High Voltage Power Circuit Breakers Lesson 1 Basic Principles

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!)

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).

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:

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

Total fault clearing time

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

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

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

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

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