What condition must hold for successful breaker interruption?
A CT ratio equals one
B Busbar current remains constant
C Gap withstands recovery voltage
D Trip coil stays energized
Answer: C. Gap withstands recovery voltage. After arc extinction, the recovering contact gap must withstand the transient and power-frequency voltage.
Which TRV parameter stresses dielectric recovery speed?
A Peak fault load
B CT burden value
C Rate of rise
D Spring charge level
Answer: C. Rate of rise. A steep rate of rise can challenge the recovering dielectric strength between separating contacts.
Why can current chopping create overvoltage?
A Busbar resistance becomes zero
B Trip battery changes polarity
C Inductive energy is released
D CT ratio increases suddenly
Answer: C. Inductive energy is released. Interrupting inductive current before natural zero releases stored magnetic energy into circuit capacitance.
What can a restrike during capacitive switching produce?
A Lower CT burden
B Reduced busbar voltage permanently
C Slower breaker operation
D High transient overvoltage
Answer: D. High transient overvoltage. Reignition across a capacitive circuit can interact with trapped charge and create high transient voltage.
Which SF6 property supports arc interruption?
A Electron attachment
B High thermal expansion only
C Magnetic attraction
D Low dielectric strength
Answer: A. Electron attachment. SF6 captures free electrons and has strong dielectric and arc-quenching properties.
Why handle SF6 arc by-products carefully?
A Some can be hazardous
B They increase CT accuracy
C They lubricate breaker contacts
D They improve gas purity
Answer: A. Some can be hazardous. Arc decomposition can produce hazardous compounds, so gas inspection, recovery, and handling follow safety procedures.
What does a temperature-compensated gas monitor assess?
A Contact travel only
B Trip-coil resistance
C Busbar current balance
D SF6 density
Answer: D. SF6 density. Gas density is a more reliable measure of insulating and interruption capability than uncorrected pressure alone.
What is preferred when maintaining SF6 equipment?
A Recover gas for reuse
B Vent gas routinely
C Mix it with air
D Release gas after each trip
Answer: A. Recover gas for reuse. Recovery equipment limits emissions and permits gas processing or reuse according to site procedures.
What carries the arc in a vacuum interrupter?
A Metal vapor plasma
B SF6 gas column
C Transformer oil bubble
D Compressed air jet
Answer: A. Metal vapor plasma. Vacuum arcs are sustained by metal vapor from the contacts and extinguish around current zero.
Why can vacuum breakers be prone to current chopping?
A Slow spring charging
B High oil pressure
C Weak trip battery
D Rapid dielectric recovery
Answer: D. Rapid dielectric recovery. Rapid dielectric recovery can interrupt small inductive currents before natural current zero.
What does point-on-wave switching control?
A CT secondary burden
B Breaker operation timing
C Busbar thermal expansion
D Relay password access
Answer: B. Breaker operation timing. Controlled switching selects breaker closing or opening instants to reduce specific switching transients.
What can cause very fast transient overvoltages in GIS?
A Slow thermal overload
B CT secondary testing
C Disconnector switching
D Battery charger ripple
Answer: C. Disconnector switching. Fast disconnector operations in GIS can produce very steep, very fast transient overvoltages.
What is the role of a surge arrester near switchgear?
A Limit transient overvoltage
B Increase interrupting current
C Measure contact timing
D Charge closing springs
Answer: A. Limit transient overvoltage. A surge arrester limits transient voltage by conducting surge current and then returning to a nonconducting state.
Why use a closing resistor in some EHV breakers?
A Increase fault-current peak
B Reduce CT ratio
C Hold isolator contacts closed
D Dampen switching overvoltage
Answer: D. Dampen switching overvoltage. A pre-insertion closing resistor can damp certain energization transients on high-voltage systems.
Controlled switching of a shunt reactor aims to reduce what?
A Busbar insulation resistance
B Switching transients
C Relay coil burden
D Breaker making capacity
Answer: B. Switching transients. Selecting suitable switching instants can reduce transient overvoltage when reactors are energized or de-energized.
How does first-pole-to-clear affect three-phase interruption?
A Changes CT polarity
B Stops DC offset immediately
C Sets breaker rated current
D Changes pole recovery voltage
Answer: D. Changes pole recovery voltage. The first pole to interrupt can experience a recovery voltage influenced by system grounding and phase coupling.
What breaker duty applies when two live systems differ in phase?
A Out-of-phase switching duty
B Capacitor-bank duty only
C Busbar earth-fault duty
D Motor overload duty
Answer: A. Out-of-phase switching duty. Out-of-phase duty covers interruption or closing when system voltages on breaker sides are not synchronized.
Why is autoreclose dead time specified?
A Recharge the CT core
B Measure busbar resistance
C Allow temporary arc deionization
D Raise SF6 density
Answer: C. Allow temporary arc deionization. Dead time gives the arc path time to deionize before a line is re-energized.
Why must a breaker TRV rating match its application?
A All lines have equal voltage
B System transient conditions vary
C CT ratios set recovery voltage
D Relay settings erase transients
Answer: B. System transient conditions vary. System grounding, line length, and switching duty shape TRV, so the breaker must meet the applicable envelope.
What does asynchronous breaker pole operation risk?
A Lower CT burden
B Phase unbalance and transients
C Improved busbar cooling
D Reduced insulation stress always
Answer: B. Phase unbalance and transients. Unequal pole timing can create temporary phase unbalance and additional switching stress.
What does an anti-pumping circuit prevent?
A Unwanted CT saturation
B Busbar differential restraint
C Repeated closing on held command
D SF6 density reduction
Answer: C. Repeated closing on held command. Anti-pumping logic prevents repeated close-open cycles while a closing command remains continuously applied.
Why require a charged spring before breaker closing?
A Raise the busbar voltage
B Measure fault direction
C Open the earth switch
D Ensure stored operating energy
Answer: D. Ensure stored operating energy. The operating mechanism needs adequate stored energy to complete a reliable closing operation.
What should busbar short-time withstand rating cover?
A Only normal load current
B Fault current until clearance
C Relay voltage range
D Breaker contact travel
Answer: B. Fault current until clearance. Busbar withstand capability must cover the prospective fault current for the protection clearing interval.
Why monitor individual GIS gas compartments?
A Measure busbar temperature only
B Locate leakage or low density
C Set relay time multiplier
D Control generator speed
Answer: B. Locate leakage or low density. Separate density monitoring helps identify which GIS compartment has lost insulating gas.
What does a trapped-key interlock enforce?
A Safe switching sequence
B CT ratio correction
C Arc voltage regulation
D Automatic relay calibration
Answer: A. Safe switching sequence. A trapped-key scheme permits switching only in an intended sequence, helping prevent unsafe access or grounding.