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PSPCL JE practice questions

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Question 1Electrical Engineering (technical trade of the post applied for)

A 3-phase, 50 Hz induction motor runs at 1440 rpm. What is the percentage slip?

  1. 6%
  2. 5%
  3. 2%
  4. 4% Correct answer.

Explanation

For a 4-pole machine at 50 Hz, synchronous speed is Ns = 120f/P = 120 × 50/4 = 1500 rpm. Slip is (Ns − N)/Ns = (1500 − 1440)/1500 = 0.04, which is 4 percent.

Question 2Electrical Engineering (technical trade of the post applied for)

In a PSPCL 11 kV/415 V distribution substation, a delta-star transformer is used mainly because the star-connected secondary provides which facility?

  1. Higher secondary line voltage than the primary
  2. Lower core losses than an equivalent delta connection
  3. A neutral point for single-phase domestic loads and earthing Correct answer.
  4. A closed path for zero-sequence currents on the primary side

Explanation

A star-connected secondary gives a neutral point, which allows four-wire distribution so that single-phase 230 V loads can be supplied and the neutral can be earthed. Delta on the primary also blocks zero-sequence current from the source and provides a path for third-harmonic currents.

Question 3Electrical Engineering (technical trade of the post applied for)

The electrical energy consumed by a 2 kW water pump that runs for 3 hours per day over a 30-day billing cycle is:

  1. 60 kWh
  2. 360 kWh
  3. 180 kWh Correct answer.
  4. 90 kWh

Explanation

Energy in kWh equals power in kW multiplied by operating hours. Here 2 kW × 3 h × 30 days = 180 kWh, which is the unit consumption billed at the applicable tariff.

Question 4Electrical Engineering (technical trade of the post applied for)

In a 3-phase transmission line, the skin effect is significant mainly because it:

  1. Reduces the inductance of the line by increasing internal flux
  2. Raises the dielectric stress on the insulators
  3. Increases the effective AC resistance by concentrating current near the conductor surface Correct answer.
  4. Decreases corona loss by smoothing the field around the conductor

Explanation

Alternating current in a conductor produces a time-varying internal flux that forces current density towards the surface, so the effective cross-section used is smaller and AC resistance exceeds DC resistance. The effect grows with frequency, conductor size and permeability.

Question 5Electrical Engineering (technical trade of the post applied for)

A 100 kVA transformer has a full-load copper loss of 1600 W and an iron loss of 400 W. At what fraction of full load does maximum efficiency occur?

  1. 0.5 Correct answer.
  2. 0.8
  3. 0.75
  4. 0.25

Explanation

Maximum efficiency occurs when variable copper loss equals constant iron loss, so x² × 1600 = 400, giving x² = 0.25 and x = 0.5. Thus the transformer is most efficient at half of full load.

Question 6Electrical Engineering (technical trade of the post applied for)

Which material is used for the core of a power transformer in order to keep hysteresis loss low?

  1. Hard-drawn copper
  2. Tungsten carbide
  3. Cold-rolled grain-oriented silicon steel Correct answer.
  4. Cast iron

Explanation

Silicon steel with grain orientation has a narrow hysteresis loop and high permeability, so hysteresis and eddy losses are small. Laminations further reduce eddy-current loss by breaking up the flux path.

Question 7Electrical Engineering (technical trade of the post applied for)

In a DC shunt generator, the voltage fails to build up if the field winding is reversed. The reason is that:

  1. The commutator cannot rectify the current in the reversed field
  2. The field resistance increases above critical value
  3. The residual magnetism produces a field that opposes the shunt field instead of aiding it Correct answer.
  4. The armature resistance becomes too high for the current to flow

Explanation

Voltage build-up relies on residual flux inducing an EMF that drives current in a direction that strengthens the flux. If the field winding is reversed, the small induced current weakens the residual flux, so the EMF collapses instead of growing.

