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6/19/2015 CircuitBreakers:Operationofcircuitbreakersandtypesofcircuitbreakers:OilCircuitbreakerandaircircuitbreaker

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Circuit Breaker Theory

Circuit Breaker MCQ's (/circuit-breakers/circuit-breakers-mcqs.htm)

Circuit Breaker True - False (/circuit-breakers/circuit-breakers-true-false.htm)

Circuit Breaker Fill in the blanks (/circuit-breakers/circuit-breakers-fill-blanks.htm)

Circuit Breakers- TheoryPage 1 (/circuit-breakers/circuit-breakers.htm)Circuit Breakers- TheoryPage 2 (/circuit-breakers/circuit-breakers-2.htm)Circuit Breakers- TheoryPage 3 (/circuit-breakers/circuit-breakers-3.htm)



Circuit Breakers : Concepts

Circuit breakers:These are heavy-duty devices capable of interrupting comparatively large electric currents safely. They are designed not onlyto carry and interrupt the normal load currents flowing in circuit but also to interrupt any abnormally high current that mayflow under fault conditions such as short circuit. Circuit breakers are constructed mechanically strong enough to withstandthe forces set-up by enormous short-circuit currents.

The ability of a breaker to withstand short circuit forces is expressed in Volt-Amperes, i.e., product of nominal circuit voltageand the short-circuit current for which the breaker is designed.

Specifications of circuit breakers:

1. Operating voltage of the circuit :It determines the insulating requirements.

2. Normal operating or maximum load current :It determines requirements of the normal or load-carrying parts.

3. Maximum abnormal or fault current that must be interrupted :It determines the mechanical requirements of the breaker itself and of its supporting structure.

Commonly used insulations are:

l. Oil

2. Vacuum

3. Inert gas e.g. sulphur hexafluoride.

Principle of operation of Circuit Breakers :



The circuit breaker actually makes a physical separation in the current-carrying or conducting element by inserting aninsulating medium sufficient to prevent current from continuing to flow. In so doing, the persistence of an arc across the gapis prevented. The circuit is usually opened by drawing out an arc between contacts until the arc can no longer support itself.The arc formed when the contacts of a circuit breaker move apart to interrupt of a circuit is a conductor made up of ionizedparticles of the insulating materials.

Whenever voltages and currents are large other forms of insulation are used in place or air to extinguish the arc as quickly aspossible.

Whenever fault occurs in the circuit breaker, relay connected to the current transformer CT actuates and closes its contacts.Current flows from the battery in the trip circuit As soon as the trip coil of the circuit breaker gets energized the circuit breakeroperating mechanism is actuated and it operates for the opening mechanism.

Thus relay forms a vital part of a circuit breaker. Various types of relays used are given below:

Relay Operation1. Differential relay Responds to vector difference between two or more similar electrical quantities.2. Impedance relay Operates when the impedance between relay point and fault point is below a specified value.3. Over current relay Responds to increase in current. The relay operates when the current exceeds a present level4. Instantaneous relay A quick operating relay (less than 0.2-second operating times).5. Static relay A relay without moving parts. In this the measurement is performed by a stationary circuit.

Types of Circuit Breakers:

Type Medium Range1. Air break C.B. Air at atmospheric pressure Low voltage Up to l000 V2. Tank type oil circuit breaker Dielectric oil Up to 33 kV

3. Minimum oil circuit breaker Dielectric oil36 kV, 1500 MVA

132 kV, 3000 MVA

4. Air blast C.B. Compressed air (pressure 20 to 30 atmospheres)132 kV, 220 kV

400 kV, 760 kV

5. SF C.B. SF gas132 k V, 220 kV

400 kV, 760 kV.6 6



Circuit Breaker > Theory Page 1 (/circuit-breakers/circuit-breakers.htm)

Circuit Breaker > Theory Page 3 (/circuit-breakers/circuit-breakers-3.htm)

6. Vacuum C.B. Vacuum 11 kV. 33 kV7. High voltage direct current C.B. Vacuum or SF 500 kV DC.

1. Air circuit breakers :An air circuit breaker employs air as the interrupting insulation medium. Of all the insulating media mentioned, air is the mosteasily ionized and, hence, arcs formed in air tend to be serve and persistent.

The switching elements for an air current, breaker, consists of main and auxiliary contacts. The auxiliary contacts open beforethe main contacts do, and the arc is drawn on them, thereby avoiding serve pitting of the main contacts.

2. Oil Circuit Breakers :

Oil circuit breakers have their contacts immersed in insulating oil. They are used to open and close high-voltage circuitscarrying relatively large currents in situations where air circuit breakers would be impractical because of the danger of theexposed arcs that might be formed. When the contacts are drawn apart, the oil covering them tends to quench the arc by itscooling effect and by the gases thereby generated, which tend to "blow out" the arc. At the instant the contacts part, the arcformed at each contact not only displaces the oil but decomposes it, creating gas and a carbon residue. If these carbonparticles were to remain in place, as a conductor they would tend to sustain the arc formed. However, the violence of the gasand the resulting turbulence of the oil disperse these particles and they eventually settle to the bottom of the tank. Theinsulating oil normally used as a dielectric strength of around 30 kV per one tenth of an inch (compared to a similar value of 1kV for air). Oil is also an effective cooling medium.

Discuss Electric Circuit >Theory : Page 2 !!!

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Circuit Breaker Theory

Circuit Breaker MCQ's (/circuit-breakers/circuit-breakers-mcqs.htm)

Circuit Breaker True - False (/circuit-breakers/circuit-breakers-true-false.htm)

Circuit Breaker Fill in the blanks (/circuit-breakers/circuit-breakers-fill-blanks.htm)

Circuit Breakers- TheoryPage 1 (/circuit-breakers/circuit-breakers.htm)Circuit Breakers- TheoryPage 2 (/circuit-breakers/circuit-breakers-2.htm)Circuit Breakers- TheoryPage 3 (/circuit-breakers/circuit-breakers-3.htm)

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