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Past Questions
The resistance of the windings of a generator is known as:
A)
generated resistance
B)
field resistance
C)
internal resistance
D)
terminal resistance
In the context of electrical generators, the resistance offered by the windings (armature windings) to the flow of current is referred to as the internal resistance. This resistance causes a voltage drop (IR drop) within the machine, meaning the terminal voltage (the voltage available at the output) is always slightly less than the generated electromotive force (EMF) when the generator is loaded.
Why other options are incorrect:
- a. 'Generated resistance' is not a standard technical term in electrical machine theory.
- b. 'Field resistance' refers specifically to the resistance of the field windings used to create the magnetic flux, not the overall resistance of the generator's power-producing windings (armature).
- d. 'Terminal resistance' usually refers to the resistance of the load connected to the terminals, or the contact resistance at the terminals, rather than the resistance of the internal windings themselves.
A ‘short circuit’ is that which:
A)
Uses short pieces of wire
B)
goes only a short distance
C)
is used for dimming light
D)
offer very low resistance path for current to flow
In electrical engineering, a 'short circuit' occurs when two nodes of an electrical circuit are connected by a path with very low impedance (resistance). According to Ohm's Law (I = V/R), if the resistance (R) approaches zero while the voltage (V) remains constant, the current (I) increases dramatically. This excessive current flow can lead to overheating, melting of insulation, or triggering protective devices like fuses and circuit breakers.
Analysis of options:
- Option (a) is incorrect because the term 'short' refers to the electrical path's resistance, not the physical length of the wire.
- Option (b) is incorrect because a short circuit can occur across a long distance if the insulation fails between two conductors.
- Option (c) is incorrect because dimming light is achieved using a rheostat or a dimmer switch (which increases resistance), whereas a short circuit is a fault condition.
- Option (d) is correct as it accurately describes the physical phenomenon of a short circuit.
Periodic Maintenance required for circuit breaker in a home is:
A)
regular cleaning and lubrication
B)
frequent testing and re setting
C)
visual inspection periodically, looking for signs of damage or over heating
D)
no maintenance is needed
Option C is correct: visual inspection periodically, looking for signs of damage or over heating.
The function of a silica gel is to absorb moisture from the air when transformer:
A)
breaths out
B)
breaths in
C)
breaths in and breaths out
D)
none of the above
In a transformer, the oil expands and contracts due to temperature changes during loading and unloading. This process is referred to as 'breathing'. When the transformer cools down, the oil contracts, creating a partial vacuum in the conservator tank. To fill this vacuum, air from the outside atmosphere is drawn into the tank (the transformer 'breathes in'). Since atmospheric air contains moisture (water vapor), it must be dried before it enters the transformer to prevent the degradation of the insulating oil's dielectric strength. The silica gel breather is placed in the air path so that as the air 'breathes in', the silica gel absorbs the moisture. When the transformer 'breathes out' (during heating), the air is pushed out of the tank and does not pass through the silica gel in a way that requires moisture removal from the internal oil. Therefore, the specific function of the silica gel is to protect the oil during the 'breathing in' phase.
Which component needs regular lubrication for smooth operation in an elevator?
A)
car frame
B)
step chain
C)
hoist ropes and guide rails
D)
cabin interior walls.
In elevator systems, lubrication is critical for components that experience constant friction and mechanical wear.
1. **Guide Rails**: The elevator car and counterweight slide along guide rails. To reduce friction, prevent vibration, and ensure a smooth ride, these rails must be lubricated (either via automatic lubricators or manual application).
2. **Hoist Ropes**: Steel wire ropes require lubrication to reduce internal friction between the strands and to protect the metal from corrosion and wear, which extends the lifespan of the ropes.
Why other options are incorrect:
- **a. Car frame**: This is a structural steel component that does not have moving parts requiring lubrication.
- **b. Step chain**: While chains in escalators require lubrication, standard elevators do not use 'step chains' as a primary moving component.
- **d. Cabin interior walls**: These are aesthetic/structural panels (usually stainless steel or laminate) and do not require lubrication.
Refrigeration and air -conditioning involve heat transfer using:
A)
water
B)
ice and water
C)
refrigerants
D)
none of the above
Refrigeration and air-conditioning systems operate on the principle of heat transfer, where heat is absorbed from a low-temperature region (the space to be cooled) and rejected to a high-temperature region (the surroundings). This process is facilitated by a specialized working fluid known as a 'refrigerant'. Refrigerants are chosen for their specific thermodynamic properties, most notably a low boiling point, which allows them to evaporate at low temperatures and absorb significant amounts of heat (latent heat of vaporization) from the environment. While water is often used as a secondary coolant (e.g., in chilled water pipes or condenser cooling towers), the primary medium that enables the refrigeration cycle (compression, condensation, expansion, and evaporation) is the refrigerant. Ice and water were used in primitive cooling methods, but modern HVAC systems rely on chemical refrigerants (like R-134a, R-410A, etc.).
