NABTEB GCE 2025 ADVANCED ELECTRICAL INSTALLATION & MAINTENANCE WORKS ANSWERS (ESSAY – TYPE A)

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ADVANCED ELECTRICAL INSTALLATION ANSWERS (TYPE A)

SECTION A
INSTRUCTIONS: ANSWER ALL QUESTIONS.

(1)
The superposition principle states that in a linear electrical circuit with multiple independent sources, the total current or voltage at any point in the circuit is the algebraic sum of the individual currents or voltages produced by each source acting alone, with all other independent sources replaced by their internal impedances.
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(2)
A fire detector is used to sense the presence of smoke, heat, or flames and provide an early warning to prevent loss of life and damage to electrical equipment and property. It helps initiate safety procedures such as alarms, shutdown of equipment, and activation of fire-fighting systems.
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(3)
(Draw the diagram)

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(4)
(i) Availability of adequate space for equipment installation and future expansion.
(ii) Safety of personnel and the public, including proper grounding and fencing.
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(5)
Protective multiple earthing is a system of earthing in which the neutral conductor of a supply network is connected to earth at several points to ensure safety by reducing the risk of electric shock and maintaining the neutrality of the system.
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(6)
The equalizer ensures balanced current distribution among the parallel paths of the armature winding, preventing circulating currents and reducing sparking at the brushes.
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(7)
Oil is used in transformer cooling because it absorbs heat from the windings and core, and also provides electrical insulation, helping to prevent electrical breakdown.
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(8)
Luminous flux is the measure of the total amount of visible light emitted by a source per unit time, expressed in lumens (lm).
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(9)
(Draw the diagram)

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(10)
Newton’s Second Law states that the acceleration of a body is directly proportional to the net force acting on it and inversely proportional to its mass.
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SECTION B
INSTRUCTIONS: ANSWER FIVE QUESTIONS ONLY.

(11a)
(i) Proper insulation of cables: Ensures that all wires are well-insulated to prevent moisture, animals, or farm tools from causing electric shocks or short circuits.
(ii) Earthing of electrical equipment: Provides a safe path for leakage current, reducing the risk of electric shock to workers and preventing damage to equipment exposed to wet or metallic farm environments.

(11b)
(Draw the diagram)

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(12a)
(i) Location: The transformer should be installed in a dry, well-ventilated and easily accessible area to enhance cooling efficiency, ensure safety and simplify maintenance activities.
(ii) Foundation strength: The supporting platform must be strong and stable to bear the weight of the transformer and to prevent vibration that could lead to mechanical damage.
(iii) Ventilation and cooling: Adequate airflow must be ensured around the transformer to allow efficient heat dissipation and to prevent overheating during operation.
(iv) Protection devices: Proper protective equipment such as circuit breakers, relays and lightning arresters must be provided to safeguard the transformer against faults, overloads and lightning surges.
(v) Earthing: A reliable earthing system must be installed to provide a safe path for fault currents and to reduce the risk of electric shock and equipment damage.

(12b)
(i) A multi substation system supplies power to several load centres from multiple substations WHILE a unit substation system serves a specific local load area from a single compact substation.
(ii) A multi substation system offers higher reliability due to multiple supply points WHILE a unit substation system provides simpler installation with fewer supply routes.
(iii) A multi substation system is more suitable for large industrial or urban networks WHILE a unit substation system is ideal for localized loads such as residential or commercial buildings.
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(14ai)
P(out) = 25 kW
P(out) = 25000 W
η = 0.9
pf = 0.8
V(line) = 400 V
P(in) = Pout/η
P(in) = 25000/0.9
P(in)= 27777.78 W
S = P(in)/pf
S = 27777.78/0.8
S = 34722.22 VA
Line Current I(line) = S/[(√3)xV(line)]
I(line) = 34722.22/(1.7320508×400)
I(line) = 50.12 A

(14aii)
For Delta Connection:
I(phase) = I(line)/(√3)
I(phase) = 50.12/1.7320508
I(phase) = 28.94 A

(14b)
(Draw the diagram)

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SECTION B
INSTRUCTIONS: ANSWER FIVE QUESTIONS ONLY.

(15a)
(Draw the diagram)

Swinburne’s method finds the losses of a shunt motor by running it on its normal supply with no mechanical load so that the measured electrical input represents mainly the constant losses of the machine. From the current and voltage readings in this no-load condition, the input power is calculated, and after accounting for the small armature copper loss, the remaining power corresponds to the motor’s constant losses, which can then be used to estimate efficiency at any load.

(15b)
(i) To provide electrical connection: Brushes supply current from the external power source to the rotating commutator so the armature windings can be energized.
(ii) To control current direction in the armature: They work with the commutator to ensure that current in each armature coil reverses at the correct moment to produce continuous unidirectional torque.
(iii) To achieve proper commutation: Brushes allow smooth switching of coil connections as they pass the neutral plane, reducing sparking and preventing damage to the commutator.
(iv) To maintain reliable contact during rotation: Their spring-loaded pressure keeps them firmly in contact with the commutator surface, ensuring steady current flow even at high speeds.
(v) To support efficient motor operation: By providing low-resistance, stable conduction, brushes help minimize losses and maintain consistent motor performance.
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(16a)
(Draw the diagram)

A potentiometer works by dividing a fixed input voltage across a long, uniform resistive element. As the sliding contact moves along this element, it taps off a portion of the total voltage. The voltmeter connected to the wiper reads this variable potential difference, which changes smoothly with the wiper’s position, allowing precise comparison or measurement of voltage.

(16b)
(i) Vertical (VOLTS/DIV) Control: This adjusts the vertical sensitivity of the oscilloscope. It determines how many volts are represented by each vertical division on the screen. Increasing the sensitivity makes smaller signals appear larger, while decreasing it compresses the waveform.
(ii) Horizontal Time Base (TIME/DIV) Control: This sets the amount of time represented by each horizontal division on the display. It effectively controls how stretched or compressed the waveform appears along the time axis, allowing you to analyze slow or fast-changing signals.
(iii) Trigger Control: This stabilizes the waveform on the screen by setting the exact voltage level and slope at which the oscilloscope begins drawing the trace. Proper triggering prevents the signal from drifting horizontally and ensures a steady, stationary display.
(iv) Intensity (Brightness) Control: This regulates how bright the trace appears on the screen. It ensures the waveform is easily visible without being overly bright, which could distort the shape or cause unnecessary strain on the display.
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(17a)
Cathodic protection is a technique used to prevent metal structures such as pipelines, tanks, or ship hulls from corroding by making the metal act as a cathode of an electrochemical cell. This is achieved by supplying electrons to the metal either from a more reactive sacrificial metal or from an external power source. By ensuring the protected metal always receives electrons rather than losing them, corrosion is greatly reduced or completely stopped.

(17b)
(Draw the diagram)

In the sacrificial-anode method, a more reactive metal such as zinc or magnesium is attached to the metal structure you want to protect. Because this added metal oxidizes more readily, it corrodes in place of the structure and continually supplies electrons to it. The protected metal therefore remains the cathode and does not corrode, while the sacrificial anode gradually wears away.
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