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Past Questions — 2025
The minimum size of single core copper conductor used for power socket wiring is
A)
1.5mm2
B)
2.5mm2
C)
6.0mm2
D)
4.0mm2
In standard electrical installation practices and according to the Indian Standard (IS) and National Electrical Code (NEC) guidelines for residential house wiring:
1. Lighting circuits typically use a minimum conductor size of 1.5 mm² because the current load for LED or CFL bulbs is relatively low.
2. Power socket circuits (which are intended for appliances like irons, mixers, or vacuum cleaners) carry higher current loads. To prevent overheating and voltage drop, a minimum conductor size of 2.5 mm² is mandated for power sockets.
3. 4.0 mm² and 6.0 mm² conductors are generally reserved for high-power appliances like air conditioners, geysers, or the main incoming supply line to the distribution board.
Therefore, for general power socket wiring, 2.5 mm² is the standard minimum requirement.
1(one) Ton of refrigeration is equal to
A)
15,000 BTU/hr
B)
13,000 BTU/hr
C)
12,000 BTU/hr
D)
none of above
A 'Ton of Refrigeration' (TR) is a unit of power used to describe the cooling capacity of air conditioning and refrigeration systems. It is defined as the amount of heat required to melt one short ton (2,000 lbs) of pure ice at 0°C (32°F) in 24 hours.
1. The latent heat of fusion for ice is approximately 144 BTU/lb.
2. For one ton of ice: 2,000 lbs × 144 BTU/lb = 288,000 BTU.
3. To find the hourly rate: 288,000 BTU / 24 hours = 12,000 BTU/hr.
Therefore, 1 TR = 12,000 BTU/hr. Options (a) and (b) are numerically incorrect based on this standard thermodynamic definition.
Transformer breaths in when
A)
load on it reduces
B)
load remains constant
C)
load on it increases
D)
none of the above
The 'breathing' of a transformer refers to the expansion and contraction of the insulating oil due to temperature changes.
1. When the load on a transformer increases, the internal temperature of the oil rises due to copper losses (I²R) and iron losses. This causes the oil to expand, pushing air out of the conservator tank (the transformer 'breathes out').
2. Conversely, when the load on the transformer reduces or is removed, the temperature of the insulating oil decreases.
3. As the oil cools, it contracts, leading to a decrease in volume.
4. To fill this void and maintain pressure, air from the atmosphere is drawn into the conservator tank. This process is known as 'breathing in'.
Therefore, option (a) is correct. Option (b) is incorrect because constant load results in constant temperature and no breathing. Option (c) is incorrect because increasing load causes the transformer to breathe out.
Megger is operated by
A)
an external voltage source
B)
an internal hand driven generator
C)
internal battery
D)
all of above
A Megger (Mega-Ohmmeter) is a specialized instrument used to measure very high electrical resistances, typically the insulation resistance of cables and motor windings. To measure high resistance, a high DC voltage is required. Traditionally, the Megger is designed with an internal hand-driven DC generator (a magneto) that produces the necessary high voltage when the operator turns a handle. While modern digital insulation testers use batteries, the classic 'Megger' referred to in standard electrical engineering exam contexts is defined by its hand-cranked generator. Option (a) is incorrect because the instrument is designed to be self-contained for safety and portability. Option (c) refers to modern insulation testers, but the fundamental definition of a Megger in technical textbooks emphasizes the hand-driven generator.
Which is better earth resistance-
A)
1 ohm
B)
50 ohm
C)
100 ohm
D)
0.5 ohm
In electrical earthing systems, the primary goal is to provide a low-impedance path for fault currents to flow safely into the earth. According to Ohm's Law (V = I * R), for a given fault current (I), a lower resistance (R) results in a lower voltage (V) appearing on the equipment chassis or the ground surface. This minimizes the risk of electric shock to personnel and ensures that protective devices (like circuit breakers or fuses) trip quickly by allowing a high enough fault current to flow. Therefore, the lower the earth resistance, the more effective and 'better' the earthing system is. Comparing the given options: 0.5 ohm < 1 ohm < 50 ohm < 100 ohm. Thus, 0.5 ohm is the best (lowest) resistance value among the choices provided.
