ADVANCED NABTEB GCE 2023 ELECTRICAL INSTALLATION ANSWERS

(ii) Narrow energy band gap: Semiconductors have a small energy band gap between the valence and conduction bands. This narrow gap allows them to be easily manipulated to change their electrical conductivity.

(iii) Temperature dependence: The conductivity of pure semiconductors is highly temperature-dependent. As temperature increases, the number of charge carriers (electrons or holes) increases, leading to enhanced conductivity.

(iv) Photoconductivity: Pure semiconductors can exhibit photoconductivity, where exposure to light increases their electrical conductivity. This property makes them useful in light-detection devices such as solar cells and photodiodes.

(v) Doping sensitivity: Pure semiconductors are sensitive to impurities or doping. By intentionally adding specific impurities, the electrical properties of semiconductors can be altered, enabling the creation of p-type or n-type semiconductors with distinct conductivity characteristics.
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(2)
Draw the diagram

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(3)
(PICK TWO ONLY)

(i) Electrode Separation: Earth electrodes should not overlap to ensure proper separation between them. This separation helps maintain the integrity and effectiveness of each electrode.

(ii) Uniform Current Distribution: By avoiding overlap, the current distribution in the soil is more uniform. This prevents concentration of current in specific areas, which could lead to uneven potential gradients and ineffective grounding performance.

(iii) Maximum Soil Contact: When electrodes do not overlap, they can be placed at different locations to maximize soil contact. This allows for optimal dissipation of electrical energy into the ground.

(iv) Reduce Grounding Resistance: Overlapping electrodes can increase the overall grounding resistance due to mutual interference. By ensuring separate, non-overlapping electrodes, the resistance can be reduced, resulting in better grounding performance.

(v) Eliminate Interference: Overlapping electrodes can create interference and create unwanted electrical connections between different grounding systems. This interference can disrupt the functioning of electrical equipment and compromise safety.

(vi) Compliance with Standards: Many electrical standards and codes specify that earth electrodes should not overlap. By adhering to these standards, you ensure that your grounding system meets the required safety and performance criteria.
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(4)
We use lightning arresters for the purpose of protecting electrical and electronic equipment from the damaging effects of lightning strikes. Lightning is a natural occurrence that generates extremely high voltage levels and can cause significant damage to power lines, buildings, and electronic devices.

Lightning arresters are devices designed to divert the harmful effects of lightning strikes away from sensitive equipment and structures. They provide a low-impedance path for the lightning current to flow, effectively redirecting the surge of electrical energy to the ground.
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(5)
Lenz’s law states that the current induced in a circuit due to a change in a magnetic field is directed to oppose the change in flux and to exert a mechanical force which opposes the motion.
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(6)
Linear velocity refers to the rate of change of an object’s position in a straight line over time. It is a vector quantity that measures how fast an object is moving in a particular direction.
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(7)
Draw the diagram

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(8)
Circular motion refers to the movement of an object in a circular path around a central point or axis. It is characterized by a constant distance between the object and the center of rotation, as well as a constant speed or angular velocity.
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(9)
The function of a damper winding is to minimize or dampen the effects of any electromotive force (EMF) that may be induced in the rotor of an electric machine, such as a generator or an alternator.
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(10)
The principle of conversion of energy states that energy cannot be created or destroyed in an isolated system. This means that the total amount of energy within a system remains constant over time. Energy can only be transferred or transformed from one form to another.
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SECTION B
ANSWER FIVE QUESTIONS ONLY

(12a)
(PICK TWO ONLY)

(i) High Accuracy: Moving coil instruments provide precise and accurate measurements, making them suitable for applications where precise readings are essential.

(ii) Wide Frequency Range: These instruments have a wide frequency range, allowing them to measure both AC and DC currents and voltages effectively.

(iii) Low Power Consumption: Moving coil instruments require minimal power for operation, making them energy-efficient and suitable for battery-powered applications.

(iv) Damping Mechanism: They possess a built-in damping mechanism that ensures rapid settling of the pointer, enabling faster and more accurate readings.

(v) Long Lifespan: Moving coil instruments have a long operating life due to their rugged construction and minimal wear and tear.

(vi) Compact Design: These instruments are usually compact and lightweight, making them easy to handle, install, and transport.

(12b)
Draw the diagram

A drop test in a DC armature is carried out to assess the durability and shock resistance of the armature. It involves subjecting the armature to controlled impacts or drops from a specified height. During the test, the armature is securely mounted and then released to freefall onto a hard surface. The impact generates mechanical stresses that simulate real-world operating conditions and potential mishaps. The armature’s performance, integrity, and any damage or deformation are evaluated after the drop test to determine its ability to withstand accidental impacts and maintain functionality.
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(13a)
The IEE regulation governing the installation of electric motors in relation to no-volt protection is specified in Regulation 537.6.3. It states that all electric motors with a rating of 3 kW or higher must be provided with no-volt release protection.

