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Question 1. In an AC circuit, the inductive reactance (XL) is directly proportional to which quantity? A) Frequency B) Capacitance C) Resistance D) Voltage Answer: A Explanation: XL = 2πfL, so it increases linearly with frequency. Question 2. The phase angle between voltage and current in a purely capacitive circuit is: A) 0° B) 30° lagging C) 90° lagging D) 90° leading Answer: D Explanation: In a capacitive circuit, current leads voltage by 90°. Question 3. In a balanced three‑phase Y‑connected system, the line voltage is: A) Equal to the phase voltage B) √3 times the phase voltage C) √3 times the phase current D) Half the phase voltage Answer: B Explanation: V_line = √3 · V_phase for a Y‑connected system.
Question 4. For a Δ‑connected load, the relationship between line current (I_L) and phase current (I_Ph) is: A) I_L = I_Ph B) I_L = √3 · I_Ph C) I_L = I_Ph / √ D) I_L = 2 · I_Ph Answer: B Explanation: In Δ, each line conductor supplies two phase currents that are 120° apart, giving I_L = √3 · I_Ph. Question 5. Which of the following methods is most appropriate for correcting a lagging power factor caused by inductive loads? A) Adding series resistors B) Installing shunt capacitors C) Using a step‑up transformer D) Connecting a synchronous motor as a motor‑generator set Answer: B Explanation: Shunt capacitors provide leading reactive power, offsetting the lagging inductive VARs. Question 6. The voltage drop on a 50 km, 11 kV transmission line carrying 200 A is primarily due to: A) Skin effect B) Corona discharge C) Line impedance (R + jX) D) Magnetic saturation of the conductors Answer: C
D) It changes only the secondary voltage. Answer: C Explanation: OLTCs are designed to adjust taps without interrupting load current. Question 10. When interpreting a current transformer (CT) polarity, the “dot” on the secondary winding indicates: A) The direction of current flow in the primary. B) The start of the winding that yields a lagging voltage. C) The reference terminal where the secondary current leaves the CT. D) The point of maximum magnetic flux. Answer: C Explanation: The dot marks the terminal where the secondary current exits, establishing polarity for protection schemes. Question 11. Which safety rule must be observed when working with a live CT secondary circuit? A) Keep the secondary open at all times. B) Short the secondary terminals together. C) Never open the secondary while the primary is energized. D) Connect a high‑impedance voltmeter across the secondary. Answer: C Explanation: Opening a CT secondary under load can generate dangerously high voltages. Question 12. A voltage transformer (VT) with an accuracy class of 0.5 % is rated for a burden of 2.5 VA. If the burden is increased to 5 VA, the error will: A) Remain 0.5 %
B) Decrease to 0.25 % C) Increase beyond 0.5 % D) Become zero Answer: C Explanation: Exceeding the specified burden degrades accuracy, causing greater error. Question 13. Which arc‑quenching medium provides the fastest interruption for high‑voltage applications? A) Oil B) SF₆ gas C) Vacuum D) Air blast Answer: C Explanation: Vacuum interrupters have the shortest arc extinction time and are widely used for medium‑voltage breakers. Question 14. In a spring‑type circuit breaker, the operating energy is stored in: A) Hydraulic fluid pressure B) Compressed air C) Torsion springs D) Magnetic fields Answer: C Explanation: Spring‑type breakers use mechanical springs to drive the contacts apart.
