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Punjab PSEB Matric NSQF Power study materials
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Punjab PSEB Matric NSQF Power practice questions

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Question 1NSQF Vocational Trade Theory (Power)

In a power distribution system, what is the standard function of a transformer?

  1. It increases the frequency of the supply
  2. It stores electrical energy for later use
  3. It changes the voltage level while keeping frequency nearly constant Correct answer.
  4. It converts alternating current into direct current

Explanation

A transformer transfers electrical energy between two windings through mutual induction, raising or lowering voltage according to the turns ratio. The supply frequency stays essentially unchanged because the same alternating flux links both windings. Conversion to DC needs a rectifier, and energy storage needs a battery or capacitor bank.

Question 2NSQF Vocational Trade Theory (Power)

Which instrument is connected in series with a circuit to measure current?

  1. Ammeter Correct answer.
  2. Wattmeter
  3. Voltmeter
  4. Megger

Explanation

An ammeter must carry the circuit current, so it is connected in series and designed with very low resistance to avoid disturbing the circuit. A voltmeter is connected in parallel across the points whose potential difference is required. A megger measures insulation resistance and a wattmeter has both current and voltage coils.

Question 3NSQF Vocational Trade Theory (Power)

Why is the earth wire connected to the metal body of an electrical appliance?

  1. To increase the operating voltage of the appliance
  2. To reduce the electricity bill of the appliance
  3. To improve the power factor of the appliance
  4. To give a low-resistance path so that fault current trips the protective device Correct answer.

Explanation

If a live conductor touches the metal body, the earth connection provides a low-resistance return path. The resulting large fault current operates the fuse or MCB quickly and limits the body potential to a safe value. Earthing has no role in billing, voltage boosting or power-factor correction.

Question 4NSQF Vocational Trade Theory (Power)

A 100 W lamp is used for 5 hours daily. What energy does it consume in 30 days?

  1. 10 kWh
  2. 5 kWh
  3. 30 kWh
  4. 15 kWh Correct answer.

Explanation

Daily energy equals power multiplied by time: 100 W × 5 h = 500 Wh = 0.5 kWh. Over 30 days this becomes 0.5 × 30 = 15 kWh. The other values arise from arithmetic slips in the multiplication.

Question 5NSQF Vocational Trade Theory (Power)

In a three-phase system, what is the phase difference between successive phase voltages?

  1. 360 degrees
  2. 90 degrees
  3. 180 degrees
  4. 120 degrees Correct answer.

Explanation

Three-phase supplies are generated by three windings displaced by 120 electrical degrees in space, giving three voltages equal in magnitude and 120 degrees apart. This arrangement produces a constant total power and rotating magnetic field. Angles of 90 or 180 degrees do not give a balanced three-phase set.

Question 6NSQF Vocational Trade Theory (Power)

Which material is most commonly used for the conductor of an overhead transmission line because of its good conductivity and moderate cost?

  1. Tungsten
  2. Aluminium Correct answer.
  3. Copper
  4. Silver

Explanation

Aluminium is lighter and cheaper than copper and has adequate conductivity, so it is standard for overhead conductors, often as ACSR. Its lower weight reduces tower loading. Silver and tungsten have other uses and are far too costly for long lines.

Question 7NSQF Vocational Trade Theory (Power)

What is the purpose of a fuse in an electrical circuit?

  1. To smooth the ripple in a DC supply
  2. To step down the supply voltage
  3. To open the circuit when current exceeds a safe value Correct answer.
  4. To correct the power factor

Explanation

A fuse element melts when the current exceeds its rated value, breaking the circuit and protecting conductors and equipment from overload or short circuit. Voltage stepping, ripple smoothing and power-factor correction are done by transformers, filters and capacitors respectively.

Question 8NSQF Vocational Trade Theory (Power)

Which type of power station converts the kinetic energy of falling water into electrical energy?

