Electrics Page 39-44 Flashcards
What is the meaning of the term insulated neutral?
In a neutral insulated system, a single earth fault occurring in the system would not cause any earth current to flow in the ship’s hull. This is because a single earth fault current does not provide a complete circuit for earth current to flow. So no protective trip will operate and system will continue to function normally.
How can the location of an earth fault be found?
Earth fault location is traced by process of elimination. Personnel whose space may be affected must be informed. Power is then switched off in a pre-determined order while monitoring the earth fault monitor to see if the fault is cleared. An insulated resistance test is carried out to locate the exact location of the faulty circuit.
What safety precautions should be taken before investigating faulty electrical equipment?
Correct risk assessments are to be carried out by a competent person to put in place control measures. Electrical circuits must be safely isolated, which includes switching off the power, securing the power switch, posting appropriate notices, and proving the circuit is dead.
List five protective devices fitted to a main electrical switchboard.
- Earth Fault Lamps
- Over-current circuit-breaker
- Reverse power trip
- Under-voltage and over-voltage trip
- Under and over frequency trip
What is an earth fault and why is it undesirable in electrical installations?
An earth fault is when electrical current starts flowing in an unintended direction due to weakness in insulation. If not rectified, it may lead to failures such as short circuits, which can cause fire, explosion, severe injuries, or death.
List five precautions to be taken before any work is carried out on electrical equipment.
- Risk assessment to be done
- Isolate mechanically and electrically, Lock out
- Prove dead with certified tool such as multimeter
- Correct PPE to be worn
- Competent person doing the work or supervised
What are three additional precautions when working near live equipment?
- All tools must be of insulated type VDE certified up to 1000 volts AC
- No loose metal jewellery on one’s person
- If possible, install an insulated approved barrier between you and live parts.
What checks are required prior to starting the emergency generator engine?
Cooling water level, oil sump level, fuel supply is on, check for leaks, turbocharger oil level, check starting mechanism (batteries or compressed air).
Describe the routine testing of an emergency generator.
The emergency generator is started and allowed to run while checking for abnormal behavior such as leaks, vibrations, and noise. All running parameters of the prime mover and alternator are checked. A blackout is simulated to ensure the generator is connected to the emergency switchboard within 45 seconds. Approximately 80% of max load is placed on the generator for 45 minutes to 1 hour, monitoring all parameters.
In this generator test the endurance is tested.
What are three underlying causes of electrical equipment failure?
Overloading, insulation failure, misuse.
What precautions should be taken before working on the switchboard?
Carry out a risk assessment, prove dead using a multimeter, lock out, inform other engineers, use correct PPE, have a competent engineer present, set up safety barriers with appropriate notices, place safety mats on the floor, and issue a permit to work with isolations if possible.
What is the meaning of the term ‘earth fault’?
An earth fault occurs when electrical current flows in an unintended direction due to weakness in insulation, which can no longer contain the electrical current.
What are four possible causes of earth faults in an electric motor?
Dampness in the motor,
weakened insulation,
high temperature,
dirty internal connection box.
What are three undesirable consequences of earth faults?
If not rectified, earth faults may lead to short circuits, which can cause fire, explosion, severe injuries, or death.
Describe one method of detecting earth faults.
Earth fault location is traced by process of elimination. Personnel must be informed before power is switched off in a pre-determined order while monitoring the earth fault monitor. Once the general location is identified, an insulated resistance test is carried out to locate the exact fault location.
State ONE publication dealing with the storage of ships batteries.
COSWP section 22.16 Storage Batteries
List SEVEN precautions that should be taken when working in an area where batteries are stored.
- Correct risk assessment carried out (PTW)
- Ventilation of the space, ensure fans are running and vents open
- NO SMOKING, HOT WORKS, NAKED LIGHTS near batteries
- Rinse away acid spills with plenty of fresh water
- Avoid wearing metal near battery terminals due to potentially dangerous voltage
- Wear goggles, gauntlet gloves, and rubber apron when handling batteries or acid
- Only use approved containers for handling battery acid
With reference to secondary cell batteries, state the two types.