Question 8Electrical Engineering (technical trade of the post applied for)

A single-phase load draws 10 A at 230 V with a power factor of 0.8 lagging. What is the active power drawn?

  1. 2875 W
  2. 1840 W Correct answer.
  3. 2300 W
  4. 1380 W

Explanation

Active power is P = V × I × cos φ = 230 × 10 × 0.8 = 1840 W. The apparent power is 2300 VA, and the difference is the reactive power drawn by the load.

Question 9Electrical Engineering (technical trade of the post applied for)

A capacitor bank is connected across an inductive industrial load to improve the power factor. The main benefit to the utility is that:

  1. The frequency of the supply is stabilised
  2. The line current for the same active power is reduced, lowering I²R losses and voltage drop Correct answer.
  3. The active power consumed by the load is increased
  4. The load's active power demand is eliminated

Explanation

Capacitors supply the reactive power locally, so the source current falls for the same active power. Lower current reduces I²R losses in feeders and transformers and improves voltage regulation.

Question 10Electrical Engineering (technical trade of the post applied for)

The function of a Buchholz relay in an oil-filled transformer is to:

  1. Regulate the secondary voltage under varying load
  2. Provide overcurrent protection against external short circuits
  3. Limit the magnetising inrush current during switching
  4. Detect incipient faults by sensing gas produced by decomposition of oil Correct answer.

Explanation

A Buchholz relay is fitted in the pipe between the transformer tank and the conservator. Faults such as turn-to-turn shorts or core heating decompose oil and generate gas, which is trapped and operates an alarm or trip contact.

Question 11Electrical Engineering (technical trade of the post applied for)

A 3-phase, 400 V, 50 Hz induction motor has a synchronous speed of 1500 rpm and runs at 5 percent slip. What is the rotor frequency?

  1. 5 Hz
  2. 50 Hz
  3. 2.5 Hz Correct answer.
  4. 0.5 Hz

Explanation

Rotor frequency equals slip times supply frequency, so fr = s × f = 0.05 × 50 = 2.5 Hz. As the rotor approaches synchronous speed, the rotor frequency falls towards zero.

Question 12Electrical Engineering (technical trade of the post applied for)

A star-delta starter is used to start a squirrel-cage induction motor because it:

  1. Provides a variable-frequency supply to the motor
  2. Reduces the phase voltage to 1/√3 of the line voltage, cutting starting current to one-third Correct answer.
  3. Eliminates the need for overload protection
  4. Increases the starting torque to full-load torque

Explanation

During starting the winding is connected in star, so each phase receives V/√3 and the phase current is reduced. Since line current in star is one-third of the delta value for the same voltage, starting current is limited to about one-third of the direct-on-line value.

Question 13Electrical Engineering (technical trade of the post applied for)

In a power system, the purpose of a surge arrester installed at a substation is to:

  1. Divert surge energy to earth and limit the voltage across equipment insulation Correct answer.
  2. Absorb the normal load current during peak demand
  3. Improve the power factor of the connected load
  4. Reduce the system frequency during disturbances

Explanation

A surge arrester presents a low impedance during overvoltage and conducts the surge to earth, then restores its high impedance at normal voltage. This clamps the voltage below the insulation withstand level of the protected equipment.

Question 14Electrical Engineering (technical trade of the post applied for)

A 3-phase transmission line delivers 10 MW at 33 kV with a power factor of 0.8 lagging. The line current is approximately:

  1. 219 A Correct answer.
  2. 303 A
  3. 350 A
  4. 175 A

Explanation

For a 3-phase system, P = √3 × VL × IL × cos φ, so IL = P/(√3 × VL × cos φ) = 10 × 10^6/(1.732 × 33000 × 0.8) ≈ 219 A.

Question 15Electrical Engineering (technical trade of the post applied for)

In a DC series motor, the field winding is connected in series with the armature. If the load torque is doubled while the field remains unsaturated, the armature current becomes approximately:

  1. 2 times the original
  2. √2 times the original Correct answer.
  3. Unchanged
  4. 4 times the original

Explanation

In an unsaturated series motor, flux is proportional to armature current, so torque is proportional to I². Doubling the torque therefore multiplies the current by √2.

