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Past Questions — 2016
In air blast circuit breakers, the pressure of air is of the order of
A)
100 mm Hg
B)
1 kg/cm2
C)
20 to 30 kg/cm2
D)
200 to 300 kg/cm2
In an air blast circuit breaker, compressed air is used at a high pressure, typically around 20 to 30 kg/cm2, to extinguish the arc quickly. This high-pressure air blast cools and de-ionizes the arc path. Option (a) 100 mm Hg and option (b) 1 kg/cm2 are far too low for effective arc interruption, while option (d) 200 to 300 kg/cm2 is excessively high and not the usual operating range.
For economy in generation power
A)
diversity factor should be high
B)
plant utilisation factor
C)
load factor should be high
D)
load factor and diversity factor should be low
For economical generation of power, the average load should be as close as possible to the maximum demand. A high load factor means better utilization of the installed capacity, lower fixed cost per unit, and hence more economical generation. Diversity factor also helps the system economically, but the standard direct condition for economy in generation is a high load factor. Option (a) is not the main criterion, and (d) is incorrect because low load factor and low diversity factor both worsen economy.
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 dielectric constant (relative permittivity) of the insulation: 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 of the original value. Options (b) and (c) are incorrect because lowering permittivity reduces capacitance, not increases it.
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 vulnerable to corrosion due to exposure to moisture, air, and environmental conditions. Termites and borers attack wood, not steel. Therefore, the correct protection required for steel poles is against corrosion.
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 device used in oil-immersed transformers. It is installed in the pipe between the transformer main tank and the conservator so it can detect internal faults by sensing gas accumulation and oil flow disturbance. It is not placed inside the main tank, conservator tank, or circuit breaker.
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 sends a trip signal to the circuit breaker. Thus, its function is to close its contacts at a predetermined value of the actuating quantity so that the circuit breaker can open the faulty circuit. Option (a) is incorrect because relays do not detect stray voltages specifically. Option (c) is incorrect because limiting arcing current is not the relay’s function. Option (d) is incorrect because the relay does not merely provide extra safety; it initiates protection by tripping the breaker.
A system having connected load of 100 kW, peak load of 80 kW, base load of 20 kW and average load of 40 kW, will have a load factor of
A)
40%
B)
50%
C)
60%
D)
80%
Load factor = Average load / Peak load × 100. Here, average load = 40 kW and peak load = 80 kW. So, load factor = (40/80) × 100 = 50%. The connected load and base load are not needed for calculating load factor. Hence, option (b) is correct.
Most of the generators in thermal power plants run at
A)
3000 rpm
B)
1500 rpm
C)
1000 rpm
D)
750 rpm
In thermal power plants, generators are usually coupled to steam turbines. For a 50 Hz system, synchronous speed is given by N = 120f / P. A commonly used turbine-generator unit has 4 poles, so N = 120 × 50 / 4 = 1500 rpm. Hence, most generators in thermal power plants run at 1500 rpm. 3000 rpm is for 2-pole machines, while 1000 rpm and 750 rpm are lower-speed synchronous speeds used in other applications.
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 factors: the rate of flux buildup in the magnetic circuit, the armature core air gap, and the spring tension opposing the motion. A faster flux buildup increases attraction sooner, a smaller air gap generally improves speed, and greater spring tension slows operation. Therefore, all listed factors affect relay speed.
Surge diverters are
A)
non-linear resistors in series with spark gaps which act as fast switches
B)
are quenching devices
C)
shunt reactors to limit the voltage rise due to Ferranti effect
D)
over-voltages of power frequency harmonics
A surge diverter (lightning arrester) protects equipment from high-voltage surges by providing a low-resistance path to earth during a surge. It typically uses non-linear resistors and spark gaps; the spark gap acts as a fast switch when the voltage exceeds a set value. Therefore, option (a) is correct. Options (b), (c), and (d) do not describe surge diverters.
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 ratio-type instrument. When the test terminals are open-circuited, the insulation resistance is effectively infinite, so no current flows through the current coil. With no current in the current coil, the deflecting torque becomes zero and the pointer moves to infinity. The pressure coil still carries current from the internal generator, so option (b) is incorrect. Options (c) and (d) are also incorrect because they do not describe the operating principle of a megger.
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. This is because the magnetic flux is proportional to current, and the induced torque depends on the interaction of fluxes, giving a relation approximately proportional to I^2. Hence, the torque is not linear, not square-root based, and not inversely proportional to current.
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 large deflection angle, commonly up to about 300° or even more. They are suitable for high-voltage measurements and have a very wide scale. PMMC instruments usually have limited deflection, induction type instruments do not typically provide such large deflection, and hot wire instruments also do not produce a 300° full-scale deflection.
430 km transmission line carrying power at 33 kV is known as
A)
short transmission line
B)
long transmission line
C)
high power line
D)
ultra high voltage line
Transmission lines are classified by length. A line up to about 80 km is a short transmission line, about 80–250 km is a medium transmission line, and above about 250 km is a long transmission line. Since the given line length is 430 km, it is a long transmission line. The 33 kV rating does not change the length-based classification.
