Auxiliary 1 Electrical

Question 1

State three devices fitted to the main breakers to protect a.c. genertators that are able to run in parallel

Answer 1

Overcurrent Protection: This device safeguards the generator from excessive current, which could result from faults or overloads.

Reverse Power Relay: It prevents power from flowing back into the generator, which could damage the prime mover.

Under/Over Voltage Protection: This ensures the generator operates within safe voltage limits, protecting both the generator and connected equipment.

Question 2

With reference to a.c. generators explain why they must be synchronised before connecting in parrellel

Answer 2

Synchronization ensures that multiple generators work together without causing electrical disturbances or mechanical stress. The key reasons for synchronization are:
✅ Matching Voltage Levels
✅ Frequency Alignment
✅ Phase Matching
✅ Load Sharing –
✅ Avoiding Mechanical Stress

Question 3

With reference to a.c. generators list two devices to ensure synchronisingis correct.

Answer 3

1. Synchroscope: This device indicates the phase difference between the incoming generator and the busbar, helping to align them correctly before connection.
2. Synchronizing Lamps: A simpler method where lamps connected in a specific configuration indicate whether the generator is in phase with the busbar.

Question 4

State what determines the circuit’s that can not be connected to the preference trip

Answer 4

✅ Essential Services – Circuits powering navigation equipment, emergency lighting, steering systems, and communication devices must remain operational.

✅ Safety-Critical Systems – Fire pumps, bilge pumps, and emergency generators cannot be disconnected as they are vital for ship safety.

✅ Control & Monitoring Systems – Automation and alarm circuits monitoring engine performance and ship stability must stay active.

✅ Legal & Regulatory Requirements – MARPOL compliance systems, emergency shutdown mechanisms, and other mandated circuits must remain powered.

Question 5

State the requirements for the positioning of the emergency source of electrical power

Answer 5

The positioning of the emergency source of electrical power is crucial for ensuring its functionality during emergencies. Here are the key requirements:

1. Above the Uppermost Continuous Deck: The emergency power source must be located above the uppermost continuous deck to remain operational even if lower decks are flooded or damaged.
2. Accessible from an Open Deck: It should be readily accessible from an open deck to facilitate maintenance and operation during emergencies.
3. Aft of the Collision Bulkhead: Ideally, it should be positioned aft of the collision bulkhead to minimize the risk of damage from head-on collisions.
4. Separated from Machinery Spaces: The emergency power source should not be contiguous to machinery spaces or spaces containing the main source of electrical power. This ensures that a fire or casualty in these areas does not interfere with the emergency power supply.

These requirements are designed to ensure the reliability and safety of the emergency power source in critical situations.

Question 6

Outline the neccessary precautions as stated in the code of safe working practices when working near live electrical equipment when it is essential for the safety for the safety of the ship or for testing purposes

Answer 6

When working near live electrical equipment, as outlined in the Code of Safe Working Practices for Merchant Seamen, the following precautions are essential to ensure safety:

Preparation and Planning:
1. Permit to Work: Obtain a formal permit detailing the task, potential hazards, and safety measures.
2. Competent Supervision: Ensure a qualified electrician or a designated competent person supervises the work.
3. Risk Assessment: Conduct a thorough assessment to identify hazards and implement mitigation measures.
4. Isolation Attempts: If possible, isolate the equipment using approved switches or lockout/tagout procedures.
5. Personal Protective Equipment (PPE): Use insulated gloves, footwear, and eye protection rated for the voltage.
6. Safe Working Area: Establish a clear, restricted zone to prevent accidental contact with live components.
7. Warning Signs: Display clear warnings to alert others of the ongoing work.
8. Insulated Tools: Use tools specifically designed for electrical work to minimize risks.

During Work:
1. Controlled Movements: Work deliberately to avoid accidental contact with live parts.
2. One-Hand Rule: Keep one hand behind your back to reduce the risk of completing a circuit.
3. Continuous Monitoring: Have a designated observer monitor the worker and the situation for safety.
4. Emergency Shutdown Plan: Be prepared to de-energize the equipment immediately in case of an emergency.

