Electrical Flashcards
With reference to electrical motors:
a. state the routine maintenance that is necessary: (8 marks)
b. describe the tests carried out to prove good electrical condition. (8 marks)
(a) Routine Maintenance Necessary (8 marks – 1 mark each for 8 points)
1. Clean external casing and air vents – to ensure proper cooling and prevent overheating.
2. Inspect and clean the commutator or slip rings – to maintain good contact and reduce sparking.
3. Check and clean brushes – ensure correct brush pressure and that they are not worn down.
4. Lubricate bearings – using correct grade and quantity of grease or oil to avoid wear or overheating.
5. Check for abnormal noise or vibration – may indicate worn bearings or misalignment.
6. Tighten terminal connections – to prevent loose connections and arcing.
7. Inspect insulation and wiring for damage – to detect early signs of deterioration or overheating.
8. Check motor mounting and alignment – to avoid mechanical stress and coupling issues.
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(b) Tests to Prove Good Electrical Condition (8 marks – 2 marks each for 4 tests)
1. Insulation Resistance Test (Megger Test):
• Measures resistance between windings and earth.
• High resistance (usually >1 MΩ) indicates good insulation.
2. Continuity Test:
• Confirms winding continuity and absence of open circuits.
• Ensures current can flow properly through motor windings.
3. Earth Fault Test:
• Detects leakage paths from windings to the motor casing.
• Ensures safety by confirming there’s no fault to earth.
4. No-Load Test Run:
• Motor is run without load to observe smooth operation, noise, vibration, and current draw.
• Confirms mechanical and electrical health under basic conditions.
With reference to main distribution systems fitted with preference trips:
(a) state why the preference trip is fitted.
(b) explain the operation of a THREE stage trip.
(c) state THREE circuits that can not be connected to the preference trip giving a reason for
EACH (6)
- Why the Preference Trip is Fitted (2 marks)
• Purpose:
A preference trip is fitted to prevent total blackout by shedding non-essential loads when the electrical load exceeds the generator capacity.
• Explanation:
It ensures the electrical system remains stable and operational by prioritizing essential services and avoiding generator overload or tripping.
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- Operation of a THREE-Stage Trip (4 marks)
A three-stage preference trip system disconnects loads in a priority-based sequence:
1. First Stage – Non-Essential Load Shedding:
• Trips comfort or hotel loads (e.g. air conditioning, laundry) when load approaches generator capacity.
2. Second Stage – Semi-Essential Load Shedding:
• Disconnects auxiliary machinery (e.g. ballast pumps, deck machinery) if overload persists after first stage.
3. Third Stage – Emergency Load Shedding:
• Trips additional circuits or starts standby generator if system is still overloaded, preventing total generator trip.
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- THREE Circuits That Cannot Be Connected to the Preference Trip (6 marks – 2 marks each)
- Steering Gear:
• Reason: Essential for navigational safety; required to be operational at all times under SOLAS. - Main Propulsion Control Systems:
• Reason: Loss of propulsion control could lead to loss of maneuverability, posing a collision or grounding risk. - Fire Detection and Alarm Systems:
• Reason: Critical for crew and ship safety; must remain active to detect and respond to fire hazards.
- Steering Gear:
a. State the conditions that must be satisfied before paralleling an a.c. generator with the main electrical switchboard. (6 marks)
b. For EACH condition stated in Q(a) explain EACH of the following:
i. How it is monitored; (4 marks)
ii. How it is adjusted. (6 marks)
Conditions that Must be Satisfied Before Paralleling an A.C. Generator with the Main Electrical Switchboard
When paralleling an alternating current (A.C.) generator with the main electrical switchboard, the following conditions must be satisfied to ensure a smooth and safe operation:
1. Voltage Match: The voltage of the generator must be equal to the voltage of the electrical switchboard.
2. Frequency Match: The frequency of the generator must match that of the switchboard.
3. Phase Sequence Match: The phase sequence of the generator must match the phase sequence of the switchboard.
4. Synchronizing: The generator must be synchronized with the main busbars.
5. Excitation and Governor Control Set Correctly: The generator’s excitation and governor control should be set properly to avoid overloads and underloads.
6. Circuit Breaker Readiness: The circuit breaker for the generator should be ready to close and be in good condition for parallel operation.
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For Each Condition, Explanation of Monitoring and Adjustment
- Voltage Match
i. How it is monitored:
• Monitoring: Voltage is monitored using a voltmeter connected to both the generator and the switchboard. A voltmeter will show the exact value of the generator’s output and the busbar voltage.
