The principle and extinguishing method of arc generation in electronic devices such as fuses

What is an electric arc?

When the fuse in the circuit is blown, when the voltage and current reach a certain value, the fuse link has just melted and disconnected, and an arc will occur between the just separated fuse links, which is called an arc. It is due to the strong electric field, which ionizes the gas and causes the current to pass through the normally insulating medium. The use of electric arcs can have many applications, such as welding, electric arc furnaces in steel plants, etc. But if the arc is generated in an uncontrolled state, it will cause damage to power transmission, distribution, and electronic equipment. So we must understand and control the arc.

Composition of electric arc

1. Arc Column Zone

The arc column region is electrically neutral and composed of molecules, atoms, excited atoms, positive ions, negative ions, and electrons. Among them, positively charged ions are almost equal to negatively charged ions, so it is also called plasma. Charged particles move directionally in plasma without consuming much energy, which is why they can transmit high currents under low voltage conditions. The main charged particles that transmit current are electrons, accounting for approximately 99.9% of the total number of charged particles, with the rest being positive ions. Due to the extremely short length of the cathode and anode regions, the length of the arc column region can be considered as the arc length. The electric field strength in the arc column region is relatively low, usually only 5-10V/cm.

2. Cathode area

The cathode is considered the source of electrons. It provides 99.9% of charged particles (electrons) to the arc column. The ability of the cathode to emit electrons has a significant impact on the stability of the arc. The length of the cathode region is 10-5-10-6cm. If the cathode voltage drop is 10V, the electric field strength of the cathode region is 106-107V/cm.

3. Anode area

The anode region is mainly responsible for accepting electrons, but it should also provide 0.1% of charged particles (positive ions) to the arc column. The length of the anode region is usually 10-2-10-3cm, so the electric field strength of the anode region is 103-104V/cm. Due to the significant impact of anode material and welding current on the voltage drop in the anode region, it can vary between 0 and 10V. For example, when the current density is high and the anode temperature is high, causing the anode material to evaporate, the anode voltage drop will decrease, even to 0V.

Characteristics of electric Arcs

1. The arc voltage required to maintain stable combustion of the arc is very low, and the voltage of a 1cm DC arc column in the atmosphere is only 10-50V.

2. A large current can pass through the arc, ranging from a few amperes to several thousand amperes.

3. The arc has a high temperature, and the temperature of the arc column is uneven. The center temperature is the highest, reaching 6000-10000 degrees, while the temperature decreases away from the center.

4. Electric arcs can emit strong light. The wavelength of light radiation from the arc is (1.7-50) × 10-7m. It includes three parts: infrared, visible light, and ultraviolet light

Classification of electric arcs

1. According to the type of current, it can be divided into AC arc, DC arc, and pulse arc.

2. According to the state of the arc, it can be divided into free arc and compressed arc (such as plasma arc).

3. According to the electrode material, it can be divided into: melting electrode arc and non melting electrode arc.

The hazards of electric arcs

1. The presence of arcs prolongs the time for switchgear to disconnect faulty circuits and increases the probability of short circuits in the power system.

2. The high temperature generated by the arc melts and evaporates the contact surface, burning out the insulation material. Oil filled electrical equipment may also pose risks such as fire and explosion.

3. Due to the fact that electric arcs can move under the action of electric and thermal forces. It is easy to cause arcing short circuits and injuries, leading to the escalation of accidents.

The principle of six extinguishing arcs

1. Arc temperature

The arc is maintained by thermal ionization, and lowering the temperature of the arc can weaken thermal ionization and reduce the generation of new charged ions. At the same time, it also reduces the velocity of charged particles and enhances the composite effect. By rapidly elongating the arc, blowing the arc with gas or oil, or bringing the arc into contact with the surface of a solid medium, the arc temperature can be reduced.

2. Characteristics of the medium

The characteristics of the medium in which the arc burns largely determine the strength of dissociation in the arc. Including thermal conductivity, heat capacity, thermal free temperature, dielectric strength, etc.