  1. Nuclear power station
  2. Hydroelectric power station Correct answer.
  3. Diesel power station
  4. Thermal power station

Explanation

In a hydroelectric station, water stored at a height flows through a turbine, and the turbine drives an alternator. Thermal, nuclear and diesel stations instead release heat to raise steam or expand hot gases. The hydro plant therefore depends on rainfall and reservoir level.

Question 9NSQF Vocational Trade Theory (Power)

In a lead-acid battery, which material forms the positive plate?

  1. Sponge lead
  2. Lead sulphate
  3. Lead peroxide Correct answer.
  4. Antimony

Explanation

The charged positive plate of a lead-acid cell is lead peroxide, a dark brown material, while the negative plate is spongy lead. During discharge both plates are converted towards lead sulphate. Antimony is added to grid alloys for strength, not as the active material.

Question 10NSQF Vocational Trade Theory (Power)

A transformer has 200 turns on the primary and 1000 turns on the secondary. If the primary is supplied at 230 V, what is the secondary voltage (ignoring losses)?

  1. 575 V
  2. 46 V
  3. 1150 V Correct answer.
  4. 230 V

Explanation

The voltage ratio equals the turns ratio: V2 = V1 × (N2/N1) = 230 × (1000/200) = 230 × 5 = 1150 V. This is a step-up transformer because the secondary has more turns. Dividing instead of multiplying gives the wrong lower values.

Question 11NSQF Vocational Trade Theory (Power)

Which starting method reduces the starting current of a squirrel-cage induction motor by connecting the stator winding in star during starting?

  1. Star-delta starter Correct answer.
  2. Rotor-resistance starter
  3. Auto-transformer starter
  4. Direct-on-line starter

Explanation

In a star-delta starter the stator is connected in star at start, so each phase receives 1/√3 of the line voltage and the starting current falls to about one third of the direct-on-line value. After the motor accelerates, the winding is switched to delta for full voltage running. Direct-on-line starting gives the highest current.

Question 12NSQF Vocational Trade Theory (Power)

What is the synchronous speed of a 4-pole induction motor supplied at 50 Hz?

  1. 1000 rpm
  2. 1500 rpm Correct answer.
  3. 750 rpm
  4. 3000 rpm

Explanation

Synchronous speed is Ns = 120f/P = 120 × 50/4 = 1500 rpm. The actual rotor speed is slightly lower because of slip. Using the wrong pole number gives the other listed speeds.

Question 13NSQF Vocational Trade Theory (Power)

Why is the armature of a DC machine laminated?

  1. To reduce eddy-current losses Correct answer.
  2. To improve the insulation resistance of the winding
  3. To reduce the weight of the commutator
  4. To increase the mechanical strength of the shaft

Explanation

The armature core carries a rotating flux, so solid iron would allow large eddy currents and heavy heating. Laminating the core with thin insulated sheets breaks the eddy-current paths and reduces loss. Shaft strength, winding insulation and commutator weight are unrelated to lamination.

Question 14NSQF Vocational Trade Theory (Power)

What is the main advantage of transmitting power at high voltage?

  1. It reduces the current and hence the I-squared-R line loss Correct answer.
  2. It allows the use of thinner insulators
  3. It removes the skin effect completely
  4. It eliminates the need for transformers

Explanation

For a given power, raising the voltage lowers the current, and line loss equals current squared times resistance. High-voltage transmission therefore improves efficiency and regulation. Transformers become essential at both ends, and insulation requirements actually increase.

Question 15NSQF Vocational Trade Theory (Power)

Which device converts electrical energy into mechanical energy?

  1. Transformer
  2. Alternator
  3. Electric motor Correct answer.
  4. Rectifier

Explanation

An electric motor takes electrical input and produces shaft torque through the interaction of current-carrying conductors with a magnetic field. An alternator and a transformer work in the reverse or non-mechanical direction, and a rectifier only changes AC to DC.