• Synchronization Equipment: Automatic voltage regulators (AVRs) and synchronizing panels provide real-time feedback on voltage values.
ii. How it is adjusted:
• Adjustment: The generator’s voltage can be adjusted by manipulating the exciter control (using the AVR or manual adjustments). If the generator voltage is too high or low compared to the switchboard, the exciter field current is adjusted to match the required voltage.
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- Frequency Match
i. How it is monitored:
• Monitoring: Frequency is monitored using a frequency meter, which indicates the generator’s frequency and the main electrical system’s frequency.
• Synchronizing Equipment: Automatic synchronizers or manual synchroscopes are used to continuously monitor the frequency of both systems.
ii. How it is adjusted:
• Adjustment: Frequency can be adjusted by adjusting the governor of the generator. The speed of the engine is controlled to either increase or decrease the frequency of the generator to match the system frequency.
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- Phase Sequence Match
i. How it is monitored:
• Monitoring: A phase sequence indicator is used to check the phase sequence of both the generator and the main switchboard. This device ensures that the phases are aligned in the correct order.
ii. How it is adjusted:
• Adjustment: If the phase sequence is incorrect, it can be reversed by switching the connections of the phase leads at the generator terminals or on the main switchboard, ensuring both systems have the same phase rotation.
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- Synchronizing
i. How it is monitored:
• Monitoring: Synchronization is monitored using a synchronoscope or an automatic synchronizer. The synchronoscope shows the relative phase difference between the generator and the system.
ii. How it is adjusted:
• Adjustment: The generator’s speed (via the governor) is adjusted to bring the generator into synchronization with the main system. Synchronizing is considered complete when the synchronoscope needle is centered, indicating that both frequency and phase are matched.
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- Excitation and Governor Control Set Correctly
i. How it is monitored:
• Monitoring: Excitation is monitored via an ammeter measuring the excitation current, while the governor control is monitored through indicators that show the speed or load of the generator.
ii. How it is adjusted:
• Adjustment: Excitation can be adjusted using the AVR to control the generator’s output voltage. The governor setting can be adjusted manually or via an automatic governor to maintain the correct speed and output frequency.
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- Circuit Breaker Readiness
i. How it is monitored:
• Monitoring: The readiness of the circuit breaker is checked by observing the breaker status indicator, which shows whether it is in the open or closed position. Also, a functional test of the breaker can be performed.
ii. How it is adjusted:
• Adjustment: If necessary, the circuit breaker trip settings (overload and short-circuit protection) are adjusted according to the generator’s characteristics and the load being supplied. This adjustment is usually done in accordance with the generator’s rating and the ship’s electrical system specifications.
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These conditions are essential for ensuring the safety and efficiency of paralleling an A.C. generator with the main electrical switchboard. Each condition ensures that the generator operates smoothly without causing electrical faults or damage to the system.
With reference to electrical motors:
a. state the routine maintenance that is necessary: (8 marks)
b. describe the tests carried out to prove good electrical condition. (8 marks)
Routine Maintenance for Electrical Motors
Routine maintenance of electrical motors is essential to ensure they operate efficiently, prevent failures, and extend their service life. The following are the necessary routine maintenance tasks:
1. Visual Inspection:
• Check for any visible damage, wear, or signs of overheating.
• Inspect the motor casing for cracks, dirt accumulation, or moisture ingress.
2. Cleaning:
• Clean the motor casing, vents, and cooling fans to prevent overheating and maintain ventilation.
• Ensure that the motor is free from dust, dirt, or any foreign objects that may obstruct its operation.
3. Lubrication:
• Lubricate the motor’s bearings as per the manufacturer’s specifications to reduce wear and prevent overheating.
• Ensure that grease fittings and oil cups are properly lubricated and maintained.
4. Vibration Check:
• Perform regular vibration analysis to detect any imbalances, misalignments, or other mechanical issues.
• Use vibration meters to check the rotor, stator, and bearing conditions.
5. Insulation Inspection:
• Inspect the motor insulation for any signs of degradation or damage that may cause short circuits or electrical faults.
• Ensure that insulation is intact and suitable for the operating environment.
6. Check Motor Alignment:
• Ensure that the motor shaft is properly aligned with the connected load to prevent mechanical stress and wear.
• Perform shaft alignment checks using alignment tools or laser alignment systems.
7. Check Motor Connections:
• Inspect all electrical connections for signs of looseness or corrosion.
• Tighten any loose connections and clean corrosion from terminals to prevent power loss or electrical failures.
