Advanced scientific principles Flashcards
What major changes occurred in the English electricity industry in 1990?
Privatisation of the 12 regional electricity boards
Which of the following organisations owns and operates power stations that generate electricity from various sources?
Generators
What is the primary role of the Transmission System Owner in the UK power system?
Owning and maintaining the high-voltage transmission system
How does the system operator balance electricity demand and generation output?
By asking generators to increase output
What is distributed generation (DG) in the power system context?
Generation connected to the distribution network
In a three-phase AC generator, how are the windings on the stator arranged to produce a three-phase output?
Windings are connected in a star configuration
What is the purpose of step-up transformers in the UK power system?
To increase the voltage for long-distance transmission
Which type of electricity generation method is known for its ability to produce electricity without significant transmission losses over long distances?
High Voltage Direct Current (HVDC)
What is the primary source of heat for most electricity generation methods in the UK?
Fossil fuels and nuclear fission
What is the purpose of combined heat and power (CHP) generation systems?
To maximize waste heat by using it for heating or production processes
What is the term for a chemical solution that contains many ions and is used in battery production?
Electrolyte
Which two companies took over the CEGB’s big coal-fired power stations in 1991 in the UK?
National Power and Powergen
In the UK, which organization is responsible for the transportation of electricity between power stations and from region to region?
National Grid
What type of electricity generation method uses the Sun’s heat energy transferred to fluids to produce electricity?
Solar thermal
Which type of electricity generation system captures waste heat or thermal energy for heating schemes or production processes?
Combined heat and power (CHP)
What is the primary purpose of the interconnectors between the UK and Europe?
To pool capacity and diversity of supplies
Which of the following is NOT a current interconnector between the UK and Europe?
750 MW between England and Ireland
What type of network carries electricity from primary substations to customers at low voltage levels in urban and rural areas?
Secondary distribution network
Which type of transformer is used to decrease the voltage from high voltage transmission systems to a more manageable level for local distribution?
Step-down transformer
What is the name for the network of circuits, overhead lines, underground cables, and substations that link power stations and distribute electricity around the country?
Power grid
Which of the following is NOT a common voltage supply system used for electricity distribution in work premises?
Two-phase AC
In the UK, where is direct current (DC) occasionally used for public electricity supplies?
Links between England, the Netherlands, Ireland, and France
What is the primary purpose of DC battery systems in modern installations?
To store excess electricity for vehicle charging points
Why is alternating current (AC) the distribution system of choice for electricity suppliers worldwide?
AC allows for a wider range of circuit arrangements and supply voltages.
In a single-phase AC supply, what are the two conductors commonly referred to as?
Line conductor and neutral conductor
What is the nominal supply voltage for a single-phase AC supply in the UK?
230 V
In a balanced three-phase system, why is there no need to have a star-point connection to neutral?
The phases have a cancellation effect on each other.
What is the primary advantage of a star (Y) connected load in a three-phase system?
It provides a balanced load across all phases.
In a delta (Δ) connected load in a three-phase system, how are the three coil windings connected to each other?
In a triangle or delta configuration.
What is the line current (IL) in a delta-connected load if the phase current (IP) is 100 A?
173.2 A
Which of the following statements is true regarding neutral conductors in a three-phase AC system?
The neutral conductor carries the same current as the line conductors.
In a star (Y) connected load in a three-phase system, what is the relationship between the line current (IL) and the phase current (IP)?
IL = IP
Which voltage configuration is commonly used for domestic premises and lighting within work premises in the UK?
Single-phase and neutral AC system
What is the primary advantage of using a multi-phase arrangement, such as three-phase, in industrial and commercial premises?
It allows for the use of very large conductors.
In a delta-connected load, if the phase voltage (VP) is 400 V, what is the line voltage (VL)?
400 V
What is the purpose of a star (Y) connected load in a three-phase system?
To ensure a balanced load across all phases.
In a balanced three-phase system, what happens to the star-point voltage naturally?
It remains at zero voltage.
Which conductor carries any out-of-balance current in a single-phase load within a three-phase system?
Neutral conductor (N)
What does the term “equilateral triangle” refer to when discussing neutral current in a three-phase system?
A triangle with sides representing phase currents.
In a delta-connected system, how many wires are used to obtain a three-phase supply?