After Work:
1. Final Inspection: Ensure the equipment is secure, tools are removed, and the area is safe.
2. Permit Cancellation: Cancel the work permit once all safety measures are terminated.

These precautions prioritize safety while allowing essential tasks to be carried out near live electrical equipment.

Question 7

Give reason why the following devices are fitted to the main breakers to protect ac generators that are able to run in parallel

Over current protection relay
Reverse power protection relay
Under/over voltage relay

Answer 7

When AC generators operate in parallel, it’s crucial to protect them from electrical faults or issues that could damage the system.

1. Overcurrent Protection Relay: This device prevents damage caused by excessive current, such as during short circuits or overloads.
2. Reverse Power Protection Relay: Ensures that no generator feeds power backward into another generator, which could cause mechanical damage or destabilize the system.
3. Under/Over Voltage Relay: Protects the system by monitoring voltage levels and disconnecting the generator if the voltage strays beyond safe limits.

These devices help maintain system reliability and safeguard equipment.

Question 8

a. State the factor that determines the minimum number of available generators required for a vessel to put to sea

b. State 7 essential services for the operation of the vessel

Answer 8

a) The factors that determine the minimum number of generator required are dictated by

• Regulatory Requirements
• Operational Load Demand
• Redundancy for Safety

b) Here are seven essential services required for a vessel’s safe operation:

1. Steering Gear – Ensures the vessel can maneuver safely.
2. Propulsion System – Includes engines, fuel systems, and associated components.
3. Electrical Power Supply – Monitoring system and safety systems.
4. Fire Detection & Suppression – Essential for onboard safety and emergency response.
5. Bilge pumps & Ballast Systems – Maintains stability and prevents flooding.
6. Navigation & Communication Equipment – Ensures safe passage and coordination with other vessels.
7. Emergency Systems – Includes lifeboats, alarms, and evacuation procedures.

These services are critical for maintaining operational integrity at sea.

Question 9

Explain the term signal phasing in an electrical motor (2)

Answer 9

This is when one of the three phase is missing in operation of a 3 phase motor.

Question 10

State the effects on a motor suffering from single phasing (6)

Answer 10

Effects of Signal Phasing on a Motor:

• Torque Loss – Uneven phasing reduces torque and may cause vibrations.
• Overheating – Increased current draw leads to insulation damage.
• Voltage Imbalance – Reduced efficiency and higher energy consumption.
• Mechanical Wear – Excessive vibration accelerates bearing failure.
• System Trips – Protection relays may shut down the motor.
• Speed Instability – Loss of synchronization affects performance.

Question 11

state how single phasing protection is achieved in a starter motor circuit
(3)

Answer 11

Single phasing protection in a starter motor circuit is typically achieved through

• Thermal Overload Relays – These monitor current levels and trip the circuit if excessive heating occurs due to single phasing.
• Single-Phasing Preventers – These detect phase imbalance and disconnect the motor to prevent damage.
• Negative Sequence Filters – Used in advanced systems to identify phase loss and trigger protective actions.

Question 12

Ohms Law

Answer 12

Ohm’s law states that the amount of current flowing in a circuit made up of pure resistance’s is
directly proportional to the electromotive force impressed on the circuit and inversely proportional
to the total resistance of the circuit.

Question 13

Describe the sequence that occurs to restore power should No.1 generator suffers a sudden failure (10)

Answer 13

• When the generator fails the vessel will black out, the link between the main and emergency switch board will open and the this starts the emergencey swith board within 45seconds.
• No. 2 main generator will have started and connected to the main switchboard.
• Switch the emergency generator to the manual position open the emergency switch board circuit breaker.
• Close the main swithboard feed to the emergency board circuit breaker.
• Close the emergency to the main switchboard feeder circuit breaker ensuring the lighting is on.
• Shut down the emergency generator
• Ensure the Auto/manual switch is replaced into the auto poisition

Question 14

list the items that would be included on a checklist for supply of power to the vessel (7)

Answer 14

• check cable condition
• check the insuation resistance and continuity of the cable provided for the shore supply
• check the insulation resisitance of the shore supply box
• Check the polarity of the shore supply using a voltmeter
• check and ensure the frequecy and voltage is correct
• Check the phase sequence of the shore supply using phase sequence tester
• Check the tightness of the shore power plug and connectors.
• Ensure the emergency generator is in manual mode

Question 15

Explain how the generator reverse power trip is tested for correct operation.