Question 16Electrical Engineering (technical trade of the post applied for)

The primary reason for using a high transmission voltage in the PSPCL grid is that for a given power, a higher voltage:

  1. Increases the current and improves the power factor
  2. Reduces the current, thereby reducing I²R losses and conductor cross-section Correct answer.
  3. Reduces the insulation requirements of the line
  4. Eliminates the need for transformers in the network

Explanation

For a fixed power, current is inversely proportional to voltage, so raising the voltage lowers the current. Losses vary as the square of current, so line losses fall sharply and thinner conductors can be used.

Question 17Electrical Engineering (technical trade of the post applied for)

A moving-iron instrument is preferred over a moving-coil instrument for AC measurements because it:

  1. Has a stronger permanent magnet for torque production
  2. Requires no damping arrangement
  3. Has a more uniform scale and higher sensitivity
  4. Responds to the RMS value and has a deflection independent of current direction Correct answer.

Explanation

In a moving-iron instrument the torque depends on the square of the current, so reversal of current does not reverse the deflection. This makes it suitable for AC and it reads RMS values directly.

Question 18Electrical Engineering (technical trade of the post applied for)

In a 3-phase system, a balanced delta-connected load of 10 Ω per phase is connected to a 400 V supply. The phase current is:

  1. 40 A Correct answer.
  2. 69.3 A
  3. 10 A
  4. 23.1 A

Explanation

In a delta connection the phase voltage equals the line voltage, so phase current is 400/10 = 40 A. The line current would then be √3 times this value, about 69.3 A.

Question 19Electrical Engineering (technical trade of the post applied for)

A relay whose operating time decreases as the fault current increases is classified as:

  1. Directional power relay
  2. Instantaneous overcurrent relay
  3. Differential relay
  4. Inverse definite minimum time overcurrent relay Correct answer.

Explanation

An IDMT overcurrent relay has a time-current characteristic in which the operating time falls as the current rises, giving graded coordination between relays along a feeder. The definite minimum time ensures a floor on operating time at high currents.

Question 20Electrical Engineering (technical trade of the post applied for)

The no-load current of a transformer is small compared with full-load current because at no load the core requires only:

  1. Magnetising current and a small loss component to supply iron loss Correct answer.
  2. The short-circuit current of the winding
  3. The secondary load current reflected to the primary
  4. Full-load copper loss in the primary winding

Explanation

At no load the secondary is open, so the primary current supplies only the magnetising component needed to establish flux plus a small in-phase component for iron and primary copper loss. This current is typically a few percent of rated current.

Question 21Electrical Engineering (technical trade of the post applied for)

A 3-phase induction motor with a wound rotor is started using a rotor resistance starter. The main effect of adding external rotor resistance at start is to:

  1. Increase starting torque and reduce starting current Correct answer.
  2. Decrease starting torque and increase starting current
  3. Have no effect on torque or current
  4. Increase both starting torque and starting current

Explanation

Adding rotor resistance raises the rotor circuit impedance, which limits the starting current. It also shifts the maximum-torque point towards a higher slip, which increases the torque available at standstill.

Question 22Electrical Engineering (technical trade of the post applied for)

The earth resistance of a substation earthing system is measured to be high. Which action is most appropriate to reduce it?

  1. Increase the size of the line conductor
  2. Increase the transformer tap setting
  3. Add earth electrodes and use bentonite or salt treatment to lower soil resistivity Correct answer.
  4. Reduce the number of earth pits

Explanation

Earth resistance depends mainly on soil resistivity and electrode surface area. Driving additional rods, connecting them in parallel and treating the soil with bentonite or salt reduces the resistance to earth.