Three wires
What is the purpose of Earth Fault Loop Impedance (Zs) verification?
To ensure the safety of electrical installations
Which elements are part of the earth fault current loop in a typical electrical system?
Protective conductors and earthing terminals
In TN-C-S systems, what is the typical maximum declared value for earth fault loop impedance external to the installation?
0.35Ω
In TN-S systems, how do earth fault loop impedances typically compare to TN-C-S systems?
They are higher
In TT systems, why do residual current devices (RCDs) need to be adopted for protection against electric shock?
Because they operate at lower earth fault currents
What is the fundamental principle of transformers?
Induction of magnetic flux in two coils
Which type of transformer is commonly used for transmitting electrical energy from power stations at high voltages?
Power transformer
What is the primary purpose of isolation transformers?
To provide electrical separation
Which type of transformer is often used in medical applications to prevent first-fault failures?
Isolation transformer
Which type of transformer is used to provide a physical neutral for power transformers with a delta-connected secondary?
Earthing transformer
what is the primary function of a step-down transformer in an electrical system?
To decrease voltage
In a step-up transformer, the number of turns in the secondary winding is _____ compared to the primary winding.
More
What is the primary function of an insulation transformer?
To provide electrical separation
Which type of transformer is commonly used for power transmission purposes due to its ability to transmit electricity over long distances efficiently?
Power transformer
What is the purpose of the magnetic core in a transformer?
To induce a magnetic field
In a current transformer, which winding has only one turn?
Primary winding
Which type of transformer is commonly used in domestic bathrooms to power shaver sockets safely?
Isolation transformer
What is the typical purpose of earthing transformers in electrical systems?
To provide a physical neutral for delta-connected transformers
Which type of transformer is used to measure high currents accurately and is commonly found in electrical metering applications?
Current transformer
What is the primary purpose of using thin, insulated laminations in transformer cores?
To reduce eddy currents
Eddy current loss in a transformer is primarily a form of which type of loss?
Core loss
What is hysteresis loss in a transformer core primarily related to?
Magnetization direction changes
What is the main cause of copper losses in a transformer?
Resistance in windings
How are copper losses in transformer windings typically calculated?
Using the formula I^2R
What is the primary function of an isolation transformer?
To provide electrical separation
Which type of transformer is commonly used in electrical metering applications to accurately measure high currents?
Current transformer
What type of inductance occurs when a primary winding induces a magnetic field that induces a current into a secondary winding?
Mutual inductance
What does the turns ratio of a transformer indicate?
The relationship between primary and secondary voltages
How is a transformer typically rated in terms of power?
In kilovolt-amperes (kVA)
What is the primary purpose of using laminations in transformer cores?
To reduce hysteresis loss
Which type of loss in a transformer core occurs due to the magnetic material’s need to be re-magnetized as the supply alternates direction?
Hysteresis loss
What is the primary cause of copper losses in a transformer?
Resistance in the windings
In transformers, what is the main factor contributing to core loss?
a) Eddy currents
Hysteresis loss
What is the primary function of an isolation transformer in electrical systems?
To provide electrical separation
Which type of transformer is commonly used to convert high voltage to low voltage for safe use in households?
Self-inductance
What does the turns ratio of a transformer determine?
The relationship between primary and secondary currents
How are copper losses in transformer windings related to the load current?
Quadratically proportional
In what unit is a transformer’s power rating typically expressed?
Kilovolt-amperes (kVA)
What is the primary function of a resistor in an AC circuit?
What is the primary function of a resistor in an AC circuit?
To restrict current flow proportionally to voltage
In a resistive AC circuit, what is the phase relationship between voltage and current?
In-phase (unity)
What types of loads in AC circuits are typically resistive?
Incandescent lamps and heaters
What does an inductor primarily introduce in an AC circuit?
Inductive reactance
In an ideal inductive circuit, how much does the current lag behind the voltage
90 degrees
What is the unit of inductance used to measure inductors in AC circuits?
Henry (H)
In a capacitive AC circuit, how does the current phase relationship differ from that in an inductive circuit?
Leading current
What is the primary effect of a capacitor in an AC circuit?
Leading current
How is capacitive reactance (XC) calculated in an AC circuit?
