Circuit Breakers : Concepts

3. Vacuum Circuit Breaker :

Here the contacts are drawn apart in a chamber from which air has been evacuated. The electric arc is essentially an electric conductor made up of ionized air. Thus, if there is no air, theoretically the arc cannot form. In practice, however, a perfect vacuum is not likely to be obtained. The small residual amount of air that may exist permits only a small arc to be formed and one of only a very short direction. The same vacuum however, will not dissipate the heat generated as readily as other insulating media. This type of breaker has certain advantages in terms of its size and simplicity.

4. Sulphur hexafluoride (SF₆) breaker :

This type of breaker is similar to the vacuum types of breaker except that the vacuum is replaced by an inert non-toxic, odorless gas-sulphur hexafluoride (SF₆). This gas, extinguished the arc so rapidly as almost to prevent its formation. It is also excellent heat-dissipating characteristics, and its dielectric strength is very much greater than that of oil.

The breakers are constructed to modules capable of operation at voltages from 34.4 kV at gas pressure of 45 psi to 362 kV at 240 psi. By connecting two or three such modules in series, breaker capable of operating at 800 kV at 240 psi can be constructed with two-three-cycle interrupting time. Features of sulphur hexafluoride interrupting module.

Terms associated with circuit breakers:

1. Arcing time :

Time between separation of circuit breaker contacts and Final current zero.

2. Dead time :

Time between final current zero of first opening and contact touch during subsequent re closing.

3. De ionizing time :

Time for de ionizing the arc space after opening of circuit breaker.

4. Fault clearing time :

Time between occurrence of fault and closing of tripping contacts or energizing of shunt trip release.

5. Opening time :

Time between energizing of shunt trip release and opening of circuit breaker contacts.

6. Rated Short circuit breaking circuit :

The rated short circuit breaker current of a circuit breaker is the highest value of short circuit which a circuit breaker is capable of breaking under specified conditions of recovery voltage and power frequency recovery voltage.

7. Relay time :

Time between occurrence.of fault and closure of relay contacts.

from ur's -- Bellapuri saikumar
(www.facebook.com/saikumar544)

Monday, 15 June 2015