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What is a fuse and what is its function?

A fuse is also called a fuse, and the IEC127 standard defines it as a "fuse-link". It is an electrical component installed in the circuit to ensure the safe operation of the circuit. The function of the fuse is: when the circuit breaks down or is abnormal, the current will continue to rise, and the rising current may damage some important or valuable components in the circuit, or it may burn the circuit or even cause a fire. If the fuse is correctly installed in the circuit, the fuse will fuse and cut off the current when the current rises to a certain height abnormally, thereby protecting the safe operation of the circuit. The earliest fuse was invented by Edison more than one hundred years ago. Since the incandescent lamp was very expensive because of the underdeveloped industrial technology at that time, it was originally used to protect the expensive incandescent lamp.

How does the fuse work?

We all know that when current flows through a conductor, the conductor will generate heat because of the resistance of the conductor. And the calorific value follows this formula: Q=0.24I2RT; where Q is the calorific value, 0.24 is a constant, I is the current flowing through the conductor, R is the resistance of the conductor, and T is the time for the current to flow through the conductor; according to this formula It is not difficult to see the simple working principle of the fuse. Once the material and shape of the fuse are determined, its resistance R is relatively determined (if the temperature coefficient of resistance is not considered). When current flows through the fuse, it will heat up, and its heat will increase over time. The current and resistance determine the speed at which the fuse generates heat. The structure of the fuse and its installation status determine the speed of heat dissipation. If the rate of heat generation is less than the rate of heat dissipation, the fuse will not blow. If the rate of heat generation is equal to the rate of heat dissipation, it will not fuse for a long time. If the rate of heat generation is greater than the rate of heat dissipation, then more and more heat will be generated. And because it has a certain specific heat and quality, the increase in heat is manifested in the increase in temperature. When the temperature rises above the melting point of the fuse, the fuse blows. This is how the fuse works. We should know from this principle that we must carefully study the physical properties of the materials you choose when designing and manufacturing fuses, and ensure that they have consistent geometric dimensions. Because these factors play a crucial role in the normal operation of the fuse. Similarly, when you use it, you must install it correctly.

What is the structure of the fuse? What are the effects of each? What are the requirements?

Generally, a fuse is composed of three parts: one is the melt part, which is the core of the fuse, which cuts off the current when it is blown. The melt of the same type and specification of the fuse must have the same material, the same geometric size, and the resistance value. As small as possible and consistent, the most important thing is to have the same fusing characteristics; the second is the electrode part, usually there are two, it is an important part of the connection between the melt and the circuit, and it must have good conductivity and should not produce obvious The third is the bracket part. The melt of the fuse is generally slender and soft. The function of the bracket is to fix the melt and make the three parts a rigid whole for easy installation and use. It must have good mechanical strength, Insulation, heat resistance and flame retardancy, there should be no breakage, deformation, burning and short circuit during use; the fuse used in power circuits and high-power equipment not only has the three parts of general fuse, but also Arc extinguishing device, because the circuit protected by this type of fuse not only has a large operating current, but also has a high voltage at both ends when the melt is blown. It often appears that the melt has melted (fused) or even vaporized, but the current does not There is no cut, the reason is that under the action of voltage and current at the moment of fusing, an arc phenomenon occurs between the two electrodes of the fuse. The arc extinguishing device must have strong insulation and good thermal conductivity, and be negative. Quartz sand is a commonly used arc extinguishing material. In addition, some fuses have fusing indicating devices. Their function is to change their appearance after the fuse is activated (blown), which is easy to be found by maintenance personnel, such as glowing, discoloration, and pop-up solid indicators.

What types of fuses are there?