XC = 1 / (2πfC)
What is the primary unit used to measure capacitance in AC circuits?
Farad (F)
What is the primary factor that determines the reactance of an inductor in an AC circuit?
Frequency of the supply (f)
In an ideal capacitive circuit, what is the phase shift between current and voltage?
What is the unit of measurement for reactance in AC circuits, whether inductive or capacitive?
Ohm (Ω)
Which type of load is most commonly associated with power factor correction using capacitors?
Inductive loads
What is the primary purpose of a transformer in an electrical circuit?
To change voltage and current levels
When an AC voltage is applied to a transformer’s primary winding, what is induced in the secondary winding?
Voltage
What does the turns ratio of a transformer represent?
The ratio of primary to secondary windings
Which component in a transformer is responsible for reducing eddy current losses?
Core material
What is the unit used to rate the capacity of a transformer?
Volt-amperes (VA)
In what unit is capacitance typically measured?
Farad (F)
What is impedance in an electrical circuit?
The combination of resistance and reactance (inductive, capacitive, or both)
Which component causes a leading or lagging current in a circuit?
Both B and C
How is impedance (Z) calculated in a circuit?
Z = R² + X²
In a circuit with both inductance and capacitance, how is the resulting reactance calculated?
By subtracting the smaller reactance from the larger
What is the purpose of an impedance triangle in electrical circuits?
To visualize the relationship between resistance and reactance
In a series RL circuit, how does the voltage across the components change with respect to the current?
Voltage decreases across components
What is the phasor sum of voltages in a series RL circuit used to determine?
Impedance (Z)
In an RLC series circuit, when do the inductive and capacitive reactance cancel each other out?
When the circuit is purely resistive
When components are connected in parallel in an electrical circuit, what becomes the common component?
Voltage
What is the total current in a parallel circuit equal to?
The sum of currents through each component
In an RLC series circuit, what happens to the power factor when the inductive reactance and capacitive reactance cancel each other out?
The power factor becomes unity (1)
Which formula is used to calculate capacitive reactance (XC) in an AC circuit?
XC = 1 / (2πfC)
When components in a circuit are connected in series, what happens to the total resistance compared to individual component resistances?
It is equal to the sum of the individual resistances
In a parallel RL circuit, how does the total impedance (Z) compare to the individual impedances of the resistor and inductor?
It is the sum of the impedances of the resistor and inductor
What happens to the voltage across components connected in parallel in an AC circuit?
It remains constant
In an RLC series circuit, if the inductive reactance (XL) is greater than the capacitive reactance (XC), what is the overall phase relationship between current and voltage?
Current leads voltage
In a parallel RC circuit, what happens to the total current (I) compared to the individual currents through the resistor and capacitor?
It is equal to the sum of the individual currents
What is the primary purpose of constructing a phasor diagram in AC circuits?
To visualize the phase relationships between components
In a series RL circuit, if the inductance (L) increases while the resistance (R) remains constant, how does this affect the impedance (Z)?
Impedance increases
When components in a circuit are connected in parallel, how does this arrangement affect the total resistance compared to individual component resistances?
It decreases
In a phasor diagram for a parallel circuit, which component serves as the reference line?
Voltage (V)
In circuits with both series and parallel components, what is often the arrangement for inductance and resistance?
Series for inductance and parallel for resistance
What is the purpose of a capacitor in a circuit designed for power factor correction?
To offset reactive power and improve power factor
How is the apparent power related to true power and reactive power in a power triangle?
Apparent power is the sum of true power and reactive power
What does the power factor represent in an electrical circuit?
The cosine of the angle between voltage and current
In the example calculation provided, what is the initial power factor rating of the motor before correction?
0.4
What is the formula for calculating capacitive reactance (XC) in an AC circuit?
XC = 1 / (2πfC)
What is the significance of a power factor of 1 (unity) in a circuit?
It indicates a purely resistive circuit
In a series RL circuit, if the inductive reactance (XL) is greater than the resistance (R), what is the phase relationship between current and voltage?
Current leads voltage
What is the formula for calculating power factor (cosθ) in a circuit using resistance (R) and impedance (Z)?
cosθ = R / Z
In a phasor diagram for a series RL circuit, which component typically lags behind the other in terms of phase?
