ISRO HSFC Technician B Electrician Solved Question paper

Answer: "1/√5 "

To determine the new power factor of an RL circuit when the frequency is doubled, we can follow these steps:

Given Information

Initial power factor $ cos phi = frac{1}{sqrt{2}}$

Step 1: Understanding the Power Factor

The power factor for an RL circuit is given by:

$cos phi = frac{R}{Z}$

where:

R is the resistance,
Z is the impedance.

The impedance Z  for an RL circuit is given by:

$Z = sqrt{R^2 + (X_L)^2}$

where X_L is the inductive reactance, which is defined as:

X_L = ω_L

ω = 2πf

where f  is the frequency

Step 2: Initial Conditions

From the initial power factor:

$cos phi = frac{R}{Z} = frac{1}{sqrt{2}}$

This implies that:

Z = R √2

Using the impedance formula:

$Z^2 = R^2 + (X_L)^2$

Substituting for Z :

$(R sqrt{2})^2 = R^2 + (X_L)^2$

$2R^2 = R^2 + (X_L)^2$

$(X_L)^2 = R^2$

Thus, we find that:

X_L = R

Step 3: Doubling the Frequency

When the frequency is doubled, the new inductive reactance  X_L = R becomes:

$X_L’ = 2X_L = 2R$

Step 4: New Impedance Calculation

Now, we calculate the new impedance  Z’:

$Z’ = sqrt{R^2 + (X_L’)^2} = sqrt{R^2 + (2R)^2} = sqrt{R^2 + 4R^2} = sqrt{5R^2} = Rsqrt{5}$

Step 5: New Power Factor Calculation

Now we can find the new power factor $cos phi’$:

$cos phi’ = frac{R}{Z’} = frac{R}{Rsqrt{5}} = frac{1}{sqrt{5}}$

Conclusion

Thus, when the frequency is doubled, the new power factor of the RL circuit becomes:

$frac{1}{sqrt{5}}$

Answer: "250 V "

The Bureau of Indian Standards (BIS) specification IS 1293-2019 pertains to plugs and socket-outlets for household and similar purposes with a rated voltage of up to and including 250 V and a rated current of up to and including 16 A. This standard applies to both fixed and portable socket-outlets, with or without earthing contact, suitable for indoor or outdoor use.

Key Points about IS 1293-2019:

• Rated Voltage: Increased from 240 V in the previous version (IS 1293-2005) to 250 V in the revised version to align with the prevailing voltage conditions in India.
• Scope: The standard covers:
  • Plugs and socket-outlets intended for household use.
  • Both fixed and portable designs.
  • Applications that may be used indoors or outdoors.
• Specifications: The standard outlines various requirements, including:
  • Electrical strength
  • Mechanical strength
  • Protection against electrical shock
  • Temperature rise
  • Insulation resistance

Answer: "Becomes double "

In electrolysis, if the current passing through the electrolyte doubles, the total mass of ions liberated at the electrode in every second also doubles. This is governed by Faraday’s first law of electrolysis, which states:

The amount of substance (mass) liberated at an electrode during electrolysis is directly proportional to the quantity of electricity passed through the electrolyte.

Therefore, if the current (the rate at which electricity flows) doubles, the amount of substance liberated (the mass of ions) also doubles within the same time frame.

Mathematically:

Mass of substance deposited (m) ∝ Quantity of electricity passed (Q)

Or,

m = ZQ

Where:

• m = Mass of the substance deposited
• Q = Quantity of electricity passed (in Coulombs)
• Z = Electrochemical equivalent (a constant for a particular substance)

In the given scenario:

If the current (I) is doubled, the quantity of electricity (Q) passed through the electrolyte in a given time (t) will also double (Q = It).

Since the mass of the substance deposited is directly proportional to the quantity of electricity, doubling the current will double the mass of ions liberated at the electrode.

Answer: "Both kinetic and potential energy of sea water "

Both kinetic and potential energy of sea water are utilized in tidal power plants.

• Kinetic Energy: This is derived from the movement of water as tides rise and fall. Tidal turbines capture this kinetic energy from the fast-flowing tidal streams.
• Potential Energy: This comes from the difference in water levels between high tide and low tide. Tidal barrages utilize this potential energy by allowing water to flow through turbines as it moves from a higher elevation (high tide) to a lower elevation (low tide).

Thus, tidal power harnesses both forms of energy to generate electricity.

Answer: "Current drawn from the charging source is constant "

In an RC series circuit, the voltage across the capacitor increases linearly under the condition that the current drawn from the charging source is constant.

