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问题 1 从 45
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A ship has a 3 phase electrical network with a neutral point earthed through a 200 Ω resistor. The generator phase voltage is 3 000 V and the synchronous impedance of the generator is 0,1 Ω. What will be the potential fault current if an earth fault occurs on the generator output cable?
15 Amps.
1 500 Amps.
3 000 Amps.
30 000 Amps.
A ship has a 3 phase electrical network with star connected generators and an earthed neutral point. The network line voltage is 440 V and phase voltage 254 V. If an earth fault occurs on one phase through a circuit of 1 Ω impedance what will be the potential fault current?
25,4 Amps.
0,004 Amps.
440 Amps.
254 Amps.
Earth leakage circuit breakers are commonly used ashore to protect people from earth faults. Which of the following best describes why earth leakage circuit breakers have not been commonly used on ship’s electrical networks in the past?
Continuity of supply is preferred.
Earth leakage circuit breakers are expensive.
Earth leakage circuit breakers are not reliable.
Earth leakage circuit breakers do not work on insulated neutral networks.
Even on a large passenger ship which generates electricity at 6 000 Volts lighting is normally supplied at voltages of 220 Volts or less. Which one of the following best describes why lighting is not supplied at a higher voltage?
Lighting switches are operated by people with no electrical training.
Electrical loads for lighting are low enough to allow lower voltages to be used without excessive cable losses.
Light fittings are not available for electrical supplies over 220 Volts.
High voltage lights have an annoying flicker.
High voltage ships tend to use earthed neutral electrical networks. Which one of the following best describes how excessive fault currents are prevented in the event of an earth fault on these ships?
An impedance is placed in the neutral earthing line.
The risk of earth faults is reduced by extra planned maintenance.
Earth leakage circuit breakers are used on every circuit.
An impedance is placed in each output cable from the generators.
If the earth lamps on your ship looked as shown which of the following would you assume was the most likely problem?
An earth fault on phase “S”.
An earth fault on phase “R”.
An earth fault on phase “T”.
An faulty lamp on phase “S”.
On shore it is normal practice to connect all exposed metalwork and the neutral point of the supply to earth. Which of the following best describes what normally happens on shore when an earth fault occurs?
A small current flows causing an earth alarm to sound.
A large current flows causing an earth alarm to sound.
A large current flows causing the overcurrent protection to disconnect the supply.
A very small current flows until the overcurrent protection disconnects the supply.
On the main switchboard cabinet for each generator there are normally instruments fitted. Which one of the following best describes the purpose of the 4 position switch often found beneath the generator voltmeter?
It is to check the balance of voltage between the generator’s phases.
It is to turn off the meter as it is not rated for continuous use.
It is to allow one meter to read the voltage of 4 different generators.
It is to allow one meter to read the voltage of 2 different generators.
The kilowatt load on the ship’s only running generator is steady but the power factor meter shows a fall from 0,9 to 0,75. Which one of the following best describes the risk that this change in power factor poses for the ship’s electrical network?
The fuel consumption of the generator will be increased and fuel could run out before arrival in port.
The increase in apparent power may cause an overcurrent trip to operate.
The decrease in reactive power could cause supply cables to get hotter.
The galley stove might burn out.
When a conductor is moved through a magnetic field an EMF is produced. Which one of the following is a way to increase the value of EMF produced?
Move the conductor more quickly.
Move the conductor in circles.
Change the conductor for one with a lower resistance.
Change the conductor for one with a lower resistivity.
When talking about electrical power generation on ships we often refer to the “prime mover”. Which one of the following best describes a “prime mover”?
The rotor of the main generator.
The rotor of the exciter for the main generator.
The pump that supplies fuel during the starting of a generator after a total blackout.
The device where the energy is first converted to mechanical energy.
Which of the following best describes what the result would be if earth faults occurred on 2 different phases of a ship’s 3 phase distribution system at the same time?
A short circuit.
Two earth fault alarms sound.
Two earth faults cannot occur at the same time.
A long circuit.
Which of the following best describes why the risk of serious electric shock is higher on board ships than it is ashore?
A ship’s crew are not as well trained as people ashore.
The large amount of metal can provide a low resistance path to earth.
Voltages used on ships are higher than those used ashore.
Ship’s tend to use an insulated neutral distribution system.
Which one of the following best describes how someone on a ship could be killed by electricity while just walking around the engine room?
By touching metalwork which has becomes live as a result of damaged insulation.
By walking on metal floor plates without shoes.
By walking on metal floor plates with rubber soled shoes.
By touching metalwork such as starter boxes.
Which one of the following best describes the fundamental difference in the typical construction of a single phase generator and a 3 phase electrical generator?
A three phase generator has 2 more rotors than a single phase generator.
A three phase generator has 2 more pairs of stator coils spaced 120° apart than a single phase generator.
A three phase generator has a commutator and a single phase generator does not.
A three phase generator has 2 more pairs of rotor coils than a single phase generator.