8. Check Cooling System:
• Inspect fans, air inlets, and cooling ducts to ensure proper airflow.
• Verify that there is no obstruction in the cooling system to avoid overheating.
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Tests to Prove Good Electrical Condition
To ensure that an electrical motor is in good electrical condition, a number of tests can be carried out:
1. Insulation Resistance Test:
• Purpose: To check the condition of the motor’s insulation.
• Method: Use a megger or insulation resistance tester to measure the resistance between the motor windings and the ground. A high resistance value (usually above 1 MΩ) indicates good insulation. If the resistance is low, the motor’s insulation may be compromised and require repair or replacement.
2. Winding Continuity Test:
• Purpose: To ensure there are no open circuits or broken windings.
• Method: Use a multimeter or an ohmmeter to check for continuity between the motor windings. Any discontinuity or infinite resistance would indicate a problem in the windings that needs attention.
3. Earth Leakage Test:
• Purpose: To detect any leakage current to the earth.
• Method: Use a leakage current tester or clamp meter to measure leakage between the motor casing and the ground. Excessive leakage indicates insulation failure or grounding issues.
4. Stator Resistance Test:
• Purpose: To verify the integrity of the stator windings.
• Method: Measure the resistance of the stator windings using an ohmmeter. A significant deviation from the manufacturer’s specified resistance could indicate a fault in the windings, such as shorted or open turns.
5. Rotor Test (e.g., Locked Rotor Test):
• Purpose: To check for faults in the rotor windings or bars.
• Method: Measure the rotor resistance using a suitable meter to ensure the rotor bars are intact and free from any breaks or shorts. A locked rotor test can also be performed to check for motor starting issues or rotor winding problems.
6. Runout Test:
• Purpose: To verify the mechanical integrity of the rotor.
• Method: Use a dial indicator or laser alignment tool to check the rotor runout. Excessive runout can cause uneven wear, excessive vibration, or even motor failure.
7. Voltage and Current Test:
• Purpose: To ensure the motor is receiving correct voltage and that current is within the rated limits.
• Method: Use a voltmeter to check the supply voltage and a clamp meter to check the current drawn by the motor. Compare the readings to the motor’s rated voltage and current specifications to ensure proper operation.
8. Performance Test (Load Test):
• Purpose: To check the motor under load conditions.
• Method: The motor is run under a known load, and its performance is evaluated by measuring parameters such as speed, torque, power output, and efficiency. Any significant deviation from rated performance could indicate mechanical or electrical issues.
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By carrying out these routine maintenance tasks and tests, the condition of the motor can be monitored and any issues can be addressed before they lead to more serious problems. Regular inspections and testing help ensure that the motor operates safely and efficiently.
State the procedure to be carried out if you found someone you suspected of having received an electric shock. (16 marks)
- Ensure Your Own Safety (4 marks)
• Disconnect the power source: Immediately disconnect the power supply to the circuit causing the shock, if possible. If you cannot disconnect the power, do not touch the victim directly.
• Use non-conductive materials: If the power cannot be disconnected, use a non-conductive object (such as a wooden stick, rubber gloves, or dry clothing) to move the victim away from the electrical source.
• Avoid touching the victim: Ensure you don’t touch the person directly if they are still in contact with the electrical source. - Check the Victim’s Condition (4 marks)
• Assess consciousness: Check if the person is conscious by calling their name or gently shaking them. If they do not respond, they may be unconscious and need urgent help.
• Look for signs of breathing and pulse: Check if the victim is breathing by looking for chest movements and feeling for a pulse on the neck or wrist. If the person is not breathing or doesn’t have a pulse, immediate resuscitation may be needed.
• Examine for burns: Check for any visible burns, especially entry and exit burns where the current may have entered or exited the body. - Call for Help (2 marks)
• Call emergency services immediately: Provide the exact location, the condition of the victim, and details of the electric shock (such as the suspected voltage, if known). Time is critical.
• Request medical help: It is important to inform emergency responders that the person has received an electric shock. - Perform Basic Life Support (BLS) if Needed (4 marks)
• CPR (Cardiopulmonary Resuscitation): If the person is not breathing or has no pulse, begin CPR immediately. Perform chest compressions at a rate of 100-120 compressions per minute, pressing down hard and fast in the center of the chest, and provide rescue breaths if trained.
• Use an AED (Automated External Defibrillator): If available, attach an AED to the victim and follow its instructions. The AED can determine if a shock is needed to restore a normal heart rhythm. - Provide First Aid (2 marks)
• Treat burns: If the person has burns, cover the burned area with a clean, non-stick bandage or cloth. Avoid applying ice or ointments.