Answer 15

• Reduce load of the generator
• then slow the generator speed down until the reverse power trip is activated.
• (done alongside for safety reasons)

Question 16

Explain how the generator over current alarm is tested for proper operation.

Answer 16

Gradually reduce the set point unit to the alarm activates then reset it to a normal value.

This is done alongside due to safety reasons.

Question 17

Explain how the generator over current trip is tested to ensure it is working correctly.

Answer 17

The circuit breaker is isolated from the main bus bar and a set of calibrated currents from a current injection set are fed directly through the closed circuit breaker or through the overcurrent relay. The results are checked again the manufactures characteristics.

Question 18

Explain how the emergency generator auto start up is tested.

Answer 18

this can be tested by shutting down power to main switch board or by opering the interconnector feeder breaker that links the main and emergency switchboards.

Question 19

Explain what preferential tripping is.

Answer 19

Preferential tripping is a protective mechanism in a marine electrical system designed to disconnect non-essential loads when there is an overload or partial failure of the power supply. This prevents the main circuit breaker from tripping and ensures that essential services—such as navigation, steering, and communication—remain operational.

Question 20

Explain Sequential starting

Answer 20

Sequential starting is a method used to start multiple motors or electrical loads in a controlled sequence rather than all at once. This prevents excessive current draw and reduces stress on the power system. This is after a full or partial blackout.

Question 21

Explain the term, fuse backup protection

Answer 21

Fuse backup protection is a system designed to provide additional protection in case a primary fuse fails to clear a fault properly. It ensures that electrical circuits remain safeguarded against short circuits, overloads, and equipment failures.

Question 22

define the term discrimination in a electrical system

Answer 22

In an electrical system, discrimination refers to the ability of protective devices, such as circuit breakers or relays, to selectively isolate only the faulty section of the system while keeping the rest operational. This ensures minimal disruption and enhances system reliability.

Question 23

Explain the term non-essential consumer in an electrical system.

Answer 23

In an electrical system, a non-essential consumer refers to a load or device that is not critical for the system’s operation and can be disconnected without affecting essential functions. These consumers are typically powered when sufficient energy is available but can be shed during power shortages or emergencies to prioritize essential loads.

Question 24

list ten routine maintenance checks that should be carried out on a vented type lead acid engine starting battery system

Answer 24

• Check electrolyte levels – Top up with distilled water if needed.
• Clean terminals – Remove corrosion for better conductivity.
• Inspect vent caps – Ensure proper gas release.
• Test voltage – Check individual cell voltages.
• Measure specific gravity – Use a hydrometer to assess charge state.
• Monitor charge cycles – Prevent sulfation and battery damage.
• Inspect cables – Look for loose or damaged connections.
• Check temperature – Avoid overheating that accelerates wear.
• Examine casing – Look for cracks, leaks, or swelling.
• Perform load test – Verify starting power reliability.

Question 25

Describe the construction of a squirrel cage motor

Answer 25

A **squirrel cage induction motor** consists of two main parts: the **stator** and the **rotor**. **Stator** - Made of laminated iron core to reduce eddy current losses. - Contains slots where **three-phase windings** are placed. - When energized, it produces a **rotating magnetic field**. **Rotor (Squirrel Cage Rotor)** - Cylindrical laminated iron core with **conductive bars** (usually copper or aluminum) placed in slots. - Bars are **short-circuited** at both ends with **end rings**, forming a cage-like structure. - The bars are often **skewed** to reduce noise and torque pulsations. **Key Features** - **No external electrical connections** to the rotor. - **Self-starting** due to induced currents from the stator’s magnetic field. - **Durable and low maintenance**, making it ideal for industrial applications.

