User:Amitpandey87

Amitpandey87 14:20, 25 May 2007 (UTC)AMIT PANDEY

CIRCUIT-BREAKERS
IMPORTANT TERMS

1.Arc Voltage : It is the voltage that appears across the    contacts of the CB during the arching period.

2.Restriking Voltage : It is the transient value that appears across the contacts at or near current zero during arching period.

3.Recovery Voltage : It is the normal frequency (50 Hz) r.m.s voltage that appears across the contacts of the CB after final arc extinction. It is approximately equal to the system voltage.

CLASSIFICATION OF CIRCUIT BREAKERS The most general way of classification is on the basis of medium used for arc extinction e.g. oil, air, SF6 or vacuum. •	Oil Circuit Breaker •	Air-blast circuit breakers •	Sulpur hexafluroide circuit breakers •	Vacuum circuit breakers

OIL CIRCUIT BREAKERS

In such CB’s, some insulating oil (e.g. transformer oil) is used as an arc quenching medium.The contacts are opened under oil and an arc is struck between them.The heat of the arc evaporates the surrounding oil and dissociates it into a substantial volume of gaseous hydrogen at high pressure.The hydrogen occupies about 1000 times that of the oil decomposed.The oil therefore pushed off from the arc and an expanding hydrogen gas bubble surrounds the arc region and adjacent portions of the contacts.The arc extinction is facilitated mainly by two process firstly the hydrogen gas has high heat conductivity and cools the arc, thus aiding the de-ionization of the medium between the contacts secondly the gas sets up turbulence in the oil and forces it into the space between contacts, thus eliminating the arching products from the arc path. The result is that arc is extinguished and circuit current is interrupted.