Which one of the following best describes the general principle used when a designer is deciding the positions of automatic disconnection devices within a ship’s electrical network?
The lower “branches” of the network “tree” must be protected against a fault on the higher parts.
The higher parts of the network “tree” must be protected against a fault on the lower “branches”.
Every cable must be protected at the point where power is supplied to it.
Every load must be protected at the point where power is supplied to it.
Which one of the following best describes the main consideration when a designer selects the voltage level for a particular area of a ship’s electrical network?
The maximum voltage level that the equipment can withstand.
The maximum voltage level available from a ship’s generator.
The minimum voltage level available from a ship’s generator.
The frequency of that area of the network.
Which one of the following best describes the main purpose if fitting reverse power protection to an electrical generator’s circuit breaker?
To prevent the generator being run backwards.
To prevent the generator circuit breaker being closed if the generator output cables are incorrectly connected.
To prevent the generator driving the prime mover.
To prevent circulating currents between 2 or more generators.
Which one of the following best describes the main purpose of fitting under voltage protection to an electrical generator’s circuit breaker?
It prevents the connection of a “dead” generator to a “live” switchboard.
It prevents the synchronisation of 2 generators with different output voltages.
It prevents overcurrent’s being taken from a generator due to low voltage.
It corrects the output voltage of a generator after a heavy load has started.
Which one of the following best describes the main reason why ships with high voltage electrical networks do not usually have a floating neutral system?
High voltage equipment is not designed for floating neutral systems.
It tends to produce voltage stresses on the system cables.
It is essential to use earth leakage circuit breakers on high voltage supplies.
High voltage networks on ships just copy shore practice.
Which one of the following best describes the meaning of “discrimination” when applied to electrical distribution systems on ships?
Protection devices are to “discriminate” between high voltage circuits and low voltage circuits, shutting off only the low voltage circuits.
Protection devices are to “discriminate” between healthy circuits and circuits with faults, shutting off only the faulty circuits.
Colour coding is used to “discriminate” between cables with flame retardancy and those without.
Colour coding is used to “discriminate” between cables made with PVC and those of other materials.
Which one of the following best describes the need for cooling of 3 phase electrical transformers?
They need cooling due to their internal iron and copper losses.
They need cooling due to their moving parts.
They need cooling due to the magnetic interaction between the 3 coils.
They only need cooling when the ambient temperatures go above 50 °C.
Which one of the following best describes the primary function of a ship’s electrical distribution system?
To ensure that at any point in the system there is always some electricity available.
To ensure that at any given point in the system there is always a voltage present.
To ensure that the voltage, current and frequency are always the same at any point in the system.
To ensure that at any given point in the system the voltage and frequency are of almost rated value.
Which one of the following best describes why few merchant ships have been fitted with nuclear electrical power generation despite the increasing price of oil fuels?
Nuclear powered ships find it hard to meet SOLAS requirements.
Nuclear ships need regular supplies of fuel which is hard to obtain in some countries.
Nuclear ships do not contain high value materials and fetch a low price when scrapped.
The generation plant is quite big and very costly.
Which one of the following best describes why it is preferable to use electromagnets when constructing an electrical generator, rather than permanent magnets?
We can control the amount of flux that electromagnets produce.
The electromagnets are bigger and therefore more robust.
The electromagnets are easier to shape.
We need conductors around the magnets to carry the generator output.
Which one of the following best describes why many electrical transformers are not fitted with overcurrent protection on the secondary circuit output?
The output from the secondary circuit is limited by the impedance of the coil.
The output from the secondary circuit is limited by the magnetic inertia of the transformer core.
The output from the secondary circuit is fixed in relation to the input to the transformer so it is protected by the overcurrent trip on the supply.
No overcurrent protection on the secondary circuit output is used when the output is taken from more than one voltage tapping.
Which one of the following best describes why many modern ships are fitted with a computerised Power Management System (PMS)?
It can reduce crew workload and allow more precise control of the network.
A PMS has been a requirement of SOLAS since 1995 for ships over 1 600 GT.
SOLAS requires ships with electrical main propulsion have a PMS fitted.
A PMS must be fitted where ships have 3 or more generators.
Which one of the following best describes why most ships have 3 or more generators which can provide electrical power?
SOLAS requires at least 3 generators to be provided.
Three smaller generators are usually cheaper than 2 large ones.
Three generators allow better balance of the ship and reduced permanent list.
It provides greater reliability of supply and allows more efficient operation.
Which one of the following best describes why physical forces can be created from the interaction of 2 magnets?
The magnets move to make the lines of magnetic flux as short as possible.
The magnets move because of an electrical charge stored within them.
The magnets move to even out the flux density.
The magnets move if the lines of magnetic flux are forced to cross.
Which one of the following best describes why the frequencies of 3 phase electrical generators being connected together must be “in phase”?
So that the firing of No. 1 cylinders on each driving diesel engine coincide.
So that the output voltage peaks coincide.