Question 26

Explain the term short circuit

Answer 26

A short circuit occurs when an unintended low-resistance connection forms between two points in an electrical system, allowing excessive current to flow. This bypasses the normal load, leading to overheating, equipment damage, and potential fire hazards

Question 27

State the dangers of a short circuit

Answer 27

1. **Overheating** – Can melt insulation and damage components. 2. **Fire Hazard** – Sparks or heat may ignite nearby materials. 3. **Electrical Shock** – Exposed live wires pose serious risks. 4. **Equipment Damage** – Voltage surges can destroy electronics. 5. **Breaker Failure** – Repeated faults may weaken protections. 6. **Explosion Risk** – Sparks can ignite flammable substances.

Question 28

Describe two methods that may be used on a main switchboard to detect an earth fault

Answer 28

- Earth Fault Indication Lamps – This is a simple and cost-effective method. Three lamps are connected to the three phases of the system. Under normal conditions, all three lamps glow with equal brightness. If an earth fault occurs on one phase, the lamp connected to that phase will either dim or turn off, while the other two lamps will glow brighter. This happens because the potential difference between the earth and the faulty phase becomes zero, preventing current flow. - Insulation Resistance Monitoring (Ohm Meter) – This method uses an instrument that measures the insulation resistance of the system with respect to the earth. A small DC voltage is injected into the system, and the meter displays the insulation resistance in kilo-ohms or mega-ohms. If the insulation resistance drops significantly, it indicates the presence of an earth fault. This system often includes visual and audible alarms to alert operators.

Question 29

Describe the construction of a cylindrical a.c generator rotor

Answer 29

A cylindrical AC generator rotor is a smooth, solid steel cylinder designed for high-speed operation in steam and gas turbine-driven generators. It has: - Field windings embedded in slots, excited by DC current to produce a rotating magnetic field. - Slip rings and brushes to transfer excitation current. - Cooling system, often hydrogen-cooled, to manage high temperatures. - Damping windings to stabilize rotor oscillations. This rotor type ensures efficient performance at 1500–3000 RPM, with a uniform air gap reducing flux variations. Would you like any additional clarification?

Question 30

explain the rotational speed required for a 4 pole rotor to supply a frequency of 60hz

Answer 30

The rotation speed of a 4 pole rotor is determined by the speed formular RPM = 120 x frequecy / poles 120 x 60 / 4 = 1800rpm

Question 31

list 8 areas that must have emergency lighting services on a vessel

Answer 31

- Escape Routes & Corridors - Stairwells - Engine Room - Bridge & Control Room - Lifeboat Stations - Crew & Passenger Cabins - Galley & Mess Areas - Deck Areas & Muster Stations - Embarcation and dis-embarcation areas.

Question 32

With reference to shore supplies, a) What are three necessary checks before connecting b) expain the possible consequences fo connecting to an incorrect shore supply

Answer 32

a) Voltage, frequency and phase rotation b) Consequences - Voltage mismatch – High voltage damages equipment; low voltage causes overheating. - Frequency issues – Incorrect frequency (e.g., 50Hz vs. 60Hz) leads to motor failures. - Phase sequence errors – Motors may run in reverse, damaging components. - Earthing problems – Poor grounding can cause electrical shocks or faults. - System overload – Insufficient current supply can trip breakers or cause voltage drops. - Paralleling issues – Accidental paralleling with generators may lead to electrical faults.

Question 33

Explain the following symbol

Answer 33

Type of Protection ‘i’ (Intrinsic Safety) The apparatus is designed in such a way as to restrict the energy within the apparatus to a level below that at which can cause ignition by either sparking or excessive temperatures.

Question 34

Explain how to recover the main switch board after a blackout

Answer 34

Recovering the Main Switchboard after a Blackout Prior to attempting a recovery of the main power system from the emergency system, the cause must be investigated and rectified. With regards to the emergency generator the following takes place: * Reinstate the 24 V shutdown relay (if fitted). * Switch the emergency generator auto/manual switch to the ‘Manual’ position. * Start the main generator ensuring all temperatures and pressures are correct. * Open the emergency switchboard circuit breaker. * Close the main switchboard circuit breaker. * Ensure that the main switchboard volts and frequency are correct and adjust as necessary. * Close the emergency to the main switchboard feeder circuit breaker ensuring the lighting is on. * Shutdown the emergency generator. * Ensure the auto/manual switch is replaced into the ‘Automatic’ position.

