Electrical Flashcards
Megger testing? Minimum values? Where is it listed?
Used to measure the insulation resistance of electrical equipment. It involves applying a high voltage between the conductors and the ground and measuring the resistance to current flow.
Min 1 megga ohm
The Institute of Electrical and Electronics Engineers (IEEE)
IEC (nternational Electrotechnical Commission standards
Describe a megger test of your main alternators?
- Disconnect the main alternators
- Connect the terminals, one the the alternator winding, one to the alternator frame/ ground
- Apply the test voltage
- Record insulation resistance reading
- Compare to guidelines
What is phasing?
In three-phase power systems, there are three alternating currents (AC) that are 120 degrees out of phase with each other. Each phase is represented by a sinusoidal waveform, and they are labeled as phase A, phase B, and phase C.
What classification requirements are there for electrics?
- Compliance with international standards such as IEC (International Electrotechnical Commission) 60092 series, which covers electrical installations in ships
- Adequate insulation and protection against electrical shock hazards
- Protection against overcurrent, short circuits, and overvoltage conditions to prevent equipment damage and ensure system safety
- Requirements for emergency power systems
- Implementation of explosion-proof, flameproof, or intrinsically safe electrical equipment
How to megger test a motor?
- Disconnect the motor
- Connect the terminals, one the the motor winding, one to the motor frame/ ground
- Apply the test voltage
- Record insulation resistance reading
- Compare to guidelines
what types of earthing systems are fitted onboard a vessel?
- Return Earth
- Grounding to ships hull
Earth fault alarms?
Earth fault alarms continuously monitor the electrical circuit for any imbalance in current flow between the phases and the earth. It then alarms once above the limit. Onboard ours are set at 7amps and high high is 10amps.
Describe some common electrical faults?
- Short circuit
- Earth fault
- Overload
- Corrosion
- Blown fuses
- Connections vibrating loose
- Water
How would you test for common electrical faults them?
- Visual inspections
- Insulation resistance testing with a meggar tester
- Continuity test with multimeter
- Earth fault half test
- Load testing equipment
- Thermal imagining for hot spots
How would you know you have an earth fault?
Onboard we have alarms and a reading of how big our earth leak is. Some vessels will have earth fault lights that show there is a fault.
In some cases the breaker may trip, if this repeats its self multiple times, this could indicate an earth fault.
Describe your main electrical system?
Onboard we have the MSB and ESB. Our vessels can take upto 318amp, 400v and 50hzs via shore power
Our generators are 250kw, 400v, 50hz, 450amp each, we have 2.
The emergency generator 106 kW
What purpose does the neutral leg have in a 4 wire system?
In a 3-phase 4-wire system, which typically consists of three hot wires (phases) and one neutral wire, the neutral helps balance the load between the phases. this providing a factor of safety.
How is an appliance earthed with a 3 phase 4 wire system?
In addition to the three phase wires (L1, L2, L3) and the neutral wire (N), there is often a separate earth (ground) wire. This earth wire is connected to the metal parts of the appliance and to the ground connection in the electrical panel or distribution board.
Why do we use an isolation transformer?
It will isolate the vessels 400v power supply from the shore powers 400v supply. This removing any risk of galvanic corrosion occurring
What is the acceptable limit for ground faults?
Onboard moonsand we have two alarm limits, high is set at 7amps and high high is at 10amps. But we keep a good practice of trying to keep it at zero or as close to it as possible.
What is a continuity test?
A continuity test is a basic electrical test used to determine if there is a complete (unbroken) path for current to flow in a circuit or component. It checks whether an electrical circuit can carry current by measuring the resistance between two points.
What electrical maintenance do we carry out on board?
- Visual inspections
- Checking connections
- Cleaning cabinet filters
- Checking calibration
- Testing preferential trips, reverse power, over speed, under and over voltage
- Battery tests
- Inspect and cleaning windings
- Lighting inspections
- AVIT inspections
- Alarm monitoring
- Ground faults
How would you trace an earth fault?
- Isolate a distribution board and see if it disappears
- bring everything on the board back online one at a time till it returns
- Split that system in half, repeating till the component is found
- Rectify the problem
- Record findings into PMS
Why is it important to fix earth faults quickly?