Question 16NSQF Vocational Trade Theory (Power)

In a single-phase circuit the power factor is 0.8 lagging and the apparent power is 10 kVA. What is the active power?

  1. 12.5 kW
  2. 6 kW
  3. 8 kW Correct answer.
  4. 10 kW

Explanation

Active power equals apparent power multiplied by power factor: 10 kVA × 0.8 = 8 kW. The remaining 6 kvar is reactive power. Dividing by the power factor instead of multiplying gives the incorrect larger value.

Question 17NSQF Vocational Trade Theory (Power)

What does the abbreviation MCB stand for in a distribution board?

  1. Motor Control Board
  2. Miniature Circuit Breaker Correct answer.
  3. Meter Calibration Bridge
  4. Main Cable Box

Explanation

An MCB is a miniature circuit breaker that trips automatically on overload or short circuit and can be reset without replacing any part. It replaces the rewirable fuse in modern boards. The other expansions are invented terms not used in practice.

Question 18NSQF Vocational Trade Theory (Power)

A technician finds that the insulation resistance of a motor winding measured with a megger is very low. What does this indicate?

  1. The supply frequency is incorrect
  2. The insulation has deteriorated and the winding may fail or leak to earth Correct answer.
  3. The winding resistance is low and the motor will draw more current normally
  4. The motor is running at synchronous speed

Explanation

A megger applies a high DC test voltage to measure insulation resistance between winding and earth. A low reading means moisture, dirt or damaged insulation allows leakage current, so the machine must be dried or repaired before use. Winding DC resistance and supply frequency are different quantities.

Question 19NSQF Vocational Trade Theory (Power)

Which of these is a renewable source of electrical energy?

  1. Natural gas
  2. Coal
  3. Diesel
  4. Solar photovoltaic Correct answer.

Explanation

Solar photovoltaic panels convert sunlight directly into electricity and the resource is naturally replenished. Coal, natural gas and diesel are fossil fuels with finite reserves and produce emissions when burnt. Renewable sources also include wind, hydro and biomass.

Question 20NSQF Vocational Trade Theory (Power)

In a DC series motor, why must the motor never be started without a mechanical load?

  1. The field current would become zero and the motor would stop
  2. The commutator would reverse its polarity
  3. The speed would rise to a dangerously high value Correct answer.
  4. The armature resistance would increase suddenly

Explanation

In a series motor the flux is proportional to armature current, so on light load the flux is very small and the speed rises sharply, risking mechanical damage. Series motors are therefore coupled to loads such as cranes and traction drives. Field failure in a shunt motor is the case that causes runaway from zero flux.

Question 21NSQF Vocational Trade Theory (Power)

Two 100 ohm resistors are connected in parallel. What is the equivalent resistance?

  1. 200 ohm
  2. 50 ohm Correct answer.
  3. 100 ohm
  4. 25 ohm

Explanation

For two equal resistors in parallel the equivalent is half of one value: (100 × 100)/(100 + 100) = 50 ohm. The parallel combination is always less than the smallest branch resistance. Adding the values gives the series result, which is 200 ohm.

Question 22NSQF Vocational Trade Theory (Power)

A transmission line experiences corona discharge. Which condition makes corona more likely?

  1. Low voltage and large conductor spacing
  2. High voltage and small conductor diameter Correct answer.
  3. Direct current instead of alternating current
  4. Low humidity and smooth conductors

Explanation

Corona occurs when the surface voltage gradient of the conductor exceeds the breakdown strength of air. High voltage and small conductor radius increase the gradient, so corona appears first at sharp points and thin wires. Bundled conductors and larger diameters reduce the effect.

Question 23NSQF Vocational Trade Theory (Power)

What is the role of the commutator in a DC generator?