Question 35

explain why an a.c. motor may need a star-delta starter

Answer 35

An AC motor may require a star-delta starter to manage its high inrush current during startup. When an induction motor starts, it draws a large current—often 6 to 8 times its rated current—which can cause voltage drops and excessive stress on electrical components. The star-delta starter mitigates this by initially connecting the motor in a **star configuration**, reducing voltage to each winding to **one-third** of the full supply voltage. This limits the starting current and torque, providing a smoother startup. Once the motor reaches a sufficient speed (typically around 70-80% of full speed), the starter switches to the **delta configuration**, allowing the motor to operate at its full voltage and normal torque. This method is commonly used for **large motors (above 5-10 kW)** where direct-on-line starting would be impractical due to excessive current draw.

Question 36

Using the sketch below sketch how the motor would be wired in both star and delta

Question 37

Describe with the aid of a sketch the features of a main circuit breaker that minimises contact damage when tripping on overload

Answer 37

- Arc Chutes: These help extinguish the arc that forms when the breaker trips, preventing excessive wear on the contacts. - Spring-Loaded Mechanism: Ensures a rapid separation of contacts, reducing the duration of arcing and minimizing damage. - Silver Alloy Contacts: Used to enhance durability and reduce erosion caused by repeated tripping. - Magnetic and Thermal Protection: A combination of magnetic trip mechanisms (for short circuits) and thermal elements (for overloads) ensures precise operation. - Contact Wiping Action: Some designs incorporate a slight sliding motion when opening, which helps clean the contact surfaces and maintain conductivity.

Question 38

Describe the routine maintenance of storage batteries and the safety precautions that should be observed when carrying out maintenance

Answer 38

Routine maintenance of storage batteries ensures their longevity and safe operation. Here’s what should be done: **Routine Maintenance** - **Regular Inspection**: Check for signs of corrosion, leaks, or swelling. - **Cleaning Terminals**: Remove dirt and corrosion using a baking soda solution. - **Electrolyte Level Check**: Ensure proper electrolyte levels in lead-acid batteries and top up with distilled water if needed. - **Charge Monitoring**: Maintain an optimal charge level to prevent deep discharge. - **Ventilation**: Ensure proper airflow to prevent gas buildup. - **Connection Checks**: Inspect cables and terminals for tightness and wear. **Safety Precautions** - **Wear Protective Gear**: Use gloves and safety goggles to prevent exposure to corrosive substances. - **Avoid Sparks and Flames**: Batteries can emit hydrogen gas, which is highly flammable. - **Proper Ventilation**: Work in a well-ventilated area to prevent gas accumulation. - **Use Insulated Tools**: Prevent accidental short circuits. - **Dispose of Batteries Correctly**: Follow environmental regulations for disposal.

Question 39

State with reasons 5 advantages of VRLA batteries

Answer 39

Here are five advantages of **Valve-Regulated Lead-Acid (VRLA) batteries**, along with reasons why they are beneficial: 1. **Maintenance-Free Operation** – Unlike traditional flooded lead-acid batteries, VRLA batteries do not require periodic water refilling, reducing upkeep. 2. **Reduced Ventilation Requirements** – Since VRLA batteries are sealed, they emit minimal gases, making them safer for enclosed spaces. 3. **Longer Lifespan** – These batteries typically have a longer service life, often lasting up to 10 years, due to their efficient recombination of gases. 4. **Flexible Installation** – Their sealed design allows them to be installed in various orientations without risk of leakage, making them ideal for compact spaces. 5. **Economical and Reliable** – VRLA batteries are cost-effective as they can be store close to the units cutting down on cable size.