According to the protection form, it can be divided into: overcurrent protection and overheating protection. The fuse used for over-current protection is the usual fuse (also called current-limiting fuse). The fuse used for overheating protection is generally called "thermal fuse". Thermal fuse is divided into low melting point alloy type, temperature sensitive trigger type, memory alloy type and so on. (Thermal fuse is to prevent heating appliances or heat-prone appliances from overheating and protect them, such as: hair dryers, electric irons, rice cookers, electric stoves, transformers, motors, etc.; it responds to the rise in the temperature of electrical appliances , Does not care about the working current of the circuit. Its working principle is different from "current limiting fuse".) According to the scope of use, it can be divided into: power fuse, machine tool fuse, electrical instrument fuse (electronic fuse), automobile fuse. According to the volume, it can be divided into: large, medium, small and micro. According to the rated voltage, it can be divided into: high voltage fuse, low voltage fuse and safety voltage fuse. According to the breaking capacity, it can be divided into: high and low breaking capacity fuses. According to the shape, it can be divided into: flat-head tubular fuses (also can be divided into internal welding fuses and external welding fuses), pointed tubular fuses, guillotine fuses, spiral fuses, blade fuses, flat fuses, wrap-around fuses Fuses, chip fuses. According to the fusing speed, it can be divided into: super slow fuse (usually represented by TT), slow fuse (usually represented by T), medium-speed fuse (usually represented by M), fast fuse (generally represented by F), special Fast fuse (usually represented by FF). According to the standard points, it can be divided into: European standard fuse (VDE), American standard fuse (UL), Japanese standard fuse (PSE). Ando Company specializes in the production of various fuse tubes, fuses, fuse holders, electrical fuses, lead fuses, electric vehicle fuses, circuit board fuses, switching power supply fuses, energy-saving lamp fuses, fuse holders, automobile fuses, automobile fuses, fuse, SMD fuses, self-recovery fuses, current fuses, thermal fuses, automobile fuse holders, fuse holders and other electronic components, welcome you to buy our fuses and other products!

 The fuse protects the electronic equipment from overcurrent damage, and can also avoid serious damage caused by the internal failure of the electronic equipment. Therefore, each fuse has a rated specification, and the fuse will blow when the current exceeds the rated specification. If the fuse is correctly installed in the circuit, the fuse will fuse and cut off the current when the current rises to a certain height abnormally, thereby protecting the safe operation of the circuit.

Automotive fuses are small in size, easy to install, and have much faster response; self-resetting fuses have significant advantages such as small size, low resistance, and fast response.

What is a slow fuse?

Slow-speed fuse is also called time-delay fuse. Its time-delay characteristic shows that the circuit remains intact when there is a non-fault pulse current and can provide protection against long-term overload. The current of some circuits at the moment of switching is greater than several times the normal operating current. Although this current has a high peak value, it appears for a short period of time. We call it pulse current or impulse current or surge current. Ordinary fuses cannot withstand this current. If you use an ordinary fuse in such a circuit, you may not be able to start normally. If you use a larger fuse, it will not be protected when the circuit is overloaded. The melt of the time-delay fuse is specially processed. It has the function of absorbing energy. Adjusting the amount of energy absorption can make it not only able to withstand the impact current but also provide protection against overload. The standard has provisions on delay characteristics. If the specified characteristics of the standard cannot meet your requirements, you can contact the manufacturer to get a solution.

Is the rated current of the fuse the current that causes the fuse to blow?

no. It should only be regarded as a nominal specification, and how big the current flowing through the fuse is and when it will blow is detailed in the fuse product standard, and it is specified in different standards. The difference. The fuse has a "fuse coefficient" whose value is greater than "1" (usually between 1.1 and 1.5), which is the ratio of "normal no-fuse current" to "rated current". It can be seen that even if the current flowing through the fuse is greater than its rated current and does not exceed the conventional non-fusing current, the fuse should not be fused.

How to understand the rated voltage of the fuse?

Whether the fuse blows or not depends on the magnitude of the current flowing through it, and has nothing to do with the operating voltage of the circuit. The rated voltage of the fuse is proposed from the perspective of safe use of the fuse. It is the highest working voltage of the circuit where the fuse is in a safe working state. This means that the fuse can only be installed in a circuit whose working voltage is less than or equal to the rated voltage of the fuse. Only in this way can the fuse work safely and effectively, otherwise, the phenomenon of continuous arcing and voltage breakdown will damage the circuit when the fuse is blown.

What does the voltage drop of the fuse indicate?

The voltage drop of the fuse is the voltage drop across the fuse under the rated current condition. It reflects the internal resistance of the fuse, and its value should not be too large. If a fuse with excessive internal resistance (voltage drop) is installed in the circuit, it will affect the system parameters of the circuit and make the circuit unable to work normally. The standard not only stipulates the upper limit of the voltage drop, but also stipulates its consistency.

What is the significance of studying the temperature rise of fuses?

The temperature rise of the fuse refers to the temperature rise value of the fuse when 1.1 times (110%) of the rated current flows through the fuse, that is, the value of the actual measured temperature minus the ambient temperature. The UL standard stipulates its upper limit at 75Co. Because the melt of the fuse is more sensitive to temperature, its melting point and impedance will change under the action of a certain high temperature for a long time, and this change will affect the accuracy of the fuse. This is commonly referred to as fuse aging. The aging fuse is very dangerous when used in the circuit. Therefore, we should pay attention to the temperature rise of the fuse when making and using the fuse. For the same reason, we should also note that even if the fuse has not been blown after a long period of use, it may have been aging, and it is best to replace it at this time.