Lead-Acid type & alkaline type
State four applications onboard for secondary cell batteries.
- Emergency generator
- Emergency lighting
- Communication
- Alarm systems
- Navigation lights
Describe the maintenance required for secondary cell batteries.
- Keep terminal connections clean and tight (Vaseline used to protect connections)
- Keep batteries securely fastened in their place
- Replenish batteries’ electrolyte with distilled water
- Regularly check specific gravity or relative density
- Keep vents clear
- Conduct regular load tests
What is the first step in running up and paralleling an alternator onto the main switchboard?
Check that there is sufficient lubrication oil in the sump of the engine.
What should be done after starting the lubrication-priming pump?
Ensure correct pressure is reached and check for leaks.
What valves need to be open before starting the engine?
All cooling valves, fuel valves, and starting/control air valves.
What is the purpose of turning the engine with the turning gear?
To clear any water build-up within the cylinders.
What should be done after turning the engine for two complete revolutions?
Monitor indicator cocks for liquid expelled due to cracked liner
Monitor any compressed air being expelled
What should you check before starting the engine locally?
Ensure that no debris or persons are around the flywheel and that the lube oil-priming pump is still running.
What should be monitored once the engine is running?
Check for leaks, abnormal noise, or misfire.
What should be done after placing the lubrication oil-priming pump in auto?
Return to the engine control room and log when the engine was started; allow the engine to warm watching exhaust gas temperature to approximately 200°C.
How should the frequency of the new generator be adjusted?
Increase the frequency to just above that of the bus-bar using the frequency gauge and governor controls.
What should be done at 10 degrees before the top on the synchroscope?
Press the breaker close button.
What should be monitored after closing the breaker for the new generator?
Ensure frequency and voltage stay constant.
What is a critical warning when operating an alternator?
Never allow an alternator to run at low load as it may begin to motor in the event of a load change.
What is the first condition that must be satisfied before paralleling an A.C. generator?
Frequency of the incoming machine must be slightly above that of the bus-bar.
How is the frequency condition achieved?
Using the generator governor.
What is the second condition for paralleling an A.C. generator?
The voltage of the incoming machine should be the same or slightly more than that of the bus-bar.
How is the voltage condition achieved?
Using the trimming adjustment on the AVR control.
What is the third condition for paralleling an A.C. generator?
The phase rotation should be the same as the bus-bar.
How is the phase rotation condition achieved?
By correct wiring of the phases.
What is the fourth condition for paralleling an A.C. generator?
The incoming generator may only have its circuit-breaker closed when in phase synchronism.
How is phase synchronism monitored?
Using a synchroscope rotating no faster than rev/seconds or phase lamps when the top lamp is illuminated.
What are the conditions that must be satisfied before paralleling an A.C. generator?
Voltage, Phase sequence, Frequency.
How is voltage monitored?
Using the Main Switchboard Voltage Meter.
How is phase sequence monitored?
Using the Main Switchboard Synchroscope.
How is frequency monitored?
Using the Main Switchboard Synchroscope.
How is frequency adjusted?
Using the generator governor control on the main switchboard.
How is voltage adjusted?
Using the AVR manual adjustment facility.
What is the first step in running up and paralleling an alternator onto the main switchboard?
Check that there is sufficient lubrication oil in the sump of the engine.
What should be done after starting the lubrication-priming pump?
Ensure correct pressure is reached and check for leaks.
What valves need to be open before starting the engine?
All cooling valves, fuel valves, and starting/control air valves.
What is the purpose of turning the engine with the turning gear?
To clear any water build-up within the cylinders.
What should be done after turning the engine for two complete revolutions?
Close all indicator cocks.
What should you check before starting the engine locally?
Ensure that no debris or persons are around the flywheel and that the lube oil-priming pump is still running.
What should be monitored once the engine is running?
Check for leaks, abnormal noise, or misfire.
What should be done after placing the lubrication oil-priming pump in auto?