E.m.f. equation of a generator is (Where is flax per pole, N is speed in r.p.m., Z is number of conductor, P is number of pole and A is number of parallel paths)
A)
e.m.f = NA P / 60 z volts
B)
e.m.f. = AZ P / 60 N volts
C)
e.m.f. = NZ P / 60 A volts
D)
none of the above
The electromotive force (e.m.f.) generated in a DC generator is derived from Faraday's Law of Electromagnetic Induction. The standard formula for the generated e.m.f. (E) is: E = (ΦZN P) / (60A).
Breaking down the variables:
- Φ (Phi): Flux per pole (Webers)
- Z: Total number of armature conductors
- N: Speed of armature in revolutions per minute (r.p.m.)
- P: Number of poles
- A: Number of parallel paths (A = P for lap winding, A = 2 for wave winding)
- 60: Conversion factor to change r.p.m. to revolutions per second.
Comparing this to the options:
- Option (a) incorrectly places P in the numerator and Z in the denominator.
- Option (b) incorrectly places A in the numerator and N in the denominator.
- Option (c) correctly follows the formula: e.m.f. = (Φ * N * Z * P) / (60 * A).
The speed for a residential elevator is between
A)
1.5-2.5 m/s
B)
0.5-2.5 m/s
C)
1.0-2.0 m/s
D)
none of above
In elevator engineering, the rated speed is determined by the height of the building and the intended use of the lift. Residential elevators (used in low to mid-rise apartment buildings) typically operate at lower speeds to ensure passenger comfort and safety, and because the travel distance is relatively short. Standard industry ranges for residential/low-rise elevators generally fall between 0.5 m/s (for very low-rise/home lifts) and 2.5 m/s (for mid-rise residential complexes). Speeds above 2.5 m/s are typically reserved for high-rise commercial buildings or skyscrapers to reduce travel time. Therefore, option (b) provides the most comprehensive range that covers both basic residential lifts and those in larger residential complexes.
Iron core of power transformer is laminated
A)
to reduce copper losses
B)
to reduce hysteresis losses
C)
to reduce eddy current losses
D)
due to (b) and (c)
Step-by-step reasoning:
1. **Understanding Eddy Currents**: When a varying magnetic flux passes through a solid conductive core, it induces small circulating currents within the core material itself. These are known as eddy currents.
2. **The Problem**: These currents flow in loops and, due to the electrical resistance of the core material, they generate heat (I²R losses), which leads to energy inefficiency and potential overheating of the transformer.
3. **The Solution (Lamination)**: To prevent these currents from flowing in large loops, the core is constructed from thin sheets (laminations) of silicon steel, each coated with a thin layer of insulating varnish.
4. **Mechanism**: The insulation increases the electrical resistance between the sheets, effectively breaking the paths of the eddy currents and confining them to very small areas. This significantly reduces the total eddy current loss.
Analysis of other options:
- **Option (a)**: Copper losses occur in the windings (conductors), not the iron core. Lamination of the core has no effect on copper losses.
- **Option (b)**: Hysteresis losses are caused by the friction of magnetic domains flipping during magnetization. These are reduced by using high-permeability materials like **Silicon Steel**, not by laminating the core.
- **Option (d)**: Since lamination specifically targets eddy currents and not hysteresis losses, this option is incorrect.
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 wire to two positions
D)
connection of two wire to one positions
A one-way single pole switch (also known as a Single Pole Single Throw or SPST switch) is the simplest type of switch used in house wiring. It has two terminals: one for the incoming live wire and one for the outgoing wire to the load. Its primary function is to 'break' or 'make' the connection of a single conductor (the phase/live wire) to a single position (the load).
- Option (a) is correct because the switch controls the flow of one wire (the live wire) to one specific destination/position (the light or appliance).
- Option (b) is incorrect because controlling one wire to two positions would describe a selector switch or a change-over switch.
- Option (c) and (d) are incorrect because a single pole switch does not control two wires simultaneously; controlling two wires (Live and Neutral) would require a double-pole switch.
Induction type of instruments are used on-
A)
d.c. supply only
B)
a.c. supply only
C)
both a.c. and d.c. supply
D)
battery supply
Induction-type instruments operate on the principle of electromagnetic induction. They require a changing magnetic field to induce eddy currents in a conducting disc (usually aluminum). According to Faraday's Law of Electromagnetic Induction, a changing magnetic flux is necessary to produce these currents.
1. Why AC only: In an Alternating Current (AC) supply, the current changes direction and magnitude periodically, creating the necessary time-varying magnetic flux to induce eddy currents and produce a deflecting torque.
