Circuit Breaker

Basically, It is a mechanical device which is used to make or break the electric Circuit under normal and abnormal condition. It is designed to close or open the contact members which is connected to the electric Circuit which has to be protected. A circuit breaker is very essential in power System to protect the machines, equipment, Transmission lines from fault.

Working Principle of Circuit Breaker :-

The circuit breaker consists of essentially of current carrying contacts known as electrodes. During normal conditions these are closed but, under abnormal situation separates to interrupt the circuit.

Breaking of a circuit means the rapid conversion of predetermined section of circuit from a conductor to insulator i.e. to isolate the faulty section from healthy one in order to ensure the safety measures. When the current carrying contacts/electrodes are separated the arc will produce across the contact (Which contains an ionized gas).

The produced arc may harm the circuit breaker and other equipment, so this arc extinction is very needed and may be extinguish by high Resistance and low resistance methods.

The insulating medium in which the contacts are placed and arc intrupption takes place must have some medium to increase the arc extinction rate so that, arc extinguished with minimum time.

The Insulating medium plays following roles mentioned below :-

• It extinguish the arc produced across the contacts/electrodes when the circuit breaker opens.
• It provides insulation between the contacts and from each contact to earth.

The most insulating fluids or mediums used for circuit breaker

1. Air (at atmospheric Pressure)
2. Compressed Air
3. Oil (which offers hydrogen for arc extinction)
4. Vacuum
5. Sulphur Hexafluoride (SF₆) Gas

These fluids are chosen depends upon the rating, type of circuit breaker and rating of the line for which circuit breaker is to be installed.

Following Gases which have been considered for breakers are :-

• Simple Gases :- Air, Oxygen, Nitrogen, Carbon-Dioxide.
• Electronegative Gases :- Sulphur Hexafluoride, Arcton

Terms related to Circuit Breaker :-

1. Recovery Voltage :- It is normal frequency (50Hz) rms value voltage that appears across the contacts of the circuit breaker after final arc extinction. It is approximately equal to the system voltage.

2. Restriking Voltage :- It is the transient voltage that appears across the contacts at or near current zero during arcing periods.

At zero current, a high frequency transient voltage appears across the contacts and is caused by rapid distribution of energy between the magnetic and electric field associated with plant and transmission lines of the system.

3. Making Capacity :- It is defined as the maximum current capacity of a circuit breaker that can flow during first cycle when the contacts are closing even if the circuit is short circuited or facing the short circuit fault without any damages or . Very high Current ( in kA) flows through the contacts, So the designers of the breakers assumes that there is short circuit and this much of current will flow and contacts have to withstand.

Making Capacity is expressed in Peak Value of current instead of r.m.s. value. The peak value of current (including d.c components) during first cycle of current wave after closure of circuit breaker is known as Making Capacity.

Making Capacity = 2.55 × Symmetrical Breaking Capacity

4. Breaking Capacity :- It is the current (r.m.s) that a Circuit Breaker is capable of breaking at given recovery Voltage and under specified condition (e.g. power factor, rate of rise of restriking Voltage).

Breaking Capacity = √3 × V × I × 10^-6 MVA

5. Normal Current Rating :- As the name suggest It is the r.m.s value of current which the circuit breaker is capable of carrying continuously at it’s rated frequency under specified condition such as temperature.

6. Service Voltage :- It is voltage on which the circuit breaker is made to operate.

Types of Circuit Breaker :-

1. Air Circuit Breaker
   • Plain Air Circuit Breaker
   • Air Blast Circuit Breaker
     1. Axial Blast Type
     2. Radial Blast Type
     3. Cross Blast Type
2. Oil Circuit Breaker
   • Minimum Oil Circuit Breaker
   • Bulk Oil Circuit Breaker
3. Vacuum Circuit Breaker
4. Sulphur Hexafluoride (SF₆) Circuit Breaker
5. Water Type Circuit Breaker