What helps extinguish an AC breaker arc at current zero?
A Contact resistance rises
B Busbar voltage falls
C Trip-coil current reverses
D Dielectric strength recovers
Answer: D. Dielectric strength recovers. At current zero the arc can extinguish if the gap dielectric strength recovers faster than the transient voltage.
What does transient recovery voltage describe?
A Normal load voltage before switching
B Voltage across contacts after interruption
C CT secondary voltage at pickup
D Voltage drop along the busbar
Answer: B. Voltage across contacts after interruption. TRV is the transient voltage appearing across breaker contacts after current interruption.
What does rate of rise of TRV measure?
A Peak current at closing
B Relay trip delay
C Contact erosion depth
D Speed of voltage rise
Answer: D. Speed of voltage rise. The rate of rise indicates how quickly the transient recovery voltage builds after interruption.
What is restriking voltage associated with?
A Normal voltage before opening
B Voltage after arc reignition
C CT ratio error
D Spring charging current
Answer: B. Voltage after arc reignition. If the contact gap cannot withstand recovery voltage, the arc may restrike and a restriking voltage appears.
Recovery voltage after interruption is mainly what?
A CT burden voltage
B Power-frequency voltage across contacts
C Trip-coil voltage
D Arc-chute pressure
Answer: B. Power-frequency voltage across contacts. Recovery voltage is the power-frequency voltage across breaker contacts after the transient component subsides.
Current chopping is most associated with interruption of what?
A Large resistive fault current
B Balanced busbar load
C Small inductive current
D DC control current
Answer: C. Small inductive current. Some breakers can extinguish small inductive current before natural current zero, producing current chopping and overvoltage.
What risk can restriking create during capacitive switching?
A Lower CT burden
B Reduced contact travel
C Transient overvoltage
D Busbar thermal relief
Answer: C. Transient overvoltage. Restrikes while switching capacitive loads can create steep transient overvoltages.
Breaker making duty must account for which current?
A Peak asymmetrical short-circuit current
B Average load current only
C Relay coil current
D Transformer magnetizing voltage
Answer: A. Peak asymmetrical short-circuit current. The first peak can be elevated by DC offset, so breaker making duty is specified for peak current.
What causes asymmetry in the first fault-current cycles?
A Busbar insulation color
B Breaker contact resistance only
C Neutral voltage regulation
D DC offset component
Answer: D. DC offset component. A DC component can offset the AC fault waveform and increase the first current peak.
What does breaker interrupting time include?
A Relay sensing time only
B Spring charging time only
C Operator response time
D Contact parting to current extinction
Answer: D. Contact parting to current extinction. Interrupting time includes the interval from contact separation until the arc is finally extinguished.
The first-pole-to-clear factor accounts for what?
A First relay pickup level
B Effect of first pole opening
C Initial CT ratio
D First busbar expansion
Answer: B. Effect of first pole opening. In a three-phase interruption, the first pole to clear may experience a different recovery voltage than later poles.
A breaker short-circuit operating duty specifies what?
A Required operation sequence
B Panel paint thickness
C Relay password policy
D Cable trench dimensions
Answer: A. Required operation sequence. A duty cycle states the opening and closing sequence the breaker must perform under specified fault conditions.
A breaker intended for auto-reclosing needs which capability?
A Repeated operating duty
B Manual-only isolator action
C Permanent earth connection
D No closing mechanism
Answer: A. Repeated operating duty. Auto-reclosing duties require the breaker to open and reclose according to the specified sequence.
What does pole-discrepancy protection detect?
A Wrong CT polarity
B Low busbar frequency
C Unequal pole operating times
D Excessive oil acidity
Answer: C. Unequal pole operating times. Pole-discrepancy logic detects that breaker poles do not occupy the same state within an allowed interval.
What triggers breaker-failure backup protection?
A Voltage rises after reclosing
B A relay resets normally
C The busbar is unloaded
D Current persists after trip command
Answer: D. Current persists after trip command. Breaker-failure protection acts when a breaker fails to interrupt current after a trip command.
Trip-circuit supervision checks what?
A Trip path readiness
B Busbar mechanical strength
C Breaker interrupting rating
D Cable route length
Answer: A. Trip path readiness. Trip-circuit supervision checks continuity of the DC trip path and associated contacts.
How is very low breaker contact resistance commonly measured?
A Micro-ohmmeter test
B Frequency sweep
C Oil acidity test
D Distance-relay injection
Answer: A. Micro-ohmmeter test. A micro-ohmmeter measures the low resistance across closed breaker contacts.
An insulation-resistance test checks what?
A Breaker operating time
B Busbar current sharing
C Insulation leakage resistance
D Relay pickup direction
Answer: C. Insulation leakage resistance. An insulation-resistance tester evaluates leakage through insulation between isolated conductors and earth.
What does a dielectric withstand test evaluate?
A Insulation withstand capability
B Closing spring charge
C Relay timing curve
D CT ratio accuracy
Answer: A. Insulation withstand capability. A withstand test applies a specified test voltage to assess insulation integrity.
What does a breaker timing test measure?
A Busbar voltage regulation
B CT excitation current
C Opening and closing times
D Arc gas composition
Answer: C. Opening and closing times. Timing tests record operating times and often compare pole simultaneity against manufacturer limits.
What can primary injection testing verify?
A Primary current path and protection
B Relay logic alone only
C Busbar paint condition
D Switchgear compartment pressure
Answer: A. Primary current path and protection. Primary injection can check the current path through CTs, wiring, relays, and associated trip action.
Secondary injection testing primarily checks which part?
A Main contact interrupting duty
B Busbar support strength
C Isolator visible gap
D Relay inputs and logic
Answer: D. Relay inputs and logic. Secondary injection applies test signals directly to relay circuits to verify settings and logic.
What does an SF6 density monitor help detect?
A Breaker contact wear directly
B Gas density reduction
C Busbar phase imbalance
D Low CT secondary burden
Answer: B. Gas density reduction. Density monitoring detects gas loss that could reduce the breaker or GIS insulation and interruption capability.
Partial-discharge monitoring in GIS can indicate what?
A Low spring charge
B Incorrect breaker timing
C Insulation defects
D Open trip-coil circuit
Answer: C. Insulation defects. Partial discharges are localized insulation events that can reveal defects before complete breakdown.
GIS differs from AIS by using what?
A Oil-filled busbar supports
B Gas-insulated metal enclosure
C Open-air breaker contacts only
D Vacuum-insulated cable trenches
Answer: B. Gas-insulated metal enclosure. Gas-insulated switchgear encloses conductors and switching parts in grounded metal compartments with insulating gas.