Circuit Protection and Polyphase Motors Assignment

Circuit Protection and Polyphase Motors - Assignment Example These techniques result in a high arc voltage to control over current and voltage during a fault in operation. Types of Air Blast Circuit Breakers: The Air blast circuit breakers can be divided into the following three types: (a). Axial Blast ABCB (b). Axial Blast ABCB with Side Moving Contact (c). Cross Blast ABCB Construction: (a). Axial Blast ABCB The axial blast air ABCB consists of two contacts; one is fixed and the other is movable. The fixed contact has an arcing chamber with a spring loading mechanism, and a nozzle orifice over which the movable contact rests under the normal closed condition. In case a fault arises, a high Figure 1.Axial Blast ABCB (Courtesy:www.electrical4u.com) air pressure in the arcing chamber causes the spring to deform. This produces an effect of lengthening and cooling of the air column in the chamber which pushes out the movable contact. As a result of these changes an arc voltage is produced which is much higher than the system voltage. The disparit y in voltage levels results in the quenching of the arc produced. (b). Axial Blast ABCB with side moving contact: Figure 2. ... As a result, an arc is drawn between the fixed and movable contact, which is passed over to the arcing electrode. This causes the surge to be quenched. (c). Cross Blast ABCB Figure 3. Cross Blast ABCB (Courtesy:www.electrical4u.com) The cross blast ABCB has a fixed blast pipe. The moving contact’s movement is perpendicular to the direction of the air blast from the blast pipe. An exhaust chamber having arc splitters is fitted on the same alignment as the blast pipe. When the movable contact is detached from the fixed contact an arc is established. High pressure air coming from the blast pipe enters the exhaust chamber after passing through the contact breaker, forcefully taking the arc into the exhaust chamber. This results in the quenching of the arc. 1. b. OIL-FILLED CIRCUIT BREAKER Working Principle: The oil-filled circuit breaker is one of the oldest types of circuit breakers. It employs mineral oil as the insulating medium to quench the surging arc. The fixed as well as t he movable contacts are immersed in oil; as a result the arc forms a bubble in the oil. The energy of the arc is utilized to decompose the oil into hydrogen gas. As a result arc quenching is obtained. Types of Oil Circuit Breakers: Oil circuit breakers can be broadly classified into two categories: (a). Bulk Oil Circuit Breaker (b). Minimum Oil Circuit Breaker (a). Bulk Oil Circuit Breaker Construction: Figure 4. Conceptual view of the Bulk Oil Circuit Breaker. (Courtesy:www.electrical4u.com) The bulk oil circuit breakers employ transformer insulating oil as the insulating and arc quenching medium. The current carrying contacts (fixed and movable) and the earthed parts of the circuit breaker are placed in a large quantity of oil in a closed tank or vessel. The oil