2. Why not DC: In a Direct Current (DC) supply, the magnetic field produced is stationary (constant). A constant magnetic field cannot induce eddy currents in a stationary disc, meaning no torque is produced and the instrument will not deflect.
3. Battery supply: Since batteries provide DC, they are unsuitable for induction-type instruments.
Therefore, induction-type instruments (such as induction wattmeters and energy meters) are exclusively used for AC measurements.
If voltage supply to the energy meter is more than the rated value, energy meter will run
A)
slow
B)
fast
C)
neither slow or fast
D)
none of above
An induction-type energy meter operates based on the interaction of magnetic fields produced by the voltage coil (pressure coil) and the current coil. The driving torque of the energy meter is proportional to the product of the voltage and the current (V × I). Specifically, the torque is proportional to the square of the voltage (V²) if the load is constant (since I = V/R). When the supply voltage increases beyond the rated value, the magnetic flux produced by the voltage coil increases. This leads to an increase in the driving torque acting on the aluminum disc, causing the disc to rotate faster than it would at the rated voltage. Consequently, the energy meter records more energy than is actually being consumed, meaning it runs 'fast'. Conversely, if the voltage were lower than the rated value, the meter would run 'slow'.
The maximum permissible voltage drop from supply terminals to any point in the installation for light wiring is
A)
3%+1V
B)
2%+1V
C)
4%+2V
D)
none of above
According to standard electrical installation guidelines (such as those derived from the Indian Electricity Rules and the National Electrical Code), the permissible voltage drop is limited to ensure that electrical appliances operate efficiently and safely. For light wiring (lighting circuits), the maximum permissible voltage drop from the supply terminals (the point of entry/distribution board) to any point in the installation is generally specified as 3% of the nominal voltage plus a constant of 1V. This allows for a small margin of error in low-voltage scenarios while maintaining a strict percentage limit for higher nominal voltages to prevent flickering of lights or malfunction of electronic ballasts. Options b, c, and d are incorrect as they do not align with the standard regulatory specifications for lighting installations.
The formula Power= VI Cos is applicable to 3-phase circuits having-
A)
star connections
B)
delta connections
C)
port above connections
D)
none of the above connections
The formula for total active power in a balanced 3-phase system is P = √3 × V_L × I_L × cos φ, where V_L is the line-to-line voltage and I_L is the line current.
1. In a Star (Y) connection: The total power is 3 × V_phase × I_phase × cos φ. Since V_L = √3 × V_phase and I_L = I_phase, substituting these into the formula gives P = 3 × (V_L/√3) × I_L × cos φ = √3 × V_L × I_L × cos φ.
2. In a Delta (Δ) connection: The total power is 3 × V_phase × I_phase × cos φ. Since V_L = V_phase and I_L = √3 × I_phase, substituting these into the formula gives P = 3 × V_L × (I_L/√3) × cos φ = √3 × V_L × I_L × cos φ.
Because the formula P = √3 VI cos φ depends on line values (V_L and I_L), it is universally applicable to both star and delta connections. Note: 'port above connections' is a typographical error in the exam paper for 'both above connections'.
Resistance of 250 V,100W lamp will be-
A)
0.4ohm
B)
25ohm
C)
625ohm
D)
2.4 ohm
To find the resistance of the lamp, we use the electrical power formula that relates Power (P), Voltage (V), and Resistance (R):
Formula: P = V² / R
Given values:
Voltage (V) = 250 V
Power (P) = 100 W
Rearranging the formula to solve for Resistance (R):
R = V² / P
Step-by-step calculation:
1. Square the voltage: 250² = 250 * 250 = 62,500
2. Divide by the power: 62,500 / 100 = 625
Therefore, the resistance is 625 ohms.
Analysis of options:
- Option (a) 0.4 ohm: Incorrect. This would be the result if one incorrectly divided power by voltage squared (100/62500).
- Option (b) 25 ohm: Incorrect.
- Option (c) 625 ohm: Correct based on the calculation.
- Option (d) 2.4 ohm: Incorrect.
Three lamps of following ratings are connected in series across 230 V supply. Which of the lamp will glow the brightest?
