What is the arc characteristic of how arcs are produced?Hazard of arcing and arc extinguishing measures

This article mainly introduces the arc, first introduced the reason and schematic diagram of the arc generation, then introduced the arc characteristics and uses, characteristics, and finally elaborated the arc hazards and arc extinguishing measures, follow the specific Xiaobian together to understand it .

In the contact electric appliance, the process of connecting and breaking the current of the contact is often accompanied by the phenomenon of gas discharge--the generation and extinguishing of the arc, and the arc has certain damage to the electrical apparatus.

Arcs are a form of gas discharge. Gas discharge is divided into self-sustained discharge and non-self-sustained discharge. Arc belongs to arc discharge in gas self-sustained discharge. Tests have shown that when a circuit whose voltage exceeds 10V and current exceeds 100MA is opened or closed in the atmosphere, a very high temperature, extremely bright and conductive gas is generated in the contact gap (or arc gap). Called arc. Due to the high temperature and strong light of the arc, it can be widely used in welding, melting, chemical synthesis, strong light sources and space technology. For contacted electrical appliances, since the arc is mainly generated when the contact breaks the circuit, the high temperature will burn the contacts and the insulation. In severe cases, it may even cause phase short circuits and electrical explosions, causing fire and endangering the safety of personnel and equipment. . Therefore, from the perspective of electrical appliances to study the arc, the purpose is to understand its basic rules, find a way to allow the arc to extinguish as soon as possible in the appliance.

We carefully observe the arc with a certain instrument, and we can see that in addition to the two poles (contacts), it is clearly divided into three regions, namely the near-cathode region, the near-anode region, and the arc column region.

The length of the near-cathode region is approximately equal to the mean free path of the electrons. Under the action of the electric field force, the positive ions move toward the cathode, causing a large amount of positive ions to accumulate in this region and forming a positive space charge layer, so that high electric field strength is formed near the cathode. The positive space charge layer forms a cathode pressure drop, and its value varies with the cathode material and the gaseous medium, but does not change much, and is about 10-20V.

The length of the near anode region is approximately equal to several times that of the near cathode region. Under the action of the electric field force, free electrons move toward the anode. They collect near the anode and are continuously absorbed by the anode to form an electric current. In this area, a large amount of electrons accumulate to form a negative space charge layer, resulting in an anode pressure drop. The value varies with the anode material but does not change much, being slightly less than the cathode voltage drop. Since the length of the near anode region is longer than that of the near cathode region, its electric field intensity is small.

The values ​​of the cathode pressure drop and the anode pressure drop are almost independent of the size of the current and can be considered as constant after the material and medium are determined.

The length of the arc column area is almost the same as the distance between the electrodes. It is the highest temperature and brightest area in the arc. Due to the approximate cylindrical shape in the free state, it is called the arc column area. In this area, the number of positive and negative particles is the same, and the plasma area is called. Because there is no space charge, the entire arc region is similar to a metal conductor. The voltage drop per unit arc length is equal. Its potential gradient E. Also for a constant, the potential gradient is related to the electrode material, current size, type of gas medium, and gas pressure.

The arc is divided into a long arc and a short arc according to its shape. The difference between length and length is generally determined by the ratio of arc length to arc diameter. A long arc is much longer than the arc length. The voltage of the long arc is the sum of the near-pole voltage drop (cathode pressure drop and anode pressure drop) and arc cylinder pressure drop. If the arc length is smaller than the arc diameter, an arc with a very short distance between two poles (for example, a few millimeters) is called a short arc. At this point the thermal effects of the two poles are strong, and the process in the near-polar region plays a major role. The voltage drop of the arc is dominated by the near-pole voltage drop, with almost no change in current.

The arc generation principle diagram is also a simple high-voltage generator circuit. A fixed-frequency pulse width modulation circuit TL494 is used to generate a square-wave signal to control the MOS transistor Q1. The alternating current on Q1 is boosted when the black-white television ignition coil T is connected in series. After 2k~10kV, after boosting, the high-voltage rectifier diode is half-wave rectified by the high-voltage package secondary series, and the output high-frequency high-voltage with DC component (or DC high voltage with high-frequency ripple is also acceptable, both are the same thing) ).