  1. It converts the alternating EMF in the armature coils into unidirectional output Correct answer.
  2. It increases the generated voltage by magnetic induction
  3. It regulates the field current automatically
  4. It cools the armature winding during operation

Explanation

The EMF induced in a rotating armature coil reverses every half cycle. The commutator and brushes act as a mechanical rectifier so that the external circuit receives unidirectional voltage. Voltage build-up depends on field resistance and speed, while cooling and field regulation are separate functions.

Question 24NSQF Vocational Trade Theory (Power)

A consumer's meter records 600 kWh over 30 days. If the tariff is Rs 7 per unit, what is the bill?

  1. Rs 4200 Correct answer.
  2. Rs 2100
  3. Rs 7000
  4. Rs 6000

Explanation

One unit is one kilowatt-hour, so the energy is 600 units. Multiplying by the tariff gives 600 × 7 = Rs 4200. Multiplying by 7 and then by 30, or using the wrong unit conversion, produces the other figures.

Question 25NSQF Vocational Trade Theory (Power)

Why are overhead lines usually preferred over underground cables for long-distance power transmission?

  1. Overhead lines carry direct current more easily
  2. Overhead lines have lower inductance than cables
  3. Overhead lines need no insulation at all
  4. Overhead lines have lower installation cost and easier fault location Correct answer.

Explanation

Overhead construction is much cheaper per kilometre, and faults can be seen and repaired from the ground. Underground cables need costly insulation and excavation, although they are safer and need less right of way in cities. Cable inductance is actually lower than that of overhead lines.

Question 26NSQF Vocational Trade Theory (Power)

In a thermal power station, which component raises the pressure and temperature of steam before it enters the turbine?

  1. The boiler superheater and economiser arrangement Correct answer.
  2. The electrostatic precipitator
  3. The coal conveyor
  4. The cooling tower

Explanation

Steam leaving the boiler drum is passed through a superheater to raise its temperature and through an economiser that recovers heat from flue gases, improving cycle efficiency before the turbine. Cooling towers reject waste heat from the condenser, precipitators trap fly ash, and conveyors only handle fuel.

Question 27NSQF Vocational Trade Theory (Power)

A hydroelectric plant has a net head of 50 m and a discharge of 10 cubic metres per second. Taking water density as 1000 kg per cubic metre and g as 9.81 m/s squared, what is the hydraulic power available?

  1. About 49 MW
  2. About 4.9 MW Correct answer.
  3. About 490 kW
  4. About 0.49 MW

Explanation

Hydraulic power equals density times g times head times discharge, that is 1000 x 9.81 x 50 x 10, giving 4,905,000 W or roughly 4.9 MW. This is the power in the water before turbine and generator efficiencies are applied.

Question 28NSQF Vocational Trade Theory (Power)

In a solar photovoltaic module, what is the function of the bypass diode connected across a group of cells?

  1. It provides an alternate current path so that shaded cells do not overheat or block the string output Correct answer.
  2. It converts direct current into alternating current
  3. It stores surplus energy during peak sunshine
  4. It boosts the module voltage above the open-circuit value

Explanation

When cells are shaded they stop generating and act as a load, causing hot spots and reducing string current. A bypass diode gives current an alternate route around the affected group, protecting the cells and preserving most of the string output.

Question 29NSQF Vocational Trade Theory (Power)

Why is the tower of a wind turbine generator built very tall, often over 80 m?

  1. To reach stronger and steadier wind speeds at greater height above ground Correct answer.
  2. To avoid the need for a gearbox
  3. To allow the blades to turn more slowly
  4. To reduce the weight of the nacelle

Explanation

Wind speed and steadiness increase with height above ground because surface friction and turbulence decrease. A taller tower therefore captures more energy, and power in the wind varies with the cube of wind speed, so even a modest gain in speed gives a large gain in output.

Question 30NSQF Vocational Trade Theory (Power)

In a nuclear power reactor, what is the role of the moderator?