Inductance (L
When determining the impedance of a circuit with both inductive and capacitive components, what value is used for reactance (X)?
The difference between inductive and capacitive reactances
What is the unit of measurement for apparent power in a power triangle?
Volt-amperes (VA)
How does the angle between true power and apparent power change when power factor is improved?
The angle decreases
In a power triangle, what does the angle θ represent?
The phase angle between voltage and current
In an RL series circuit, if the impedance (Z) is greater than the resistance (R), what is the phase relationship between current and voltage?
Voltage leads current
What is the formula for calculating capacitive reactance (XC) in an AC circuit?
XC = 1 / (2πfC)
If the power factor of a circuit is 0.8, what is the angle by which the current lags or leads the voltage?
30 degrees
In a power triangle, what does the cosine of the angle (cosθ) represent?
Power factor
What is the power factor of a purely resistive circuit?
1 (unity)
What does the angle formed by the R and Z lines in an impedance triangle represent?
The angle by which the current leads or lags the voltage
How is the power factor defined in terms of the impedance triangle?
It is the cosine of the angle formed by the R and Z lines.
In the given circuit with components in series and in parallel, what is the impedance (Z) value?
30.14 Ω
What is the formula for capacitive reactance (XC) in the circuit?
XC = 1 / (2πfC)
What is the power factor of a circuit when the angle of lagging current is 66.55 degrees?
0.398
Why is power factor correction important in electrical installations?
It helps avoid overheating of equipment and cables.
What is a common method for power factor correction in electrical installations?
Using synchronous motors
How is power factor correction typically achieved using capacitors?
By adding capacitive kVAr to cancel out inductive kVAr
What is load profiling used for in electrical supply monitoring?
To monitor times of high demand
In a balanced three-phase system, what happens to the star-point voltage?
It is naturally at zero current.
In an impedance triangle, if the current leads the voltage, what type of circuit is typically present?
Inductive
How is capacitive reactance (XC) affected when the frequency of the AC power supply increases?
XC decreases
What is the formula for calculating apparent power (S) in an AC circuit?
S = VI
What is the angle between the voltage and current in a purely resistive circuit?
0 degrees (in-phase)
What is the primary purpose of power factor correction equipment in electrical systems?
To increase the power factor
Which type of equipment is rarely used today for power factor correction in large factories?
Synchronous motors
What does “kVAr” stand for in the context of power factor correction?
Kilovolt-Ampere Reactive
How is power factor correction typically achieved in fluorescent luminaires?
By connecting capacitors in parallel
What does load profiling help electrical systems anticipate?
Times of excessive consumption
In a balanced three-phase system, what is the relationship between the three phases?
They are 120 degrees out of phase.
What are the three basic features of DC machines?
Magnetic-field system, conductors, and commutator
What part of a DC machine is responsible for setting up the magnetic field?
Field windings
In a DC generator, what is the function of the commutator?
To reverse current flow in the armature
What type of current does a DC generator produce?
Direct current (DC)
What is the primary difference between series and shunt DC motors?
Shunt motors have a common current for field and armature.
Which type of DC motor is known for its ability to be used on both alternating current (AC) and direct current (DC)?
Universal motor
In a balanced three-phase star-connected system, if the line voltage (VL) is 480 V, what is the phase voltage (VP)?
240 V
Why is there no need for a star-point connection to neutral in a balanced three-phase star-connected system?
Because the current drawn by any one phase is taken out equally by the other two
In a delta (Δ) connected load, if the phase current (IP) is 50 A, what is the line current (IL)?
100√3 A
What is the primary advantage of using a delta-connected system in transmission?
Reduced line current
Which part of a DC machine is responsible for reversing the direction of current flow in the armature?
Commutator
What is the main advantage of DC generators over DC motors?
DC generators can convert mechanical energy into electrical energy.
What is the function of the armature in a DC motor?
To convert electrical energy into mechanical power
In a compound DC motor, what is the advantage of having both series and shunt field windings?
Better speed control
Which type of DC motor is commonly used in applications where high starting torque is required, such as electric drills?
Series motor
Which type of DC motor is best suited for applications requiring excellent torque characteristics and used in dragline excavators?
Series-wound motors
In which application is a shunt motor most suitable due to the requirement for constant speed and torque maintenance?