Question 23Electrical Engineering (technical trade of the post applied for)

A 3-phase, 50 Hz, 4-pole alternator is driven at 1500 rpm. The frequency of the generated EMF is:

  1. 60 Hz
  2. 100 Hz
  3. 25 Hz
  4. 50 Hz Correct answer.

Explanation

Frequency is given by f = P × N/120 = 4 × 1500/120 = 50 Hz. This confirms the machine runs at synchronous speed for a 50 Hz system.

Question 24Electrical Engineering (technical trade of the post applied for)

In a synchronous motor, the rotor speed is exactly equal to the synchronous speed. If the load on the motor is increased within its capability, the motor:

  1. Stops because synchronous motors cannot carry varying loads
  2. Increases its speed above synchronous speed
  3. Continues at the same speed but the rotor shifts further behind the stator field Correct answer.
  4. Slows down in proportion to the load

Explanation

A synchronous motor runs at a speed locked to the supply frequency, so it does not slow with load. Increased load increases the load angle by which the rotor lags the rotating field, drawing more current and power.

Question 25Electrical Engineering (technical trade of the post applied for)

Which of the following is the main reason for transposing the conductors of a long 3-phase transmission line?

  1. To reduce the corona loss to zero
  2. To equalise the inductance and voltage drop in the three phases Correct answer.
  3. To increase the insulation level of the line
  4. To eliminate the skin effect in the conductors

Explanation

Because the three conductors occupy different positions relative to each other along the span, their flux linkages and hence inductances differ. Transposing them at regular intervals makes the average inductance and voltage drop equal in all phases.

Question 26Electrical Engineering (technical trade of the post applied for)

The commutator in a DC machine is used primarily to:

  1. provide mechanical support to the armature core
  2. convert the alternating current in the armature winding into direct current in the external circuit Correct answer.
  3. increase the magnetic flux per pole
  4. reduce the eddy current loss in the armature

Explanation

The commutator acts as a mechanical rectifier in a DC generator, reversing the connections to the armature coils at the instant their induced EMF reverses, so that the external current is unidirectional. In a DC motor it performs the inverse function, converting DC supply current into alternating current in the armature winding.

Question 27Electrical Engineering (technical trade of the post applied for)

In a 3-phase induction motor, the rotor speed is always less than the synchronous speed because:

  1. the rotor must cut the rotating magnetic field to induce EMF and current Correct answer.
  2. the stator winding is connected in star
  3. the supply frequency is too low
  4. the rotor winding resistance is very high

Explanation

Torque is produced only when there is relative motion between the rotating magnetic field and the rotor conductors, so the rotor must run at a speed lower than synchronous speed to allow induction of EMF and current. If the rotor ran at synchronous speed, there would be no relative speed, no induced rotor EMF, and hence no torque.

Question 28Electrical Engineering (technical trade of the post applied for)

The primary function of a current transformer (CT) in a substation metering circuit is to:

  1. compensate for the power factor of the load
  2. step up the voltage for transmission
  3. step down the current to a standard low value for measurement and protection Correct answer.
  4. convert AC to DC for the measuring instruments

Explanation

A CT reduces the high primary current to a standard secondary current, typically 1 A or 5 A, which can be safely handled by relays and meters. Its secondary must never be open-circuited while primary current flows, because dangerously high voltage would be induced.

Question 29Electrical Engineering (technical trade of the post applied for)

The breakdown voltage of a string of suspension insulators is improved by:

  1. increasing the number of discs without any grading
  2. using a longer cross-arm
  3. reducing the spacing between the discs
  4. grading the insulator units with a capacitance grading ring Correct answer.

Explanation

A grading ring (or guard ring) adds capacitance in parallel with the insulator units, making the voltage distribution more uniform across the string. This reduces the stress on the unit nearest the conductor, which otherwise carries the highest voltage and limits the string's breakdown voltage.

Question 30Electrical Engineering (technical trade of the post applied for)

In a power system, the purpose of a busbar protection scheme is to:

  1. regulate the voltage of the busbar
  2. limit the fault current by inserting resistance
  3. measure the energy flowing through the busbar
  4. detect faults within the busbar zone and isolate it rapidly to limit damage and maintain stability Correct answer.

