What forms when live breaker contacts separate under current?
A Electric arc
B Magnetic bearing
C Oil film
D Busbar joint
Answer: A. Electric arc. Current continues through ionized gas between separating contacts, forming an arc.
What is the purpose of an arc-quenching medium?
A Increase load current
B Measure contact resistance
C Close the breaker spring
D Cool and deionize the arc
Answer: D. Cool and deionize the arc. The medium helps remove heat and charge carriers so the arc can extinguish.
What is the main contact designed to do?
A Measure system frequency
B Trip the breaker
C Carry normal current
D Ground the busbar
Answer: C. Carry normal current. Main contacts carry current during normal service and separate during breaker operation.
What is the role of an arcing contact?
A Increase busbar voltage
B Protect main contacts from arcing
C Measure protection current
D Charge the closing spring
Answer: B. Protect main contacts from arcing. Arcing contacts take much of the arc erosion during switching, preserving the main contacts.
A vacuum breaker extinguishes its arc in what condition?
A Compressed oil
B Open air blast
C High vacuum
D Hydrogen pressure
Answer: C. High vacuum. A vacuum interrupter uses a sealed high-vacuum gap where the arc extinguishes near current zero.
SF6 gas in a breaker is used mainly for what?
A Spring charging
B Current measurement
C Busbar support
D Arc quenching and insulation
Answer: D. Arc quenching and insulation. Sulfur hexafluoride has strong insulating and arc-quenching properties in suitable switchgear designs.
Oil in an oil circuit breaker provides what function?
A Trip-circuit supervision
B Current transformation
C Busbar coupling
D Insulation and arc quenching
Answer: D. Insulation and arc quenching. Oil provides insulation and helps cool or decompose around the arc in oil-breaker designs.
An air-blast breaker uses what to interrupt an arc?
A Compressed air
B Transformer oil
C High vacuum only
D Hydraulic fluid
Answer: A. Compressed air. An air-blast breaker directs compressed air across the arc to cool and deionize it.
How does a circuit breaker differ from an isolator?
A Breaker interrupts fault current
B Isolator measures fault current
C Breaker gives visible isolation only
D Isolator trips on relay command
Answer: A. Breaker interrupts fault current. A circuit breaker interrupts load and fault current within its rating; an isolator is for off-load separation.
A disconnector is another name for which device?
A Trip relay
B Isolator
C Bus coupler
D Current transformer
Answer: B. Isolator. A disconnector or isolator provides a visible off-load separation point.
What can a fuse striker pin operate?
A CT secondary winding
B Busbar support
C Breaker timing circuit
D Associated switch mechanism
Answer: D. Associated switch mechanism. Some fuse-switch combinations use the fuse striker to release a mechanism and open all phases.
An HRC fuse is designed to safely interrupt what?
A Low frequency voltage
B High fault current
C Relay timing pulses
D Normal control signals
Answer: B. High fault current. High-rupturing-capacity fuses are designed to interrupt substantial fault currents safely.
What does a withdrawable breaker allow?
A Live busbar disassembly
B Relay current amplification
C Automatic voltage regulation
D Removal from its compartment
Answer: D. Removal from its compartment. A withdrawable breaker can be moved between connected, test, and isolated positions for operation or maintenance.
In a breaker test position, what is commonly disconnected?
A All control wiring always
B Protective earth permanently
C Primary power connections
D The busbar itself
Answer: C. Primary power connections. The test position isolates the breaker primary circuit while allowing specified control and functional checks.
Why divide metal-clad switchgear into compartments?
A Increase fault level
B Separate major circuit parts
C Remove protection relays
D Change system frequency
Answer: B. Separate major circuit parts. Compartmentalization separates areas such as busbars, breaker units, and cable terminations.
What does an arc chute do in a low-voltage breaker?
A Measures earth resistance
B Stores closing energy
C Splits and cools the arc
D Changes phase sequence
Answer: C. Splits and cools the arc. Arc plates lengthen, split, and cool the arc to help extinguish it.
Arc-flash protection aims primarily to reduce what?
A Transformer losses
B Arc exposure and injury
C Relay burden
D Cable capacitance
Answer: B. Arc exposure and injury. Arc-flash mitigation reduces exposure to thermal energy and pressure from an electrical arc.
What does breaker racking move?
A Breaker between positions
B Current between phases
C Voltage into a CT
D Arc gas into a tank
Answer: A. Breaker between positions. Racking moves a withdrawable breaker between service, test, and isolated positions.
Why should an isolator open only after its breaker?
A Isolator has no contacts
B Breaker raises line voltage
C Breaker interrupts the current
D Isolator measures current
Answer: C. Breaker interrupts the current. Opening the breaker first interrupts current so the isolator can then provide off-load separation.
What condition should permit an earthing switch to close?
A Breaker carrying load
B Busbar energized normally
C Relay pickup exceeded
D Circuit safely isolated
Answer: D. Circuit safely isolated. Interlocking should prevent grounding a circuit that has not been isolated and verified de-energized.
What does a mimic diagram show on a switchgear panel?
A Power circuit arrangement
B Cable insulation resistance
C Relay firmware version
D Breaker oil chemistry
Answer: A. Power circuit arrangement. A mimic diagram gives a simplified visual representation of busbars, breakers, and connected circuits.
What does a stored-energy spring provide?
A Mechanical operating energy
B CT excitation voltage
C Arc-quenching gas
D Transformer oil pressure
Answer: A. Mechanical operating energy. The charged spring supplies mechanical energy for breaker opening or closing.
What is a switch-fuse combination used to provide?
A Distance measurement only
B Voltage transformation only
C Load switching and fault protection
D Busbar differential restraint
Answer: C. Load switching and fault protection. A switch-fuse unit combines switching with fuse-based protection for suitable circuits.
Which item provides insulation between a live conductor and support?
A Insulator
B Bus coupler
C Trip coil
D Closing relay
Answer: A. Insulator. Insulators support live conductors while isolating them electrically from grounded structures.
What is the purpose of a breaker position indicator?
A Measure fault current
B Show open or closed state
C Test insulation strength
D Adjust trip timing
Answer: B. Show open or closed state. Position indicators give operators a local indication of whether breaker contacts are open or closed.