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ELECTRICAL INSTALLATION
01-10: ABDADDCACD
11-20: CADBDCDDBB
21-30: CBDCBBACBB
31-40: ACDBDBDABC
COMPLETED
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PART I: ANSWER TWO QUESTIONS ONLY
(2ai)
(i) A joint box is used to connect two or more conductors in a wiring system, to join wires together and ensure continuity of the circuit, while a junction box is used to house and protect the electrical connections or junctions, serving as an enclosure for connecting or branching conductors in a circuit.
(ii) A joint box specifically serves to connect wires or cables, while a junction box provides a protective enclosure for electrical connections, preventing exposure to the environment.
(2b)
(PICK THREE ONLY)
(i) Fuses: A fuse is a protective device that contains a metal wire or strip that melts when the current exceeds a certain level, thereby disconnecting the circuit to prevent damage from overcurrent.
(ii) Circuit Breakers: A circuit breaker is an automatically operated electrical switch that interrupts the flow of current when it detects an overcurrent, protecting the circuit from damage.
(iii) Overcurrent Relays: An overcurrent relay is a protective device that detects overcurrent conditions and triggers a circuit breaker to interrupt the current, often used in industrial settings.
(iv) Thermal Overload Relays: A thermal overload relay protects motors from overheating due to overcurrent by using a bimetallic strip that bends when exposed to excessive current, causing the relay to trip and disconnect the circuit.
(v) Molded Case Circuit Breakers (MCCBs): MCCBs are similar to circuit breakers but are specifically designed for higher current ratings and offer adjustable trip settings for overcurrent protection.
(vi) Residual Current Devices (RCDs): An RCD detects leakage currents caused by faults such as ground faults and trips the circuit to prevent electrical shock, thus offering overcurrent protection in some cases.
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(3ai)
SERIES MOTOR
(3aii)
SHUNT MOTOR
(3aiii)
COMPOUND MOTOR
(3b)
(i) Clean the conduit ends.
(ii) Measure and mark the conduit for cutting.
(iii) Cut the conduit to the required length.
(iv) Apply cutting oil to the conduit.
(v) Use a pipe threader to cut threads onto the conduit.
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PART II: ANSWER TWO QUESTIONS ONLY
(5)
(PICK FIVE ONLY)
(i) PVC Insulated Cables: These are flexible cables with PVC insulation, widely used in residential and commercial wiring. They are commonly installed in dry, indoor environments where there is minimal exposure to heat or moisture.
(ii) Armoured Cables: These cables have a layer of steel wire armor for protection, making them suitable for outdoor and industrial environments. They are commonly used in underground installations or areas where cables are subject to mechanical stress.
(iii) XLPE Insulated Cables: Cross-linked polyethylene insulation cables are used for high-temperature and high-voltage applications. They are commonly installed in power distribution networks and industrial systems due to their high thermal and electrical resistance.
(iv) Rubber-Insulated Cables: These cables have rubber insulation and are flexible, making them ideal for portable equipment and temporary installations. They are commonly used in construction sites and outdoor areas where flexibility and durability are required.
(v) Low Voltage Cables: Used for electrical installations in residential, commercial, and industrial buildings, these cables are suitable for low-voltage applications. They are installed in lighting circuits, power distribution, and appliances.
(vi) High Voltage Cables: These cables are designed for the transmission of high-voltage electricity and are typically used in substations, power plants, and overhead transmission lines.
(vii) Cat 5/6 Cables: These twisted-pair cables are primarily used for data communication and networking, most commonly installed in office buildings, homes, and data centers for computer networking and internet connections.
(viii) Coaxial Cables: These cables consist of a central conductor, insulation, and a shielding layer, typically used for television and broadband connections. They are commonly installed in residential areas for cable television or internet connections.
(ix) Twin and Earth Cables: These cables are often used for household wiring, providing both a live and neutral conductor, with an earth conductor for safety. They are commonly used in residential electrical circuits.
(x) Fire-Resistant Cables: These cables are designed to maintain their function during a fire, ensuring the circuit remains operational for a certain period. They are commonly installed in safety-critical areas such as fire alarm systems, emergency lighting, and evacuation routes.
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(6a)
A cell is a single electrochemical unit that converts chemical energy into electrical energy through a chemical reaction between its electrodes and electrolyte.
(6b)
(PICK TWO ONLY)
(i) Leclanché cell
(ii) Zinc-carbon cell
(iii) Alkaline cell
(iv) Mercury cell
(v) Lithium cell
(6c)
(PICK TWO ONLY)
(i) Corrosion of the zinc electrode: This occurs when the zinc electrode is consumed over time. To rectify this, the zinc electrode can be replaced or the cell can be kept in a cool, dry place to slow corrosion.
(ii) Drying of the electrolyte: The electrolyte can evaporate, causing a loss in power. To rectify this, the cell should be stored in a sealed container to prevent moisture loss.
(iii) Increased internal resistance: This happens due to the buildup of unwanted chemical products inside the cell. To rectify this, replace the cell when its internal resistance becomes too high to maintain performance.
(iv) Leakage of electrolyte: Leakage may occur if the cell casing is damaged, leading to loss of electrolyte and reduced performance. The solution is to replace the cell casing or ensure cells are stored and handled carefully to prevent physical damage.
(v) Voltage drop under load: A voltage drop occurs when the cell is used beyond its design capacity. To rectify this, use the cell within its recommended current range or replace it with a new one if it is too depleted.
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PART III: ANSWER TWO QUESTIONS ONLY
(7)
(i) Grounds:
In motors, a ground refers to an unintended path of electricity to the ground or chassis of the motor. This can cause the motor to malfunction or not work at all.
Possible cause: A ground can be caused by a fault in the insulation of the windings, a loose or corroded connection, or a damaged bearing that allows the rotor to touch the stator.
(ii) Open:
An open in a motor refers to a break or interruption in the electrical circuit of the motor. This can prevent the motor from working.
Possible cause: An open can be caused by a broken wire, a loose or corroded connection, or a faulty switch or contactor.
(iii) Shorts:
A short in a motor refers to an unintended path of electricity between two or more points in the motor’s electrical circuit. This can cause the motor to overheat, malfunction, or not work at all.
Possible cause: A short can be caused by a fault in the insulation of the windings, a loose or corroded connection, or a damaged bearing that allows the rotor to touch the stator.
(iv) Reverse:
Reverse in a motor refers to a condition where the motor is rotating in the opposite direction to what is intended. This can cause the motor to malfunction or not work at all.
Possible cause: Reverse rotation can be caused by incorrect wiring or connections, a faulty starter or contactor, or a problem with the motor’s internal wiring or windings.
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(8a)
The purpose of a brush is to maintain electrical contact between the stationary and rotating parts of an electrical machine, such as a motor or generator. The brush rides on the rotating commutator or slip ring, allowing current to flow into or out of the machine.
(8b)
(i) Identify the brush holders: Locate the brush holders on the machine, which are designed to house the brushes and ensure proper alignment with the commutator or slip rings.
(ii) Insert the brushes: Place the brushes into the holders, ensuring they are properly oriented so that they make contact with the commutator/slip rings.
(iii) Connect the brush leads: Securely attach the brush leads (cables or connectors) to their designated terminals.
(iv) Adjust spring tension: The brushes are typically held against the rotating component by spring tension. Adjust the springs to ensure proper pressure—too much pressure can cause excessive wear, while too little pressure can result in poor electrical contact.
(v) Inspect alignment and movement: Ensure the brushes are free to move slightly within their holders and are aligned to make consistent contact with the commutator/slip rings.
(8c)
Fault: Excessive wear or damage to the brush.
Repair: Replace the worn-out brush with a new one. Ensure the new brush is of the correct size and material for the specific machine. Additionally, inspect the commutator or slip rings for signs of damage or wear, as this can cause uneven wear on the brushes. If needed, clean or recondition the commutator or slip rings.
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