commonly used components. There are also some places where fuses are suitable. 1) Good selectivity. As long as the rated current of upper and lower fuse links meets the requirement of overcurrent selection ratio of 1.6: 1 specified in national standard and IEC standard, that is, the rated current of upper fuse links is not less than 1.6 times of that of lower fuse links, it is considered that the upper and lower fuse links can selectively cut off the fault current; 3) When one-phase fuse occurs, the adverse consequences of two-phase operation of three-phase motor can be compensated by fuse with alarm signal, and one-phase fuse can disconnect three phases; 3) Some circuit breakers have low breaking capacity. If the circuit breaker with low rated current is installed near the large-capacity transformer, the breaking capacity will be insufficient. The existing products with high breaking capacity can meet the requirements, but the price is higher. 2) It has multiple protection functions, including long time delay, instantaneous, short time delay and ground fault protection (including zero sequence current and residual current protection), and can realize overload, open circuit delay, large short circuit current instantaneous action and ground fault protection respectively. The protection sensitivity is extremely high, it is convenient to adjust various parameters, and it is easy to meet various protection requirements of distribution lines. In addition, it can have cascade protection function, which has better selective action performance; 3) Nowadays, most products have intelligent features. Besides protection functions, they also have power measurement, fault recording and communication excuses to realize centralized monitoring and management of power distribution devices and systems.