When connecting wattmeter to a load circuit consuming large current, it is necessary to use:
A)
potential transformer
B)
isolation transformer
C)
power shunts
D)
current transformer
A wattmeter typically consists of two coils: a current coil (CC) and a potential coil (PC). The current coil is connected in series with the load. Standard wattmeters have current coils designed for relatively low current ratings (e.g., 5A or 10A). When the load circuit consumes a 'large current' that exceeds the rated capacity of the wattmeter's current coil, it is impractical and unsafe to pass the full load current through the meter. To solve this, a Current Transformer (CT) is used. The CT steps down the high primary current to a lower, manageable value (typically 5A) while maintaining the proportional relationship, allowing the wattmeter to measure the power safely and accurately.
Why other options are incorrect:
- a. Potential Transformer: Used when the voltage is too high for the potential coil, not for large currents.
- b. Isolation Transformer: Used for electrical isolation and safety, not for scaling current for measurement.
- c. Power Shunts: While shunts can be used to bypass current, in high-power industrial or departmental electrical measurements, Current Transformers are the standard and preferred method for accuracy and safety (providing galvanic isolation).
Which of the following is a good conductor of electricity ?
A)
germanium
B)
glass(not molten)
C)
glass(molten)
D)
mica
To determine the correct answer, we must analyze the electrical conductivity of each material provided in the options:
1. **Germanium (a):** Germanium is a semiconductor. While it conducts electricity better than an insulator, it is not considered a 'good conductor' in the same sense as metals or ionic liquids; its conductivity is limited and depends heavily on temperature and doping.
2. **Glass (not molten) (b):** Solid glass is an excellent electrical insulator because its electrons are tightly bound in a covalent network, preventing the flow of electric current.
3. **Glass (molten) (c):** When glass is melted, the rigid network structure breaks down, and ions (such as sodium ions in soda-lime glass) become mobile. This mobility of ions allows the molten glass to conduct electricity effectively. Therefore, in its molten state, glass acts as a conductor.
4. **Mica (d):** Mica is a mineral known for its exceptional insulating properties and is frequently used as a dielectric in capacitors.
Comparing the options, molten glass is the only material listed that exhibits significant electrical conductivity due to ionic mobility.
25mm conduit size means that:
A)
the mean diameter of the conduit is 25mm
B)
the external diameter of the conduit is 25mm
C)
the internal diameter of the conduit is 25mm
D)
none of the above
In electrical installations and plumbing, the nominal size of a conduit refers to its internal diameter (ID). This is because the internal space is the critical dimension that determines the capacity for wires or cables to be pulled through the pipe without exceeding the permissible fill ratio (as per NEC or IS standards). The external diameter varies depending on the material (PVC, GI, or EMT) and the wall thickness (schedule/class) of the conduit, but the nominal size remains a reference to the internal clearance.
According to I.E. rules, the supply voltage fluctuation at consumer’s terminal should not be more than:
A)
2% above or below of declared nominal voltage
B)
3% above or below of declared nominal voltage
C)
5%above or below of declared nominal voltage
D)
none of the above
According to the Indian Electricity (IE) Rules, specifically under the guidelines regarding the quality of supply and voltage variations, the permissible limit for voltage fluctuation at the consumer's terminal is generally maintained within ±6% for low voltage distribution in many standards, but the specific regulatory benchmark often cited in departmental examinations for IE Rules regarding the standard permissible variation is ±6% or ±5% depending on the specific category of supply. However, in the context of standard multiple-choice questions for JE exams based on IE Rules, the accepted standard for nominal voltage fluctuation is ±6% (which is closest to 5% among the given options) or specifically ±6% as per CEA (Central Electricity Authority) regulations which superseded parts of the old IE rules. Given the options provided (2%, 3%, 5%), 5% is the standard technical limit often tested as the maximum permissible deviation for stable consumer equipment operation. Therefore, option (c) is the most accurate choice among the provided options.
The most important key element in performing any periodic maintenance and services on elevator is:
A)
cost reduction
B)
speed of work
C)
safety
D)
all of the above
In the context of elevator maintenance and engineering, safety is the paramount priority. Elevators are high-risk mechanical systems that transport people vertically; any failure during operation or maintenance can lead to catastrophic accidents or fatalities.
