12KV 50HZ Hv Vacuum Circuit Breaker Permanent Magnetic Mechanism Three Phase

Vacuum Arc Stability:

In a 50 Hz ac circuit the current passes through zero value 100 times in a second (i.e., once every 10 ms). It is desirable that the current is interrupted when it is passing through zero value, otherwise overvoltage will be induced due to current chopping. Therefore, for successful arc interruption it is necessary that the arc be stable for half-cycle duration and particularly that it continues to exist at currents approaching zero.

The arc stability is found to depend upon:

(i) The contact material and its vapour pressure and

(ii) Circuit parameters such as voltage, current, capacitance and inductance.

In low current circuits most of the evaporation takes place at discrete points, called the cathode spots, while at higher currents the evaporation takes place from cathode and anode spots. In addition to these sources gas is added to the enclosure of the contacts when it is stripped from other parts of the enclosure because of high temperature and impinging metal vapour.

Vapour pressure and arc stability are found to be interrelated. The higher the vapour pressure at lower temperature, the better is the stability of the arc i.e., the longer the life time of the arc. There are certain metals like Zn, Bi, which provide such characteristics and are better electrode materials for vacuum circuit breakers. Besides the vapour pressure, the thermal conductivity of the metal also affects the current chopping level.

A good heat conducting metal will cool very rapidly and its contact surface temperature will fall. This will reduce the evaporation rate and the arc will be chopped because of insufficient vapour. On the other hand, a bad heat conductor will maintain its temperature and vaporization for a longer time and the arc will be more stable.

Shunting the contacts with different values of capacitance reduces the average life time of the arc. The higher the value of the capacitance, the lower is the average life time of the arc. An inductance in series, on the other hand increases the lifetime of the arc. Similarly, higher the system operating voltage, longer is the duration of the arc. This is because more restoring voltage is available to keep the arc alive.