What is the main purpose of electrical insulation?
A Increase conductor resistance always
B Store shaft energy
C Limit current to safe paths
D Set grid frequency
Answer: C. Limit current to safe paths. Insulation separates conductors and limits current to intended paths, protecting people and equipment.
What is dielectric breakdown?
A Increase in capacitance only
B Normal charging current
C Reduction in conductor length
D Loss of insulating strength
Answer: D. Loss of insulating strength. Breakdown occurs when an electric field causes an insulating material to conduct or fail.
What is a partial discharge?
A Localized insulation discharge
B Complete circuit short
C Normal load current
D Full system flashover
Answer: A. Localized insulation discharge. A partial discharge is a localized electrical discharge that does not completely bridge the insulation between conductors.
What is corona around a high-voltage conductor?
A Oil circulation
B Solid insulation puncture
C Battery electrolysis
D Partial gas ionization
Answer: D. Partial gas ionization. Corona is a partial discharge in the gas near a conductor when the local electric field is high enough to ionize the gas.
What is flashover on an insulator?
A Surface discharge path
B Internal metal fracture
C Normal leakage current
D Cable conductor melting
Answer: A. Surface discharge path. Flashover is a disruptive discharge across an insulating surface or through the surrounding gas.
What is puncture of a solid insulator?
A Surface contamination only
B Conductor sag
C Internal dielectric breakdown
D Air-gap corona only
Answer: C. Internal dielectric breakdown. Puncture is breakdown through the body of the solid dielectric.
What does electrical clearance measure?
A Shortest air distance
B Surface creepage path
C Conductor length
D Grounding resistance
Answer: A. Shortest air distance. Clearance is the shortest distance through air between conductive parts.
What does creepage distance measure?
A Shortest distance through air
B Cable core diameter
C Ground electrode depth
D Path along insulation surface
Answer: D. Path along insulation surface. Creepage distance is measured along an insulating surface between conductive parts.
Which instrument commonly measures insulation resistance?
A Wattmeter
B Megohmmeter
C Tachometer
D Frequency meter
Answer: B. Megohmmeter. A megohmmeter applies a DC test voltage and measures insulation resistance.
What does dielectric strength describe?
A Thermal conductivity only
B Mechanical hardness only
C Cable current rating
D Field withstand before breakdown
Answer: D. Field withstand before breakdown. Dielectric strength is the electric field a dielectric can withstand before breakdown under specified conditions.
What does dielectric loss tangent indicate?
A Dielectric energy loss
B Conductor length
C Surge crest factor
D Air-gap distance
Answer: A. Dielectric energy loss. The loss tangent tan delta relates active dielectric loss to its reactive capacitive behavior.
What does a lightning impulse test simulate?
A Steady DC loading
B Lightning-type transient stress
C Normal motor starting
D Battery charging
Answer: B. Lightning-type transient stress. A lightning impulse test applies a short, steep transient to evaluate insulation response to lightning-type surges.
What does a switching impulse test represent?
A Switching surge stress
B Normal ripple voltage
C Steady thermal loading
D Radio interference only
Answer: A. Switching surge stress. Switching impulses represent transient overvoltages caused by operations such as line or breaker switching.
What does a sphere gap measure in HV testing?
A Average load current
B Insulation resistance
C Peak voltage
D Power factor only
Answer: C. Peak voltage. A calibrated sphere gap gives a disruptive-discharge voltage related to the peak applied voltage.
What does a voltage divider do in HV measurement?
A Stores surge energy
B Raises insulation resistance
C Changes waveform frequency
D Scales voltage for instruments
Answer: D. Scales voltage for instruments. A divider reduces a high voltage to a lower proportional signal suitable for measurement.
What does a surge arrester protect against?
A Long-term overload only
B Low power factor
C Transient overvoltage
D Rotor imbalance
Answer: C. Transient overvoltage. A surge arrester limits transient overvoltage by conducting surge current and protecting insulation.
What is a common nonlinear arrester material?
A Metal-oxide varistor
B Copper winding
C Silicon steel
D Carbon brush
Answer: A. Metal-oxide varistor. Metal-oxide surge arresters have strongly nonlinear voltage-current characteristics and conduct heavily during surges.
What is insulation coordination?
A Matching motor and load speeds
B Balancing three-phase currents
C Matching withstand and protection
D Setting transformer turns ratio
Answer: C. Matching withstand and protection. Insulation coordination selects equipment withstand levels and protective devices to achieve reliable protection with suitable margins.
Where does a lightning surge arrester send surge current?
A Into the load winding
B Toward earth
C Into control logic
D Back through the gate
Answer: B. Toward earth. A surge arrester provides a low-impedance path for surge current to the grounding system.
What is one effect of a sharp electrode edge?
A Lower field everywhere
B Higher local electric field
C Higher creepage distance
D Lower system voltage
Answer: B. Higher local electric field. Field concentration at sharp edges raises local electric stress and can initiate corona or discharge.
What does grounding a test object help provide?
A Higher dielectric strength
B Defined reference potential
C Lower capacitance always
D A faster impulse front
Answer: B. Defined reference potential. Grounding establishes a reference potential and provides a controlled return path during high-voltage testing.
What is the usual unit of electric-field strength?
A Amperes per second
B Volts per metre
C Coulombs per kilogram
D Watts per metre
Answer: B. Volts per metre. Electric field is voltage change per distance and is commonly expressed in volts per metre.
What does a lightning-protection system intercept?
A Normal load current
B Motor torque
C Lightning current
D Battery ripple
Answer: C. Lightning current. Lightning protection provides preferred paths to intercept and conduct lightning current safely to earth.
What does insulation withstand testing assess?
A Specified voltage endurance
B Motor speed accuracy
C Conductor color
D Grid frequency
Answer: A. Specified voltage endurance. A withstand test applies a specified voltage for a stated duration to check that insulation does not fail.
What commonly accelerates insulation ageing?
A Low conductor resistance
B Stable temperature only
C Short cable length
D Electrical, thermal and moisture stress
Answer: D. Electrical, thermal and moisture stress. Insulation ageing can be accelerated by electrical stress, heat, moisture, contamination and mechanical damage.