Question 40

Explain the term flame proof enclosure

Answer 40

A **flameproof enclosure** (often designated as **Ex d** in hazardous area classifications) is a protective housing designed to contain an explosion within itself, preventing flames or hot gases from escaping and igniting the surrounding atmosphere. **Key Features of a Flameproof Enclosure** - **Explosion Containment**: If an internal explosion occurs, the enclosure prevents it from spreading outside. - **Flamepath Design**: The enclosure has precisely engineered gaps that allow gases to escape while cooling them down to prevent ignition outside. - **Robust Construction**: Made from materials like cast iron, stainless steel, or aluminum to withstand high pressures. - **Ingress Protection**: Typically rated **IP66 or higher**, ensuring protection against dust and moisture. - **Certified Components**: External parts like cable glands must be certified to maintain the integrity of the enclosure.

Question 41

Explain why an insulated neutral is preferred to an earthed neutral

Answer 41

An insulated neutral is preferred over an earthed neutral because it allows continued operation during a single earth fault without immediate shutdown. As there is the possibilty to single short to either from phase to either earth or the neutral before shut down will occur. This also - Reduces shock risk by preventing unintended grounding. - Minimizes fire hazards in explosive environments. - Prevents large fault currents that could damage equipment. - Improves fault detection for proactive maintenance.

Question 42

how are essential circuits are protected should a bus bar overload occur

Answer 42

Essential circuits are protected during a **bus bar overload** using several key methods: 1. **Preferential Tripping** – Non-essential loads are disconnected first, ensuring critical circuits remain operational. 2. **Load Shedding** – Automated systems reduce power consumption by shutting down non-essential equipment. 3. **Circuit Breakers & Fuses** – Overcurrent protection devices trip to isolate faults and prevent damage. 4. **Busbar Differential Protection** – Detects faults within the busbar and isolates affected sections. 5. **Backup Protection** – Ensures redundancy in case primary protection fails.

Question 43

state 3 necessary checks before connecting shore supply to the vessel and expain the possible of connecting to an incorrect supply

Question 44

describe the construction of a cylindrical ac generator rotor and explain the rotational required for a 4 pole rotor to supply a frequency of 60hz

Answer 44

cylindrical rotor is commonly used in high-speed synchronous generators, particularly in steam and gas turbine-driven applications. Its construction includes: - Rotor Core – Made of laminated steel to reduce eddy current losses. - Field Windings – Placed in axial slots along the rotor surface and excited by DC current to produce a rotating magnetic field. - Smooth Cylindrical Shape – Ensures uniform air gap and minimizes flux variations. - Shaft & Bearings – Supports the rotor and allows smooth rotation. - Cooling System – Uses air or hydrogen cooling to dissipate heat efficiently. Rotational Speed for a 4-Pole Rotor at 60Hz The synchronous speed (Ns) of an AC generator is determined by the formula: f = Supply frequency (60 Hz) P = Number of poles (4) RPM = 120 \ poles (4) x 60 = 1800

Question 45

Describe witg the aid of a sketch, an arrangement for a lead acid batteries that maintains the batteries in a ready state and enables automatic connection to emergency supplies in the even of power failure

Answer 45

The battery is connected to the charger by contacts that are held close by a NC solenoid which is powered from the main supply. When this fails the spring in the solenoid opens the charger contacts and closes the interlocked contacts from the battery to the emergency supply.

Question 46

Draw a Synchronising lamp system.

Answer 46

Question 47

Describe the construction of a squirrel cage motor

Answer 47

**Key Points:** - Stator Core – Made of silicon steel laminations (0.4–0.5mm thick) to reduce hysteresis loss, housing the three-phase windings. - Rotor Construction – Composed of insulated steel laminations to minimize eddy currents and ensure rigidity. - Squirrel Cage Design – Large motors use copper bars brazed to end rings, while small ones use cast aluminum with built-in fan blades. - Slot Skewing – Reduces starting torque ripple and ensures quiet operation. - Bearings & Air Gap – Precision-mounted with ball/roller bearings for minimal stator-rotor clearance. **

Question 48

With reference to induction motors 1. explain why the contactor may not protech against short circuit currents. 2. state how pretection against short circuit currents is achieved. 3. explain the operation of the device stated in part (2)

Answer 48

1. The breaker may not operate due to the contacts being welded together as a result of a high overload current caused by the short. 2. fuse backup protection operates when a short circuit fault or stalled motor current which arise from a control circuit failure or a welded contactor. 3. high current heats up the fuse element and melts it, the sand filter in the enclosed fuse becomes moten sticks together to form glass tube containing the energy within the body.