What does the breaking capacity of a fuse mean?

When the current between the conventional non-fusing current and the rated breaking capacity (current) specified by the relevant standards is applied to the fuse, the fuse should operate satisfactorily without endangering the surrounding environment. The expected fault current of the circuit where the fuse is placed must be less than the rated breaking capacity current specified in the standard, otherwise, when the fuse is blown in the fault, there will be continuous arcing, ignition, blown fuse, melting together with the contacts, and the fuse mark cannot be recognized Phenomenon. Of course, the breaking capacity of inferior fuses does not meet the requirements of the standard, and the above-mentioned hazards will also occur during use. Fuse selection In order to facilitate users to select suitable fuse tubes for the components, circuits or equipment to be protected, this guide is specially formulated.

The selection of fuse tube can follow the following process:

Factors to consider

The safety certification of the fuse tube is determined according to the safety certification required by the whole machine, and the fuse tube can be preliminarily determined to be the IEC standard or the UL standard.

1. The space limitation in the circuit during design.

2. Installation method.

The rated voltage should be greater than or equal to the effective circuit voltage, and the breaking capacity should be greater than the maximum fault current in the circuit. Whether there is a starting current in the circuit when the whole machine is switched on and off. The starting current is normal in some circuits. In this case, time-delay and medium-delay fuse tubes should be used. The current and duration that the fuse tube must cut off (this condition is determined by the designer according to the protection requirements of the specific circuit). Refer to the I-T curve of the corresponding model, and take the maximum rated current that meets the requirements as the upper limit A1.

1, and the stable current through the fuse tube (depending on the specific circuit).

2. For the difference between the rated current of IEC standard and UL standard fuse, please refer to "Stable Current" for details.

3. For the influence of ambient temperature on the carrying capacity of the fuse tube, see "Ambient Temperature" for details.

4. The impact of pulse (impact current, surge current, starting current and current transient value) on the life of the fuse tube, see "Pulse" for details.

5. The starting current and duration are compared with the I-T curve of the corresponding model.

After comprehensively considering the above five factors, the minimum rated current that meets the requirements is selected as the lower limit A2.

After comprehensively considering the above factors, select the most suitable model and rated current.

When A1>A2, choose the corresponding type fuse tube with rated current A2.

When A1≤A2, select the corresponding type fuse tube with rated current A1. The sample should be tested in the actual circuit

Steady-state current

There are different conditions in practical applications and between laboratories, such as:

A. Sometimes a fuse box is used;

B. The cross-sectional area of the wires in the circuit;

C. The contact resistance of the fuse clip, etc.

Considering the above factors, the fuse tube selected under the condition of 25℃ should meet the following conditions to make the fuse tube continue to work reliably:

IEC specification: the rated current of the fuse tube In=steady-state current/0.9.

UL specification: the rated current of the fuse tube In=steady-state current/0.75.

Ambient temperature

The current carrying capacity test of the fuse tube is carried out at an ambient temperature of 25℃, and the current carrying capacity of the fuse tube is affected by the ambient temperature. The higher the ambient temperature, the shorter the life of the fuse tube, and the lower the carrying capacity. Therefore, when selecting a fuse tube, the ambient temperature around the fuse tube should be considered. The influence of ambient temperature on the carrying capacity of various fuse tubes is shown in the following figure: (II) indicates that the ambient temperature has a bearing capacity and 5In for fast-blow and wire-wound fuse tubes. The effect of fusing time


The pulse generates thermal cycles, which in turn produces mechanical fatigue and affects the life of the fuse tube. The design should make the pulse I2T much smaller than the nominal melting heat I2T of the fuse tube. Refer to Table 1 for the relationship between the life of the fuse tube (the number of pulse cycles that can withstand) and U (U = the ratio of the pulse I2T value to the fuse tube I2T value). This catalog provides various specifications of fuse tube melting heat I2T for reference. Table 2 provides approximate calculation formulas for I2T values of various typical pulse waveforms:

Can withstand the number of pulses U( %)
100,000 times 20%
10,000 times 30%
1,000 times 40
Note: The pulse interval must be long enough to dissipate the heat generated by the previous pulse.

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