- Risk of electric shock
- Fire risk
- Preventing damage
- Maintaining system reliability and performance
How to test reverse power trip and why?
Reverse power trip can be tested by increasing the load on one generator and decreasing on the other until the generator starts to become a motor
What is the load of your bowthruster motor, how is it run?
150kw, 400v, 50hz by a slip ring motor/ squirrel cage motor. A Variable frequency drive is installed to allow the motor to gradually increase and decrees
How does a frequency drive work? How do you calculate frequency?
An electronic device that controls the speed and torque of an electric motor by varying the frequency and voltage supplied to the motor.
- AC converts to DC power using the rectifier
- Capacitors then smooth out the DC power
- The smoothed DC is then converted back to AC power with a variable frequency and voltage using an inverter
- The control circuit then adjusts the values as needed
- The motor will now spin at a speed until changed
Frequency Counter: Use a frequency counter, an instrument that directly measures the frequency of an electrical signal. This is the most straightforward method and provides an accurate reading.
Safety devices fitted to bow thruster / board?
- Circuit breaker
- Thermal overload relays
- Temperature sensors
- Oil level sensors
- Interlocking systems
- Emergency stop
- Ground fault protection
Direct online starter / Star Delta start, what are the safeties, how do they work?
DOC
- Circuit breakers
- Thermal overload relays
- Emergency stop
- Under voltage protection
When the start button is pressed, the control circuit energizes the contactor, closing the main contacts and supplying full voltage to the motor. Safety devices like thermal overload relays continuously monitor the current and temperature.
Star-Delta Starter
- Time relay
- Circuit breaker
- Thermal overload Rely
- Interlock system
- Emergency stop
- Under voltage protection
When the start button is pressed, the control circuit energizes the star contactor, connecting the motor windings in star configuration, which reduces the voltage and current. After a preset time, the timer relay switches to delta configuration by de-energizing the star contactor and energizing the delta contactor, applying full voltage to the motor.
Drawing of ammeter on one phase, what is it? How does it work?
see one note
Want to check Shaft earth system, what would happen if it wasn’t working, potential faults etc?
If it wasn’t working there is a possibility that the current being produced by the propeller will result in spark corrosion on the shaft. also know as galvanic corrosion.
The faults could be caused by
- Brush wear
- Poor electrical contact
- Corrosion and fouling
- Broken earth cable
Minimum requirement for Megger test?
1 meggar ohm
How to find pump ground fault?
- Lockout system
- Inspect for damage and connections
- Isolate the motor
- Conduct meggar test
- Conduct continuity test
- Check control circuit
- Isolate sections of the circuit until found
What causes the ground fault?
- Insulation breakdown
- Salt
- Vibration
Earth fault, how to find?
- Lockout system
- Inspect for damage and connections
- Isolate the motor
- Conduct meggar test
- Conduct continuity test
- Check control circuit
- Isolate sections of the circuit until found
Other types of test on an electrical motor?
- Megger test
- Dielectric Absorption test - To assess the quality of the motor’s insulation over time by apply a DC voltage for a prolonged period and measure the resistance at different intervals.
- High-Potential (Hi-Pot) Test - To check the insulation strength by applying a high voltage by apply a voltage much higher than the motor’s operating voltage for a short duration and check for insulation breakdown.
- Winding Resistance Test - To measure the resistance of the motor windings by use a precision ohmmeter to measure the resistance of each phase winding. Compare with manufacturer specifications to detect issues like short circuits or imbalances.
What information would you give when requesting shore power?
Voltage, Amps, Herts, Plug connections, Where connections are
What if you were 50hz and supplied with 60hz?
- Equipment will run at faster speeds
- Have reduced torque
- Nosier due to increase speeds
- Increased temperatures/ overheating
- Possible malfunctions
How do you check phase rotation?
By connecting the terminals to a phase rotation tester will be able to confirm if the rotation is correct
What is an AVR and how does it work?