Question 49

With reference to storage batteries explain each of the following 1. boost charge 2. slow charge 3. trickle charge 4. float charge

Answer 49

1. **Boost Charge** – A high-current charge used to quickly restore battery capacity, often applied when a battery is deeply discharged. 2. **Slow Charge** – A low-current charge over an extended period, reducing stress on the battery and prolonging lifespan. usually less then 10amps. 3. **Trickle Charge** – A continuous low-current charge that maintains battery capacity without overcharging, ideal for long-term storage. equal to it discharge when the battery has not load. 4. **Float Charge** – A regulated voltage charge that keeps the battery fully charged while supplying power to connected loads.

Question 50

Describe the response of the voltage output of an a.c. alternator when a sudden, large increase in load occurs.

Answer 50

When a sudden, large increase in load occurs on an AC alternator, the voltage output initially drops due to the increased current demand. This phenomenon is known as transient voltage dip. The alternator’s Automatic Voltage Regulator (AVR) then reacts by increasing excitation current to restore voltage to its preset level. If the load increase is excessive, the alternator may struggle to recover, leading to flickering lights, unstable frequency, or even system failure

Question 51

1. Descripe with the aid of sketch a cartidge type fuse. 2. state why a fuse used in a motor circuit differs from a a fuse used in a lighting circuit.

Answer 51

1. A cartridge fuse consists of a heat resisting ceramic body filled with silica sand, having metal end caps on which a silver current carrying element is soldered through the middle. 2. Quick-acting fuses are typically used in lighting circuits which have a smal inrush current or where over current or short circuits must be quickly interupted. Slow-blow fuses are used for electric motor protection where in rush current of up to 10times the running current on normal startup.

Question 52

With reference to a.c generators 1. state why the excitation is direct current 2. explain why excitation is required

Answer 52

**1. Why AC Generators Use DC Excitation** AC generators require **DC excitation** because a **steady magnetic field** is needed to induce AC voltage in the stator windings. If AC were used for excitation, the polarity of the magnetic field would constantly change, disrupting stable power generation. **2. Why Excitation is Required** Excitation provides the **magnetic field** necessary for power generation The rotor's magnetic field rotates, cutting through the stator windings, inducing an electric current. This follows **Faraday’s Law**, producing alternating voltage, which is then transmitted as electrical power.

Question 53

With reference to AVR's explain the effects that would be observed should an AVR give a low output when the generator is running in parallel with a second generator whose AVR has no fault

Answer 53

If an **Automatic Voltage Regulator (AVR)** gives a **low output** while a generator is running in **parallel** with another, several issues can arise: **Effects of Low AVR Output in Parallel Operation** 1. **Unequal Reactive Power Sharing** – The generator with low AVR output will supply less **reactive power**, causing the other generator to compensate, leading to **imbalance**. 2. **Voltage Instability** – The bus voltage may fluctuate as the system struggles to maintain stability. 3. **Increased Load on the Other Generator** – The second generator may experience **higher current draw**, potentially **overloading** its components. 4. **Power Factor Issues** – The affected generator may operate at a **poor power factor**, reducing efficiency and increasing losses. 5. **Possible System Trips** – If the voltage deviation is severe, **protective relays** may activate, causing a generator shutdown.

Question 54

With reference to AVRs explain the purpose

Answer 54

An **Automatic Voltage Regulator (AVR)** ensures stable voltage output in **AC generators** by adjusting excitation levels. Its key purposes include: - **Voltage Stabilization** – Maintains a consistent generator output despite load variations. - **Reactive Power Control** – Balances reactive power when generators run in parallel. - **Surge Protection** – Prevents damage from voltage fluctuations and overloads. - **System Efficiency** – Optimizes generator performance and prevents excessive wear.