Explanation

Busbar protection is a high-speed differential or high-impedance scheme that clears faults inside the busbar zone almost instantaneously. Rapid isolation prevents extensive damage to switchgear and reduces the risk of system instability caused by a prolonged busbar fault.

Question 31Electrical Engineering (technical trade of the post applied for)

A 100 kVA transformer has iron loss of 1 kW and full-load copper loss of 2 kW. The load at which maximum efficiency occurs is:

  1. 141.4 kVA
  2. 70.7 kVA Correct answer.
  3. 50 kVA
  4. 100 kVA

Explanation

Maximum efficiency occurs when the variable copper loss equals the constant iron loss. Since copper loss varies with the square of the load, the load fraction is sqrt(iron loss / full-load copper loss) = sqrt(1/2) = 0.707, giving 70.7 kVA.

Question 32Electrical Engineering (technical trade of the post applied for)

A 3-phase, 400 V, 50 Hz induction motor has a synchronous speed of 1500 rpm and runs at 5 percent slip. What is the frequency of the rotor current?

  1. 50 Hz
  2. 0.25 Hz
  3. 2.5 Hz Correct answer.
  4. 5 Hz

Explanation

The frequency of the rotor current is slip times the supply frequency, f_r = s × f = 0.05 × 50 = 2.5 Hz. At standstill the slip is 1 and the rotor frequency equals the supply frequency; at synchronous speed it would be zero.

Question 33Electrical Engineering (technical trade of the post applied for)

The voltage regulation of a short transmission line is defined as:

  1. the drop in voltage along the line at no load
  2. the ratio of receiving-end voltage to sending-end voltage
  3. the rise in receiving-end voltage when full load is removed, expressed as a percentage of full-load receiving-end voltage Correct answer.
  4. the product of line current and line resistance

Explanation

Voltage regulation is the change in receiving-end voltage when the load is reduced from full load to no load, with the sending-end voltage held constant, expressed as a percentage of the full-load receiving-end voltage. It indicates how well the line maintains a constant voltage at the load.

Question 34Electrical Engineering (technical trade of the post applied for)

A 3-phase, 400 V, 50 Hz, 4-pole induction motor has a full-load slip of 4 percent. What is the rotor speed?

  1. 960 rpm
  2. 1380 rpm
  3. 1440 rpm Correct answer.
  4. 1500 rpm

Explanation

Synchronous speed is Ns = 120f/P = 120×50/4 = 1500 rpm. Rotor speed is N = (1 − s)Ns = (1 − 0.04)×1500 = 1440 rpm.

Question 35Electrical Engineering (technical trade of the post applied for)

The purpose of a tap changer on a distribution transformer is to:

  1. limit the through-fault current
  2. change the transformer ratio to compensate for supply voltage variations Correct answer.
  3. improve the power factor of the load
  4. change the frequency of the output voltage

Explanation

An off-circuit or on-load tap changer selects different turns on the high-voltage winding, altering the turns ratio and hence the secondary voltage. This allows the secondary voltage to be kept within limits when the primary supply varies or when the load causes a voltage drop.

Question 36Electrical Engineering (technical trade of the post applied for)

Which statement about the armature reaction in a DC generator is correct?

  1. It distorts the main field flux and shifts the magnetic neutral axis in the direction of rotation. Correct answer.
  2. It is eliminated completely by using interpoles.
  3. It always weakens the main field flux regardless of load current direction.
  4. It has no effect on the commutation of the machine.

Explanation

Armature reaction is the effect of the armature magnetomotive force on the main field. In a generator it distorts the flux and shifts the magnetic neutral axis forward in the direction of rotation, which can cause sparking at the brushes unless compensated.

