What is Circuit Breaker?
Circuit Breaker is switch capable of making or breaking the circuit under no-load as well as on-load condition. It can make or break circuit either manually or by remote control. A Circuit Breaker in conjunction with Relay can break the circuit under fault condition.
Operating Principle of Circuit Breaker:
A Circuit Breaker CB consists of two contacts which are called electrodes, one of which remain fixed, called fixed contact and another moving contact. Under normal operating condition, this contact will remain closed to supply power but as soon as fault is sensed by the Relay, trip coil of Circuit Breaker energizes and the moving contact of CB is pulled apart by some mechanism to open the CB.
When contacts of CB are separated under fault condition, an arc is stuck between the fixed and moving contacts. The current is thus able to continue till the arc persists. The production of arc not only delays the current interruption but it also produces huge amount of heat which if exceeds a limit may damage the system or CB itself. Therefore, the design of CB is done in such a way to minimize the arcing period so that
1) Heat produced during arcing may not exceeds the dangerous value.
2) To have fast fault clearing.
It is worth here to mention that a typical Breaker opening and closing time remain around 30-35 ms and 60-70 ms respectively. Notice that CB opening time is less than the closing time to ensure fast fault clearing.
Arcing Phenomenon in Circuit Breaker:
When a short circuit occurs, heavy current flows through the contacts of circuit breaker before they are opened by the protective system. At the instant when the contacts begin to open after getting trip command from the Relay, the contact area decreases rapidly and large fault current causes increased current density and hence rise in temperature. The heat produced in the medium in between the contacts is sufficient enough to ionize the medium. This ionized medium acts as a conductor and arc is stuck in between the contacts of the circuit breaker. It shall be noted here that the potential difference between the fixed and moving contacts is quite small and just enough to maintain the arc. This arc provides a low resistance path to the current and thus due to arcing the current in the circuit remain uninterrupted as long as arcing persists.
During the arcing period the current flowing through the contacts of circuit breaker deepens upon the arc resistance. The greater the arc resistance the smaller will be the current flowing through the contacts of CB. The arcing resistance depends upon the following factors:
Degree of Ionization:
The more the ionization of medium between the contacts, the less will be the arcing resistance.
Length of Arc:
The arc resistance increases as the length of arc increases i.e. as the separation between the contacts of Breaker increases the arcing resistance also increases.
Cross Section of the Arc:
The arcing resistance increases with decrease in the cross sectional area of the arc.
Principle of Arc Extinction:
As we discussed earlier in this post, ionization of medium in between the contacts and potential difference across the contacts are responsible for the production and maintenance of arc. Thus for arc extinction, we can increase the separation between the contacts to such an extent that potential difference across the contacts is not sufficient enough to maintain the arc. But this philosophy is impractical as in EHV (Like 220 kV, 400 kV, 765 kV etc.) system; the separation between the contacts to extinguish the arc will be many meters which is not practically achievable.
Another way for extinction of arc is to demonize the medium in between the contacts. If the arc path is demonized the arc extinction will definitely be facilitated. This may be achieved by cooling the arc or by quickly removing the ionized particles from the space in between the contacts. This principle of arc extinction is used in all modern Circuit Breakers.