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1、 Characteristics and working principle of fuse
Fuse, also known as current fuse, is defined as "fuse link" in iec127 standard and is mainly used for overload protection.
The first fuse was invented by Edison more than 100 years ago. It was originally used to protect expensive incandescent lamps because of the value of underdeveloped incandescent lamps.
As its original design, it is an electrical component installed in the circuit. When the harmful current exceeds a certain period of time, it will sacrifice itself to ensure the safe operation of the circuit.
We all know that the current in physics has a thermal effect, that is, when the current flows through the conductor, the conductor will rise because of the resistance of the conductor. The calorific value follows the formula: q ≤ 0.24i2rt, where q is the calorific value, I is the current flowing through the conductor, r is the resistance of the conductor, and t is the time when the current flows through the conductor. When determining the material and shape of the fuse, its resistance R is relatively determined (if its resistance temperature coefficient is not considered).
When the current flows through it, it will rise, and its calorific value will increase with time. The size of current and resistance determines the heat dissipation rate, while the structure and installation of fuse determine the heat dissipation rate. If the heat generation speed is less than the heat dissipation speed, the fuse will not be blown; If the heat generation speed is equal to the heat dissipation speed, the fuse will not blow for a long time; If the heat generation speed is greater than the heat dissipation speed, more and more heat will be generated. Because it has a certain specific heat and mass, the increase of heat is reflected in the increase of temperature. When the temperature rises above the melting point of the fuse, the fuse will blow, which is its working principle.
2、 Classification of fuses
1. It can be divided into overcurrent protection and overheat protection according to the protection mode;
Fuses used for current protection are often referred to as fuses (also known as current limiting fuses).
Fuses for overheating protection are usually called temperature fuses. Temperature fuses are divided into low melting point alloy shapes, temperature triggered shapes, and memory alloy shapes. The temperature fuse is used to prevent the heater or electrical appliances that are easy to be heated from over temperature protection, such as hair dryer, iron, rice cooker, electric stove, transformer, motor, etc; It responds to the increase of the temperature rise of the electrical appliance, regardless of the working current of the circuit. Its working principle is different from that of the "current limiting fuse".
2. According to the fusing speed, it can be divided into: super slow fuse (usually expressed in t), slow fuse (usually expressed in t), medium speed fuse (usually expressed in m), fast fuse (usually expressed in f), and super fast fuse (usually expressed in ff);
On the fuse surface, especially the small glass tube fuse, we only marked "f5a/250V", that is, the fast fuse with rated current of 5A and rated voltage of 250V.
Fast fuse or ultrafast fuse is used when it contains components that are particularly sensitive to overcurrent, while slow fuse is usually used in circuits with large current. It is necessary to say two more words about slow fusion. We generally believe that fuses are used for short circuit protection rather than overload protection, but slow fuses can provide overload protection. Also called delay fuse, its delay characteristic can protect overload for a long time when the circuit has fault free pulse current.
In some circuits, the current at the switching time is several times more than the normal working current. Although the peak value of this current is very high, its occurrence time is very short. We call it pulse current or surge current. Ordinary fuses cannot bear this current. If ordinary fuses are used, they may not start normally. If larger fuses are used, they cannot protect the circuit when it is overloaded.
3. According to the scope of use, it can be divided into the following types: power fuse, machine fuse, electrical instrument fuse (electronic fuse), automobile fuse;
4. According to volume integration, it can be divided into: large, medium, small and micro;
5. According to the rated voltage, it can be divided into: high-voltage fuse, low-voltage fuse and safety voltage fuse;
The high-voltage fuse is applicable to 3kV-35kV, and the low-voltage fuse is applicable to AC 220V, 380V, DC 220V, 440V. 500V 750V 1000V 1500V
6. According to breaking capacity, it can be divided into: high breaking capacity fuses and low breaking capacity fuses;
7. According to the shape, it can be divided into: flat head tubular fuse (or internal welding fuse and external welding fuse), pointed head tubular fuse, guillotine fuse, spiral fuse, plug fuse, flat fuse, winding fuse, patch fuse, etc
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