A)
15 W 230 V lamp
B)
100 W 230 V lamp
C)
60 W 230 V lamp
D)
200 W 230 V lamp
Step-by-step reasoning:
1. **Understanding Resistance**: The power rating of a lamp is given by P = V²/R. Therefore, the resistance of a lamp is R = V²/P. Since all lamps are rated for the same voltage (230V), the lamp with the lowest power rating (15W) will have the highest resistance, and the lamp with the highest power rating (200W) will have the lowest resistance.
2. **Series Connection**: In a series circuit, the current (I) flowing through every component is the same.
3. **Actual Power Dissipation**: The brightness of a lamp depends on the actual power it dissipates when connected to the circuit, which is calculated by P_actual = I² * R.
4. **Comparison**: Since the current (I) is constant for all lamps in series, the power dissipated is directly proportional to the resistance (P ∝ R).
5. **Conclusion**: The 15W lamp has the highest resistance among the given options. Consequently, it will dissipate the most power (I²R) when connected in series, making it glow the brightest. Conversely, the 200W lamp has the lowest resistance and will glow the dimmest.
The electric motor helps the elevator car to
A)
move downward
B)
move upward
C)
both (a) and (b)
D)
none of above
In an elevator system, the electric motor (typically an AC or DC motor) is the primary prime mover. It drives a hoisting machine (sheave) that moves the cables. To move the elevator car upward, the motor rotates in one direction to pull the car up. To move the elevator car downward, the motor either rotates in the opposite direction or controls the descent (braking/regulating speed) to ensure a safe landing. Regardless of whether the system is traction-based or hydraulic, the motor is essential for initiating and controlling movement in both vertical directions. Therefore, the motor helps the car move both upward and downward.
The unit of e.m.f is
A)
Watt
B)
Volt
C)
Ampere
D)
Kilowatt-hour
Step-by-step reasoning:
1. Understanding e.m.f: e.m.f stands for Electromotive Force. Despite the word 'force', it is not a mechanical force but rather the energy provided by a cell or generator per unit charge.
2. Definition: e.m.f is defined as the potential difference between the terminals of a source when no current is flowing.
3. Unit Identification: Since e.m.f is a measure of electrical potential difference, its SI unit is the Volt (V).
Analysis of other options:
- a. Watt: This is the unit of Power (P = VI).
- c. Ampere: This is the unit of Electric Current (I).
- d. Kilowatt-hour: This is a unit of Electrical Energy (E = P × t).
The standard door width for hospital elevators are
A)
1,100mm
B)
1,300mm
C)
1,200mm
D)
1,400mm
In the design of elevators for healthcare facilities (Hospital Elevators), the primary requirement is to accommodate stretchers, hospital beds, and medical personnel simultaneously. Standard architectural and engineering guidelines for hospital bed elevators typically specify a clear door opening width of at least 1,200 mm (approximately 48 inches). This width is essential to ensure that a standard hospital stretcher (which is typically around 700-900mm wide) can pass through easily with accompanying medical staff or equipment without obstruction. Options a (1,100mm) is generally too narrow for efficient stretcher movement, while options b (1,300mm) and d (1,400mm) are non-standard widths for basic hospital requirements, although larger custom elevators may exist, 1,200mm remains the industry standard benchmark for minimum clear opening.
Continuity of an electrical circuit is usually checked by –
A)
ammeter
B)
voltmeter
C)
ohm meter
D)
all of the above
Continuity testing is the process of verifying whether an electrical path is complete (closed) or broken (open).
1. **Why Ohm meter is correct**: An ohmmeter measures electrical resistance. In a continuity test, the meter sends a small current through the circuit; if the resistance is very low (close to zero), it indicates a continuous path. Most modern digital multimeters have a dedicated 'continuity' setting that uses the ohmmeter function to trigger a beep when a low-resistance path is detected.
2. **Why Ammeter is incorrect**: An ammeter measures the flow of current in a live circuit. While current flow implies continuity, an ammeter is not used as a standalone tool to 'check' continuity because it requires an external power source and would be dangerous or impractical for simple path verification.
3. **Why Voltmeter is incorrect**: A voltmeter measures the potential difference between two points. While a voltage drop across a break in a circuit can indicate a lack of continuity, it is not the standard instrument used to check for a complete path.
Therefore, the ohmmeter is the specific instrument designed to measure the resistance necessary to determine continuity.