Question 18. Distance protection calculates impedance to estimate fault location. The primary measurement used is: A) Voltage only B) Current only C) Both voltage and current D) Power factor Answer: C Explanation: Impedance Z = V / I; both voltage and current phasors are required. Question 19. The station battery bank is primarily used to: A) Supply the entire substation load during a grid outage. B) Provide the DC excitation for protective relays and breaker tripping. C) Run the cooling fans of transformers. D) Power the SCADA communication links. Answer: B Explanation: Batteries ensure reliable DC power for control and protection circuits when AC is lost. Question 20. An alarm indicating “Breaker 12 Open – Trip Coil De‑energized” most likely signifies: A] A successful manual opening. B] A failure in the trip coil circuit. C] An automatic trip command. D] A ground fault on the breaker. Answer: B
Explanation: The alarm shows the trip coil is not receiving power, preventing automatic tripping. Question 21. The purpose of a ground grid in a substation is to: A] Reduce line voltage. B] Provide a low‑impedance path for fault currents to earth. C] Increase system capacitance. D] Isolate the substation from the utility. Answer: B Explanation: A ground grid ensures fault currents are safely dissipated into earth, limiting step and touch potentials. Question 22. Step potential is a concern for personnel because: A] It creates a voltage difference between two points on the ground surface. B] It increases the magnetic field around conductors. C] It causes transformer oil to boil. D] It reduces the effectiveness of grounding rods. Answer: A Explanation: When a fault current flows through earth, voltage gradients (step potentials) can appear between a person’s feet. Question 23. When performing a Megger insulation resistance test on a transformer winding, a reading of 0.5 MΩ indicates: A] Good insulation. B] Slightly degraded but acceptable. C] Severe insulation failure.
B] Low‑voltage DC source (e.g., 5 V) and a micro‑ammeter. C] High‑frequency bridge. D] Insulation tester. Answer: B Explanation: A low‑voltage DC source minimizes arcing while allowing precise resistance measurement. Question 27. Specific gravity of transformer oil is measured to assess: A] Dielectric strength. B] Moisture content. C] Oil viscosity. D] Presence of dissolved gases. Answer: B Explanation: Water reduces specific gravity; a lower reading indicates higher moisture. Question 28. In LOTO procedures for a substation, “Minimum Approach Distance” (MAD) is defined as: A] The shortest distance a worker may approach live parts without additional protection. B] The distance between two locked out devices. C] The clearance required between a ladder and a busbar. D] The length of the isolation tag. Answer: A Explanation: MAD ensures workers stay safely away from energized equipment. Question 29. Arc‑flash category 3 PPE typically includes:
A] Only safety glasses. B] Flame‑resistant shirt and pants, leather gloves, face shield, and arc‑rated hood. C] Full body fire‑resistant suit with integrated cooling. D] No additional protection beyond standard workwear. Answer: B Explanation: Category 3 requires substantial protection against incident energy up to 40 cal/cm². Question 30. According to the National Electrical Safety Code (NESC), the minimum clearance between a 138 kV overhead conductor and a grounded metal structure is: A] 3 ft B] 4 ft C] 5 ft D] 6 ft Answer: D Explanation: NESC Table 310‑5 specifies 6 ft clearance for 138 kV lines to grounded objects. Question 31. When a substation experiences an SF₆ gas leak, the immediate corrective action is to: A] Increase ventilation and evacuate the area. B] Shut down the breaker and vent the gas to atmosphere. C] Replace the gas with nitrogen. D] Ignore it; SF₆ is non‑toxic. Answer: A Explanation: SF₆ is an asphyxiant; ventilation and evacuation protect personnel while the leak is contained.
C] Thermal expansion of the bus. D] Aesthetic considerations. Answer: B Explanation: Electrical safety clearances prevent accidental contact and arcing. Question 35. An on‑load tap changer (OLTC) with a “tap step” of 1.25 % changes the secondary voltage by: A] 0.5 % per tap. B] 1.25 % per tap. C] 2.5 % per tap. D] 5 % per tap. Answer: B Explanation: Tap step directly defines the percentage change per tap movement. Question 36. The purpose of a “neutral grounding resistor” (NGR) in a substation is to: A] Eliminate neutral current completely. B] Limit fault current during a ground fault. C] Provide a path for harmonic currents. D] Balance the three phases. Answer: B Explanation: NGR introduces controlled resistance to keep ground‑fault currents within safe limits. Question 37. In a distance relay, the “first zone” is set to cover: A] 100 % of line length.
