Objective Questions on SF6 CB – A Complete MCQ Series

SF6 Circuit Breaker (72 Questions)

Question 1

Which of the following is the main advantage of an SF6 circuit breaker over an oil circuit breaker?

A. Lower cost of maintenance

B. Higher dielectric strength and arc-quenching ability of SF6 gas

C. Simpler design and construction

D. Easier gas handling compared to oil filling

Question 2

What is the typical dielectric strength of SF6 gas at atmospheric pressure compared to air?

A. 1.5 times

B. 2.5 times

C. 3.5 times

D. 5 times

Question 3

In a puffer-type SF6 circuit breaker, what is the primary mechanism for arc extinction?

A. Thermal blast from auxiliary arc

B. High-pressure gas blast from compression cylinder

C. Natural convection cooling

D. Magnetic field deflection

Question 4

What is the critical pressure of SF6 gas?

A. 22.1 bar at -45.6°C

B. 37.6 bar at 45.6°C

C. 50.2 bar at 60°C

D. 15.8 bar at 25°C

Question 5

In modern SF6 circuit breakers, what is the typical SF6 gas pressure maintained at 20°C for rated voltage operation?

A. 1-2 bar gauge

B. 4-6 bar gauge

C. 8-10 bar gauge

D. 12-15 bar gauge

Question 6

What is the primary disadvantage of SF6 gas from an environmental perspective?

A. High toxicity to humans

B. Ozone depletion potential

C. Extremely high global warming potential

D. Acid rain formation

Question 7

During arc interruption in SF6 breakers, which decomposition product is most corrosive to internal components?

A. SF4

B. SOF2

C. SO2F2

D. HF (Hydrofluoric acid)

Question 8

What is the minimum percentage of SF6 purity typically required for circuit breaker applications?

A. 95%

B. 97%

C. 99%

D. 99.9%

Question 9

In a self-blast or auto-expansion SF6 circuit breaker, what provides the energy for arc extinction?

A. External pneumatic system

B. Thermal energy from the arc itself

C. Spring-loaded compression mechanism

D. Hydraulic actuator

Question 10

What is the typical interrupting time for a modern 145 kV SF6 circuit breaker from fault initiation to arc extinction?

A. 10-15 ms

B. 25-35 ms

C. 50-60 ms

D. 80-100 ms

Question 11

Which property of SF6 makes it superior for current interruption compared to air or oil?

A. High electronegative nature

B. Low molecular weight

C. High thermal conductivity only

D. Low density

Question 12

What is the recommended action when SF6 gas pressure drops below the minimum operating pressure alarm level?

A. Continue operation and monitor

B. Immediately trip and isolate the breaker

C. Reduce load current by 50%

D. Switch to backup breaker within 24 hours

Question 13

In double-pressure SF6 circuit breakers, what is the typical pressure ratio between high-pressure and low-pressure chambers?

A. 1.5:1

B. 3:1

C. 10:1

D. 20:1

Question 14

What is the liquefaction temperature of SF6 at atmospheric pressure?

A. -63.8°C (sublimation point)

B. -50.8°C

C. -20°C

D. 0°C

Question 15

Which contact material is most commonly used in modern SF6 circuit breakers for its arc resistance?

A. Pure copper

B. Silver-tungsten alloy

C. Copper-tungsten (CuW) alloy

D. Aluminum-bronze

Question 16

What is the primary purpose of molecular sieves or desiccants in SF6 circuit breaker gas compartments?

A. Absorb oxygen contamination

B. Remove moisture and decomposition products

C. Increase gas pressure

D. Prevent SF6 leakage

Question 17

At what rate does the dielectric strength of SF6 increase with pressure?

A. Linear up to 5 bar, then saturates

B. Logarithmic throughout the range

C. Linear throughout the practical range

D. Exponential above 3 bar

Question 18

What is the typical contact travel distance in a 245 kV SF6 puffer circuit breaker?

A. 50-80 mm

B. 150-200 mm

C. 300-400 mm

D. 500-600 mm

Question 19

Which international standard specifies the testing requirements for high-voltage SF6 circuit breakers?

A. IEC 60044

B. IEC 62271-100

C. IEEE 519

D. IEC 61000

Question 20

What phenomenon occurs when a circuit breaker attempts to interrupt current near its maximum rating with insufficient arc energy?

A. Thermal runaway

B. Prestrike failure

C. Current chopping

D. Inadequate self-blast pressure

Question 21

What is the typical annual leakage rate specified for sealed SF6 circuit breakers?

A. 10% per year

B. 5% per year

C. 1% per year

D. 0.5% per year

Question 22

In rotating arc SF6 circuit breakers, what creates the arc rotation?

A. External motor drive

B. Magnetic field from axial coils

C. Tangential gas flow

D. Thermal buoyancy effects

Question 23

What is the main advantage of GIS (Gas Insulated Switchgear) using SF6 over conventional air-insulated substations?

