Introduction
The primary function of an electrical circuit breakers is to provide opening and closing the current carrying contacts. Although it is seems to be very simple . But we should remember , that one circuit breaker remains at its closed position for maximum period of its life span. Very occasionally it is required to operate a circuit breaker for opening and closing contact. Hence , circuit breaker operation must be very reliable without any delay or sluggishness. For achieving this reliability the circuit breaker operating mechanism becames more complex than it was first thought.
Opening and closing distance, as well as stroke between contacts and velocity of moving contacts during operation, are the most important parameters to be considered during designing circuit breaker. Contact gap, traveling distance of moving contacts and their velocity are determined by types of arc quenching medium, current and voltage rating of a circuit breaker.
Circuit Breaker opening operation requirement
1. The circuit breaker is desired to be at open position as fast as possible . It is because of limiting contacts erosion and to interrupt faulty current as rapidly as possible. But total travel distance of the moving contact is not determined only by necessity of interruption of faulty current, but rather the contacts gap needed to withstand the normal dielectric stresses and lighting impulse voltage appears across the contacts when the CB is at open position.
2. The need for carrying the continuous current and for withstanding a period of arc in CB, make it necessary to use two sets of contacts in parallel one the primary contact which is always made of high conductive material such as copper and the other is arcing contact , made of arc resistance material such as tungstan or molybdenum , which has much lower conductivity than primary contacts. During opening circuit breaker operation, the primary contacts open before the arcing contact.
3.However, due to the difference in the electrical resistance and the inductor of the electrical paths of the primary and arcing contacts, a finite time is required to attain total current commutation, from main contact to arcing contact.
4. So when the moving contact starts travelling from closed position to open position the contact gap gradually increases and after some time a cricital contact position reaches which indicates the minimum conduct gap required for preventing re- arcing after very next current zero.
Circuit Breaker closing operation requirement
1. The moving contact must travel towards fixed contact at sufficient speed to prevent pre- arcing phenomenon. As the contact gap reduces arcing may start before contacts are closed finally.
2. During closing of contacts, the medium between contacts is replaced, hence sufficient mechanical power to be supplied during this CB operation to compress dielectric medium in the arcing chamber.
3. After hitting fixed contact, the moving contact may bounce back, due to repulsive force which is not at all desirable. Hence sufficient mechanical energy to be supplied to overcome repulsive force due to closing operation on fault.