Five ideal Marx stages each charge to 80 kV. Find ideal impulse output.
A 80 kV
B 400 kV
C 160 kV
D 500 kV
Answer: B. 400 kV. In series discharge, five equal stage voltages add to 5 times 80 kV, or 400 kV, before circuit losses.
A 2000:1 divider reads 50 V. Estimate the high voltage.
A 25 kV
B 50 kV
C 100 kV
D 200 kV
Answer: C. 100 kV. The high-side voltage is the divider ratio times its output: 2000 times 50 V equals 100 kV.
For coaxial insulation, where is radial electric field greatest?
A At outer conductor surface
B Midway through dielectric
C At inner conductor surface
D Outside the cable only
Answer: C. At inner conductor surface. In a coaxial geometry E varies inversely with radius, so the field is greatest at the smallest radius, the inner conductor.
How does increasing cable insulation outer radius affect inner-surface field at fixed inner radius and voltage?
A It raises the field
B It has no effect
C It reverses polarity
D It lowers the field
Answer: D. It lowers the field. The coaxial field is V divided by a ln(b/a); increasing b increases ln(b/a) and lowers the field at the inner radius.
What happens to Marx-generator output when load capacitance rises?
A Peak voltage doubles
B Wavefront becomes infinite
C Peak voltage generally falls
D Output becomes DC
Answer: C. Peak voltage generally falls. The test object loads the impulse generator; higher capacitance generally reduces attainable peak voltage and changes the waveform.
What does a chopped lightning impulse test apply?
A Steady power-frequency voltage
B Impulse interrupted by discharge
C Pure DC voltage
D Low-frequency current
Answer: B. Impulse interrupted by discharge. A chopped impulse is interrupted by a disruptive discharge, testing insulation response to a steep transient collapse.
What does the tail resistor mainly influence in an impulse circuit?
A Test frequency
B Ground potential
C Wave decay time
D Electrode diameter
Answer: C. Wave decay time. The tail resistor and equivalent capacitances mainly govern the impulse decay or time to half value.
Why place a surge arrester close to protected equipment?
A Increase conductor length
B Raise residual voltage
C Increase earth resistance
D Reduce lead-inductance voltage
Answer: D. Reduce lead-inductance voltage. Long connecting leads add L di/dt voltage during surge current, so short leads reduce the voltage at the equipment terminals.
What voltage can lead inductance add during a steep surge?
A I squared R
B L di over dt
C V divided by C
D C dv over dt
Answer: B. L di over dt. The inductive voltage is L times the rate of current change, which can be significant during a steep surge.
What is the main coordination comparison for an arrester?
A Rated current versus motor speed
B Creepage versus conductor length
C Frequency versus capacitance
D Protective level versus withstand
Answer: D. Protective level versus withstand. The arrester protective level must be sufficiently below the equipment withstand level, including lead and grounding effects.
What is apparent charge in partial-discharge measurement?
A Total stored energy
B Peak withstand voltage
C Insulation volume
D Equivalent terminal charge transfer
Answer: D. Equivalent terminal charge transfer. Apparent charge is an equivalent charge transfer at the test terminals that would create the measured response; it is not the exact local cavity charge.
How does PD extinction voltage usually compare with inception voltage?
A It is usually lower
B It is always higher
C It is always equal
D It is unrelated
Answer: A. It is usually lower. Once discharge has started, it often continues down to a lower voltage before extinguishing, though behavior depends on the defect and test conditions.
Why can a Schering bridge detect insulation ageing?
A Loss tangent can rise
B Pole count changes
C Conductor length shrinks
D Grid frequency doubles
Answer: A. Loss tangent can rise. Ageing, moisture or contamination can increase dielectric loss, changing tan delta measured by the bridge.
What does a resonant AC test set provide for a capacitive load?
A High current at zero voltage
B High voltage, low source power
C Pure DC output
D A chopped impulse only
Answer: B. High voltage, low source power. Resonance circulates reactive energy within the test circuit, allowing a comparatively small source to sustain high test voltage.
Why does a cascade test transformer need stage insulation?
A All stages are grounded equally
B Stages sit at different potentials
C Frequency is multiplied
D Output is always DC
Answer: B. Stages sit at different potentials. Upper transformer stages are elevated above ground and must be insulated for their common-mode potential.
What is the dominant field issue at a sharp electrode tip?
A Local field enhancement
B Uniform field everywhere
C Lower electric stress
D Higher air density
Answer: A. Local field enhancement. The small radius of curvature concentrates electric field and can initiate corona or breakdown.
What does a guard or grading ring improve on an insulator string?
A Conductor ampacity
B Voltage and field distribution
C Grounding resistance
D Power frequency
Answer: B. Voltage and field distribution. A grading ring reshapes the field and can improve voltage distribution along the insulator string.
Why is a polluted insulator more vulnerable when wet?
A Conductive surface film forms
B Surface resistance becomes infinite
C Air gap disappears
D Voltage polarity reverses
Answer: A. Conductive surface film forms. Moisture dissolves contaminants into a conductive film, allowing leakage current and dry-band arcing.
What does the BIL rating express?
A Continuous operating current
B Insulation resistance
C Lightning impulse withstand level
D Thermal class
Answer: C. Lightning impulse withstand level. Basic insulation level is a specified lightning-impulse withstand level used in insulation coordination.
What does switching impulse testing emphasize in EHV equipment?
A Slow-front overvoltage withstand
B DC leakage current only
C Motor starting torque
D Radio-frequency output
Answer: A. Slow-front overvoltage withstand. Switching surges have slower fronts and can be important insulation stresses for high-voltage and extra-high-voltage equipment.
How does a sphere gap's reading change with reduced air density?
A Breakdown voltage decreases
B Breakdown voltage doubles
C Gap becomes a divider
D Voltage is unchanged always
Answer: A. Breakdown voltage decreases. Lower air density generally lowers the disruptive-discharge voltage, so atmospheric correction is required.
What can a capacitive divider require for accurate impulse measurement?
A A motor tachometer
B A battery charger
C A water valve
D Damping and compensation
Answer: D. Damping and compensation. A practical capacitive divider may need damping and compensation to control oscillation and preserve impulse response.
Why is a PD-free setup needed during PD testing?
A Increase test frequency
B Short the test object
C Avoid background discharge signals
D Raise ambient humidity
Answer: C. Avoid background discharge signals. Background discharges from test equipment or connections can mask or be mistaken for activity in the test object.
What is a common effect of corona above onset voltage?
A Lower conductor voltage always
B Higher insulation resistance
C No acoustic noise
D Power loss and interference
Answer: D. Power loss and interference. Corona consumes power and can produce audible noise, radio interference and chemical by-products.
What is the practical purpose of a withstand test?
A Verify insulation under specified stress
B Measure motor speed
C Find turbine efficiency
D Set line frequency
Answer: A. Verify insulation under specified stress. A withstand test checks that an insulation system survives a defined voltage and duration without disruptive failure.