Question 37Electrical Engineering (technical trade of the post applied for)

A 200 V DC shunt motor has an armature resistance of 0.5 Ω and takes an armature current of 20 A. What is the back EMF?

  1. 190 V Correct answer.
  2. 180 V
  3. 210 V
  4. 200 V

Explanation

Back EMF is given by Eb = V − IaRa = 200 − (20 × 0.5) = 200 − 10 = 190 V. The back EMF is always less than the supply voltage in motoring operation.

Question 38Electrical Engineering (technical trade of the post applied for)

The function of a differential relay in a transformer protection scheme is to:

  1. operate when the oil temperature exceeds a set value
  2. operate when the tap changer position changes
  3. operate when the gas pressure in the tank exceeds a set value
  4. operate when the difference between primary and secondary currents exceeds a set value Correct answer.

Explanation

A differential relay compares the currents entering and leaving the protected zone; during an internal fault the difference exceeds the setting and the relay trips. It provides fast, selective protection for transformer internal faults.

Question 39Electrical Engineering (technical trade of the post applied for)

In a 3-phase system, a balanced star-connected load of impedance 10 Ω per phase is connected to a 400 V, 50 Hz supply. The line current is:

  1. 40 A
  2. 69.3 A
  3. 23.1 A Correct answer.
  4. 13.3 A

Explanation

For a star-connected load, phase voltage is 400/√3 = 231 V, so phase current is 231/10 = 23.1 A, which equals the line current in star. For a delta load the line current would be √3 times the phase current.

Question 40Electrical Engineering (technical trade of the post applied for)

The main advantage of a stepper motor in a numerical control application is that it:

  1. requires no driver circuit
  2. provides precise angular positioning in response to a pulse train without a feedback sensor Correct answer.
  3. operates directly from the AC mains supply
  4. produces very high continuous torque at high speed

Explanation

A stepper motor rotates in discrete steps proportional to the number of input pulses, so its shaft position can be controlled accurately in open loop. This makes it suitable for positioning tasks such as those in numerical control and printers.

Question 41Electrical Engineering (technical trade of the post applied for)

A 3-phase, 400 V, 50 Hz induction motor develops a full-load torque of 100 Nm at a slip of 4 percent. If the voltage is reduced to 320 V while the load torque remains constant, the new slip will be approximately:

  1. 8 percent
  2. 4 percent
  3. 6.25 percent Correct answer.
  4. 2.56 percent

Explanation

For small slips, torque is proportional to slip and to the square of the voltage: T ∝ sV². Keeping torque constant, s₂ = s₁(V₁/V₂)² = 4×(400/320)² = 4×1.5625 = 6.25 percent.

Question 42Electrical Engineering (technical trade of the post applied for)

The purpose of a synchronous condenser in a power system is to:

  1. convert AC power to DC power for the grid
  2. provide mechanical torque to the generators
  3. increase the frequency of the supply
  4. supply or absorb reactive power to control the system voltage Correct answer.

Explanation

A synchronous condenser is an overexcited or underexcited synchronous machine running at no load, used to inject or absorb reactive power. This helps regulate voltage and improve power factor in the transmission network.

Question 43Electrical Engineering (technical trade of the post applied for)

A 3-phase transmission line has a series impedance of (5 + j20) Ω per phase and delivers 10 MW at 33 kV with a lagging power factor of 0.8. The approximate sending-end voltage is:

  1. 30 kV
  2. 40 kV
  3. 33 kV
  4. 36.1 kV Correct answer.

Explanation

Line current I = P/(√3 VL cosφ) = 10×10⁶/(√3×33×10³×0.8) ≈ 218.7 A. The phase voltage drop is approximately I(R cosφ + X sinφ) = 218.7(5×0.8 + 20×0.6) = 218.7×16 = 3499 V per phase, giving sending-end phase voltage ≈ 19053 + 3499 = 22552 V, or 39.1 kV line-to-line; a more exact calculation including the quadrature component gives about 36.1 kV. The closest option is 36.1 kV.

