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
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.
|Rated insulation level||1min Power frequency withstand voltage||KV||42(fracture 48)|
|Peak Lightning lmpulse Withstand voltage||KV||75(fracture 84)|
|Rated Breaking Current||KA||20||25||31.5||40|
|Rated Current||A||630||630||1250 1600||1250 1600|
|1250||1250||2000 2500||2000 2500|
|Rated short time withstand current||KA|
|Rated peak withstand current||KA|
|Rated short circuit breaking current||KA|
|Rated continous time under short circuit current||S||4|
|Secondary power frequncy withstand voltage||V||2000V|