Circuit Breakers
Circuit breaker is an equipment, which can make or break a circuit manually or automatically under all conditions i.e. no load, full load and short circuit conditions.
OPERATING PRINCIPLE
It consists of fixed and moving contacts which remain closed under normal condition, when fault occur the trip coil are energized and the moving contacts are pulled apart. When contacts get separated an arc will be struck between the contacts, which will not only delay current interruption process but also generate enormous heat, which may cause damage to the system or to the circuit breaker itself. So the main problem with circuit breaker is to extinguish the arc within the shortest possible time.
ARC PHENOMENON
When fault occurs current flow through contacts before they are being separated. At the instant when contact separates, the contact area decreases and the large fault current cause increased current density and hence rise in temperature occur due to which the surrounding air or oil get ionised. They acts as conductor and an arc is struck between them. The potential difference between contacts is sufficient to maintain the arc. The arc provides a low resistance path and current flows till the arc persists.
The current flow depends on the arc resistance. As the resistance increases current flow between the contacts decreases. Arc resistance depends on:
(a) Degree of ionisation – arc resistance increases with decrease in number of ionised particles.
(b) Length of arc – arc resistance increases with length of the arc i.e. separation of contacts.
(c) Cross-section of arc – arc resistance increases with decrease in the area of x-section of the arc.
FACTORS RESPONSIBLE FOR MAINTENANCE OF ARC
(a) Potential difference between contacts – if contacts have small separation the potential difference between them is sufficient to maintain to maintain the arc. So in order to extinguish the arc the separation can be increased to such a distance that the potential difference becomes inadequate to maintain the arc.
(b) Ionised particles between contacts – the ionised particles between the contacts tend to maintain the arc. If the arc path can be deionised by cooling or removing the ionized particles arc extinction can be facilitated.
Written by arjun on October 8th, 2008 with
no comments.
Read more articles on circuit breakers and Power System and Switchgear&Protection.