(13bi)
The distribution factor (Kd):
Given:
Number of poles (P) = 4
Number of phases (m) = 3
Total number of slots (Zs) = 84
Kd = sin(πZs/N)
Where:
Zs = Total number of slots
N = Number of phases x Number of poles
Kd = sin[π x 84/(3×4)]
Kd = 0.935

(13bii)
The number of slots per pole per phase (Y):
Y = Zs/(P x m)
Y = 84/(4 x 3)
Y = 7

(13biii)
The number of electrical degrees between adjacent slots (β):
β = 360°/Zs
β = 360°/84
β = 4.286°

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(14a)
Phasing of transformer winding refers to the arrangement of the winding connections in a transformer. Winding phasing determines the relative positions and polarities of the primary and secondary windings, as well as the direction of the currents flowing through them. The primary winding is connected to the power source, while the secondary winding is connected to the load. The phasing of the windings ensures that there is proper energy transfer between the primary and secondary sides.

(14b)
(PICK FOUR ONLY)

(i) Space allocation: Ensure sufficient space is available for installing the transformer, considering factors such as access, clearance, and future expansion.

(ii) Foundation design: Properly design and construct a sturdy foundation to support the weight of the transformer and minimize vibration.

(iii) Cooling system: Determine the cooling requirements of the transformer and install appropriate cooling systems, such as radiators or fans, to maintain optimal operating temperatures.

(iv) Electrical connections: Make secure and reliable electrical connections, including power cables, busbars, and control wiring.

(v) Protection systems: Install protective devices such as circuit breakers, relays, and surge arresters to safeguard the transformer from faults, overloading, and voltage fluctuations.

(vi) Ventilation and fire protection: Ensure proper ventilation and install fire detection and suppression systems to prevent overheating or fire hazards.

(vii) Safety measures: Implement safety measures like fencing, signage, and interlocks to restrict access to authorized personnel and mitigate potential risks.

(viii) Testing and commissioning: Perform comprehensive testing and commissioning procedures to verify the transformer’s performance, insulation resistance, and protective schemes before energizing it.

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(15a)
Draw the diagram

(i) A varley loop test arrangement is set up. This involves connecting the power source, a set of resistors, and a voltmeter in a loop configuration.

(ii) The loop is then connected at one end to the faulted point in the network, while the other end is connected to a known healthy point, the source or a reference point.

(iii) A voltage drop is measured across the resistors in the loop using the voltmeter. The resistors are varied to obtain different voltage readings.

(iv) These voltage readings are recorded and plotted against the respective resistor values.

(v) By analyzing the voltage and resistor data, a graph is obtained. The slope of this graph can be used to determine the approximate distance to the fault, as the slope is inversely proportional to the distance.

(vi) The fault location can then be estimated by comparing the obtained slope with a known reference slope or using mathematical algorithms specifically designed for fault location calculation.

(15b)
(PICK TWO ONLY)

(i) Proper Grounding: Electrical installations must have a reliable and effective grounding system to minimize the risk of electrical shocks and equipment damage.

(ii) Low Impedance: The bonding system needs to have low impedance connections to ensure efficient fault current flow, fault clearing, and proper operation of protective devices.

(iii) Equipment Bonding: All non-current carrying metal parts of electrical equipment need to be bonded together to prevent potential differences and to ensure safe operation.

(iv) Bonding Conductors: Bonding conductors should be sized adequately to carry fault currents and meet the specified short-circuit requirements.

(v) Bonding Electrodes: Proper bonding electrodes, such as grounding rods or conductive metal water pipes, must be used and properly connected to provide efficient grounding.

(vi) Continuity: The bonding conductors and connections should have continuous electrical conductivity, avoiding any breaks or corrosion that may compromise the system’s effectiveness.
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(17a)
(PICK FOUR ONLY)

(i) Display Screen: This is where the waveform is displayed. It typically uses a cathode ray tube (CRT) or a liquid crystal display (LCD).

(ii) Input Channels: Oscilloscopes can have one or multiple input channels. Each channel allows you to connect a signal source to measure and display its waveform.

(iii) Vertical Amplifier: The vertical amplifier amplifies the input signal and adjusts its amplitude for display on the screen. It also controls the vertical position of the waveform.

(iv) Timebase Control: The timebase control sets the speed or the time scale at which the waveform is displayed horizontally on the screen. It enables you to adjust the sweep speed or time per division.

(v) Trigger Controls: The trigger controls allow you to synchronize the display of the waveform with a specific part of the signal. It ensures that the waveform is stable and repeatable.

(vi) Horizontal Amplifier: The horizontal amplifier processes the synchronized signal received from the trigger circuit and controls the horizontal deflection on the screen.

(vii) Probes: Probes are used to connect the oscilloscope to the circuit or signal source under test. They typically have a ground lead and a probe tip for accurate voltage measurements.

(viii) Controls and Knobs: These include various controls and knobs that enable adjustments to the vertical and horizontal scales, trigger levels, timebase settings, and other parameters for precise signal analysis.