Automatic Voltage Regulator - Their purpose is to regulate and stabilize the output voltage of the generator within its limits. This is due to when load is increased, voltage will drop so the AVR will measure this drop and in response increase the excitation.
The earth fault would not go after finding?
You have a second earth fault
Why does a synchroscope go clockwise?
A visual indication to the operator that the generator’s speed is leading the grid frequency ( Going faster)
If the online gen is at 1500rpm, would the incoming gen be faster or slower? Why?
It would be slightly faster because when the load is applied it will dip slightly while the governor reacts to increase the speed to 1500
A motor won’t start, everything points to the motor, what would you do?
- Visual inspection of the power supply, wiring and motor
- Check voltage, continutiy and capacitor is appliacalbe
- Rotate shaft manually
- No mechanical obstructions
- Check starter circuit
- Check overloads and trips
- Perform meager test
How does star delta work and components?
When the start button is pressed, the control circuit energizes the star contactor, connecting the motor windings in star configuration, which reduces the voltage and current. After a preset time, the timer relay switches to delta configuration by de-energizing the star contactor and energizing the delta contactor, applying full voltage to the motor.
Tag out procedure for electrical?
Follow our onboard SMS electrical checklist procedure
- Inform affected personnel
- Stop and secure equipment
- Locate and lock power feed
- Install lock and tag
- Engineer witness lockout
- Reverse for un-isolating
Switchboard safety devices?
- Circuit breakers
- Residual Current Devices (RCD)
- Earth fault
- Fuses
- Surge protection
- Thermal overloads
- Reverse power
- Preferential trips
- Over and under voltage
- Synchronizes
- Emergency stops
How do we get 220v for lighting systems from 400v supply?
Step down transformers
Set points for reverse power trips?
Around 20% of its full load
Motor protection devices?
- Thermal overloads
- Circuit breakers
- Motor protection relays
- Phase failure relays
- Earth faults
- Surge protection
- Unload and overload protection
- Temperature sensors
Singe phasing and how to fault find?
Single phasing refers to a condition in a three-phase electrical system where one of the phases is lost or disconnected, leaving the motor or equipment operating with only two phases instead of three. This condition can lead to several issues, including reduced motor torque, overheating, increased current draw, and mechanical stress on the motor.
Use a multimeter to measure voltage across each phase of the motor. Compare the voltage readings for all three phases.
What is 3 phase and what are its benefits?
Three-phase power is a type of electrical power transmission and distribution system that utilizes three alternating currents (phases) of equal magnitude and frequency, with a phase difference of 120 degrees between them.
- Increased power capacity
- Balanced load distribution
- Smoother power delivery
- Efficient power transition
- Motor performance
- Better harmonic profile
Motor components?
- Stator
- Rotor
- Windings
- Bearings
- Shaft
- Housing
- End covers
- Fan
- Terminal box
- Name plate
Motor starting arrangements?
- DOL
The motor is directly connected to the power supply (typically through a contactor) without any additional starting devices. This arrangement is suitable for small to medium-sized motors with low starting torque requirements. - Star/ Delta Starter
The motor windings are initially connected in a star configuration for starting, and then switched to a delta configuration once the motor reaches a certain speed. This arrangement reduces the starting current and torque. - Soft starter
A soft starter is a solid-state device that gradually ramps up the voltage supplied to the motor during starting, reducing the starting current and torque. It provides smooth acceleration and minimizes mechanical stress on the motor and connected equipment. - VFD
A VFD controls the speed and torque of the motor by varying the frequency and voltage of the power supply. It allows for precise control of motor speed, acceleration, and deceleration, as well as energy savings and improved efficiency.
What can be damaged caused by reverse power trip failing to operate?
- Mechanical damage to the generator
- Rotor damage
- Overheating
Most likely places to find earth faults?
- Junction boxes
- Motors
- Transformers
- Sacrifical anodes
- Navigation lights
- Winches
- Crane remotes
How does a thermocouple work?
If two dissimilar metal wires are joined together to form a closed circuit and one junction is maintained at a higher temperature than the other, then a small e.m.f. is generated and a milliamp current will flow through the wires. The size of the current is directly related to the difference in temperature between the two junctions.