• Keep the victim calm and warm: If the victim is conscious, keep them calm and in a comfortable position. Avoid moving them unless necessary. Keep them warm by covering them with a blanket to prevent shock. - Monitor the Victim (2 marks)
• Monitor for further symptoms: Even if the victim appears to be fine, electrical shock can cause delayed effects, such as cardiac arrhythmias, internal injuries, or neurological effects.
• Avoid giving food or drink: Do not offer the victim food or drink, especially if they are unconscious or semi-conscious, as this could lead to choking or other complications. - Transport to Medical Facility (2 marks)
• Ensure the victim is taken to the hospital: Regardless of the severity of the shock, the victim should be evaluated by medical professionals. Electrical shock can cause hidden injuries that may not be immediately apparent, such as internal injuries or damage to the heart.
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Additional Considerations:
• Electrical shock can affect the heart: Even if the victim appears stable, electrical shock can cause arrhythmias (irregular heart rhythms) that may not be apparent immediately.
• Delayed symptoms: Victims of electrical shock may not show symptoms right away but may experience symptoms like dizziness, chest pain, shortness of breath, or nausea later. Always get them checked by a doctor.
By following these steps, you can ensure that the situation is handled effectively and that the victim receives the necessary care.
State the effect and consequences if EACH of the following faults occur on a Generator but the protection devices fail to operate.
a. Short circuit.
(4 Marks)
b. AVR Failure.
(4 Marks)
c. Loss of residual magnetism. (4 Marks)
d. Overload.
(4 Marks)
a. Short Circuit (4 Marks)
• Effect: A short circuit on a generator occurs when there is a direct low-resistance connection between two points of different potential (such as the output terminals of the generator), causing a large amount of current to flow. Without protection, this can lead to:
• Overheating: The massive current flow generates heat, which can cause severe damage to the generator windings and insulation.
• Physical damage: The extreme currents may cause mechanical damage to the generator’s components, such as the rotor or stator.
• Potential fire: The heat generated by the short circuit may lead to a fire if the insulation material catches fire.
• Consequences:
• Permanent damage to the generator’s electrical components (windings, rotor, stator, etc.).
• Risk of equipment failure: The generator could be rendered completely inoperable, requiring significant repair or replacement.
• Possible danger to personnel due to electrical hazards, burns, or explosions.
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b. AVR Failure (4 Marks)
• Effect: The Automatic Voltage Regulator (AVR) controls the generator’s output voltage by adjusting the excitation current to the rotor. If the AVR fails, the generator’s voltage will no longer be regulated properly. This can lead to:
• Overvoltage or undervoltage conditions: The output voltage may fluctuate or stay too high or too low.
• Instability in power delivery: Voltage irregularities can cause issues with the equipment powered by the generator, potentially leading to malfunction or failure.
• Excessive wear: Prolonged overvoltage or undervoltage may cause damage to both the generator and connected equipment.
• Consequences:
• Damage to the generator’s windings and insulation due to prolonged overvoltage or undervoltage.
• Operational instability: Sensitive electrical equipment connected to the generator may be damaged or malfunction due to unstable voltage.
• Loss of synchronization with the grid if the generator is operating in parallel with other units or the utility grid.
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c. Loss of Residual Magnetism (4 Marks)
• Effect: Residual magnetism is the remaining magnetic field in the generator’s rotor after it has been de-energized. This magnetism is essential for the excitation system to start generating voltage when the generator is initially started. If residual magnetism is lost:
• No initial voltage generation: The generator will not be able to produce voltage because the excitation system relies on the residual magnetism to start the process of voltage buildup.
• Failure to build up output voltage: The generator may fail to start or fail to deliver power.
• Dependence on external excitation source: In some cases, external excitation or an external DC source is required to restore magnetism.
• Consequences:
• Generator failure to start: The generator will not produce any output power, affecting operations.
• Need for manual intervention: Additional steps, such as manually re-exciting the generator or using external equipment, are necessary to restore normal operation.
• Potential delay in operations: If the generator fails to start, it may cause delays in power supply to critical systems.
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d. Overload (4 Marks)
• Effect: An overload condition occurs when the generator is forced to provide more power than its rated capacity. If the protection devices fail, the generator will continue to operate under these conditions. This can lead to:
• Overheating: The generator’s windings and components may overheat due to the excessive current demand, which could lead to insulation breakdown and further damage.
