← Junior Engineer
Past Questions
A megger indicates infinity when test terminals are open-circuited. This is because:
A)
No current flows through the current coil
B)
No current flows through the pressure coil
C)
No current flows through the compensating winding
D)
Current does not flow through current coil and pressure coil
A megger measures insulation resistance using a hand-driven generator and a ratio-type movement. When the test terminals are open-circuited, the insulation resistance is effectively infinite, so no current flows through the pressure coil circuit. Since the instrument indication depends on the ratio of currents, the pointer moves to infinity.
Why not the others:
- (a) is incorrect because the current coil is not the reason for the infinity reading in open circuit.
- (c) compensating winding is not the deciding factor here.
- (d) is incorrect because the pressure coil current condition is the key, not both coils.
The function of protective relay in a circuit breaker is
A)
to detect any stray voltages
B)
to close the contacts when the actuating quantity reaches a certain predetermined value
C)
to limit arcing current during the operation of circuit breaker
D)
to provide additional safety in the operation of circuit breaker
A protective relay monitors an electrical quantity such as current, voltage, frequency, or power. When this quantity crosses a preset value, the relay operates and closes its contacts to trip the circuit breaker. Thus, its function is to initiate breaker operation during abnormal conditions. Option (a) is incorrect because relays do not detect stray voltages specifically. Option (c) is incorrect because limiting arcing is the job of the circuit breaker design, not the relay. Option (d) is incorrect because the relay’s primary role is protection/tripping, not general additional safety.
A string efficiency of 100% implies that
A)
shunt capacitance is 1 MF
B)
potential across each disc is same
C)
potential across each disc is zero
D)
one of the insulator discs is shorted
String efficiency is 100% when the voltage is uniformly distributed across all the discs of the insulator string. That means each disc shares the same potential. In practice, due to shunt capacitance, the lower discs usually take more voltage, so efficiency is normally less than 100%. Hence, 100% efficiency implies equal potential across each disc. The other options are incorrect: zero potential across each disc is impossible, a shorted disc would distort distribution and reduce efficiency, and '1 MF' is irrelevant.
In substation which of the following devices is a carrier communication device?
A)
CVT
B)
Earth conductor
C)
Wave trap
D)
Lightning arrestor
In substations, a wave trap (line trap) is used in carrier communication systems. It blocks high-frequency carrier signals from entering the power lines beyond the intended point while allowing power frequency current to pass. CVT is used for voltage measurement/protection, earth conductor provides grounding, and lightning arrestor protects against surges.
Buchholz relay is:
A)
Located in the conservator tank
B)
Located in the transformer tank itself
C)
Connected in the pipe connecting main tank of transformer and conservator
D)
Installed in the circuit breaker
A Buchholz relay is a gas-actuated protective relay used in oil-immersed transformers. It is fitted in the pipeline between the transformer main tank and the conservator tank so that it can detect gases formed due to internal faults and also oil surge during serious faults. Hence, option (c) is correct. Options (a) and (b) are incorrect because the relay is not placed inside the conservator or the main tank. Option (d) is incorrect because it is not a circuit breaker device.
The protection against direct lightening strokes and high voltage steep waves is provided by
A)
earthing of neutral
B)
lightening arresters
C)
ground wires
D)
lightning arresters and ground wires
Protection against direct lightning strokes and high-voltage steep waves is provided by both lightning arresters and ground wires. Lightning arresters divert surge voltage safely to earth, while ground wires intercept direct lightning strokes and protect overhead lines. Earthing of neutral is for system grounding and fault protection, not direct lightning protection.
The equipment installed in power plants to reduce air pollution due to smoke is
A)
Induced draft fans
B)
De-super heaters
C)
Electrostatic precipitators
D)
Re-heaters
Electrostatic precipitators are used in power plants to remove fine dust and ash particles from flue gas before it is released into the atmosphere, thereby reducing air pollution due to smoke. Induced draft fans only help move flue gases through the boiler system, de-superheaters control steam temperature, and reheaters increase steam temperature between turbine stages. They do not clean smoke.
Which power plant is free from environmental pollution problems?