Production line conveyor systems
What advantage do compound motors offer over other types of DC motors?
The benefits of both series and shunt motors
What is a practical use of direct current generators?
Charging batteries
What is the main principle of operation for all AC motors?
Interaction between magnetic fields
What is the primary difference between a synchronous motor and an induction motor?
Induction motors operate at a precise speed.
How does an induction motor achieve torque and rotation?
By creating slip between rotor and stator fields
What is the main reason single-phase motors require additional components for starting?
The magnetic flux components are equal and opposite.
Which type of single-phase motor uses a start winding with a different resistance to create a phase shift?
Split-phase induction motor
In a capacitor-start motor, what is the purpose of the starting capacitor?
To create a phase shift in the circuit
Where are shaded-pole motors commonly used due to their low starting torque?
Electric fans
Which type of motor is used in power tools and household appliances like food blenders?
Universal motor
How Motor Starters Operate
Switching the motor on and off
When is a star-delta starter typically used for three-phase motors?
When high starting torque is required
In which applications are AC generators or alternators commonly used?
Hybrid electrical vehicle drives
Why are cage induction motors preferred over wound rotor induction motors in many applications?
They have no brushes or slip rings.
What is the primary advantage of using a star-delta starter for three-phase motors?
Soft start to limit mechanical stress
Which component provides undervoltage protection in a Direct Online (DOL) starter?
Contactor coil
Miscellaneous Concepts
To control the speed of the motor
When is it necessary to switch a motor’s field pole pairs in and out?
To achieve synchronous speed
What is the primary function of a rotor resistance starter?
What is the primary function of a rotor resistance starter?
To reduce startup currents in the rotor of a wound rotor induction motor.
Which type of motor is suitable for a rotor resistance starter?
Wound rotor induction motor.
How do electronic motor starters achieve a soft start for AC motors during startup?
By reducing the motor’s voltage initially and then gradually increasing it.
What is the primary advantage of using variable frequency drives (VFDs) with AC induction motors?
To save energy and money by adjusting motor speed and torque.
Why should measures be taken to prevent continued reversal in motors using reverse-current braking?
To ensure safety, as continued reversal can be dangerous.
What is the purpose of protective devices like fuses and circuit breakers in electrical circuits?
To protect against overcurrent and short circuits.
What is the primary function of a relay in an electrical circuit?
To mechanically switch a set of contacts based on an electrical signal.
How do contactors differ from relays?
Contactors are used for switching lower current loads, while relays can handle higher current loads.
What is the main purpose of a fuse in an electrical circuit?
To protect against overcurrent.
How do residual current devices (RCDs) operate?
RCDs monitor the current in both line and neutral conductors and trip when an imbalance occurs.
Which of the following best describes the main function of a fuse in an electrical circuit?
To interrupt the circuit when the current exceeds a safe limit.
What is the purpose of a breaking capacity (kA) rating for a fuse?
To define the maximum fault current the fuse can safely interrupt.
What is the primary difference between miniature circuit breakers (MCBs) and moulded-case circuit breakers (MCCBs)?
MCBs are used for final circuits, while MCCBs are used for distribution circuits.
How do thermal and magnetic trip mechanisms work in a circuit breaker?
Thermal trip responds to overcurrent by bending a bimetallic strip, while magnetic trip responds to short circuits with rapid switching.
What is the primary function of an RCD in an electrical circuit?
To detect and trip in case of an earth fault.
How do RCBOs differ from standalone RCDs?
RCBOs combine overcurrent protection with earth fault protection in a single device.
What is the primary advantage of using solid-state relays over electromechanical relays?
Solid-state relays have no moving parts, increasing reliability.
When should rotor resistance starters be used?
When dealing with heavy loads.
What is the primary function of solenoids in electrical systems?
To convert electrical signals into mechanical action
Solenoids are commonly used in electromagnetic locking devices for what purpose?
To engage the locking mechanism
What safety feature is typically associated with solenoids used in locking mechanisms in the event of an electrical circuit failure?
Safe drop position
When is the term “contactor” more accurately used instead of “relay”?
When describing a device that controls both high-current and low-current circuits
In the context of motor starters like the direct online (DOL) starter, what role does the contactor play?