• Mechanical stress: The generator’s mechanical components, including the rotor and bearings, will experience increased stress, potentially leading to failure.
• Reduction in efficiency: The generator will operate inefficiently, leading to fuel wastage (if it is a fuel-driven generator) or other resource inefficiencies.
• Consequences:
• Permanent damage to generator components: Prolonged overload can cause severe damage to the windings, bearings, and other mechanical parts of the generator.
• Shutdown or failure: If the generator continues to operate under overload conditions, it may eventually shut down or fail, requiring extensive repairs.
• Risk to connected equipment: An overloaded generator can provide unstable or insufficient power to connected loads, leading to possible damage or malfunction of sensitive equipment.
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In summary, each fault represents a significant risk to both the generator and the connected systems, and the failure of protection devices exacerbates the potential damage, highlighting the importance of proper maintenance and the functioning of protective measures.
With reference to earth faults on a ships electrical system:
a. state how they are indicated; (4 marks)
b. describe how the location is be found; (6 marks)
c. state the safety precautions necessary in order to carry out repairs. (6 marks)
- How Earth Faults Are Indicated on a Ship’s Electrical System (4 Marks)
Earth faults in a ship’s electrical system are typically indicated in the following ways:
1. Earth Fault Indicator (EFI): A dedicated indicator or alarm on the ship’s electrical control panel signals the presence of an earth fault. These systems detect any imbalance in the system caused by current flowing to earth (ground). When an earth fault occurs, the EFI shows a visual alarm, such as a light or display, and an audible signal may also be triggered.
2. Earth Leakage Relay (ELR): This relay detects an abnormal flow of current to the ground and trips the circuit or activates an alarm. It continuously monitors the leakage current and triggers a response if the current exceeds a predefined threshold.
3. Voltage Measurement: Some systems use a measurement of the voltage difference between the ship’s hull (which is connected to earth) and the neutral point of the electrical system. If an earth fault occurs, the voltage may shift from zero or cause an unbalanced voltage reading.
4. Multi-functional Protective Relays: Modern electrical systems may incorporate multi-functional protection relays that can detect and indicate earth faults along with other faults such as overloads or short circuits.
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- How the Location of an Earth Fault is Found (6 Marks)
Finding the location of an earth fault involves a systematic approach:
1. Visual Inspection: Start with a visual inspection of the electrical panels, cables, and connections. Look for signs of damage, such as burns, scorch marks, or exposed conductors that could indicate where the fault might be occurring.
2. Use of Insulation Resistance Tester: This tool, also known as a megger, measures the insulation resistance of the electrical system. By testing different segments of the electrical wiring, you can determine which section of the system has lower insulation resistance, indicating a potential earth fault.
3. Use of Clamp Meters: Special clamp meters that detect leakage current can be used to pinpoint the location of an earth fault. By clamping around different sections of wiring, the meter can detect which section of the circuit has abnormal current flow due to an earth fault.
4. Segmenting the Electrical System: If the fault is not readily visible, the electrical system can be divided into smaller segments by switching off and isolating sections of the system. Testing insulation resistance after each isolation helps to narrow down the fault’s location.
5. Monitoring Earth Fault Indicators: If an earth fault indicator or relay is present, the fault might give an indication of the affected zone or circuit. This can guide technicians to the specific area where the fault is likely to be.
6. Tracing Cable Routing: Once a fault zone is identified, tracing the cable routing from the power panel or switchboard toward various machinery, lighting circuits, or other electrical systems will help identify the exact location of the fault.
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- Safety Precautions Necessary to Carry Out Repairs (6 Marks)
Before performing any repairs related to earth faults, several safety precautions must be followed:
1. Isolate the Electrical System: Before working on any electrical system, ensure that the power is completely turned off, and the relevant electrical circuit is isolated from the rest of the ship’s electrical system. Lock-out and tag-out procedures should be followed to ensure that the system cannot be accidentally re-energized.
2. Test for Voltage: Use a proper voltage tester to confirm that there is no voltage present in the affected area. This is critical to avoid electrocution or shock while working on the electrical components.
3. Wear Proper Personal Protective Equipment (PPE): Technicians should wear the appropriate PPE, such as insulated gloves, safety boots, and rubber mats to minimize the risk of electrical shock. If working on high-voltage systems, face shields or eye protection should also be worn.
4. Ensure Adequate Ventilation and Space: If repairs are being carried out in confined spaces or areas with potential for gas accumulation (e.g., near electrical cabinets or distribution boards), ensure there is adequate ventilation. Proper space should also be available for safe movement and operation of tools.