Return to the engine control room and log when the engine was started; allow the engine to warm watching exhaust gas temperature to approximately 200°C.
How should the frequency of the new generator be adjusted?
Increase the frequency to just above that of the bus-bar using the frequency gauge and governor controls.
What should be done at 10 degrees before the top on the synchroscope?
Press the breaker close button.
What should be monitored after closing the breaker for the new generator?
Ensure frequency and voltage stay constant.
What is a critical warning when operating an alternator?
Never allow an alternator to run at low load as it may begin to motor in the event of a load change.
What is the first condition that must be satisfied before paralleling an A.C. generator?
Frequency of the incoming machine must be slightly above that of the bus-bar.
How is the frequency condition achieved?
Using the generator governor.
What is the second condition for paralleling an A.C. generator?
The voltage of the incoming machine should be the same or slightly more than that of the bus-bar.
How is the voltage condition achieved?
Using the trimming adjustment on the AVR control.
What is the third condition for paralleling an A.C. generator?
The phase rotation should be the same as the bus-bar.
How is the phase rotation condition achieved?
By correct wiring of the phases.
What is the fourth condition for paralleling an A.C. generator?
The incoming generator may only have its circuit-breaker closed when in phase synchronism.
How is phase synchronism monitored?
Using a synchroscope rotating no faster than rev/seconds or phase lamps when the top lamp is illuminated.
What are the conditions that must be satisfied before paralleling an A.C. generator?
Voltage, Phase sequence, Frequency.
How is voltage monitored?
Using the Main Switchboard Voltage Meter.
How is phase sequence monitored?
Using the Main Switchboard Synchroscope.
How is frequency monitored?
Using the Main Switchboard Synchroscope.
How is frequency adjusted?
Using the generator governor control on the main switchboard.
How is voltage adjusted?
Using the AVR manual adjustment facility.
- Describe how to manually couple a second auxiliary generator to the main switchboard and balance out the load (16)
• Check that there is sufficient lubrication oil in the sump of the engine
• Start the lubrication-priming pump, and ensure correct pressure is reached. Check for leaks.
• Ensure that all cooling valves are open, fuel valves are open and starting/control air valves are open.
• With the fuel rack closed, and indicator cocks open, turn the engine with turning gear if fitted or kick on air for 2
complete revolutions in order to clear, any water build up within the cylinders. Once complete close all indicator
cocks.
• Once satisfied, disengage turning gear; ensure that no debris or persons are around the flywheel. Check the lube
oil-priming pump is still running. Open the fuel rack and start the engine locally, release the air once turning. With
the engine running, check for leaks, abnormal noise or misfire.
• Place the lubrication oil-priming pump in auto, return to the engine control room and log when the engine was started;
allow the engine to warm watching exhaust gas temperature to approximately 200oC.
• At the main switchboard, use the frequency gauge and governor controls increase the frequency of the “new”
generator to that just above that of the bus-bar, check that the voltage is the same as that of the bus-bar, check
for any engine warning alarms.
Use the synchroscope, and adjust using the governor control, the speed of the incoming machine so that the
synchroscope is rotating once every 3 seconds in the clockwise direction. At 10 degrees before the top “10 to 12
O’clock”, press the breaker close button. This generator is now on the board, decrease the running generator using the
governor control, and increase the incoming machine to match load, using the Power meter. Ensure frequency and voltage
stay constant. NEVER ALLOW AN ALTERNATOR TO RUN AT LOW LOAD AS IT MAY BEGIN TO MOTOR IN THE
EVENT OF A LOAD CHANGE
- Describe the conditions that must be satisfied before paralleling an A.C. generator with the main electrical
switchboard. For each condition describe how it is achieved (16)
• Frequency of the incoming machine must be slightly above that of the bus-bar approx generator
governor is used to achieve this
• The voltage of the incoming machine should be the same, if not slightly more than that of, the bus-
bar this is achieved by using the trimming adjustment on the AVR control
• The phase rotation should be the same as the bus-bar, this is achieved normally by correct wiring
of the phases, but if the incoming machine is too slow it may lag that of the bus bar
• The incoming generator may only have its circuit-breaker closed when in phase synchronism, this is
shown by the use of a synchroscope rotating no faster than 1rev/3seconds or phase lamps when the
top lamp is only illuminated
Describe how to manually couple a second auxiliary generator to the main switchboard and balance out the load.