Operating the start and stop controls
What is the primary purpose of BS 3036 rewirable fuses?
To act as overcurrent protection devices
What factor is applied to BS 3036 fuses due to their lack of reliability in certain conditions?
Correction factor (Cf)
What is the main advantage of BS 88 fuses with high-rupturing capacity (HRC) or high-breaking capacity (HBC)?
They can handle high prospective fault currents
What are the three common types of Miniature Circuit Breakers (MCBs) based on their trip characteristics?
Type A, Type B, and Type C
Which type of MCB is typically used for industrial applications where there are large inrush currents, such as industrial motors and welding equipment?
Type D
What is the primary purpose of Residual Current Devices (RCDs) in electrical systems?
To provide protection against electric shock
When are Arc Fault Detection Devices (AFDDs) required to be installed according to BS 7671: 2018 (2022)?
In high-risk residential buildings, houses of multiple occupation (HMOs), purpose-built student accommodation, and care homes
What is the purpose of the inverse square law in lighting calculations?
To calculate the illuminance on a surface at varying distances from a light source
What does the cosine law of illumination take into account when calculating illumination on surfaces?
The additional area illuminated when light falls obliquely on a surface
What is the lumen method used for in lighting design?
To find the number of luminaires needed for a specific average illuminance level
What best describes the function of a relay in an electrical circuit?
Controls one circuit with another circuit
In the context of electrical components, what is the primary function of a solenoid valve,
Controls the flow of fluids in a system
What does the coefficient of utilization (Uf) take into account when designing lighting systems?
The reflectivity of room surfaces
When designing a lighting system, why is it important to consider the space-height ratio?
To determine the number of luminaires required
What is the unit of measurement for luminous intensity?
Candela (cd)
Which type of circuit breaker is suitable for protecting circuits with inductive loads like motors or fluorescent luminaires?
Type C MCB
In the lumen method, what is used to quantify the efficiency of a lamp or luminaire?
Efficacy
What is the primary function of Residual Current Circuit Breakers with Overload (RCBOs) in electrical systems?
To protect against electrical shock and overloads
When are Arc Fault Detection Devices (AFDDs) recommended for installation according to BS 7671: 2018 (2022)?
In high-risk residential buildings and care homes
Which term describes the total amount of light emitted from a light source?
Luminous flux
What is the primary purpose of luminaires in lighting systems?
To produce light
In the context of lighting design, what is the space-height ratio used for?
To decide the number of luminaires needed
If the space-height ratio (Sr) is 2:1 and the mounting height (H) is 3 meters, what would be the spacing between luminaires (S)?
6 meters
What does the term “efficacy” describe in lighting systems?
The energy efficiency of a lamp in producing light
Why is it important to consider the color rendering index (CRI) when selecting luminaires for specific applications?
To ensure objects appear in their true colors
Which type of lamp is known for producing heat as an unintended byproduct?
Incandescent lamp
How does convection contribute to heating in convection panel heaters?
By circulating warm air
Which type of heating system stores heat during off-peak hours and releases it gradually throughout the day?
Storage heaters
What is the primary function of an immersion heater in a hot water system?
To heat the water directly in a vessel
What does the efficacy of a lamp indicate?
The lamp’s energy efficiency in producing light
What is the main purpose of room thermostats in heating systems?
To maintain a consistent room temperature
How do programmable time controllers enhance heating system efficiency?
By reducing energy usage during unoccupied hours
What technology allows homeowners to control heating systems remotely through smartphones and voice commands?
Home automation
Which of the following factors does the space–height ratio (Sr) depend on?
Manufacturer’s specifications
In what situations is good color rendering particularly important when selecting luminaires?
Street lighting
Which type of lamp works by passing a current through a filament to produce light?
Incandescent lamp
What gas is typically added to a tungsten-halogen lamp to prevent filament evaporation?
Argon
How do radiant heaters primarily heat spaces?
By emitting electromagnetic waves
Which type of heating system charges up at night when energy is cheaper and releases heat gradually during the day?
Storage heaters
What is the primary function of a room thermostat in a heating system?
To maintain a consistent room temperature
What devices is used to adjust heating schedules based on occupancy patterns?
Time switch
What technology allows homeowners to control heating systems and lighting remotely using voice commands?
Wi-Fi