5. Use Insulated Tools: All tools used for repairs should be insulated and rated for electrical work to minimize the risk of electric shock or short circuits while working near live components.
6. Supervised Work: Repairs should be conducted under the supervision of a qualified electrical officer or technician, especially if working on high-voltage systems. A second person should be present to act in case of emergency.
By adhering to these safety precautions, the risk of injury, equipment damage, or further faults can be minimized when repairing an earth fault.
State the effect and consequences if EACH of the following faults occur on a Generator but the protection devices fail to operate.
a. Short circuit (4 Marks)
b. AVR Failure (4 Marks)
c. Loss of excitation (4 Marks)
d. Overload (4 Marks)
- Short Circuit (4 Marks)
Effect and Consequences:
• Damage to Generator Windings: In the event of a short circuit, excessive current can flow through the generator windings, potentially causing them to overheat and sustain damage, leading to a failure of the stator and rotor windings.
• Fire Hazard: The high current resulting from the short circuit may cause overheating of the electrical components, posing a fire risk in the generator compartment.
• Mechanical Stress: The rapid rise in current can cause mechanical stress on the rotating parts, potentially leading to damage in bearings, the rotor, or the shaft.
• System-wide Impact: If the generator remains online without protection, it can damage other parts of the electrical distribution system, including transformers, circuit breakers, and cables.
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- AVR Failure (4 Marks)
Effect and Consequences:
• Voltage Fluctuations: The Automatic Voltage Regulator (AVR) controls the generator’s output voltage. If the AVR fails, the generator output voltage may either drop or rise to unsafe levels, leading to voltage instability across the electrical system.
• Overvoltage or Undervoltage: If the AVR fails and does not regulate the voltage properly, it may cause an overvoltage or undervoltage condition, which can damage sensitive equipment connected to the generator, such as control panels, motors, and computers.
• Overheating: Voltage imbalances caused by AVR failure can lead to increased heat in the generator windings and connected equipment, potentially resulting in overheating and failure of the generator or other electrical components.
• Poor Load Sharing: If the generator is part of a multiple-generator system, AVR failure can lead to poor load sharing, causing uneven load distribution, increased wear and tear on generators, and potential system instability.
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- Loss of Excitation (4 Marks)
Effect and Consequences:
• Loss of Generator Output: The excitation system provides the magnetic field needed to generate electricity. Without excitation, the generator will lose its ability to produce voltage and power, causing it to shut down.
• Potential for Generator Desynchronization: Loss of excitation can lead to a loss of synchronism, where the generator becomes out of phase with the system, which could damage the generator and the connected power network.
• Increased Rotor Heating: Loss of excitation can result in excessive current flowing through the generator rotor without a proper magnetic field, causing overheating and potential damage to the rotor windings.
• System Instability: In the case of multiple generators on the same grid, loss of excitation can destabilize the power system, leading to generator trip-offs, frequency issues, and potential blackouts.
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- Overload (4 Marks)
Effect and Consequences:
• Overheating of the Generator: When the generator operates beyond its rated capacity, it may overheat. Overheating can damage the windings, insulation, and bearings, resulting in costly repairs and downtime.
• Reduction in Efficiency: An overloaded generator operates at a lower efficiency, which means more fuel consumption for the same output, resulting in increased operational costs.
• Potential for Mechanical Failure: Overloading increases the mechanical stress on the generator’s components, including the rotor and bearings, which can lead to mechanical failure or catastrophic damage over time.
• Shutdown or Trip: If the generator continues to operate in overload conditions without protection, it may automatically shut down or be tripped by safety systems to prevent further damage. This could result in power loss and affect the continuity of the electrical supply.
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Each of these faults, if protection devices fail to operate, can lead to significant damage to the generator, operational downtime, and costly repairs, along with potential disruptions to the power supply. Proper functioning of protection devices is essential to safeguard the generator and associated systems.
State the effect and consequences if EACH of the following faults occur on a Generator but the protection devices fail to operate.
a. Short circuit (4 Marks)
b. AVR Failure (4 Marks)
c. Loss of excitation (4 Marks)
d. Overload (4 Marks)
- Short Circuit (4 Marks)
Effect and Consequences:
• Damage to Generator Windings: In the event of a short circuit, excessive current can flow through the generator windings, potentially causing them to overheat and sustain damage, leading to a failure of the stator and rotor windings.
• Fire Hazard: The high current resulting from the short circuit may cause overheating of the electrical components, posing a fire risk in the generator compartment.