3. Pressure of gas medium

The pressure of the gas medium has a significant impact on the dissociation of the arc. Because the higher the pressure of the gas, the higher the concentration of particles in the arc, the smaller the distance between particles, the stronger the composite effect, and the easier it is for the arc to extinguish. In a high vacuum environment, the probability of collision is reduced, which suppresses collision dissociation, while the diffusion effect is strong.

4. Contact Material

The contact material also affects the process of detachment. When using high-temperature resistant metals with high melting points, good thermal conductivity, and large heat capacity as contacts, it reduces the emission of hot electrons and metal vapor in the arc, which is beneficial for arc extinguishing.

The method of extinguishing the arc

1. Use the medium to extinguish the arc

The detachment of the arc gap largely depends on the characteristics of the extinguishing medium around the arc. Sulfur hexafluoride gas is an excellent arc extinguishing medium with strong electronegativity. It can quickly adsorb electrons and form stable negative ions, which is conducive to recombination and ionization. Its arc extinguishing ability is about 100 times stronger than air; Vacuum (pressure below 0.013Pa) is also a good medium for arc extinguishing. Due to the small number of neutral particles in vacuum, it is not easy to collide and dissociate, and vacuum is conducive to diffusion and dissociation. Its arc extinguishing ability is about 15 times stronger than air.

2. Use gas or oil to blow the arc

Blowing an arc causes the diffusion and cooling recombination of charged particles in the arc gap. In high-voltage circuit breakers, various forms of arc extinguishing chamber structures are used to generate enormous pressure from gas or oil and forcefully blow it towards the arc gap. There are two main ways to blow an arc: vertical blowing and horizontal blowing. Vertical blowing is the blowing direction parallel to the arc, which causes the arc to become thinner; Horizontal blowing is the blowing direction perpendicular to the arc, which elongates and cuts off the arc.

3. Use special metal materials as arc extinguishing contacts

Using high-temperature resistant metals with high melting points, thermal conductivity, and large heat capacity as contact materials can reduce the emission of hot electrons and metal vapor in electric arcs, thus achieving the effect of suppressing ionization; The contact material used simultaneously also requires high resistance to arc and welding. Common contact materials include copper tungsten alloy, silver tungsten alloy, etc.

4. Electromagnetic arc blowing

The phenomenon of electric arc moving under the action of electromagnetic force is called electromagnetic blowing arc. Due to the movement of the arc in the surrounding medium, it has the same effect as air blowing, thus achieving the purpose of extinguishing the arc. This arc extinguishing method is more widely used in low-voltage switchgear.

5. Make the arc move in the narrow slit of the solid medium

This type of arc extinguishing method is also known as slit arc extinguishing. Due to the movement of the arc in the narrow slit of the medium, on the one hand, it is cooled, which enhances the ionization effect; On the other hand, the arc is elongated, the arc diameter is reduced, the arc resistance increases, and the arc is extinguished.

6. Separate the long arc into short arcs

When the arc passes through a row of metal grids perpendicular to it, the long arc is divided into several short arcs; The voltage drop of short arcs mainly falls in the anode and cathode regions. If the number of grids is sufficient to ensure that the sum of the minimum voltage drops required to maintain arc combustion in each segment is greater than the applied voltage, the arc will extinguish on its own. In addition, after the AC current crosses zero, due to the near cathode effect, the dielectric strength of each arc gap suddenly increases to 150-250V. By using multiple arc gaps in series, a higher dielectric strength can be obtained, so that the arc will not reignite after being extinguished at zero crossing.

7. Adopt multi fracture arc extinguishing

Each phase of a high-voltage circuit breaker is connected in series with two or more breaks, which reduces the voltage borne by each break and doubles the contact breaking speed, causing the arc to quickly elongate and benefiting arc extinguishing.

8. Improve the separation speed of circuit breaker contacts

Improved the speed of elongating the arc, which is beneficial for arc cooling, recombination, and diffusion.