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

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ELECTRICAL INSTALLATION OBJ (TYPE A)
01-10: CBBBBBABDB
11-20: DCDABDABAD
21-30: CCCCABCBDB
31-40: CDBDCACBBC

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ELECTRICAL INSTALLATION & MAINTENANCE WORKS ANSWERS (TYPE A)

ANSWER SIX(6) QUESTIONS ON ALL

PART I: ANSWER (2) QUESTIONS ONLY

(2a)
(i)Switch Fuse (Switch-Fuse): This typically refers to a device that combines a switch and a fuse in a single unit. It allows for both control (on/off) and protection (fusing) of a circuit. The fuse is usually part of the switch mechanism, providing overload protection when the switch is in the “on” position.

(ii)Fuse Switch (Fuse Holder with Switch): In some contexts, this term might refer to a fuse holder that includes a switch. The primary function is to hold the fuse, and the switch provides a means to control power to the circuit. The switch might not be directly linked to the fuse holder mechanically.

(2b)
(i)Short circuit current; The current that flows when two points of different potentials are connected with very low or zero resistance, causing a very high current flow in the circuit.

(ii)Overload current; The current which exceeds the normal rated current of the electrical equipment or circuit but is less than short circuit current, causing excessive heating and potential damage.

(iii)Circuit protective conductor; It is a conductor that provides a path for fault current to flow to earth to ensure safety by preventing electric shock or fire.
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(3a)
(i)Open Type Motor Enclosure.
-Example Applications: Workshop machines, laboratory equipment

(ii)Totally Enclosed Fan Cooled (TEFC)
-Example Applications: Pumps, compressors in dusty or wet environments

(iii)Explosion Proof Enclosed.
-Example Applications: Oil refineries, chemical plants where explosive gases are present

(3b)
-Symptom: Motor fails to start or runs erratically.
-Possible Cause: Broken field winding or open armature winding.
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PART II: ANSWER (2) QUESTIONS ONLY

(4i)
methods of joining conductors.
(i)Twisting and Soldering: Conductors are twisted together and soldered to ensure a good electrical connection. Used for small wire joins.
(ii)Crimping: Conductors are inserted into a metal connector and compressed with a crimping tool to form a strong mechanical and electrical joint.
(iii)Compression Joints: Using compression connectors and a special tool to compress the conductor into the connector. Used in high voltage cables.
(iv)Bolted Joint: Conductors are joined using bolts and washers, suitable for large conductors like busbars.
(v)Welding/Brazing: Metal conductors are fused by heat to provide a permanent and strong electrical connection.

(4ii)
factors that affect the current rating of cables.
(i)Ambient Temperature: Higher temperatures reduce the current carrying capacity of cables.
(ii)Cable Installation Method: Cables buried underground or enclosed in ducts have different current ratings due to heat dissipation differences.
(iii)Conductor Cross-section: Larger cross-sectional areas allow higher current ratings.
(iv)Type of Insulation: Different insulation materials can withstand different maximum temperatures.
(v)Grouping of Cables: Multiple cables grouped together affect heat dissipation and reduce current capacity.
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(5a)
secondary cell is a rechargeable electrochemical cell that can be charged, discharged, and recharged multiple times.

(5b)
(i)Lead-acid battery
(ii)Nickel-cadmium (NiCd) battery

(5c)
Rechargeability:
(i)Primary Cells: These cells are non-rechargeable and are designed to be used until their chemical energy is depleted. Once the reactants are consumed, the cell cannot be reused.
(ii)Secondary Cells: These cells are rechargeable. They can be recharged multiple times by applying an external electrical current, which reverses the chemical reactions and allows the cell to be used again.

Life Cycle and Cost;
(i)Primary Cells: Generally have a shorter lifespan and are more expensive in the long run for applications that require long-term power. They are often used in devices with low power requirements or where replacement is easy.
(ii)Secondary Cells: Have a longer lifespan and can be more cost-effective over time because they can be recharged and reused many times. They are commonly used in applications like electric vehicles, renewable energy systems, and portable electronics.
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PART III: ANSWER (2) QUESTIONS ONLY

(7a)
(i)Distortion of the Main Field: Armature reaction causes the magnetic field to shift, leading to a distortion in the main magnetic field. This can result in a shift of the neutral plane where the brushes are positioned, potentially causing sparking and reducing efficiency.

(ii)Reduction in Main Flux: The armature reaction can weaken the main magnetic field, particularly under heavy load conditions. This reduction in flux can lead to a decrease in the generated voltage in a generator or a decrease in the torque produced by a motor.

(iii)Increased Losses: The distortion and weakening of the magnetic field can lead to increased iron losses and copper losses in the machine, reducing its overall efficiency.

(7b)
(i)Overheating and Insulation Damage: Using a wire with a smaller gauge (thicker wire) than specified can lead to increased current density in the winding, causing overheating and potential damage to the insulation. Conversely, using a wire with a larger gauge (thinner wire) than specified can lead to increased resistance and also cause overheating.

(ii)Reduced Efficiency or Machine Failure: A wrong wire gauge can lead to increased resistance (with thinner wire) or reduced slot fill (with thicker wire), affecting the machine’s performance. This can result in reduced efficiency, overheating, or even failure of the machine due to excessive current or inadequate heat dissipation.

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