The maximum permissible voltage drop from supply terminals to any point in the installation for power wiring is
A)
7.5%
B)
5%
C)
4%
D)
none of above
According to standard electrical installation codes (such as the National Electrical Code and the Indian Standard IS 732), the maximum permissible voltage drop from the point of supply (supply terminals) to any point in the installation is generally limited to ensure the efficient operation of electrical equipment. For power wiring and general installations, the standard limit is 5%. A voltage drop exceeding this limit can lead to overheating of conductors, malfunction of electronic equipment, and reduced efficiency of motors (causing them to draw more current and potentially overheat). Therefore, option (b) 5% is the correct technical standard.
Electrical energy is measured in-
A)
Ohm meter
B)
volt meter
C)
ampere meter
D)
energy meter
To determine the correct instrument for measuring electrical energy, we must analyze the function of each device listed in the options:
1. **Ohm meter (Option a):** This instrument is used to measure electrical resistance, which is the opposition to the flow of current in a circuit. Its unit is the Ohm (Ω).
2. **Volt meter (Option b):** This instrument is used to measure electrical potential difference (voltage) between two points in a circuit. Its unit is the Volt (V).
3. **Ampere meter (Option c):** This instrument is used to measure the flow of electric current through a conductor. Its unit is the Ampere (A).
4. **Energy meter (Option d):** Electrical energy is the product of power (voltage × current) and the time for which it is consumed. An energy meter (also known as a kilowatt-hour meter) is specifically designed to integrate power over time to measure the total electrical energy consumed. Its common unit is the kilowatt-hour (kWh).
Therefore, the energy meter is the only instrument among the choices designed to measure electrical energy.
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
An electric generator is an electromechanical device that operates on the principle of electromagnetic induction (Faraday's Law). It takes mechanical energy (provided by a prime mover such as a turbine, engine, or hand crank) and converts it into electrical energy.
Analysis of options:
- Option (a) describes an electric motor, which converts electrical energy into mechanical energy.
- Option (b) correctly describes a generator, which converts mechanical energy into electrical energy.
- Option (c) is incorrect because a single machine cannot be both a generator and a motor simultaneously in its primary function, although some machines (like synchronous machines) can operate in both modes depending on the input.
- Option (d) is incorrect as option (b) is the standard definition.
Voltage drop allowed in the wiring from consumer’s board at any point inside the house should be
A)
2% of declared supply voltage plus one volt
B)
3% of declared supply voltage plus one volt
C)
4% of declared supply voltage plus one volt
D)
none of above
According to standard electrical installation guidelines and the Central Electricity Authority (CEA) regulations (which govern electrical standards in India), the permissible voltage drop from the consumer's distribution board to any point in the internal wiring of a residential or commercial building is limited to ensure the efficient operation of electrical appliances. The standard limit is defined as 3% of the declared supply voltage plus one volt. This margin accounts for the inherent resistance of the conductors and ensures that the voltage at the furthest point of the installation remains within the acceptable operating range of the equipment. Option (a) is too restrictive, and Option (c) exceeds the standard safety and efficiency limits.
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 all
A power transformer is a static electrical device that operates on the principle of Faraday's Law of Electromagnetic Induction. Its primary purpose is to change the voltage level of an alternating current (AC) supply. Depending on the turns ratio of the primary and secondary windings, it can either increase the voltage (Step-up transformer) for efficient long-distance transmission or decrease the voltage (Step-down transformer) for safe distribution and utilization by consumers.
Analysis of other options:
- Option (a) is incorrect because transformers do not work with direct current (DC). DC does not create the changing magnetic flux required to induce a voltage in the secondary winding.
- 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.
- Option (d) is incorrect as option (b) accurately describes the function.
According to I.E. rules, the voltage fluctuation in residential buildings should not be more than
A)
2% of declared supply voltage for residential building
B)
3% of declared supply voltage for residential building
C)
6% of declared supply voltage for residential building
D)
none of above
According to the Indian Electricity (I.E.) Rules and the standards set by the Central Electricity Authority (CEA) regarding the quality of supply, the permissible variation in voltage for low-voltage (LV) supply to consumers is generally maintained within a specific tolerance band. For residential buildings and general domestic consumers, the standard permissible voltage fluctuation is ±6% of the declared nominal supply voltage. This ensures that electrical appliances operate efficiently without risking damage due to under-voltage or over-voltage conditions. Options a (2%) and b (3%) are too stringent and are typically not the standard for general residential distribution, while 6% is the industry and regulatory benchmark for these installations.