  1. It absorbs all neutrons to shut the reactor down
  2. It slows down fast neutrons so that they can sustain the fission chain reaction Correct answer.
  3. It shields operators from gamma radiation
  4. It cools the reactor core and drives the turbine directly

Explanation

Fission releases fast neutrons, which are far less likely to cause further fission in uranium-235. A moderator such as light water, heavy water or graphite slows neutrons to thermal energies where the fission cross-section is high, sustaining the chain reaction.

Question 31NSQF Vocational Trade Theory (Power)

A diesel generator set is rated 100 kVA at 0.8 power factor. What is its rated active power output?

  1. 80 kW Correct answer.
  2. 125 kW
  3. 20 kW
  4. 100 kW

Explanation

Active power equals apparent power multiplied by power factor, so 100 kVA x 0.8 gives 80 kW. The kVA rating limits current and heating, while the kW rating is what the prime mover can actually deliver.

Question 32NSQF Vocational Trade Theory (Power)

Which statement correctly describes the difference between a unit (kWh) and a kilowatt in an electricity bill?

  1. A unit is a rate of power, while a kilowatt is an amount of energy
  2. A unit equals one kilowatt per second
  3. A kilowatt is a rate of power, while one unit equals one kilowatt sustained for one hour Correct answer.
  4. Both terms mean exactly the same quantity

Explanation

The kilowatt measures power, the rate at which energy is used. One unit of electricity is one kilowatt-hour, the energy consumed when one kilowatt is drawn for one hour, which is why bills charge per unit.

Question 33NSQF Vocational Trade Theory (Power)

A single-phase energy meter makes 600 revolutions per kWh. If the disc makes 150 revolutions in a test, what energy has been recorded?

  1. 4 kWh
  2. 2.5 kWh
  3. 0.4 kWh
  4. 0.25 kWh Correct answer.

Explanation

Energy equals revolutions divided by the meter constant, so 150 divided by 600 gives 0.25 kWh. The meter constant expresses how many disc revolutions correspond to one kilowatt-hour.

Question 34NSQF Vocational Trade Theory (Power)

What is the purpose of earthing the metallic body of an electrical appliance?

  1. To increase the voltage supplied to the appliance
  2. To provide a low-resistance path so that fault current trips the protective device and the body stays near earth potential Correct answer.
  3. To reduce the electricity consumption of the appliance
  4. To improve the power factor of the appliance

Explanation

If insulation fails and a live conductor touches the metal body, the earth connection carries fault current back to the source. This quickly operates the fuse or MCB and holds the body at near earth potential, protecting the user from shock.

Question 35NSQF Vocational Trade Theory (Power)

Why is a residual current circuit breaker (RCCB) used in addition to an MCB in a domestic installation?

  1. It detects small earth-leakage currents that an MCB will not trip on, protecting people from shock Correct answer.
  2. It limits the maximum voltage supplied to the house
  3. It interrupts short-circuit currents faster than any MCB
  4. It corrects the power factor of the load

Explanation

An MCB responds to overload and short-circuit current but not to a small current flowing to earth through a person. An RCCB compares live and neutral currents and trips at a low leakage, typically 30 mA, giving shock protection.

Question 36NSQF Vocational Trade Theory (Power)

A cable of 4 square millimetre cross-section carries 25 A. If the same load is carried by a 2.5 square millimetre cable, what happens to the current density and heating?

  1. Current density falls and heating falls
  2. Current density stays the same because current is unchanged
  3. Current density and resistive heating per unit volume both increase Correct answer.
  4. Heating falls because resistance per metre decreases

Explanation

Current density is current divided by cross-sectional area, so reducing area from 4 to 2.5 square millimetres raises it. Resistance per metre rises with smaller area, so I squared R heating per metre also rises, which is why cable size must be chosen for the load.

Question 37NSQF Vocational Trade Theory (Power)

In a three-phase star-connected system with a line voltage of 415 V, what is the phase voltage?

