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Substation Earthing System Design

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Circuit Breaker - Operating Principle and Arcing Phenomenon

How Circuit Breaker Works?

A circuit breaker essentially consists of fixed and moving contacts, called electrodes. Under normal operating conditions, these contacts remain closed and will not open automatically until and unless the system becomes faulty. 

The contacts can be opened manually or by remote control whenever desired. When a fault occurs on any part of the system, the trip coils of the breaker get energised and the moving contacts are pulled apart by some mechanism, thus opening the circuit.


Operating principle of Circuit Breaker

When the contacts of a circuit breaker are separated under fault conditions, an arc is struck between them. The current is thus able to continue until the discharge ceases. The production of arc not only delays the current interruption process but it also generates enormous heat which may cause damage to the system or to the breaker itself. Therefore, the main problem in a circuit breaker is to extinguish the arc within the shortest possible time so that heat generated by it may not reach a dangerous value. 

Arc Phenomenon in Circuit Breaker

When a short-circuit occurs, a heavy current flows through the contacts of the circuit breaker before they are opened by the protective system. At the instant when the contacts begin to separate the contact area decreases rapidly and large fault current causes increased current density and hence rise in temperature. 

The heat produced in the medium between contacts (usually the medium is oil or air) is sufficient to ionise the air or vapourise and ionise the oil. The ionised air or vapour ,acts as conductor and an arc is struck between the contacts. The potential difference between the contacts is quite small and is just sufficient to maintain the arc. The arc provides a low resistance path and consequently the current in the circuit remains uninterrupted so long as the arc persists. 

During the arcing period, the current flowing between the contacts depends upon the arc resistance. The greater the arc resistance, the smaller the current that flows between the contacts. The arc resistance depends upon the following factors:
  1. Degree of ionisation - the arc resistance increases with the decrease in the number of ionised particles between the contacts.
  2. Length of the arc - the arc resistance increases with the length of the arc i.e. seperation of contacts.
  3. Cross section of arc - the arc resistance increase with the decrease in the area of cross section of the arc.