Vacuum Arc or Arc in Vacuum
As there is no such media the arc in vacuum circuit breaker differs from general arc in circuit breaker. In vacuum arc the electrons, ions and atoms are all derived from the electrodes itself. The absolute vacuum is not practically possible to create so there are some gases in practical vacuum chamber but the gas pressure here is so low that it does not have any significant role in conduction process during arc. In this sense the vacuum arc is therefore really a metal vapour discharge. The vacuum arc can be divided into two main regions, the cathode region and the plasma region.
Cathode Region of Arc PlasmaThe vapour necessary to sustain vacuum arc comes mainly from the cathode spots. Each spot carries a mean current, dependent on the cathode material, which is about 100 A for copper. The current density at the spots is estimated to be 1010 - 1011 A/m2, depending on the cathode material. The cathode spots move on the cathode surface. At higher current the numbers of cathodes spots is increased due to repulsion the motion of parallel spots and their movements become more random. Whenever the current carrying contacts open in a circuit breaker, cathode spots are formed depending upon the current flowing through the contacts. At high current multiple numbers of cathode spots formed which constitute the main source of vapour for the arc in vacuum circuit breaker. The cathode surface is normally not perfectly smooth and may have many micro projections on the surface.
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When current carrying contacts are being separated in a vacuum circuit breaker the current flowing in the circuit will be concentrated at those projections as they form the last point of contacts. Due to their small area of contact the projections are sufficiently heated up and they suffer explosive evaporation and supply the vapour for formation of arc in vacuum circuit breaker. The vapour which has high density at the cathode spot, expands into the vacuum and perhaps at a distance of 10 mm from the cathode. The an electron traversing the inter electrode gap experiences condition of high pressure near the cathode where the mean free path is quite less than that of low pressure in the plasma where it is in the order of 10 mm.
At low currents, the voltage drop in the plasma region of low current is negligible. The voltage gradient is less than 0.01 V/mm. At high current, the gradient may be increased up to a few V/mm.
Stability of Vacuum ArcThe power frequency current passes through the contacts in circuit breaker, crosses current zero point 100 times in a second. It is always desirable to interrupt the current during it passes the zero value otherwise there will be current chopping effect which may causes switching over voltage in the system. Therefore, it is necessary to interrupt the arc as long as it is stable for a half cycle duration particularly it should continue to exist when the current approaches to zero.
The stability of arc in vacuum circuit breaker depends upon the contact materials, pressure of metal vapor and circuit parameters such as voltage, current, inductor and capacitance. It is observe that higher vapour pressure in low temperature is better stability of arc. Some metals like Zn, Bi also show better stability of vacuum arc. Like vapour pressure thermal conductivity of contact material is also a major factor regarding stability of vacuum arc in circuit breaker. If the contact metal is good conductor of heat, the contact surface temperature will fall in faster rate thus metal vapour will be condensed fast hence due to the lack of vapor the vacuum arc will be interrupted. But if the metal used for circuit breaker contacts is bad conductor of heat, the metal vapour will not condense fast and the arc continues thus vacuum arc in circuit breaker becomes stable. For successful and safe current interruption in vacuum circuit breaker, both arc extinction at proper point of time and the stable arc are required. It is observed that the metal having high boiling and melting point gives low vapour in high temperature but in the same time it becomes poor conductor. Again the metal having low melting and boiling points gives more vapour at high temperature and in the same time it becomes good conductor. Therefore, to combine these contradictory properties in one single material, alloys of two or more metals or a metal and nonmetal have to be made. Some example of alloys used as the materials to make vacuum circuit breaker contacts are copper-bismuth, silver-lead, copper-lead etc.