  1. About 240 V Correct answer.
  2. About 415 V
  3. About 138 V
  4. About 720 V

Explanation

For a star connection the phase voltage equals the line voltage divided by the square root of three, so 415 divided by 1.732 gives about 240 V. This is why domestic single-phase supply is taken between one line and neutral.

Question 38NSQF Vocational Trade Theory (Power)

A balanced three-phase load draws 20 A per phase at 415 V line voltage with a power factor of 0.85. What is the total active power?

  1. About 21.2 kW
  2. About 36.6 kW
  3. About 7.1 kW
  4. About 12.2 kW Correct answer.

Explanation

Three-phase active power is root three times line voltage times line current times power factor, that is 1.732 x 415 x 20 x 0.85, giving roughly 12.2 kW. In a balanced star or delta load the line current equals the phase current for star, and this formula applies to both connections.

Question 39NSQF Vocational Trade Theory (Power)

Why is the neutral of a distribution transformer secondary usually earthed at the transformer itself?

  1. To allow the transformer to run without a neutral conductor
  2. To fix the neutral near earth potential so that phase-to-earth voltage stays limited and protective devices operate correctly Correct answer.
  3. To increase the secondary voltage
  4. To reduce the transformer copper loss

Explanation

Earthing the neutral establishes a reference so that each phase stays at a defined voltage to earth, and earth-fault current can return to the source to operate protection. Without this reference, an earth fault could raise the potential of exposed metalwork to a dangerous value.

Question 40NSQF Vocational Trade Theory (Power)

A distribution transformer is rated 11 kV / 415 V. What is its approximate turns ratio?

  1. About 26.5 to 1 Correct answer.
  2. About 4.15 to 1
  3. About 415 to 1
  4. About 11 to 1

Explanation

The turns ratio equals the primary to secondary voltage ratio, so 11000 divided by 415 gives about 26.5. The transformer steps the 11 kV distribution voltage down to the 415 V three-phase utilisation voltage.

Question 41NSQF Vocational Trade Theory (Power)

What is the main reason for using a ring main system in low-voltage urban distribution?

  1. It doubles the supply voltage to consumers
  2. It removes the need for any protective devices
  3. It eliminates the need for a neutral conductor
  4. A fault section can be isolated while supply continues through the other side of the ring Correct answer.

Explanation

In a ring main each consumer is fed from two directions, so if one section of cable fails or is switched out for repair, supply can still reach consumers from the other side. This gives higher reliability than a simple radial feeder.

Question 42NSQF Vocational Trade Theory (Power)

Why are stay wires used on an overhead distribution pole carrying a line at a bend?

  1. To carry the electrical load current to earth
  2. To insulate the pole from the conductors
  3. To increase the height of the conductor above ground
  4. To resist the unbalanced pull of the conductors and keep the pole vertical Correct answer.

Explanation

At an angle or terminal pole the conductor tension is not balanced, so the pole would tend to lean. A stay wire anchored in the ground takes this pull in tension and keeps the pole upright and the clearances correct.

Question 43NSQF Vocational Trade Theory (Power)

An overhead line insulator is found to have a cracked shed after a storm. What is the correct immediate action?

  1. Isolate the line and replace the damaged insulator before restoring supply Correct answer.
  2. Reduce the system voltage until the next annual shutdown
  3. Increase the clearance of the conductor to compensate
  4. Apply paint over the crack and keep the line energised

Explanation

A cracked insulator loses dielectric strength and can flash over or fail mechanically, especially in wet weather. The line must be isolated and the insulator replaced so that insulation co-ordination and clearance are restored before the line is re-energised.

Question 44NSQF Vocational Trade Theory (Power)

What does a current transformer do in a high-voltage metering installation?

