Capacitance switching constitutes
severe operating duty for a circuit breaker. At the time the breaker
opens at near current zero the capacitor is fully charged. After
interruption, when the alternating voltage on the source side of the
breaker reaches its opposite maximum, the voltage that appears across
the contacts of the open circuited breaker is at least twice the
normal line-to-neutral voltage of the circuit.
Due to the circuit constants on the
supply side of the breaker the voltage across the open contact can
reach three times the normal line-to-neutral. If a breakdown occurs
across the open contact the arc is reestablished. After it is
interrupted and with subsequent alternation of the supply side
voltage, the voltage across the open contact is even higher.
ANSI Standard C37.06 (indoor oilless
circuit breakers). The application guide ANSI/IEEE Standard C37.012
covers the method of calculation of the quantities covered by C37.06
Standard.
Note that the definitions in C37.04
make the switching of two capacitors banks in close proximity to the
switchgear bus a back-to back mode of switching. This classification
requires a definite purpose circuit breaker (breakers specifically
designed for capacitance switching).
A breaker specified for capacitor
switching should include as applicable.
1. Rated maximum voltage.
2. Rated frequency.
3. Rated open wire line charging
switching current.
4. Rated isolated cable charging and
shunt capacitor switching current.
5. Rated back-to-back cable charging
and back-to-back capacitor switching current.
6. Rated transient overvoltage factor.
7. Rated transient inrush current and
its frequency.
8. Rated interrupting time.
9. Rated capacitive current switching
life.
10.Grounding of system and capacitor
bank.
Loadbreak interrupter switches are
permitted by ANSI/IEEE Standard C37.30 to switch capacitance but they
must have tested ratings for the purpose.
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