This document discusses the selection of circuit breakers. It begins by defining a circuit breaker as a protective device that is used to automatically open the faulty part of a power system during a fault. There are two main factors considered when selecting a circuit breaker: 1) its normal working power level and fault level ratings, which are specified by the rated interrupting current or MVA, and 2) its momentary current and speed ratings. The momentary current rating must be higher than the maximum current during fault conditions, while the speed rating depends on transient fault currents and specified cycles. Multiplying factors are used to determine the circuit breaker's short circuit interrupting current from fault analysis calculations.

1. SHROFF S.R. ROTARY INSTITUTE OF CHEMICAL TECHNOLOGY
PRINCIPAL SUPPORTER & SPONSOR- UPL LTD./ SHROFF FAMILY
MANAGED BY ANKLESHWAR ROTARY EDUCATION SOCIETY
APPROVED BY AICTE, NEW DELHI, GOVT. OF GUJARAT & GTU AFFILIATED
Guided By: Mr. Ankur K. Gheewala
Assistant Professor,
Department of Electrical Engineering,
Shroff S. R. Rotary Institute of Chemical Technology, Vataria, Ankleshwar
Enrollment No. Name of Student
150990109011 Ravirajsinh Solanki
150990109012 Ashutosh Singh
Name of Course Electrical Power System-II
Course code 2160908
Name of Topic Selection Of Circuit Breaker
Group No. 5

2. CONTENT
•What Is Circuit Breaker?
•Selection Of Circuit Breaker

3. What Is Circuit Breaker?
• The Circuit Breakers Are Protective Devices Which Are Used
In Power System To Automatically Open The Faulty Part Of
The System In The Event Of A Fault.
• In Normal WorkIng Condition They Can Be Used As A Switch.

4. Hence the two functions of circuit breakers are
(i) to act as switch for normal load conditions.
(ii) to automatically isolate or open the faulty part in the event
of a fault.
The circuit breakers are normally used in power system at
places where the power level is very high.
They are used in high voltage transmission lines,
substations,
generating stations and for heavy loads in industries.

5. Since The Circuit Breakers Are Employed In Places
Where The Power Level Is High . Whenever Its Contacts
Open It Has To Interrupt Heavy Currents Both During
Load Conditions And Faulty Conditions.
Since The Power System Is Predominantly Inductive
In
Nature, The Interruption Of Current When The Circuit
Breaker Open Its Contact Is Associated With Large
Voltage Induced Across Its Contacts Which Inturn
Results In Sparking At The Contacts.
Hence In Circuit Breakers The Amount Of Current It
Has To Intrupt Is An Important Criteria.

6. SELECTION OF CIRCUIT
BREAKER

7. The Circuit Breaker For A Particular Application (Or Load) Is
Selected Based On The Following Ratings.
I. Normal Working Power Level Specified As Rated
Interrupting Current Or Rated
Interrupting Kva.
2.The Fault Level Specified As Either The Rated Short Circuit
Interrupting Current Or
Rated Short Circuit Interrupting MVA
3. Momentary Current Rating.
4. Normal Working Voltage.
5. Speed Of Circuit Breaker.

8. The speed of the circuit breaker is defined as the time
between the occurrence of the fault to the extinctiond of the
arc (i.e., when contacts are open).
It is normally specified in cycles of supply frequency.
The standard speed of circuit breakers are 8, 5, 3 or 11/2
cycles

9. The Momentary Current Rating Is The Maximum
Current That May Flow Through The Circuit Breaker For
A Small Duration Of Time.
It Is The Current That May Flow During Subtransient
Period Of Fault Condition.
In Fault Analysis The Subtransient Fault Current
Calcutated Using Subtransient Circuit Model Is The
Symmetrical Subtransient Current.
It Is Then Multiplied By A Factor Of "1.6' To Get The
Maximum Momentary Current During Fault. The Circuit
Breakers Are Selected Such That Its Momentary Current
Rating Is Less Than The Calculated Value.

10. Normally The Circuit Breaker Will Open Its
Contacts In The Transient Period And So The Short
Circuit Interrupting Current Rating Depends On
Transient Period Current.
 In Fault Analysis The Transient Fault Current
Calculated Using Transient Circuit Model Is The
Symmetrical Transient Fault Current.
It Is Then Multiplied By A Factor 1.0d To 1.5 To
Get The Maximum Interrupting Current.
The Circuit Breaker Is Selected Such That Its
Short Circuit Interrupting Current Rating Is Less
Than The Calculated Value.

11. The Multiplying Factor For Various Speeds Of Circuit
Breakers Are Shown In Table

12. . The Short Circuit Interrupting MVA Can Be Determined From
Pre-fault Voltage And Short-circuit Interrupting Current As
Follows:
Where |Vpfl| = Magnitude Of Prefault Line Voltage At The Fault
Point In kV. |Ifl|=Magnitude Of Line Value Of Short Circuit
Interrupting Current At The Fault In Ka.

13. THANK YOU