A. Lower initial cost

B. Easier maintenance access

C. Significantly reduced space requirements

D. Better fault location capability

Question 24

Which failure mode is most commonly associated with moisture contamination in SF6 breakers?

A. Contact welding

B. Dielectric breakdown in gas gaps

C. Mechanical jamming

D. Spring failure

Question 25

What is the recommended maximum moisture content in SF6 gas for circuit breaker service?

A. 500 ppmv

B. 200 ppmv

C. 100 ppmv

D. 50 ppmv

Question 26

In thermal cycle testing of SF6 breakers, what is being primarily evaluated?

A. Contact resistance over repeated operations

B. Seal integrity under temperature variations

C. Arc interruption capability at different temperatures

D. Mechanical endurance

Question 27

What is the significance of the TRV (Transient Recovery Voltage) in circuit breaker interruption?

A. It determines the minimum operating voltage

B. It represents the voltage stress across open contacts immediately after current interruption

C. It indicates the steady-state voltage rating

D. It measures the contact resistance

Question 28

Which factor has the greatest impact on the interrupting capability of an SF6 circuit breaker?

A. Contact material composition

B. Ambient temperature

C. Nozzle throat diameter and gas pressure

D. Number of interrupter units

Question 29

What is a "capacitor switching" duty in circuit breaker specifications?

A. Interrupting capacitive inrush currents during energization

B. Switching shunt capacitor banks with high restriking voltage

C. Operating with series capacitor compensation

D. Switching under no-load conditions

Question 30

Why are SF6 circuit breakers particularly suitable for frequent switching operations?

A. Low contact erosion due to superior arc quenching

B. Minimal maintenance requirements

C. Self-lubrication properties of SF6

D. Low operating energy

Question 31

What is the typical mechanical life expectancy of a modern SF6 circuit breaker?

A. 2,000 operations

B. 5,000 operations

C. 10,000 operations

D. 25,000 operations

Question 32

In condition monitoring of SF6 breakers, what does a sudden increase in SO2 concentration indicate?

A. Normal aging process

B. Recent arcing activity or overheating

C. Moisture ingress

D. Gas leakage

Question 33

What is the purpose of grading capacitors in multi-break SF6 circuit breakers?

A. Store energy for arc interruption

B. Ensure equal voltage distribution across breaks

C. Filter high-frequency transients

D. Reduce contact bounce

Question 34

Which phenomenon can cause an SF6 breaker to fail during line charging switching operations?

A. Excessive thermal loading

B. Contact welding due to inrush current

C. Prestrike causing overvoltage on transformer

D. Insufficient gas pressure

Question 35

What is the recommended gas handling procedure when servicing SF6 circuit breakers?

A. Vent to atmosphere and refill

B. Use recovery and recycling equipment to capture SF6

C. Dilute with nitrogen before venting

D. Burn off in a controlled environment

Question 36

In puffer-type SF6 breakers, what is the typical compression ratio achieved during the opening stroke?

A. 1.5:1

B. 3:1

C. 6:1

D. 12:1

Question 37

What is the primary function of the blast valve in a puffer-type SF6 circuit breaker?

A. Regulate gas pressure during closing

B. Control gas flow direction during opening operation

C. Prevent gas leakage during maintenance

D. Monitor gas density continuously

Question 38

Which component in an SF6 breaker is most susceptible to damage from arc by-products?

A. Operating mechanism springs

B. Nozzle made of PTFE or similar material

C. Support insulators

D. Control wiring

Question 39

What is the typical breaking capacity of a modern 420 kV SF6 circuit breaker?

A. 10-20 kA

B. 31.5-63 kA

C. 80-100 kA

D. 120-150 kA

Question 40

In SF6 gas handling, what is the purpose of a gas analyzer?

A. Measure gas pressure only

B. Detect moisture content and purity

C. Determine gas temperature

D. Calculate gas volume

Question 41

What is the typical response time of a density monitor in SF6 circuit breakers?

A. Instantaneous (milliseconds)

B. 1-2 seconds

C. 10-30 seconds

D. 2-5 minutes

Question 42

Which international standard governs the handling and management of SF6 gas?

A. IEC 60376

B. IEC 60480

C. IEC 62271-1

D. IEC 60038

Question 43

What is the primary advantage of hybrid circuit breakers using SF6 with vacuum interrupters?

A. Reduced SF6 usage

B. Lower manufacturing cost

C. Simpler operating mechanism

D. Higher voltage ratings

Question 44

During short-line fault interruption, what characteristic makes SF6 superior to other mediums?

A. Faster dielectric recovery

B. Higher thermal capacity

C. Better current zero behavior

D. Lower arc voltage

Question 45

What is the typical gas pressure in high-voltage GIS (Gas Insulated Switchgear) compartments?

A. 1-2 bar gauge

B. 3-4 bar gauge

C. 6-7 bar gauge

D. 8-10 bar gauge

Question 46

Which decomposition product indicates severe internal arcing in SF6 equipment?

