working principle of fuse,purpose and structure of fuse,technical parameters of fuse and physical process of work
working principle of fuse
1. the use and structure of the fuse
A fuse is a protective device that melts the melt and breaks the electrical device after the current exceeds a predetermined value for a certain period of time. It is the simplest and most effective protective device that combines induction, comparison, and execution. Short circuit and overload protection in low voltage distribution lines. Since the fuse is insensitive to overload, it is not suitable for overload protection and is mainly used for short circuit protection.
The fuse is mainly composed of a melt and a melt pipe and a fuse seat on which the melt is installed. The melt is the main part, which is both a sensing element and an actuator. The melt can be made into filamentous, flake-like, ribbon-shaped, cage-like materials. There are two types of materials: low-melting materials such as lead, zinc, tin, and tin-lead alloys; the other are high-melting-point materials such as silver and copper. Aluminum and so on. Fusible tube material is ceramic, insulated steel paper or glass fiber.
2.Main working principle and main technical parameters of the fuse
Fuse is a simple structure, easy to use, low-cost protection of electrical appliances. It consists mainly of melts and conductive parts that contain the melt, in addition to insulating seats and insulating tubes.
When used, the melt is connected in series with the protection circuit. When the circuit is under normal load current, the melt temperature is low. If the circuit is short-circuited, the circuit current increases and the melt heats up. When the melt temperature rises to the melting point, it fuses and breaks the faulty circuit to achieve the purpose of protecting the line.
3.the physical process of fuse work
1). Melt temperature rise When short-circuit current occurs in the circuit, the melt temperature rises to the melting temperature, but the melt is still in a solid state and does not begin to melt. At this point, the higher the current, the faster the temperature rises.
2). Melt Melt The melt continues to absorb heat, and some of the metal begins to change from a solid state to a liquid state. Since the melt melt needs to absorb a part of the heat, the melt temperature is always kept at the melting point during this stage.
3). Arc Generation The molten metal continues to be heated until vaporized, ie metal vapor appears. At this point, due to the appearance of a momentary small insulation gap, the current is abruptly interrupted, and the circuit voltage at this time will immediately break through this gap and generate an arc, so that the circuit is turned on again to form a second heating phase.
4). Arc Extinguishing After the arc is formed, if the energy is small, it will extinguish itself with the expansion of the melting gap; otherwise, the arc burns and spreads into the filler, so that the melt gap is further expanded, so that the arc cannot continue to burn and the arc extinguishes. Then the fuse actually cuts off the current and acts as a protective circuit.
The main features of the pre-arcing process are: temperature rise and melting of the melt, and the fuse reacts to failure.
The main features of the after-arc process: The arc containing a large amount of metal vapor spreads and burns in the gap and is finally extinguished. The duration of this process depends on the arc extinguishing capability of the fuse.
Figure 1 Protection characteristics of fuses Figure 2 Melting process
T—blown time; I—current flowing through the fuse; Ir—minimum melting current
4. commonly used fuses
1).RC1A Series Ceramic Insert Fuse
Commonly known as the porcelain plug insurance, consists of porcelain sockets and porcelain plugs. The porcelain sockets have static sockets and connection screws. The middle gap and the protruding part of the porcelain plug form an arc extinguishing chamber. When the current is large, the arc extinguishing chamber is filled with asbestos weaving to prevent melting. When the body fuses, metal particles splash.
Lead-tin alloy wire rated current 5-15; limit breaking capacity 0.25-0.5
Copper wire rated current 30-100; limit breaking capacity 1.5-3
Variable cross-section copper sheet Rated current 200; Limit breaking capacity 3
It consists of a fuse tube, a melt, and a static socket.
The melt is a zinc piece with a variable cross-section and a static socket is mounted on the insulating base plate. The fuse tube is made of toughened fibers. There is no packing in the tube. When the melt melts, the arc will not be ejected in the tube. Rated current 15-1000, breaking capacity 1.2-12
3).RT and NT filled with closed tubular fuses
Mainly consists of a melting pipe and a base. The melting pipe includes a pipe body, a melt, an indicator, a contact knife, a cover plate and quartz sand. The pipe body is made of ceramics and has a high mechanical strength and heat resistance. The pipe is equipped with a working melt. Body and indicator melt. The melt is usually punched from thin copper sheets into variable cross-sectional shapes, and the middle part is connected with a tin bridge. Generally, the fuse sheet is enclosed in a cage during assembly to increase the contact area between the melt and the quartz sand, thereby improving the breaking of the fuse. The ability to make the tube evenly heated and not easily broken. The fuse indicator is a mechanical signal device that contains copper wire in parallel with the melt. When the circuit is short-circuited, after the melt blows, the current is rapidly transferred to the copper wire and the indicator pops out immediately under the action of the spring, indicating a red signal indicating that the melt melts.
4). Fast fuses, also known as semiconductor device protectors,
Commonly used RS series filled fast fuses, RLS series spiral fast fuses, N GT series fuses for semiconductor device protection
RS0 is mainly used for short-circuit protection of silicon rectifier components and their complete sets
RS3 is mainly used for short circuit protection of thyristors and their complete sets
RLS differs from RS melt material and structure in that it is mainly used for the short-circuit protection of small-capacity silicon rectifying devices and thyristors.
NGT is used for semiconductor protection.
5). Self-resetting fuses
High-power non-linear resistive elements with good current limiting performance, often used in series with low-voltage circuit breakers to increase breaking capacity. Advantages: When the fault is removed, quickly recover, do not need to replace the melt, put into operation again.
The structure of the self-resetting fuse is mainly composed of a current terminal (also called an electrode), a mica glass (filler), an insulating tube, a melt, a piston, an argon gas, and a housing. Among them, the self-resetting fuse housing is generally made of stainless steel, and the stainless steel housing is separated from the internal yttrium oxide ceramic insulating tube by mica glass. The mica glass is both a filler and an insulator. The wiring of the self-resetting fuse, which is often used in series with the circuit breaker, itself is connected in parallel with an additional resistor to suppress the over-voltage occurring during the breaking. During normal operation, the self-resetting fuse is in a low-resistance state, and the parallel resistance flows only a small current. When the line fails, the self-resetting fuse is in a high-impedance state. The parallel resistance can absorb the over-voltage generated by it, and maintain the operating current required by the circuit breaker tripping device to ensure the reliable breaking of the circuit breaker. Therefore, the breaking current of the circuit breaker is actually the limiting current of the self-resetting fuse.