Strong conductivity, energy concentration, high temperature, high brightness, light weight, and easy to change.

The arc can be used as a strong light source such as an arc lamp, an ultraviolet source such as a sun lamp or a strong heat source such as an electric arc furnace.

Arc has a thermal effect.

1, the flame characteristics

In order to make everyone understand the characteristics of the arc more profound, first of all take everyone to recall the flame characteristics of alcohol lamps we have learned. See the figure below, we can simply summarize the characteristics of the flame:

1 The flame is produced by the combustion of alcohol through the wick, and has a certain width;

2 The flame temperature has a certain gradient, the outer flame temperature is highest, and the outer flame is used when heated;

3 The flame is susceptible to the condition of the wick and the influence of the outside air flow, causing jumps to cause instability.

2. Arc characteristics

Combine the characteristics of the flame, the following details the characteristics of the arc.

1 Arc generation and size: The arc is generated by the discharge of the tip of the electrode and has a certain width. The width of the arc is determined by the distance between the two electrodes, the current control technology, etc. The larger the distance between the electrodes, the greater the width of the arc.

2 Arc temperature characteristics: The same with the flame temperature characteristics of the welding machine discharge arc is also a temperature gradient, the wider the arc, the smaller the temperature gradient, but the difference is that the arc of the highest center temperature, the arc edge temperature is lowest, the same In the case of the current value, the larger the electrode spacing, the higher the center temperature.

3 The stability of the arc: the oxidation of the electrode, damage to the tip, and changes in ambient air pressure will affect the stability of the arc.

Severe harm caused by the arc voltage, the temperature of up to several thousand degrees Celsius, light damage to the equipment, but can cause an explosion, resulting in a fire, life-threatening and property safety. Especially in the petroleum and power industries, extra attention is needed. Due to the special nature of the industry, accidents and even casualties are more likely to occur.

In the power industry, switchgears can generate arcs because their temperatures can be as high as several thousand degrees Celsius. They can burn out contacts and even lead to contact welding. If the arc is not extinguished immediately, it may burn the operator, burn the equipment, or even cause a fire. Therefore, electrical appliances with contacts should consider the issue of arc extinguishing. Especially the high-voltage power distribution must pay attention. However, due to a faulty operation with a load, or a foreign object (conductor) inside the switch box, when the switch box is pulled out, the foreign matter momentarily connects the two poles and separates, causing the arc to be generated, resulting in an explosion phenomenon and causing injury. , Burn operators.

In the petrochemical industry, various types of equipment can lead to the generation of electric arcs, coupled with the presence of unpredictable natural factors. Therefore, in the petrochemical industry, special care must be taken to prevent explosions and fires.

Due to the special nature of the industry, the air around the company contains a certain degree of flammable and explosive gases. As long as it meets a discharge phenomenon, it may explode, resulting in a major disaster. In the electric power industry enterprises, arcing is more likely to occur. For example, when the short circuit occurs, the current is small, but because of the ground fault, the arcing may occur at the grounding point; the switch is poorly fabricated, improperly installed, or not maintained timely; and the line is poorly laid; The choice of electrical equipment and wire has not taken appropriate measures according to the environment; animal bites, scratches, etc. cause insulation damage. All of the above situations may cause arcing accidents, so they must not be taken lightly.

The main measures for arc extinguishing are mechanical arcing, narrow slit extinction, and gate arc extinguishing.

1 mechanical arc. When breaking the contacts, the arc length is rapidly increased, so that the electric field intensity of the arc burning is not enough to extinguish the arc per unit length, as shown in FIG. 1 .

2 narrow arc extinction. Relying on the action of a magnetic field, the arc is driven into narrow slits made of arc-resistant material to accelerate cooling of the arc, as shown in FIG. 2 . This kind of arc extinguishing device is mostly used for AC contactor.

3 gate arc extinguishing. When the contacts are broken, the resulting arcs are forced into a plurality of sets of copper plated steel sheets (gates) that are insulated from each other by electrodynamic forces. The arc is divided into groups of short arcs in series.

Figure 1 Mechanical arcing

1-Static contact; 2-moving contact

Figure 2 Narrow arc extinguishing device