Page 88 - Practical Power System and Protective Relays Commissioning

86 Practical Power System and Protective Relays Commissioning

b. Contact parting time is the interval between the energizing of the trip
coil and the parting of the primary arcing contacts of the breaker
c. Arcing time is the interval between the parting of primary arcing con-
tacts and the extinction of the arc on the primary contacts of the
breaker
d. Tripping time interrupting time is the interval between the energiz-
ing of the trip coil and the extinction of the arc on the primary
contacts of the breaker
e. Reclosing time is the interval between the energizing of the trip coil
when the breaker is in the close position and the making of primary
arcing contacts on the reclosing stroke
f. Trip-free time is the interval between the touching of the main con-
tacts and the parting of the main contacts when the breaker has
attempted a close into unknown fault condition
g. Breaker duty cycle: each breaker has a sequence of operation
cycles depending on the breaker mechanism reset time between the
opening and closing operation and between the closing and
opening operation
h. Closing (C)
i. Opening (O)
j. Operating sequence or duty cycle of CB.
O—0.3 s—CO—3 min—CO.

7.2 PRINCIPLE OF ARC INTERRUPTION
The CB should provide an insulating medium that is sufficient to prevent
current from continuing to flow during current interruption. The circuit is
usually opened by drawing out an arc between contacts until the arc can no
longer be sustained and is extinguished as shown in Fig. 7.1.

FIGURE 7.1 Voltage waveform of the circuit breaker interruption process. (A) Before short
circuit (SC); (B) during SC; and (C) after SC.