Page 87 - Practical Power System and Protective Relays Commissioning

Chapter 7

Circuit Breakers Theory Testing

and Commissioning

7.1 INTRODUCTION
Circuit breakers vary from simple molded case breakers used in 120-V
lighting to complex units used at 750 kV. The major factors in the design of
CB include the method of extinction of fault currents and the speed of the
operating mechanism, arc-interrupting medium, and insulation system.
CB functions are to:
open and close the electrical circuit manually and automatically and
trip in case of short circuit to isolate the faulty part in the system.
CB ratings include the following:
1. Voltage ratings
a. Nominal kV rating
b. Maximum voltage at which the breaker may be operated;
2. Insulation level withstand voltage Root Mean Square Value (RMS) kV at
50 or 60 Hz;
3. Current ratings
a. Continuous 50- or 60-Hz rating in amperes—current that the breaker
carries in normal conditions without exceeding the heating limits
b. Short-time rating of approximately 1.6 of the rated interrupting current
a. Momentary rating up to 1 second
b. Four-second rating;
4. Interrupting rating is highest the current breaker should interrupt at nor-
mal voltage, in general it is about 16 times the rated continuous current
of the breaker
a. Three-phase rated MVA—interrupting capacity
b. Amperes at rated voltage
c. Maximum amperes;
5. Operating time in cycles
a. Closing time is the interval between the energizing of the closing
circuit and the making of the arcing contacts of the breaker

Practical Power System and Protective Relays Commissioning.
DOI: https://doi.org/10.1016/B978-0-12-816858-5.00007-1 85
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