• Mechanical Stress: The rapid rise in current can cause mechanical stress on the rotating parts, potentially leading to damage in bearings, the rotor, or the shaft.
• System-wide Impact: If the generator remains online without protection, it can damage other parts of the electrical distribution system, including transformers, circuit breakers, and cables.
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- AVR Failure (4 Marks)
Effect and Consequences:
• Voltage Fluctuations: The Automatic Voltage Regulator (AVR) controls the generator’s output voltage. If the AVR fails, the generator output voltage may either drop or rise to unsafe levels, leading to voltage instability across the electrical system.
• Overvoltage or Undervoltage: If the AVR fails and does not regulate the voltage properly, it may cause an overvoltage or undervoltage condition, which can damage sensitive equipment connected to the generator, such as control panels, motors, and computers.
• Overheating: Voltage imbalances caused by AVR failure can lead to increased heat in the generator windings and connected equipment, potentially resulting in overheating and failure of the generator or other electrical components.
• Poor Load Sharing: If the generator is part of a multiple-generator system, AVR failure can lead to poor load sharing, causing uneven load distribution, increased wear and tear on generators, and potential system instability.
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- Loss of Excitation (4 Marks)
Effect and Consequences:
• Loss of Generator Output: The excitation system provides the magnetic field needed to generate electricity. Without excitation, the generator will lose its ability to produce voltage and power, causing it to shut down.
• Potential for Generator Desynchronization: Loss of excitation can lead to a loss of synchronism, where the generator becomes out of phase with the system, which could damage the generator and the connected power network.
• Increased Rotor Heating: Loss of excitation can result in excessive current flowing through the generator rotor without a proper magnetic field, causing overheating and potential damage to the rotor windings.
• System Instability: In the case of multiple generators on the same grid, loss of excitation can destabilize the power system, leading to generator trip-offs, frequency issues, and potential blackouts.
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- Overload (4 Marks)
Effect and Consequences:
• Overheating of the Generator: When the generator operates beyond its rated capacity, it may overheat. Overheating can damage the windings, insulation, and bearings, resulting in costly repairs and downtime.
• Reduction in Efficiency: An overloaded generator operates at a lower efficiency, which means more fuel consumption for the same output, resulting in increased operational costs.
• Potential for Mechanical Failure: Overloading increases the mechanical stress on the generator’s components, including the rotor and bearings, which can lead to mechanical failure or catastrophic damage over time.
• Shutdown or Trip: If the generator continues to operate in overload conditions without protection, it may automatically shut down or be tripped by safety systems to prevent further damage. This could result in power loss and affect the continuity of the electrical supply.
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Each of these faults, if protection devices fail to operate, can lead to significant damage to the generator, operational downtime, and costly repairs, along with potential disruptions to the power supply. Proper functioning of protection devices is essential to safeguard the generator and associated systems.
With reference to earthing arrangements on ships:
, (a) With the aid of a sketch, describe the difference between an insulated and an earthed system.
(12 marks)
(b) Explain why an insulated earth system would be preferred to an earthed system. (4 marks)
(a) Difference Between an Insulated and an Earthed System (12 Marks)
The electrical earthing system on ships is critical for safety, particularly when dealing with the electrical distribution system. Below, I’ll describe both an Insulated System and an Earthed System, with the aid of a sketch.
- Insulated System:
An insulated system is one in which the neutral of the electrical power supply (typically the generator or transformer) is isolated from the ship’s hull. This means that there is no direct electrical connection between the ship’s electrical system and the body (hull) of the vessel.
Sketch of Insulated System:
\+---------------------+
| Electrical |
| Generator |
\+---------------------+
|
| Neutral (Insulated)
|
\+--------------+
| Ship's |
| Hull |
\+--------------+
(No direct
connection)
• Key Features of an Insulated System:
• The neutral point of the electrical generator or transformer is isolated from the hull of the ship.
• The electrical system is insulated from the ship’s body, meaning there is no direct earth connection.
• A fault (such as a short circuit) on one phase may not immediately affect the rest of the system, which allows time for corrective actions and alarms to be triggered.
- Earthed System:
An earthed system, on the other hand, directly connects the neutral point of the ship’s electrical system (such as from the generator or transformer) to the hull of the vessel, creating a direct connection to the earth (the ship’s body).
Sketch of Earthed System:
\+---------------------+
| Electrical |
| Generator |
\+---------------------+
|
| Neutral (Earthed)
|
_/_\_
| |
| | Ship's Hull
|\_\_\_|
(Earth connection)
• Key Features of an Earthed System:
• The neutral of the electrical system is connected directly to the ship’s hull, forming an earth connection.