Ensure that no persons are working on the machine, all covers and guards are in place. Ensure sufficient lubrication oil level within the sump. Ensure that starting and control air is on, fuel supply is available and the cooling system is operational. Start the lube oil priming pump, check the engine for leaks and that the pressure has risen. Open all indicator cocks, with the fuel supply closed turn the engine on turning gear ensuring all cylinders purge now close indicator cocks.
Disengage turning gear, open fuel supply and start the engine. Ensure the priming pump has stopped and check the machine for leakage. Set the frequency of the incoming machine to that just higher of the bus bar; ensure the correct voltage and phase rotation is correct. Bring the machine into synchronism and close the circuit breaker, using the governor controls reduce the outgoing machine and increase the incoming machine. Ensuring that voltage and frequency remain constant whilst balancing.
Explain the procedure to take when taking the A.C. generator off electrical load.
Ensure that sufficient capacity remains on other generator before any load shifting takes place. Decrease the load of out-going machine using the governor control, increasing the incoming machine maintaining constant frequency and voltage. With only a fraction of the load remaining 50-100KW open the circuit breaker of the out-going machine.
What steps should be taken to isolate the machine for safe maintenance?
Lock and tag the Main Circuit Breaker keeping keys on your person. Ensure that the generator controls are placed into OFF or MANUAL on the main switchboard and place signage. Shut down lube oil priming pump, cooling water, fuel and air supply. Lock off and tag the valves and controls. Drain the pressure of all systems. Open all indicator cocks ensure that the fuel rack is in the OFF, CLOSED or STOP position. Ensure that the excitation equipment is isolated and all electrical connections proved ‘dead’.
List four maintenance criteria necessary for good operation of electrical motors.
Motor cooling fins and fan housing free from dust or debris. Electrical insulation in good condition. Bearings are in good condition and well lubricated. Shaft is well aligned.
Describe how to test for electrical faults within an electrical motor.
Risk assessment carried out. Isolate the motor mechanically and electrically, tag and lock all areas of electrical supply. Remove the electrical cover and test that there is no electrical supply (prove dead). Using an approved insulation resistance tester measure between all phases and the casings, also between all phases. Measure the resistance of the windings and log results. Balance test to make sure windings are balanced.
Describe how to overhaul an electric motor that has been flooded with sea water.
- Carry out a risk assessment. 2. Isolate the motor, tag and lock out, and prove dead. 3. Disassemble the motor, removing bearings, all covers, and rotor. 4. Wash all components with distilled water. 5. Place components in a warm dry environment to fully dry. 6. Apply varnish to the bearings and allow to dry again. 7. Reassemble the motor with new bearings, use penetrative spray like WD-40 to displace water, and check for free rotation. 8. Perform an IR test to check insulation. 9. Check insulation on windings and perform a balance test. 10. If results are normal, commission the motor for use.
How to check the insulation resistance of the motor?
Use an insulation resistance tester to test between all phases and between all phases and the casing. The test should be conducted at twice the working voltage with a minimum acceptable value of 1 mega-ohm.
What is the minimum satisfactory value of insulation resistance?
100 Mega Ohm
What instrument and voltage are used for insulation resistance testing?
Insulation Resistance Tester with a test voltage of 500V if the motor winding operational voltage is less than 1000 Volts AC/DC.
Define the regular maintenance necessary for good electric motor operation.
- Test insulation resistance of field windings. 2. Regularly lubricate shaft bearings with high-quality lubricant. 3. Routine cleaning of fan cages, blades, and casing fins. 4. Routine balancing of rotor shafts. 5. Routine checks of shaft alignment and observe vibration during operation.
Outline the procedure to determine the possible cause of a small 3 phase motor failing to rotate.