A)
Thermal power plant
B)
Nuclear power plant
C)
Hydro-power plant
D)
Geothermal energy power plant
Hydro-power plants use the energy of flowing or falling water, so they do not burn fuel and therefore do not cause air pollution like thermal power plants. Nuclear power plants have radioactive waste and safety concerns, and geothermal plants may release gases and other environmental issues. Hence, among the given options, hydro-power plant is considered free from environmental pollution problems.
The operation speed of a relay depends on:
A)
Rate of flux buildup
B)
Armature core air gap
C)
Spring tension
D)
All of these
The operating speed of a relay is influenced by several mechanical and magnetic factors. A faster rate of flux buildup helps the relay pick up quicker, a smaller armature core air gap reduces the time needed for attraction, and spring tension affects how much force must be overcome for operation. Since all three affect the relay’s operation speed, the correct answer is "All of these".
Steel poles for transmission lines need protection against
A)
termites
B)
borer
C)
corrosion
D)
all of these
Steel poles used in transmission lines are mainly exposed to atmospheric moisture, rain, and pollution, so they require protection against corrosion. Termites and borer attack timber poles, not steel poles. Therefore, the correct answer is corrosion.
The torque developed in Induction type relay is:
A)
Directly proportional to the current
B)
Directly proportional to the square of the current
C)
Directly proportional to square root of the current
D)
Inversely proportional to the current
In an induction type relay, the operating torque is produced by electromagnetic induction and is proportional to the square of the current flowing through the relay coil. This is because the induced flux and the resulting interaction depend on the current magnitude in a nonlinear way. Hence, the torque varies as I². The other options are incorrect because the torque is not linear, not proportional to square root of current, and not inversely proportional to current.
In order to increase the limit of distance of transmission line
A)
series resistances are used
B)
synchronous condensers are used
C)
shunt capacitors and series reactors are used
D)
series capacitors and shunt reactors are used.
For increasing the transmission distance of a line, compensation is used to reduce the effective series reactance and to control voltage. Series capacitors compensate the line inductive reactance, improving power transfer capability, while shunt reactors help control overvoltage due to line charging, especially on long lines. Hence, the correct choice is series capacitors and shunt reactors.
Why others are incorrect:
- (a) Series resistances increase losses and reduce efficiency, so they are not used.
- (b) Synchronous condensers are mainly used for reactive power support and voltage regulation, but this option is not the standard combined method for increasing transmission line length.
- (c) Shunt capacitors and series reactors is the opposite of what is generally used for long-distance transmission; series reactors would further increase reactance.
During load shedding
A)
system voltage is reduced
B)
system frequency is reduced
C)
some loads are switched off
D)
system power factor is changed
Load shedding means intentionally disconnecting part of the load from the power system when demand exceeds available supply. This is done to prevent system collapse and maintain stability. It does not primarily mean reducing voltage, frequency, or power factor; those may change as a consequence of system conditions, but the defining action is switching off some loads.
The relay best suited for phase fault relaying for medium transmission lines is:
A)
Mho relay
B)
Reactance relay
C)
Impedance relay
D)
None of these
For phase-fault protection on medium transmission lines, the **reactance relay** is best suited because it is less affected by **fault resistance**. Medium transmission lines often have appreciable fault resistance, and a reactance relay responds mainly to the reactive component of line impedance, making it more reliable for such faults.
Why not the others?
- **Mho relay**: Better suited for **long transmission lines** and directional protection.
- **Impedance relay**: Can be affected by fault resistance and is less suitable than reactance relay for phase faults on medium lines.
- **None of these**: Incorrect because reactance relay is the correct choice.
In the below figure, the relay circuit is divided into three parts. What does the first part consist of?
A)
Primary winding of a current CT which is connected in series with the line to be protected
B)
Secondary of the CT and the operating coil
C)
Tripping circuit
D)
All of these
In a protective relay circuit, the first part is the current transformer (CT) primary connected in series with the line to be protected. This senses the line current and feeds a proportional current to the relay circuit. The secondary of the CT and the operating coil form the next part, while the tripping circuit is the final part. So only option (a) correctly describes the first part.
The phenomenon of corona is generally accompanied by
A)
a bang
B)
a hissing sound
C)
magnetic hum
D)
all of these
Corona discharge on transmission lines is usually accompanied by an audible hissing sound, a crackling/banging noise, and often a faint buzzing or humming effect. Since all the listed effects can occur, the correct answer is 'all of these'.
