Electrician and Wireman MCQ Quiz - Objective Question with Answer for Electrician and Wireman - Download Free PDF

Explanation:

Alternating Current

Definition: Alternating Current (AC) is a type of electrical current in which the magnitude of the current changes continuously and its direction reverses periodically. This behavior contrasts with Direct Current (DC), where the current flows steadily in one direction.

Working Principle:

In AC, the flow of electrons alternates between moving forward and backward within the conductor. This alternating behavior is caused by the periodic reversal of the voltage polarity across the circuit. The frequency of the AC determines how often the current reverses direction per second. For example, in most countries, the standard frequency of AC is 50 Hz or 60 Hz, meaning the current reverses direction 50 or 60 times per second.

Characteristics:

• Magnitude: The magnitude of AC varies continuously with time, typically following a sinusoidal waveform. The voltage and current rise and fall in a smooth, cyclic manner.
• Direction: The direction of AC changes periodically, reversing back and forth, unlike DC, which flows in a single direction.

Advantages:

• Efficient Power Transmission: AC can be easily transformed to higher or lower voltages using transformers, making it ideal for transmitting electricity over long distances with minimal losses.
• Wide Application: AC is the standard form of electricity used in homes, offices, and industries because most electrical devices and appliances are designed to work with AC.
• Flexible Generation: AC can be generated using devices like alternators, which are simpler and more efficient than DC generators for large-scale electricity production.

Disadvantages:

• Complexity in Circuits: AC circuits require additional components like capacitors and inductors to manage voltage and current, which makes them slightly more complex than DC circuits.
• Safety Concerns: Due to its alternating nature, AC can be more dangerous than DC at high voltages, as the periodic reversal can cause more severe effects on the human body.

Applications:

• Domestic and industrial power supply.
• Operation of electrical appliances such as fans, refrigerators, and televisions.
• Powering large-scale machinery and equipment in factories.

Correct Option Analysis:

The correct option is:

Option 1: Changes continuously; changes periodically

This option accurately describes the behavior of an alternating current. The magnitude of AC changes continuously, typically in a sinusoidal pattern, and the direction of current flow reverses periodically, following the frequency of the AC system.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Does not change; remains the same

This description corresponds to Direct Current (DC), not Alternating Current (AC). In DC circuits, the current flows steadily in one direction, and its magnitude remains constant over time. Therefore, this option is incorrect for describing AC.

Option 3: Does not change; changes periodically

This option is inconsistent. If the magnitude of the current does not change, it cannot be categorized as AC since the defining characteristic of AC is the continuous variation in magnitude and periodic reversal in direction. This statement is contradictory and does not apply to AC.

Option 4: Changes continuously; remains the same

This option partially describes the behavior of AC but fails to account for the periodic reversal of direction. While the magnitude of AC does change continuously, its direction also reverses periodically. Hence, this option is incomplete and incorrect.

Conclusion:

Understanding the fundamental characteristics of Alternating Current (AC) is essential for differentiating it from Direct Current (DC). The correct option, as explained, accurately captures the behavior of AC, where the magnitude changes continuously and the direction reverses periodically. This unique property makes AC suitable for widespread use in power transmission, domestic applications, and industrial operations, despite its associated complexities and safety considerations.

The correct answer is option 2.

The fundamental principle for electrical installations aligns directly with international safety standards such as IEC 60364 and NFPA 70 (NEC), which emphasize:

• Protection against electric shock
• Overcurrent protection (including overloads and short circuits)
• Protection against fault currents
• Proper grounding and bonding

These are foundational principles to prevent injury, fire, and equipment damage. The other options contradict safety standards and could pose serious hazards if followed.