- Identify mechanical or electrical reasons for non-starting. 2. Check if bearings or gears are seized. 3. Ensure power supply is on and available at motor terminals. 4. If power is available, check for faults inside the motor. 5. Conduct continuity tests on phase windings. 6. If continuity is proved, perform megger tests.
Explain the procedure to remove a faulty motor requiring replacement.
- Carry out a correct risk assessment. 2. Isolate the motor and pump both mechanically and electrically. 3. Switch off power, secure it, and post notices. 4. Ensure correct PPE, tools, lighting, and lifting equipment are available. 5. Uncouple the motor from the pump and stow it safely. 6. Secure the work area until the new motor is fitted.
What is an insulated neutral system?
A system where the neutral point of the generator or transformer is not directly connected to the ship’s hull (earth)
This design allows for continued operation during a single-phase earth fault.
What happens in an insulated neutral system when a single-phase earth fault occurs?
The system continues to operate without immediate shutdown, triggering an alarm instead
This is crucial for reliability in marine electrical systems.
Why are insulated neutral systems common on ships?
To ensure continuity of supply and reliability even in case of minor faults
Marine electrical systems must be dependable.
What is the risk associated with a second fault in an insulated neutral system?
It can create a phase-to-phase short circuit, which is dangerous
This can happen if a second phase-to-earth fault occurs before the first one is corrected.
What is an earth fault?
An unintended connection between a live conductor (phase) and the ship’s hull (earth)
This can lead to serious electrical issues if not managed properly.
What occurs in an insulated neutral system during a single earth fault?
It triggers an earth fault monitoring alarm without immediate shutdown
This allows for monitoring and management of the fault.
What happens in a solidly earthed system when an earth fault occurs?
It can cause a large fault current, which may trip circuit breakers immediately
This is common in land-based electrical grids.
Why are earth faults carefully monitored on ships?
If left unchecked, they can escalate to more serious faults, leading to equipment damage or fires
Effective monitoring is essential for safety.
What is an insulated neutral?
• In an insulated neutral system, the neutral point of the generator or transformer is not directly connected to the ship’s hull (earth).
• This means that if a single-phase earth fault occurs, the system continues to operate without immediate shutdown. Instead, an alarm is triggered.
• These systems are common on ships to ensure continuity of supply, as marine electrical systems must be reliable even in case of minor faults.
• However, if a second fault (another phase-to-earth fault) occurs before the first one is corrected, it can create a phase-to-phase short circuit, which is dangerous.
What is an earth fault? Chatgpt
Earth Fault
• An earth fault occurs when there is an unintended connection between a live conductor (phase) and the ship’s hull (earth).
• In an insulated neutral system, a single earth fault does not cause an immediate shutdown but triggers an earth fault monitoring alarm.
• In a solidly earthed system (common in land-based grids), an earth fault can cause a large fault current, which may trip circuit breakers immediately.
• On a ship, earth faults are carefully monitored because if left unchecked, they can escalate to more serious faults, leading to equipment damage or fires.
Difference in insulated neutral and earth fault?
Definition
Effect
Protection
Common in
Different systems that use an insulated neutral system and why?
Main switchboard
Emergency switchboard
Navigation and bridge equipment
Communication systems
Control systems
Hazardous zone systems( tankers )
Why Use Insulated Neutral Systems on Board?
• Avoid unnecessary trips due to a single earth fault.
• Maintain critical operations and safety systems.
• Easier to locate and rectify faults using insulation monitoring devices.
• Reduces risk of arcing and potential fires, especially in hazardous areas
What is used for an insulation resistance test? What does it check?
Megohmmeter
The output high voltage 250v-1000v
A megohmeter induces 250-1000v to stress insulation to check the resistance
You use when the circuit is completely dead
1megahom per 1000v
What is used for continuity test?
Multimeter
Checks if there is a complete path of current flow
Does not induce any electricity into circuit - no voltage or current
But does induce a little voltage when 1.5v-9v when checking resistance
Should only be used on deenergised circuits