Constant Current: When a constant current flows into the capacitor, the charge (Q) on the capacitor increases linearly over time, as expressed by the equation:

Q = I⋅t

where I is the constant current and t is time.

Capacitor Voltage: The voltage (V) across a capacitor is directly proportional to the charge stored on it, given by:

V=Q/C​

where C is the capacitance of the capacitor.

Linear Increase: Since the charge Q increases linearly with time when current is constant, the voltage V across the capacitor also increases linearly:

V=I⋅t/C​

Answer: "$overline{X+Y}$ "

Since 1, 2 & 3 are grounded.

S1	S0	V
0	0	I0
0	1	I1
1	0	I2
1	1	I3

I₀ = 1, I₁ = 0, I₂ = 0, I₃ = 0

x = S₂

y = S₁

The output of the 4 x 1 MUX is:

$Y = F = bar{S_1}bar{S_2}I_0 + bar{S_1}S_2I_1 + S_1bar{S_2}I_2 + S_1S_2I_3$

Substituting the values:

$F = xy cdot 1 + xy cdot 0 + xy cdot 0 + xy cdot 0$

Simplifying:

$F =bar{X}bar{Y} = overline{X+Y}$

Answer: "Starting, Lighting, Ignition "

• In SLI Battery, S, L and I stand for Starting, Lighting, and Ignition.
• An SLI battery is a lead-acid and rechargeable type of battery that is mainly used in automobiles.
• An SLI battery is composed of six galvanic cells arranged in a series to provide a 12-volt system.
• Heavy vehicles, which run on diesel, would require two batteries for a 24-volt system.

In an SLI battery, these letters represent the three primary functions it serves in a vehicle:

• Starting: Refers to the battery’s ability to provide the initial power needed to start the engine of a vehicle.
• Lighting: Indicates the battery’s role in powering the vehicle’s lights, including headlights and interior lights.
• Ignition: Relates to the battery’s function in supplying power to the ignition system and other electrical accessories in the vehicle.

Answer: "Lowest resistance in n readings "

For a fixed voltage, when current readings are taken n times, the lowest value of resistance in ‘n’ readings is considered to be the value of earth resistance.

1. Purpose of Earthing: Earthing is a safety measure in electrical systems designed to provide a path for fault currents to flow safely to the ground. This helps prevent electric shocks and reduces the risk of damage to equipment in the event of a fault.
2. Minimum Resistance Path: Fault currents will indeed follow the path of least resistance. The lower the resistance of the earth connection, the easier it is for fault currents to dissipate harmlessly into the ground.
   • • This is crucial for the efficient operation of protective devices, such as fuses and circuit breakers, which rely on detecting abnormal currents to disconnect faulty equipment.

Answer: "Raising mains "

Raising mains is the most suitable type of distribution system for multi-story building.

Types of distribution systems:

1.) Raising mains:

• This system involves vertical supply lines (rising mains) that distribute electrical power from the ground floor to various floors in the building. It is specifically designed to handle the high demand for electricity in multistorey structures, where power needs to be efficiently delivered to multiple levels.
• When in electrical wiring, the appliances are connected through joints, is called a joint box, Raising mains system or the tree system of wiring.
• In this type of distribution system, many branch lines are taken out from the mainline.
• Each branch line works as a separate circuit.
• Phase wire to different rooms is taken from the main board, through a fuse on the main board.
• Neutral and earth wire is common to all the circuits.
• The different circuits coming from the main board is connected in parallel. If there is a short circuit in one particular circuit, then only the fuse of that circuit is broken and the power supply to other parts of the circuit will not be affected.

2.) Busbar chamber:

• Busbar Chambers are the metallic strips or bars, housed inside the switchgear, panel boards, and busway enclosures for high current power distribution and to ensure straightforward network configuration.

3.) Radial distribution:

• This system is used only when a substation or generating station is located at the center of the consumers.
• In this system, different feeders radiate from a substation or a generating station and feed the distributors at one end.
• Thus, the power flow in this distribution is in only one direction.

Answer: "Convert the induced AC into DC "

• In a DC generator, the function of the commutator is primarily to convert the induced AC (alternating current) into DC (direct current).

• Induced AC Current: In a DC generator, the armature winding generates alternating current (AC) due to the rotation of the coil in a magnetic field.
• Commutator Function: The commutator acts as a mechanical rectifier. It reverses the connections of the armature winding to the external circuit every half turn, ensuring that the output current flows in one direction (DC).

• Result: This conversion allows the generator to provide a steady direct current to the external load, which is essential for most applications requiring DC power.