• In case of a fault, a direct path to earth is established, which can result in immediate protective actions (such as tripping the circuit breaker).
• The ship’s hull becomes part of the electrical return path for the system, and any electrical fault will directly affect the hull, potentially posing a shock hazard.
⸻
Difference Between Insulated and Earthed Systems:
Feature Insulated System Earthed System
Neutral Connection Neutral is isolated from the ship’s hull (no earth connection). Neutral is directly connected to the ship’s hull (earth connection).
Fault Behavior Faults are more contained, with alarms activated before corrective action is needed. Faults create a direct path to the hull, which can lead to immediate tripping of circuit breakers.
Safety Risk Less immediate risk to personnel as there is no direct contact between electrical system and hull. Risk of electrical shock increases as the hull becomes part of the electrical return path.
System Stability Allows more time for action in case of faults, reducing the chance of electrical hazards. Allows quicker response to faults but may lead to potential issues with hull safety.
⸻
(b) Why an Insulated Earth System Would Be Preferred to an Earthed System (4 Marks)
An insulated earth system would generally be preferred to an earthed system for the following reasons:
1. Safety of Crew:
• In an insulated system, there is no direct electrical connection between the ship’s hull and the electrical system. This reduces the risk of electrical shock to the crew, as the hull is not part of the electrical circuit.
2. Control of Faults:
• In case of an electrical fault, such as a short circuit to the hull, an insulated system allows the fault to be identified and dealt with more effectively without the risk of immediate consequences. It allows time for alarms to be activated and for the crew to correct the issue before any serious consequences occur.
3. Less Risk of Corrosion:
• When the electrical system is earthed, the ship’s hull may carry a small but continuous electrical current, which could cause electrolysis and promote corrosion of the hull. An insulated system eliminates this risk, as no current flows through the hull.
4. Operational Continuity:
• An insulated system ensures that, in the event of a fault, only one part of the electrical system is affected at a time. This helps maintain the stability of the overall system, ensuring that other parts of the electrical system can continue to operate while the fault is being addressed.
Overall, an insulated earth system is generally safer and provides better control over potential faults, reducing risks to personnel and the ship’s structure while maintaining operational reliability.
What does a system drawing look like when it is earthed? In an earthed system
This system shows how an early fault from any one of the electrical lines will cause a closed circuit.
Causing the system to trip
What does the drawing look like in an insulated neutral system?
In an insulated neutral system, one earth fault connecting will not cause a closed circuit allowing equipment to still run.
For example steering gear/ fire fighting/navigational lights
What does it look like when a line conductor becomes earthed in an earthed system?
This is what it looks like when a system becomes closed and an earth fault will occur.
In an insulated neutral system what will it look like when when there is an earthed fault/trip?
I’m an insulated neutral system it requires two line conductors to come into connection with the earth point in order to cause an earth fault
What does a star and a delta system look like?
When a motor star delta system starts it switches to delta when up to speed
What does a star delta for a motor and a generator system look like?
What does an earthed neutral system look like? Image
An earthed system is when current is going to an undesierd location causing an earth fault
What does an insulated neutral look like?
Insulated neutral is where the current will go or an undesired location but current from one phase or line conductor will not cause the system to trip. For example this is used on steering gear, fire fighting appliances and navigational lights and bilge pumps.
How will the emergency battery’s power the navigational lights GMDSS on top of the bridge? Basic answer
The generator and emergency generator will fail
An interlock will activated closing the generators switch and close the switch to engage the battery’s
What do you use a meggar test?
Insulation resistance meggar test
Checking current is flowing properly
Doing U-E, V-E, W-E is the main test
Testing below will check for short circuit or insulation degradation
U-V, V-W, W-U
1 mega ohm and more is good insulation.
Negative goes to earth and positive goes to phase/line conductors
Set meggar to 500DC
What is a continuity test?
Measures resistance U1/U2 point point A-B
What are the main points of a DOL starter?
- [ ] A DOL is motor connected to the full starting voltage
- [ ] High starting current 3-8x more
- [ ] It has a high starting torque
- [ ] Used for small motors up to 5kw
- [ ] Used for water pumps, compressors, fans and conveyor belts
- [ ] Runs at 220v/110v or 24v with aid of transformer
- [ ] The electromagnetic contacter which can be opened by thermal overload relay under fault conditions
What does a DOL starter diagram look like?
What does a DOL starter diagram look like?