Biomass

• Biomass is an environmentally-friendly, renewable energy source.
• It is produced when organic matter, such as food or animal waste, is broken down by microorganisms in the absence of oxygen, in a process called anaerobic digestion.
• In a biogas power plant, biomass is burned in a boiler to produce high-pressure steam. This steam flows over a series of turbine blades, causing them to rotate. The rotation of the turbine drives a generator, producing electricity.
• Bio methane is the primary component of biogas, which is produced from the anaerobic digestion of animal waste, organic waste, and agricultural residues.
• Biomass mainly consists of methane (CH4) (50-75%), carbon dioxide (CO2) (25-50%), and traces of other gases.
• Some of the most popular biomass choices are crop residues, municipal and industrial sewage, agricultural material, livestock manures, seaweed, food-processing, and paper wastes.

Explanation
 

• Biomass does not have a high calorific value compared to fossil fuels like coal or petroleum. While it provides energy, its calorific value is lower than conventional fossil fuels.
• The other statements are scientifically valid, as biomass stores solar energy, is renewable, and carbon-neutral in the long term (although combustion releases CO₂, plants absorb CO₂ during their lifecycle).

Hence, option 1 is the correct answer.

Fencing Requirements for Outdoor Substations

• The minimum height of metallic fencing around a transformer in an outdoor substation should be 1.8 meters.
• In high-security areas, the height may be increased to 2.4 meters (8 feet) or more with barbed wire on top.
• Three strands of barbed wire should be installed at a 45-degree outward angle on top of the fence.
• The IEEE Std. 80-2013 recommends grounding every 15-30 meters (50-100 feet) along the fence.

Difference between Indoor and Outdoor Substations:

The symbols of various power electronics devices are:

IGBT (Insulated Gate Bipolar Transistor)

MOSFET (Metal Oxide Semiconductor Field Effect Transistor)

GTO (Gate Turn-Off Thyristor)

​

BJT (Bipolar Junction Transistor)

An overexcited alternator always supplies lagging current to the connected load, which means that load is of lagging nature. Lagging load take active and reactive power from the supply or alternator. Therefore, reactive power flows outwards from an over-excited alternator.

Important Points:

• Under excited alternator works at the leading power factor
• The normal excited alternator works at the unity power factor
• The overexcited alternator works at lagging power factor

Mistake PointsFor Synchronous motor its opposite of alternator,

• Under excited synchronous motor works at lagging power factor
• The normal excited synchronous motor works at the unity power factor
• The overexcited synchronous motor works at a leading power factor

The speed of the induction motor can be controlled by the following methods:

1) V/f control (or) frequency control

2) Changing the number of stator poles

3) Controlling supply voltage

4) Changing winding resistance by adding rheostat in the stator circuit

Given figure shows the symbol of DC shunt motor

Temperature rises in electrical Machines are due to power losses occurring in various parts of the machines.

The various power losses occurring are rotational loss, friction and windage loss, and electrical and magnetic losses.

The permissible temperature rise of electrical machines is determined by the type of insulation materials used for winding.​

Properties of insulating materials for electrical machine

A perfect insulating material ought to possess the subsequent properties:

• High dielectric strength.
• It should withstand high temperatures.
• Good thermal conductivity.
• It should not undergo thermal oxidation.
• It should withstand vibration, abrasion, and bending.
• Not to absorb moisture.

Concept:

The energy is given by:

\(E=P\times t\)

\(E=(V\times I)\times t\)

where, I = Current

R = Resistance

t = Time

Calculation:

Given, V = 220 V

E = 1.5 kW

t = 1 hr

\(1.5\times 10^3=220\times I\times 1\)

I = 6.82 A

Pig Tail:

This joint is also called as Pig-tail/Rat-tail/Twisted joint.

This joint is suitable for pieces where there is no mechanical stress on the conductors, as found in the junction box or conduit accessories box

Married joint:

A married joint is used in places where appreciable electrical conductivity is required, along with compactness.

As the mechanical strength is less, this joint could be used at places where the tensile stress is not too great.

Britannia joint:

This joint is used in overhead lines where considerable tensile strength is required. It is also used both for inside and outside wiring where single conductors of diameter 4 mm or more are used.