Which of the following is not the distribution system normally used?
A)
3 phase-4 wire
B)
3 phase-3 wire
C)
Single phase - 3 wire
D)
Single phase - 4 wire
In electric power distribution, the commonly used systems are 3-phase 3-wire, 3-phase 4-wire, and single-phase 3-wire. A single-phase 4-wire distribution system is not normally used because it is not a standard practical arrangement for general distribution. Hence, option (d) is the correct answer.
In a hydro-electric plant a conduct system for taking water from the intake works to the turbine is known as
A)
Dam
B)
Reservoir
C)
Penstock
D)
Surge tank
In a hydro-electric plant, the conduit/system used to carry water from the intake works to the turbine is called a **penstock**. A **dam** stores water, a **reservoir** is the stored water body, and a **surge tank** is used to absorb pressure fluctuations and prevent water hammer, not for direct water conveyance to the turbine.
In order to improve the power factor __ device is employed in the substation
A)
Synchronous condenser
B)
Synchronous reactor
C)
Series Capacitors
D)
None of these
A synchronous condenser is used in substations to improve power factor by supplying reactive power (leading vars) to the system. This helps offset the lagging reactive power drawn by inductive loads. A synchronous reactor is generally used to absorb reactive power, so it does not improve power factor. Series capacitors are used mainly for line compensation and voltage improvement, not as the standard substation device for power factor correction.
Surge impedance of transmission line is given by
A)
(L/C)^1/2
B)
(C/L)^1/2
C)
(CL)^1/2
D)
1/(CL)^1/2
The surge impedance (also called characteristic impedance) of a transmission line is given by Z0 = √(L/C), where L is the inductance per unit length and C is the capacitance per unit length. Hence option (a) is correct. Option (b) is the inverse of the correct formula, while (c) and (d) do not match the dimensional form of surge impedance.
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
When alternating current flows through a conductor, it tends to concentrate more near the surface than at the core. This phenomenon is called the skin effect. Corona is a discharge phenomenon around high-voltage lines, permeability is a magnetic property, and unsymmetrical fault is a type of power system fault.
In case of transmission line conductors with the increase in atmospheric temperature
A)
length increase but stress decreases
B)
length increases and stress also increases
C)
length decreases but stress increases
D)
both length as well as stress decreases
When atmospheric temperature increases, a transmission line conductor expands due to thermal expansion, so its length increases. At the same time, the conductor’s tensile stress (or tension) decreases because the increased temperature reduces the mechanical pull in the line. Hence, the correct statement is that length increases but stress decreases. The other options are incorrect because temperature rise does not cause contraction of the conductor, and both length and stress cannot decrease simultaneously in this case.
A circuit breaker is
A)
power factor correcting device
B)
a device to neutralize the effect of transients
C)
a waveform correcting device
D)
a current interrupting device
A circuit breaker is an automatic switching device that opens a circuit under normal or fault conditions, thereby interrupting current flow. Its main function is protection by breaking excessive fault current. It is not used for power factor correction, waveform correction, or neutralizing transients, so option (d) is correct.
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 electrical load when generation is insufficient or the system is under stress. This is done to prevent total blackout and to maintain system stability. It does not directly mean reducing voltage, frequency, or power factor, though those may be affected indirectly in some situations.
Earth fault relays are:
A)
Directional relay
B)
Non-directional relay
C)
Short operate time relay
D)
Under voltage relay
Earth fault relays are generally **non-directional relays** because they operate when an earth fault current exceeds the set value, regardless of the direction of current flow. They detect leakage or fault to earth and trip the circuit.
- (a) Directional relay: used where fault direction matters, such as in interconnected systems.
- (b) Non-directional relay: correct, since earth fault protection usually responds to magnitude of earth fault current.
- (c) Short operate time relay: this describes operating speed, not the relay type.
- (d) Under voltage relay: operates on voltage drop, not on earth faults.
The fault clearing time of a circuit breaker is usually
A)
few minutes
B)
few seconds
C)
one second
D)
few cycles of supply voltage
A circuit breaker is designed to interrupt fault current very quickly to protect the system. The fault clearing time is therefore usually only a few cycles of the AC supply, not seconds or minutes. Since one cycle is 1/50 or 1/60 second, fault clearing happens within a very short interval. Hence options (a), (b), and (c) are too long.
Measurement range of a voltmeter can be extended by using:
A)
High current resistance
B)
High series resistance
C)
Low shunt resistance
D)
Low series resistance
A voltmeter measures potential difference and has a high internal resistance. To extend its measurement range, an additional high resistance is connected in series with the voltmeter so that larger voltages can be measured without drawing excessive current. A shunt resistor is used to extend an ammeter’s range, not a voltmeter’s. Hence, the correct answer is high series resistance.