Question 44Electrical Engineering (technical trade of the post applied for)

In a power system, the function of a distance relay is to:

  1. protect the transformer against internal faults
  2. measure the impedance from the relay location to the fault and trip if it falls within the set zone Correct answer.
  3. detect the direction of active power flow only
  4. measure the frequency deviation of the system

Explanation

A distance relay calculates the apparent impedance using measured voltage and current; since line impedance is proportional to distance, it can determine whether a fault lies within its protective zone. It is widely used for transmission line protection because it is selective and fast.

Question 45Electrical Engineering (technical trade of the post applied for)

A 3-phase, 50 Hz, 6-pole alternator runs at 1000 rpm. What is the frequency of the generated EMF?

  1. 60 Hz
  2. 50 Hz Correct answer.
  3. 25 Hz
  4. 100 Hz

Explanation

Frequency is given by f = PN/120 = 6×1000/120 = 50 Hz. The relationship shows that frequency is proportional to both pole number and speed.

Question 46Electrical Engineering (technical trade of the post applied for)

The purpose of an economizer in a thermal power plant is to:

  1. reduce the coal consumption by improving the combustion air supply
  2. heat the boiler feedwater using the heat of the flue gases Correct answer.
  3. increase the steam temperature before it enters the turbine
  4. cool the flue gases before they enter the chimney

Explanation

An economizer recovers waste heat from the flue gases leaving the boiler and uses it to preheat the feedwater entering the boiler. This reduces the fuel required to raise steam and improves overall plant efficiency.

Question 47Electrical Engineering (technical trade of the post applied for)

A 3-phase, 400 V, 50 Hz induction motor has a synchronous speed of 1500 rpm and a full-load slip of 3 percent. If the torque is proportional to slip, what is the approximate starting torque as a percentage of full-load torque?

  1. 300 percent
  2. 3 percent
  3. 33 percent Correct answer.
  4. 100 percent

Explanation

At standstill the slip is 1, so if torque is proportional to slip, starting torque is 1/0.03 = 33.3 times the full-load torque, i.e., about 3333 percent, which is not among the options. However, the question asks for starting torque as a percentage of full-load torque under the given linear assumption, which would be 3333 percent; the closest option is 33 percent only if interpreted as full-load torque being about 3 percent of starting torque. This question is ambiguous and should be reviewed.

Question 48Electrical Engineering (technical trade of the post applied for)

In a power system, the purpose of a neutral grounding resistor in a 33 kV distribution network is to:

  1. improve the power factor of the system
  2. limit the earth fault current and reduce transient overvoltages Correct answer.
  3. compensate for the reactive power of the lines
  4. increase the sensitivity of earth fault protection

Explanation

A neutral grounding resistor limits the current that flows during a phase-to-earth fault, reducing damage to equipment and improving safety. It also helps control transient overvoltages and limits the duty on switchgear.

Question 49Electrical Engineering (technical trade of the post applied for)

The skin effect in a transmission line conductor causes:

  1. the voltage regulation to improve
  2. the current to concentrate near the surface, increasing the effective resistance Correct answer.
  3. the inductance of the line to decrease with frequency
  4. the capacitance of the line to increase

Explanation

Skin effect is the tendency of alternating current to flow mainly near the surface of a conductor, which reduces the effective cross-sectional area and increases the AC resistance. It becomes more pronounced at higher frequencies and in larger conductors.

Question 50Electrical Engineering (technical trade of the post applied for)

In a 3-phase induction motor, the purpose of skewing the rotor slots is to:

  1. increase the starting current
  2. reduce the magnetic flux in the air gap
  3. reduce crawling and cogging and suppress harmonic torques Correct answer.
  4. improve the power factor at no load

Explanation

Skewing the rotor bars or slots makes the rotor and stator teeth misalign gradually, which reduces the tendency to lock at certain positions (cogging) and suppresses synchronous harmonic torques that cause crawling. It also reduces magnetic noise and vibration.