In a vacuum arc-suppression device, the contacts diverge in a negative pressure environment where the air density is very low. For exapmle, vacuum interrupter of Tavrida Electric contain high vacuum (10-6 Pa). The free path of a molecule reaches 50 m, and the free path of an electron reaches 300 m. Under such conditions, the electrical breakdown between the electrodes is difficult due to the absence of charge carriers. The breakdown voltage of a 1 mm long gap in vacuum reaches 100 kV. The process of combustion and extinguishing of the arc in a vacuum at alternating current is as follows.
When the contacts open, the contact pressure continuously decreases and the transient resistance of the contacts increases and tends to infinity at a pressure equal to zero. Even at low currents at the moment of opening the contacts because of the release of large amounts of heat contact material melts and a liquid metal bridge is formed, which under the influence of high temperature heats up and evaporates. When the bridge breaks, an arc ignites, which burns in the metal vapor environment of the electrodes. The vacuum arc is characterized by a small voltage drop of 20-30 V. Almost 10 µs after the zero current has passed between the contacts, the electrical strength of the vacuum is restored. This is caused by a very fast diffusion of charges due to the large difference in particle densities in the arc and the surrounding vacuum.
A great advantage of this DU is the high rate of recovery of the electrical strength of the gap. Vacuum circuit breakers are currently the most efficient and durable. Their service life without revision reaches 25 years.The service life of the vacuum circuit breaker contacts is mainly due to the following. The electrons generated by ionization move with tremendous velocity toward the anode. As a result of the bombardment of the anode by electrons, positive ions are produced. The liberated positive ions move to the cathode and destroy it.
It should be noted that high electric field strengths at small contact distances also contribute to the formation of an arc in a vacuum due to auto-electronic emission. Degasification of the contacts is important for the normal operation of the vacuum circuit breaker, since the gases adsorbed by them are released when heated and degrade the vacuum. In order to remove the gases from the contacts, they are heated to a red glow for several hours.
The vacuum arc is characterized by current breaking (cutting off) when approaching the zero value. This is explained by the fact that as the current decreases, the vapor pressure of the metal drops. As a result, the arc becomes unstable and goes out.
A sharp decrease in current can cause overvoltages that are dangerous to the equipment being switched off. The shear current depends on both the parameters of the circuit to be disconnected and the properties of the contact material. Tungsten is weld-resistant, has a high melting point, and is wear-resistant. However, with tungsten contacts, shear current and overvoltage values are very high because tungsten vapors create low pressure. Overvoltages in copper contacts are 2.5 times lower, but they are more susceptible to welding and wear. These contradictions are eliminated today by the development and application of special metal-ceramics.
When the contacts open, the contact pressure continuously decreases and the transient resistance of the contacts increases and tends to infinity at a pressure equal to zero. Even at low currents at the moment of opening the contacts because of the release of large amounts of heat contact material melts and a liquid metal bridge is formed, which under the influence of high temperature heats up and evaporates. When the bridge breaks, an arc ignites, which burns in the metal vapor environment of the electrodes. The vacuum arc is characterized by a small voltage drop of 20-30 V. Almost 10 µs after the zero current has passed between the contacts, the electrical strength of the vacuum is restored. This is caused by a very fast diffusion of charges due to the large difference in particle densities in the arc and the surrounding vacuum.
A great advantage of this DU is the high rate of recovery of the electrical strength of the gap. Vacuum circuit breakers are currently the most efficient and durable. Their service life without revision reaches 25 years.The service life of the vacuum circuit breaker contacts is mainly due to the following. The electrons generated by ionization move with tremendous velocity toward the anode. As a result of the bombardment of the anode by electrons, positive ions are produced. The liberated positive ions move to the cathode and destroy it.
It should be noted that high electric field strengths at small contact distances also contribute to the formation of an arc in a vacuum due to auto-electronic emission. Degasification of the contacts is important for the normal operation of the vacuum circuit breaker, since the gases adsorbed by them are released when heated and degrade the vacuum. In order to remove the gases from the contacts, they are heated to a red glow for several hours.
The vacuum arc is characterized by current breaking (cutting off) when approaching the zero value. This is explained by the fact that as the current decreases, the vapor pressure of the metal drops. As a result, the arc becomes unstable and goes out.
A sharp decrease in current can cause overvoltages that are dangerous to the equipment being switched off. The shear current depends on both the parameters of the circuit to be disconnected and the properties of the contact material. Tungsten is weld-resistant, has a high melting point, and is wear-resistant. However, with tungsten contacts, shear current and overvoltage values are very high because tungsten vapors create low pressure. Overvoltages in copper contacts are 2.5 times lower, but they are more susceptible to welding and wear. These contradictions are eliminated today by the development and application of special metal-ceramics.
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