A. CF4

B. SOF2

C. SO2

D. HF

Question 47

What is the purpose of the safety valve or rupture disc in SF6 circuit breakers?

A. Prevent overpressure during normal operation

B. Relieve pressure during internal fault conditions

C. Control gas flow during interruption

D. Maintain constant density

Question 48

In a hydraulic operating mechanism for SF6 breakers, what is the typical operating pressure?

A. 50-100 bar

B. 200-350 bar

C. 500-600 bar

D. 800-1000 bar

Question 49

What is the dielectric recovery rate of SF6 gas after current zero?

A. 10-20 kV/μs

B. 30-50 kV/μs

C. 60-80 kV/μs

D. 100-150 kV/μs

Question 50

Which factor most affects the contact erosion rate in SF6 breakers?

A. Operating voltage

B. Interrupted current magnitude

C. Ambient temperature

D. Gas pressure

Question 51

What is the typical closing time for a 245 kV SF6 circuit breaker?

A. 20-30 ms

B. 40-60 ms

C. 80-100 ms

D. 120-150 ms

Question 52

In spring-operated mechanisms, what is the purpose of the charging motor?

A. Open the contacts

B. Close the contacts

C. Compress the opening/closing springs

D. Operate auxiliary switches

Question 53

What is the minimum breakdown voltage of SF6 gas at 5 bar absolute pressure for a 10 mm gap?

A. 50 kV

B. 100 kV

C. 150 kV

D. 200 kV

Question 54

Which test is performed to verify the mechanical integrity of SF6 circuit breakers?

A. Power frequency withstand test

B. Mechanical endurance test

C. Temperature rise test

D. Partial discharge measurement

Question 55

What is the primary purpose of arc contacts in SF6 breakers?

A. Carry continuous current

B. Withstand mechanical stress

C. Take arc damage during interruption

D. Provide low contact resistance

Question 56

In cold climate applications, what special provision is required for SF6 breakers?

A. Higher gas pressure

B. Gas heating systems

C. Reduced interrupting rating

D. Special contact materials

Question 57

What is the typical resistance of the opening and closing coils in SF6 breaker mechanisms?

A. 1-5 ohms

B. 10-50 ohms

C. 100-200 ohms

D. 500-1000 ohms

Question 58

Which phenomenon can cause capacitance switching overvoltages in SF6 breakers?

A. Current chopping

B. Thermal runaway

C. Gas decomposition

D. Contact welding

Question 59

What is the purpose of the operating mechanism "trip free" feature?

A. Prevent closing on faults

B. Allow manual operation

C. Enable remote control

D. Provide mechanical interlocking

Question 60

In digital monitoring systems for SF6 breakers, what parameter is typically trended for predictive maintenance?

A. Operating time

B. Contact travel speed

C. Motor charging time

D. All of the above

Question 61

What is the typical gas volume in a 145 kV SF6 circuit breaker interrupter chamber?

A. 10-20 liters

B. 30-50 liters

C. 80-100 liters

D. 150-200 liters

Question 62

Which insulation coordination principle is critical for SF6 breaker selection?

A. Matching BIL to system requirements

B. Optimizing gas pressure

C. Selecting contact materials

D. Determining operating mechanism type

Question 63

What is the primary cause of contact bounce in circuit breakers?

A. Insufficient spring force

B. Mechanical impact energy

C. Gas pressure variations

D. Electromagnetic forces

Question 64

In SF6 circuit breaker maintenance, what is the purpose of megger testing?

A. Measure gas density

B. Check insulation resistance

C. Test contact resistance

D. Verify operating time

Question 65

What is the typical pressure rise during internal arc tests in SF6 equipment?

A. 2-3 times normal pressure

B. 5-8 times normal pressure

C. 10-15 times normal pressure

D. 20-25 times normal pressure

Question 66

Which component ensures synchronous operation of multiple interrupter units in series?

A. Grading capacitors

B. Tie rods

C. Equalizing springs

D. Control relays

Question 67

What is the primary advantage of magnetic actuation in modern SF6 breakers?

A. Lower maintenance

B. Faster operation

C. Higher reliability

D. All of the above

Question 68

In SF6 gas quality assessment, what does the dew point measurement indicate?

A. Gas purity

B. Moisture content

C. Decomposition products

D. Oxygen concentration

Question 69

What is the typical storage life of SF6 gas in properly maintained cylinders?

A. 1-2 years

B. 5-10 years

C. 15-20 years

D. Indefinite if sealed properly

Question 70

Which factor determines the number of interrupters in series for EHV SF6 breakers?

A. Current rating

B. Voltage rating

C. Interrupting capacity

D. Mechanical design

Question 71

What is the purpose of the blast tube in puffer-type SF6 breakers?

A. Guide moving contacts

B. Direct gas flow to nozzle

C. Support fixed contacts

D. Cool the arc

Question 72

In condition-based maintenance of SF6 breakers, what analysis method detects partial discharge?

A. Gas chromatography

B. Acoustic emission

C. Ultraviolet imaging

D. All of the above

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