Question 55

With reference to induction motor starters. 1. state when a Star/Delta starter may be required 2. Describe the operation fo the Star/Delta Starter 3. Explain why the motor configuartion is changed from Star to Delta

Answer 55

**1. When a Star/Delta Starter May Be Required** A **Star/Delta starter** is used for **three-phase induction motors** when: - The motor has a **high starting current** that needs to be reduced. - The application requires **gradual acceleration** to prevent mechanical stress. - The motor power rating is **above 5 kW**, where direct-on-line starting may cause voltage dips. **2. Operation of the Star/Delta Starter** - **Starting Phase (Star Connection)** – The motor windings are connected in a **star configuration**, reducing voltage to **one-third** of the supply voltage, limiting inrush current. - **Transition Phase** – After reaching **about 80% of full speed**, a timer or relay switches the motor to delta mode. - **Running Phase (Delta Connection)** – The windings are reconfigured to **delta**, allowing full voltage operation for normal running conditions. **3. Why the Motor Configuration Changes from Star to Delta** - **Reduced Starting Current** – Star connection lowers voltage per phase, minimizing initial current draw. - **Full Power Operation** – Delta connection restores full voltage, enabling the motor to deliver **maximum torque**. - **Improved Efficiency** – Ensures smooth transition from startup to normal running without excessive electrical or mechanical stress.

Question 56

How does an earth fault monitor work?

Answer 56

An earth fault monitoring instrument injects a small DC voltage into the AC distribution system to measure insulation resistance. The system can monitor as little as 1 mA, significantly lower than earth lamps at 60ma, and provides direct readings in kΩ. The system offers visual and audible alarms when an earth fault occurs and can be set to trigger alerts at a predefined insulation resistance or leakage current level.

Question 57

The number of generators required for a vessel is determined by several factors:

Answer 57

- **SOLAS and Regulatory Requirements:** SOLAS rules and classification society guidelines mandate redundant power supplies, ensuring that essential and safety-critical loads remain powered in case of generator failure. - **Electrical Load and Reserve Capacity:** The total power demand—covering propulsion, navigation, safety, and auxiliary systems—requires generators sized with extra capacity to manage load changes and emergencies. - **Vessel Type and Operational Profile:** Different vessel types (e.g., passenger ships vs. cargo vessels) have varying power needs, impacting the number of generators needed for safe and reliable operations. - **Redundancy and Reliability:** Extra generators are essential to provide backups that ensure continuous operation even if one generator fails. - **Emergency Power Supply Considerations:** Additional, quickly starting emergency generators are required to support vital systems during faults or complete power loss. This combination of regulatory, operational, and safety factors ensures a reliable power system at sea.

Question 58

WIth reference to electrical maintenance. a) explain the procedure for proving a motor circuit is isolated using a multimeter b) expalin the procedure for testing the insulation resistance and earth bonding of the motor, give examples of acceptable readings.

Answer 58

**a) Proving a Motor Circuit is Isolated Using a Multimeter** - Turn off power and apply Lockout/Tagout (LOTO) to prevent re-energization. - Verify isolation by attempting to operate the motor—no response confirms disconnection. - Set up the multimeter to AC voltage mode and test on a known live circuit first. - Measure voltage across each phase to neutral/ground—zero volts confirms isolation. - Double-check by testing the multimeter again on a known live circuit. **To assess the motor’s insulation integrity and grounding, follow these steps** Insulation Resistance Test - Disconnect the Motor – Ensure the motor is completely disconnected from the power supply. - Lockout/Tagout – Prevent accidental energization. - Select a Megohmmeter – Choose an insulation tester with a suitable test voltage (typically 500V or 1000V). - Measure Resistance – Apply the test voltage between the motor windings and ground. - Interpret Results: - 10 MΩ or more – Good insulation. - 5-10 MΩ – Acceptable but may require monitoring. - 2-5 MΩ – Critical, maintenance needed. - Below 2 MΩ – Poor insulation, immediate action required. **Earth Bonding Test** - Set Up a Continuity Tester – Use a low-resistance ohmmeter. - Measure Resistance – Place one probe on the motor frame and the other on the earth connection. - Acceptable Readings – Typically less than 0.1 Ω, indicating a strong earth bond