1. Why 'safety' is correct: Periodic maintenance is specifically designed to ensure that safety devices (such as governors, buffers, limit switches, and braking systems) are functioning correctly to prevent accidents. All maintenance protocols (like Lock-Out Tag-Out - LOTO) are centered around ensuring the safety of the technician and the future passengers.
2. Why 'cost reduction' is incorrect: While efficiency is a goal, reducing costs should never come at the expense of safety or quality in elevator maintenance.
3. Why 'speed of work' is incorrect: Rushing through maintenance (speed) often leads to overlooked faults or errors, which directly compromises the safety of the lift.
4. Why 'all of the above' is incorrect: Because cost and speed are secondary objectives that must be subordinated to the absolute requirement of safety.
The main function of power transformer is:
A)
to increase or decrease the d.c. voltage
B)
to increase or decrease the a.c. voltage
C)
to convert a.c. to d.c. vice-versa
D)
none of the above
A transformer is a static electrical device that transfers electrical energy between two or more circuits through electromagnetic induction.
1. **Working Principle**: Transformers operate based on Faraday's Law of Electromagnetic Induction. They require a changing magnetic field to induce a voltage in the secondary winding. This change is provided by Alternating Current (AC).
2. **Function**: Depending on the turns ratio of the primary and secondary windings, a transformer can either increase the voltage (Step-up transformer) or decrease the voltage (Step-down transformer) while maintaining the frequency.
**Analysis of Options:**
- **Option (a)** is incorrect because transformers cannot operate on Direct Current (DC). A steady DC voltage does not create a changing magnetic flux, meaning no voltage would be induced in the secondary winding.
- **Option (b)** is correct as it describes the fundamental purpose of a power transformer in electrical grids (e.g., stepping up voltage for transmission to reduce losses and stepping it down for distribution).
- **Option (c)** is incorrect because the conversion of AC to DC (or vice-versa) is the function of a Rectifier or an Inverter, not a transformer.
A generator is a machine which converts:
A)
electrical energy into mechanical energy
B)
mechanical energy into electrical energy
C)
both (a) and (b)
D)
none of the above
A generator is an electrical machine that operates on the principle of electromagnetic induction (Faraday's Law). It takes mechanical energy as input (usually from a prime mover like a turbine or an engine) and converts it into electrical energy (voltage and current) as output.
Analysis of options:
- Option (a) describes a 'Motor', which converts electrical energy into mechanical energy.
- Option (b) correctly describes a 'Generator'.
- Option (c) is incorrect because a single machine cannot perform both conversions simultaneously in the same direction of energy flow, although some machines (like motor-generators) can be switched between modes.
- Option (d) is incorrect as option (b) is the standard definition.
The domestic refrigerator uses which type of compressor?
A)
centrifugal
B)
axial
C)
miniature sealed unit
D)
piston type reciprocating
Step-by-step reasoning: 1. Domestic refrigerators require a compressor that is compact, quiet, leak-proof, and low-maintenance. 2. A 'miniature sealed unit' (also known as a hermetic compressor) integrates the electric motor and the compressor into a single welded steel shell. This prevents refrigerant leakage and reduces noise, making it ideal for home use. 3. While the internal mechanism of a sealed unit is often a reciprocating piston, the term 'miniature sealed unit' specifically describes the assembly type used in domestic appliances. 4. Analysis of other options: (a) Centrifugal compressors are used for very large-scale industrial chilling plants due to their high volume flow. (b) Axial compressors are used in jet engines and large gas turbines, not refrigeration. (d) While a piston type is used inside, a standard 'piston type reciprocating' compressor usually refers to open-type industrial compressors that require external lubrication and seals, which are impractical for domestic refrigerators.
The maximum permissible voltage drop as per IE Rule from supply terminals to any point in the installation for light wiring is:
A)
1% of the declared supply voltage
B)
3% of the declared supply voltage
C)
4% of the declared supply voltage
D)
none of all
According to the Indian Electricity (IE) Rules and standard electrical installation practices (which are followed in PWD departmental exams), the permissible voltage drop from the point of supply (the energy meter or supply terminals) to any point in the installation is limited to ensure the efficient operation of electrical equipment. For lighting circuits, the maximum permissible voltage drop is generally restricted to 3% of the declared supply voltage. This limit is set to prevent flickering in lamps and to ensure that the voltage remains within the operating tolerance of the connected loads. Option (a) 1% is too restrictive and impractical for standard wiring; Option (c) 4% is typically associated with larger power circuits or specific industrial distributions rather than standard light wiring installations.
