1. High Voltage Power Circuit Breakers
Lesson 1
Basic Principles

2. Circuit breakers Purpose
The main purpose of a circuit breaker is to:
Switch load currents
Make (close) onto a fault
Break normal load current and fault currents
Carry fault current without blowing itself open (or up!)

3. Important Characteristics
The important characteristics from a protection point of view are:
The speed with which the main current is opened after a tripping impulse is received
The capacity of the circuit that the main contacts are capable of interrupting (rupturing capacity).

4. Important Characteristics
The first characteristic is referred to as the ‘tripping time’ and is expressed in cycles.
Modern high-speed circuit breakers have tripping times between 2 and 8 cycles.
The tripping or total clearing or break time is made up as follows:

5. Important Characteristics
Opening time: The time between instant of application of tripping power to the instant of separation of the main contacts.
Arcing time: The time between the instant of separation of the main circuit breaker contacts to the instant of arc extinction of short-circuit current.
• Total break or clearing time: The sum of the above

6. Total fault clearing time

7. Old & New Breaker Interrupting Rating
The old style method used to rate breakers was expressed in MVA
This is our friend the power formula

8. Old & New Breaker Interrupting Rating
The new method used to rate breakers is expressed in KA
40KA, 50KA 60KA etc..

9. Choosing Interrupting Capacity
The selection of the breaking capacity depends on the actual fault conditions expected in the system and the possible future increase in the fault level of the main source of supply.
Different locations on the system have different faults currents and breaker requirements
As the system grows these values can change

10. Choosing Interrupting Capacity
As the system grows these values of fault current can change
This may require breakers to be replace with ones with greater interrupting capability

11. Types of circuit breakers
The types of breakers basically refer to the medium in which the breaker opens and closes. The medium could be oil, air, vacuum or SF6