  1. It corrects the power factor of the installation
  2. It steps the line voltage down to 110 V for the meter
  3. It steps the line current down to a safe, standard value for the meter and relays Correct answer.
  4. It converts alternating current into direct current

Explanation

A current transformer has its primary in series with the line and a secondary with many turns, so the secondary current is a fixed fraction of the line current, commonly 5 A or 1 A. This isolates the meter and relays from high voltage and high current.

Question 45NSQF Vocational Trade Theory (Power)

A current transformer secondary must never be left open-circuited while the primary is energised. Why?

  1. The secondary voltage can rise to a dangerous value and the core can overheat Correct answer.
  2. The primary current will fall to zero and the load will trip
  3. The meter will read too low a value only
  4. The transformer will change from step-down to step-up action

Explanation

With the secondary open, the demagnetising secondary ampere-turns disappear and the whole primary current acts as magnetising current. Flux and core loss rise sharply, producing a very high secondary voltage that endangers personnel and insulation, so the secondary is shorted when not in use.

Question 46NSQF Vocational Trade Theory (Power)

Which relay characteristic is used so that a feeder relay does not trip for faults beyond the next downstream relay?

  1. Voltage grading only
  2. Instantaneous operation at the same setting for all relays
  3. Time grading, where each upstream relay is set with a longer time delay Correct answer.
  4. Frequency grading

Explanation

Time grading gives the relay nearest the fault the shortest operating time and each upstream relay a longer delay. This ensures the downstream breaker clears the fault first and limits the outage to the smallest possible section of the network.

Question 47NSQF Vocational Trade Theory (Power)

In a thermal power plant, why is the condenser operated at a pressure below atmospheric?

  1. It prevents the turbine from overspeeding
  2. It increases the boiler pressure directly
  3. Lower pressure lowers the saturation temperature, so more heat is extracted from the exhaust steam and cycle efficiency rises Correct answer.
  4. It removes ash from the flue gas

Explanation

Turbine exhaust steam condenses at a temperature set by the condenser pressure. Reducing that pressure lowers the condensing temperature, increasing the heat rejected per kilogram is done at a lower temperature and improving the Rankine cycle efficiency.

Question 48NSQF Vocational Trade Theory (Power)

What is the purpose of a Buchholz relay fitted in the pipe between a transformer tank and its conservator?

  1. To measure the load current of the transformer
  2. To detect gas produced by an incipient fault inside the transformer and give an alarm or trip Correct answer.
  3. To cool the transformer oil by circulation
  4. To regulate the secondary voltage of the transformer

Explanation

A fault such as a turn-to-turn short or a hot joint decomposes oil and produces gas that collects in the relay. Slow gas accumulation gives an alarm, while a severe fault causes oil surge that operates the trip contact, disconnecting the transformer before damage spreads.

Question 49NSQF Vocational Trade Theory (Power)

A technician must test the insulation resistance of a 415 V three-phase motor before commissioning. Which instrument and procedure is correct?

  1. A multimeter on the resistance range with the supply switched on
  2. A wattmeter connected across two phases with the motor energised
  3. A clamp meter around one phase conductor while the motor runs
  4. A 500 V megger applied between winding and earth with the supply isolated and the winding discharged afterwards Correct answer.

Explanation

Insulation resistance is measured with a DC test voltage higher than the working voltage, so a 500 V megger suits a 415 V machine. The supply must be isolated, the winding earthed and discharged before and after the test so that stored charge cannot shock the technician or damage the instrument.

Question 50NSQF Vocational Trade Theory (Power)

During a load test a three-phase induction motor draws a current well above its nameplate value while delivering normal mechanical load. Which cause should be investigated first?

  1. A loose earth connection at the terminal box
  2. A supply voltage that is significantly below the rated value Correct answer.
  3. An excessive supply frequency above 50 Hz
  4. A worn bearing on the driven machine

Explanation

An induction motor draws more current at reduced voltage because it must develop the same torque with a weaker air-gap flux. Low voltage is therefore a common cause of overload current, and the supply should be checked before the motor is condemned.