While calculating total lumens required in a lighting design, which of the following is of no consideration?
A)
level of illumination required
B)
floor area
C)
coefficient of Utilization
D)
amount of natural light coming through the window
To calculate the total lumens required for an artificial lighting design, the standard Lumen Method formula is used: Total Lumens = (E × A) / (CU × LLF), where E is the required illumination level (lux), A is the floor area, CU is the Coefficient of Utilization, and LLF is the Light Loss Factor.
1. 'Level of illumination required' (Option a) is the target lux level (E), which is a primary variable.
2. 'Floor area' (Option b) is the total surface area (A) to be lit, which is essential for the calculation.
3. 'Coefficient of Utilization' (Option c) accounts for the efficiency of the luminaire and the reflectance of walls and ceilings, which is critical to determine how many lumens actually reach the working plane.
4. 'Amount of natural light' (Option d) is generally not considered in the calculation of the artificial lighting system's capacity. Lighting design is typically based on the 'worst-case scenario' (nighttime or overcast conditions) to ensure the space meets safety and productivity standards regardless of external light sources. While natural light is a benefit, the artificial system must be designed to provide the required illumination independently.
A 100-W lamp gives a luminous flux of 3000 lm. Its efficiency is:
A)
30 lm/W
B)
300 lm/W
C)
3.33%
D)
30%
The luminous efficiency (also known as luminous efficacy) of a light source is defined as the ratio of the total luminous flux emitted to the total electrical power consumed.
Given data:
- Luminous flux (Φ) = 3000 lm
- Power consumption (P) = 100 W
Formula:
Efficiency (η) = Luminous Flux / Power
η = 3000 lm / 100 W
η = 30 lm/W
Analysis of options:
- Option (a) 30 lm/W: Correct, as per the calculation.
- Option (b) 300 lm/W: Incorrect, this would require a flux of 30,000 lm.
- Option (c) 3.33%: Incorrect. Luminous efficiency is typically expressed in lm/W. Even if converted to a percentage of the theoretical maximum (683 lm/W for monochromatic light at 555nm), the value would be different.
- Option (d) 30%: Incorrect. Efficiency in illumination is not measured as a simple percentage of power unless comparing it to a specific standard.
If a piece of metal wire is stretched, its resistance will increase.
A)
true
B)
false
C)
both true
D)
both false
The electrical resistance (R) of a conductor is determined by the formula R = ρ(L/A), where ρ is the resistivity of the material, L is the length, and A is the cross-sectional area. When a metal wire is stretched, two things happen simultaneously: 1) The length (L) increases, and 2) The cross-sectional area (A) decreases because the total volume of the metal remains constant (Volume = L × A). Since resistance is directly proportional to length and inversely proportional to the cross-sectional area, both the increase in L and the decrease in A contribute to an overall increase in the resistance. Therefore, the statement is true.
When resistance is added to a circuit having a constant potential, the current is?
A)
reduced in the same proportion
B)
increased in the same proportion
C)
unaltered
D)
made fluctuating
For a circuit with constant potential difference, Ohm’s law gives I = V/R. Since V is constant, increasing resistance R makes the current I decrease inversely. Therefore, when resistance is added, the current is reduced in the same proportion. Option b is incorrect because current does not increase when resistance increases. Option c is incorrect because the current changes when resistance changes. Option d is incorrect because adding resistance does not make the current fluctuate; it simply lowers the current.
It is a vertical transport vehicle that efficiently move people or goods between floors of a building:
A)
escalator
B)
elevator
C)
hydraulic pump
D)
both (a) and (b)
Step-by-step reasoning: 1. Analyze the keywords in the question: 'vertical transport vehicle', 'move people or goods', and 'between floors'. 2. Evaluate Option (a) Escalator: An escalator is a power-driven, continuous moving staircase. While it moves people between floors, it is not a 'vehicle' (it is a fixed conveyor system) and it cannot move goods/freight efficiently. 3. Evaluate Option (b) Elevator: An elevator (or lift) is a platform or enclosure (a vehicle) that moves vertically up and down a shaft to transport people or goods between floors. This perfectly matches all criteria in the question. 4. Evaluate Option (c) Hydraulic pump: This is a mechanical device used to move fluids, not a transport vehicle for people or goods. 5. Evaluate Option (d): Since only (b) fits the definition of a 'vehicle' for both people and goods, (d) is incorrect. Therefore, the correct answer is elevator.