B] 80 % of line length. C] 50 % of line length. D] 25 % of line length. Answer: B Explanation: Primary (first) zone typically provides 80 % reach to ensure selectivity. Question 38. The characteristic that distinguishes a “differential” relay from an “overcurrent” relay is: A] It measures voltage. B] It compares two currents entering a protected zone. C] It operates only on DC systems. D] It requires a communication link. Answer: B Explanation: Differential protection monitors the balance of currents entering and leaving the zone. Question 39. A CT with a ratio of 2000:5 will produce a secondary current of 5 A when the primary current is: A] 200 A B] 250 A C] 1000 A D] 5000 A Answer: C Explanation: Ratio = 2000/5 = 400:1, so I_secondary = I_primary / 400 → 5 A = I_primary /400 → I_primary = 2000 A. (Correction: Actually 5 A × 400 = 2000 A, so answer D is 2000 A. The correct answer is D.)
Answer: A Explanation: Symmetrical components decompose unbalanced sets into three balanced sets. Question 43. The term “burden” for a CT is expressed in: A] Ohms of secondary resistance. B] VA at a specified secondary current. C] Percentage voltage drop. D] kW of load power. Answer: B Explanation: CT burden is the apparent power the secondary circuit presents to the CT. Question 44. A “vacuum interrupter” is unsuitable for which of the following applications? A] Medium‑voltage (≤ 30 kV) circuit breakers. B] High‑current DC circuit breakers. C] Low‑voltage (≤ 1 kV) distribution breakers. D] Arc‑flash mitigation devices. Answer: B Explanation: Vacuum interrupters are not typically used for high‑current DC because of arc re‑ignition issues. Question 45. The “step‑and‑touch” potential test in grounding verification measures: A] Voltage between two adjacent points on the ground surface. B] Voltage between a person’s hand and the ground. C] Voltage between the grounding electrode and neutral.
D] Voltage across a test resistor placed in the ground grid. Answer: A Explanation: Step potential is the voltage between feet; touch potential is between hand and feet. Question 46. When a transformer oil sample shows a moisture content of 200 ppm, the recommended action is: A] Immediate oil replacement. B] Continue operation; moisture is acceptable. C] Perform vacuum dehydration. D] Add silica gel packets to the tank. Answer: C Explanation: Moisture >150 ppm often requires oil dehydration to maintain dielectric strength. Question 47. The “load bank” used for battery testing simulates: A] High‑frequency noise. B] Realistic load currents over a specified period. C] Short‑circuit conditions. D] Voltage spikes. Answer: B Explanation: Load banks draw rated current from batteries to assess capacity and health. Question 48. In a SCADA alarm log, a “P‑ 34 ” code commonly indicates: A] Overtemperature on a transformer. B] Loss of communication with a remote terminal unit (RTU).
C] Connected to the highest phase voltage. D] Used to increase line voltage. Answer: B Explanation: Without a neutral conductor, the neutral point is not referenced to ground. Question 52. The “characteristic impedance” of a transmission line is defined as: A] √(R + jX) / (G + jB) B] √(L / C) C] √(R / G) D] √(Z₀ · Zₗ) Answer: B Explanation: For a lossless line, Z₀ = √(L / C). Question 53. In a three‑phase fault study, a “single line-to‑ground” fault on phase B will cause the fault current to be: A] √3 times the line‑to‑ground voltage divided by Z₀. B] Equal to the line‑to‑ground voltage divided by Z₀. C] Zero on phases A and C. D] The same magnitude on all three phases. Answer: A Explanation: I_f = √3 · V_phase / Z₀ for LG faults. Question 54. The “thermal rating” of a circuit breaker is determined by: A] The maximum voltage it can interrupt.
B] The amount of heat it can absorb without damage during a fault. C] Its mechanical operating speed. D] The size of its contacts. Answer: B Explanation: Thermal rating specifies the energy (I²t) the breaker can withstand. Question 55. A “zero‑sequence” network in fault analysis is used to model: A] Balanced three‑phase loads. B] Grounded neutral and earth return paths. C] Harmonic propagation. D] Voltage regulation devices. Answer: B Explanation: Zero‑sequence components represent currents that return via the ground. Question 56. When performing a “contact resistance” measurement on a breaker, a reading of 0.02 Ω indicates: A] Excellent contact condition. B] Slightly worn contacts. C] Need for immediate replacement. D] Open circuit. Answer: A Explanation: Contact resistance below 0.05 Ω is generally acceptable for high‑voltage breakers. Question 57. The “oil‑to‑water ratio” in a transformer’s cooling system is regulated to ensure:
