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SECTION A:ANSWER ALL QUESTIONS
(1)
(i) Kirchhoff’s Current Law (KCL): The sum of currents entering a junction in a circuit is equal to the sum of currents leaving the junction.
(ii) Kirchhoff’s Voltage Law (KVL): The sum of the electrical potential differences around any closed loop or circuit is equal to zero.
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(2)
The purpose of the evaporator in a central air conditioning system is to absorb heat from the indoor air. It allows the refrigerant to evaporate by turning it from a low-pressure liquid to a gas, cooling the air as it passes over the evaporator coils.
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(3)
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(4)
(PICK TWO ONLY)
(i) Transformers
(ii) Circuit breakers
(iii) Switchgear
(iv) Busbars
(v) Isolators
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(5)
An earthing system is a safety mechanism in electrical installations that connects the electrical system to the ground to prevent electric shock hazards.
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(6)
The enclosure of an electric motor serves to protect its internal components from environmental factors such as dust, moisture, dirt, and physical damage. It also helps to contain the motor’s heat and prevent overheating by ensuring efficient airflow or cooling.
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(7)
A transformer is rated in KVA (kilovolt-amperes) rather than kilowatts (KW) because it is a measure of the apparent power, which accounts for both the real power (KW) and reactive power (KVAR) in the electrical system.
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(8)
Illuminance is the measure of the amount of light that falls on a given surface area. It is expressed in lux (lx), where one lux is equal to one lumen per square meter.
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(9)
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(10)
Newton’s First Law of Motion states that an object at rest will remain at rest, and an object in motion will continue in motion with the same speed and in the same direction unless acted upon by an unbalanced external force.
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SECTION B: ANSWER FIVE QUESTIONS ONLY
(11a)
(PICK TWO ONLY)
(i) Correct Placement of Detectors: Smoke detectors should be installed in locations where smoke is likely to accumulate, such as ceilings or high walls. Heat detectors should be placed in areas where heat buildup is common, like kitchens or mechanical rooms, avoiding areas with high humidity or airflow that could cause false alarms.
(ii) Ensure Proper Wiring: All wiring should be properly insulated, routed, and protected to prevent damage from environmental factors, physical impact, or interference. The wiring must be checked for correct polarity and secure connections to ensure reliable operation.
(iii) Test the System: Before finalizing the installation, the system must be thoroughly tested to ensure that all detectors, alarms, and control panels are working properly. This includes checking the sensitivity of detectors and verifying that the alarms can be heard or seen throughout the building.
(iv) Compliance with Codes and Standards: The fire alarm system must comply with local and national fire safety codes and standards. This ensures that the system meets safety requirements and is capable of providing adequate warning in the event of a fire.
(v) Regular Maintenance and Inspection: To maintain reliability, fire alarm systems should be subject to regular inspection and maintenance. This includes checking batteries, testing alarms, cleaning detectors, and verifying system components to ensure they function correctly at all times.
(11b)
A typical neon discharge lamp consists of a glass tube filled with neon gas at low pressure. When a high voltage is applied across the electrodes at each end of the tube, the neon gas ionizes, causing it to emit a characteristic red-orange glow. The lamp operates on the principle of electrical discharge, where electrons collide with neon atoms, exciting them and causing them to release light as they return to their lower energy states. The lamp is often used for signs and decorative lighting.
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(12a)
(PICK FOUR ONLY)
(i) Proper Spacing Between Equipment: Ensure adequate spacing between electrical equipment, such as transformers, circuit breakers, and switchgear, to reduce the risk of overheating and allow for proper ventilation. This helps prevent the accumulation of heat that could lead to a fire.
(ii) Installation of Fire-Resistant Materials: Use fire-resistant or fire-retardant materials in the construction of walls, floors, and equipment enclosures to minimize the spread of fire in case of an incident.
(iii) Grounding and Earthing: Properly ground all electrical equipment to prevent electrical faults that could lead to sparks or overheating, which can ignite fires.
(iv) Regular Inspection and Maintenance: Conduct regular inspections and maintenance of all equipment, including transformers, cables, and circuit breakers, to detect potential issues like overheating, worn-out insulation, or damaged components that may lead to fire.
(v) Adequate Ventilation: Ensure that substations are well-ventilated to prevent the buildup of flammable gases or excessive heat, which could increase the likelihood of fire.
(vi) Fire Detection and Suppression Systems: Install fire detection systems, such as smoke detectors and heat sensors, along with automatic fire suppression systems to quickly detect and control a fire.
(vii) Clear and Unobstructed Access: Maintain clear and unobstructed access routes to fire extinguishers, emergency exits, and firefighting equipment to ensure quick response in case of a fire outbreak.
(12b)
(i) Power Flow: In a ring main system, the power flows in a circular loop, allowing current to flow from either direction, while in a radial main system, power flows in a single direction from the source to the load.
(ii) Reliability: The ring main system offers higher reliability, as the failure of one section can be bypassed, while in a radial main system, a fault typically causes a power outage for all downstream users.
(iii) Cost: The ring main system is generally more expensive to install and maintain due to its complex structure and additional components, while the radial main system is simpler and cheaper to install.
(iv) Flexibility: In a ring main system, the ability to reroute power increases operational flexibility, while the radial main system lacks this flexibility, as power supply is dependent on a single path.
(v) Fault Detection and Isolation: The ring main system allows for more efficient fault detection and isolation due to its redundancy, while the radial main system may take longer to detect and isolate faults, as it has only one main supply route.
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(14a)
Resistance of each resistor R = 500Ω
Line-to-line voltage VL = 400V
Line current IL = ?
Phase Voltage Vph = VL/(√3)
Vph = 400/(√3)
Vph = 230.94V
Iph = Vph/R
Iph = (230.94)/(500)
Iph = 4.6188A
Phase Current Iph = Line Current IL
Line Current IL = 4.6188A
(14b)
Total Power Consumed = Ptotal
Ptotal = (√3) x VL x IL x cosΦ
Ptotal = (√3) x 400 x 4.6188 x 1
Ptotal = 3200W
(14c)
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(15a)
Hopkinson’s method of measuring the efficiency of two similar DC shunt machines involves running the machines in parallel, with one machine acting as a motor and the other as a generator. The power output of the generator is fed into the motor, and the total input power is measured. The efficiency of the motor and generator is calculated by comparing the losses in both machines.
(15b)
The purpose of a commutator in a DC motor is to reverse the direction of current flow through the armature windings as the motor rotates, ensuring that the torque produced by the motor remains in the same direction. It converts the alternating current induced in the armature windings into direct current, allowing the motor to produce continuous rotational motion.
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(16a)
The principle of operation of a Wheatstone bridge is based on the balance of two voltage dividers. It consists of four resistors arranged in a diamond shape, with a galvanometer connected between two opposite points. When the bridge is balanced, the ratio of resistances in one pair of opposite arms is equal to the ratio in the other pair, resulting in zero current through the galvanometer. The unknown resistance can be calculated by adjusting the other resistors until the bridge is balanced.
(16b)
(i) Sweep Frequency: Controls the rate at which the oscilloscope’s electron beam moves horizontally across the screen, affecting the time base. It determines how fast the waveform is displayed along the horizontal axis.
(ii) Focus: Adjusts the sharpness of the oscilloscope’s trace by controlling the focus of the electron beam. It ensures the displayed waveform is clear and well-defined.
(iii) Intensity: Controls the brightness of the oscilloscope’s display. It adjusts the strength of the electron beam, making the waveform either brighter or dimmer on the screen.
(iv) Synchronous Voltage Amplitude: This control sets the vertical sensitivity of the oscilloscope, determining how much the signal is amplified or attenuated vertically on the screen, affecting the height of the waveform.
(v) Horizontal Gain: Adjusts the horizontal scaling of the oscilloscope display, controlling how much the input signal is stretched or compressed along the time axis. It affects the time interval per division on the screen.
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(17a)
Corrosion in an electrical installation refers to the gradual deterioration of metal components due to chemical reactions with their environment, such as exposure to moisture, air, or chemicals. This often leads to the formation of rust or other corrosion products on metal parts like wires, connectors, and terminals, which can increase resistance, reduce conductivity, and ultimately compromise the safety and efficiency of the electrical system.
(17b)
The impressed current method of cathodic protection involves applying a continuous external electric current to a metal structure to counteract the corrosive effects of the environment. An external power source supplies the current, which flows through an anode placed near the structure, making the structure itself the cathode. This prevents the metal from losing electrons and thus protects it from corrosion.
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