A certain cable has an insulation of relative permittivity 4. If the insulation is replaced by one of relative permittivity 2, the capacitance of the cable will become
A)
one half
B)
double
C)
four times
D)
none of these
For a cable, capacitance is directly proportional to the permittivity of the insulation material (C ∝ εr). If the relative permittivity changes from 4 to 2, the capacitance changes in the ratio 2/4 = 1/2. Therefore, the capacitance becomes one half. Option (b) is incorrect because capacitance does not increase when permittivity decreases; option (c) is far from the correct proportional change; option (d) is incorrect because the exact result is listed in (a).
Potentiometer is a/an instrument:
A)
Indicating
B)
Comparison
C)
Calibrating
D)
Recording
A potentiometer is a comparison instrument because it measures an unknown voltage by comparing it with a known reference voltage under null-balance condition. It does not directly indicate the value like an indicating instrument, nor does it record continuously. Calibration is one of its applications, but its fundamental type is a comparison instrument.
Isolators are used for disconnecting a circuit when
A)
Line is energised
B)
Line is on full load
C)
Line carries no current
D)
Can be operated under any condition
An isolator is a no-load switch used to disconnect a circuit only after the current has been interrupted by a circuit breaker. It is operated when the line is de-energized and carries no current. It is not meant for opening an energized or full-load circuit. Therefore, the correct answer is (c) Line carries no current.
The fact that a conductor carries more current on the surface as compared to core, is known as
A)
skin effect
B)
corona
C)
permeability
D)
unsymmetrical fault
In an AC conductor, current tends to concentrate near the outer surface rather than flowing uniformly through the entire cross-section. This phenomenon is called skin effect. Corona is an electrical discharge around high-voltage conductors, permeability is a magnetic property, and unsymmetrical fault is a type of power system fault.
Which of the following instruments can be used for full scale deflection of 300°?
A)
PMMC
B)
Induction type
C)
Hot wire
D)
Electrostatic
Electrostatic instruments can be designed to give a very large deflection, even up to about 300° full-scale. They are suitable for high-voltage measurements and have a nearly uniform torque over a wide range.
Why others are incorrect:
- PMMC instruments usually have a maximum deflection of about 90°.
- Induction type instruments also generally have limited deflection, typically around 90°.
- Hot wire instruments do not provide such a large full-scale deflection.
The emf generated due to nth harmonic component of flux in an alternator will be
A)
n times the fundamental emf
B)
same as fundamental emf
C)
less than the value of fundamental emf
D)
none of these
In an alternator, the emf induced by the nth harmonic component is not n times the fundamental emf. Although harmonic flux has higher frequency, the winding distribution and pitch factors reduce the resultant harmonic emf. Also, the harmonic flux component itself is generally smaller than the fundamental. Therefore, the emf due to the nth harmonic is less than the fundamental emf. Options (a) and (b) are incorrect for this reason, and (d) is unnecessary because (c) is correct.
For the measurement of high pressure with high accuracy the device used is
A)
Manganin wire pressure
B)
Ionization gauge
C)
Dead weight gauge
D)
Bourdon tubes
For measuring high pressure with high accuracy, the dead weight gauge (dead-weight tester) is used because it provides a direct pressure standard based on known weights and piston area. It is highly accurate and is used for calibration. Manganin wire pressure gauges are used for very high pressures but are not as accurate as dead-weight gauges. Ionization gauges are for very low pressures (vacuum). Bourdon tubes are common for pressure measurement, but they are less accurate than dead-weight gauges.
Most of the generators in thermal power plants run at
A)
3000 rpm
B)
1500rpm
C)
1000 rpm
D)
750rpm
In thermal power plants, generators are commonly coupled to steam turbines and run at synchronous speed. For a 50 Hz system, synchronous speed is given by N = 120f/P. A 2-pole generator runs at 3000 rpm, while a 4-pole generator runs at 1500 rpm. Most thermal power plant turbo-generators are 4-pole machines, so the typical speed is 1500 rpm. The other options are less common for standard thermal station generators.
The breakdown voltage in gases depends on
A)
distance between the electrodes
B)
relative air density
C)
humidity
D)
all of these
The breakdown voltage in gases depends on the electrode spacing, the relative air density, and the humidity. These factors affect the ionization process and the dielectric strength of the gas. Therefore, all the given options are correct.