So that the current peaks coincide.
So that the frequency peaks coincide.
Which one of the following best describes why the overall system power factor is important when operating electrical generators on a ship?
It determines the total current that the generators must produce for a given active power.
It determines the kilowatt load on the generators.
It determines the fuel consumption of generators.
The overall system power factor cannot safely exceed 0,95.
Which one of the following best describes why the overall system power factors tends to change while operating electrical generators on a ship?
It increases as the generator load approaches 100 % of capacity.
It decreases as the generator load approaches 100 % of capacity.
It is the sum of the power factors of all the individual loads and these may vary.
It varies if the ship hits bad weather and pitches heavily.
Which one of the following best describes why we do not normally run generators that are supplying the ship’s main switchboard at their maximum continuous output where their fuel efficiency is usually best?
There needs to be enough reserve capacity in the generating system to deal with unexpected load increases.
Fuel efficiency of generators is unimportant compared to that of the main engine.
Starting and stopping an extra generator to supply short term increases in load uses more fuel than running it all the time.
The generators on ships are rarely able to run at maximum continuous output without breaking down.
Which one of the following describes when the under voltage trip in the starter panel of an electric motor is most likely to operate?
After a large electrical load is started nearby in the network.
As the motor is started.
During an electrical black out.
If the motor becomes seized.
Which one of the following describes where a “bus tie” might be found on a ship with a high electrical power demand?
In the main switchboard near the ends of the busbars.
In any distribution board which is subject to high vibration.
In the main switchboard near the centre of the busbars.
In any starter panel which is subject to high vibration.
Which one of the following options best describes how a single device can protect against fast and slow overcurrent faults in a ship’s distribution system?
An inverse time/current characteristic is needed.
Only a fuse can protect against both types of faults.
Two separate current sensors are fitted into the single protection device.
A single device cannot protect against both types of faults.
Which one of the following options best describes the advantage industries ashore have over ships when using electrical power?
They can rely on a supply with voltage and frequency well controlled and redundancy provided.
Their supply is close and responsive.
They can select any voltage and frequency of supply they like.
They can call in specialist electricians if they have a problem.
Which one of the following options best describes the reason for a ship’s distribution system being arranged for preferential tripping?
Low power loads are tripped in preference to high power loads.
Non-essential loads are arranged to trip in order to prevent the main generator from tripping.
Low voltage loads are tripped in preference to high voltage loads.
Final sub circuits are tripped in preference to those higher in the distribution “tree”.
Which one of the following options best describes what determines how much mechanical power the prime mover connected to an electrical generator must produce?
Enough to meet the apparent power demand and overcome mechanical losses such as friction.
Enough to meet the active power demand and overcome mechanical losses such as friction.
Enough to meet the reactive power demand and overcome mechanical losses such as friction.
Enough to meet the active power demand and overcome electrical losses such as hysteresis.
Which one of the following options best describes what will happen if the voltages of the main switchboard and incoming generator are not the same at the time of connecting the generator?
Large voltages will flow until the currents become equal.
The generator under voltage trip will operate.
The generator frequency will be forced to match the switchboard frequency.
Large currents will flow until the voltages become equal.
Which one of the following options best describes what would happen to the terminal voltage of an AC generator following a typical major load increase if no AVR was fitted?
The higher current through the generator windings will result in a fall in terminal voltage.
The higher current through the generator windings will result in a rise in terminal voltage.
The terminal voltage will not change if the generator is well designed.
The terminal voltage will rise or fall, depending on the amount of field compounding.
Which one of the following options best describes why batteries, using chemical generation, are not currently used for the main source of electricity on ships?
Batteries cannot store much energy.
Batteries are expensive to buy in the quantities required.
Batteries produce a low cell voltage.
Batteries produce DC electricity.
Which one of the following options best describes why controlling the electrical network on a ship can be more challenging than controlling a larger network ashore?
Ship’s electrical networks are subject to the movement of the ship.
Ship’s electrical networks are subject to proportionately larger variations in load demand.
Ship’s electrical networks are more complex than a network ashore.
Ship’s electrical networks use more different voltages than a typical network ashore.
Which one of the following options best describes why each feed normally leaves the section boards in a ship’s electrical network via some form of isolation?
To allow motors to be stopped remotely.
To allow for maintenance on the circuit.
To allow lights to be switched off remotely.
To protect against slow overloads.
Which one of the following options best describes why most electric motors, driving engine room pumps, are usually supplied at around 440 Volts, even if the ship uses electricity at higher voltages for the bow thruster?
The size of motors normally required for engine room pumps are only available for 440 Volt supplies.
The size of motors normally required for bow thrusters are only available for high voltage supplies.
Using higher voltages for the pump motors would require cables too thick to bend within the space available in the engine room.
They tend to be near the switchboard and therefore the supply cable losses are low.
Тест проверяет знания об электрических сетях на судне, их устройстве, работе и обслуживании, включая поиск и устранение неисправностей.
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