One-way single pole switch controls:
A)
connection of one wire to one position
B)
connection of one wire to two positions
C)
connection of two wires to two positions
D)
connection of one wire to two positions
A one-way single pole (SPST - Single Pole Single Throw) switch is the most basic type of electrical switch. It consists of two terminals. Its primary function is to connect or disconnect a single conductor (the 'hot' or 'phase' wire) to complete or break the circuit. In terms of operation, the switch allows the current from one wire to be directed to one of two states or 'positions': the 'ON' position (closed circuit) or the 'OFF' position (open circuit). Therefore, it controls the connection of one wire across two possible positions (connected or disconnected). Note: Options 'b' and 'd' in the provided list are identical, but the technical definition matches this description.
A parallel circuit is one that has:
A)
all elements connected end to end
B)
same current flowing through all elements
C)
all element connected across the power supply so that removing one element does not stop the others from working
D)
all elements placed side by side
In a parallel circuit, the components are connected across the same two nodes (the power supply), meaning each component has its own separate path for current to flow.
1. Why Option (c) is correct: Because each element is connected independently across the voltage source, the voltage across each element is the same. If one element is removed or fails (creating an open circuit in that specific branch), the other branches remain closed and continue to receive power, allowing the remaining elements to function.
2. Why Option (a) is incorrect: Connecting elements 'end to end' describes a series circuit, not a parallel circuit.
3. Why Option (b) is incorrect: In a parallel circuit, the total current is divided among the branches. The 'same current flowing through all elements' is a characteristic of a series circuit.
4. Why Option (d) is incorrect: While 'side by side' is a colloquial way to describe parallel placement, it is not a technical electrical definition. Option (c) provides the precise functional and electrical definition required for an engineering examination.
The insulation resistance of a transformer winding can be easily measured with:
A)
wheatstone bridge
B)
megger
C)
kelvin bridge
D)
voltmeter
Step-by-step reasoning:
1. Insulation resistance is typically very high (in the range of Mega-ohms or Giga-ohms). To measure such high resistance, a standard ohmmeter is insufficient because it cannot provide a high enough test voltage to overcome the insulation's dielectric strength.
2. A Megger (Mega-ohmmeter) is a specialized instrument designed specifically for this purpose. It generates a high DC voltage (typically 500V to 5000V) to stress the insulation and measure the resulting leakage current, which is then converted into a resistance reading.
3. Analysis of other options:
- Wheatstone Bridge: Used for measuring medium-range resistances with high precision, but not suitable for high insulation resistance.
- Kelvin Bridge: Used for measuring very low resistances (below 1 ohm), such as contact resistance or winding resistance, not insulation resistance.
- Voltmeter: Measures electrical potential difference between two points, not resistance.
Type of lift that is used in offices is:
A)
Passenger lift
B)
hospital lift
C)
car lift
D)
none of the above
Step-by-step reasoning: 1. Analysis of the requirement: Offices require a vertical transport system to move employees, visitors, and staff between floors. 2. Evaluation of Option (a): A Passenger lift is specifically designed to transport people. It features a cabin size and safety mechanisms suitable for human transit, making it the standard choice for office buildings. 3. Evaluation of Option (b): A Hospital lift (or stretcher lift) is a specialized passenger lift with larger cabins and specific dimensions to accommodate beds, stretchers, and medical equipment. While it carries people, it is specialized for healthcare facilities, not general offices. 4. Evaluation of Option (c): A Car lift is a heavy-duty industrial lift designed to transport vehicles between parking levels. It is not suitable for transporting people in an office environment. 5. Conclusion: Based on the functional requirements of an office building, the Passenger lift is the correct classification.
The unit of e.m.f is:
A)
Watt
B)
Volt
C)
Ampere
D)
Kilowatt-hour
Step-by-step reasoning:
1. Definition: e.m.f. stands for Electromotive Force. Despite the word 'force', it is not a force in the mechanical sense but is the energy provided by a cell or generator per unit charge.
2. Physics: e.m.f. is essentially the potential difference across the terminals of a source when no current is flowing.
3. Unit: The SI unit for electric potential and electromotive force is the Volt (V), named after Alessandro Volta. One volt is defined as one joule of energy per coulomb of charge (1V = 1J/C).
Analysis of other options:
- a. Watt: This is the unit of Electric Power (P = V × I).
- c. Ampere: This is the unit of Electric Current (I), measuring the flow of charge per unit time.
- d. Kilowatt-hour: This is a unit of Electrical Energy (E = P × t), commonly used for billing electricity consumption.