The Power dissipated in a resistor R by a current (I) passing through it is given a P=IV, where V is the potential difference across the resistor. This equation can be expressed in a number of alternative ways by substitutions using
R= V/I
Power = ( Current) I x Voltage (V)
Power Formulae
1. (Power) P=IxV
2. (Power) P= V^2/R
3. (Power) P= I^(2 )R
As (Current) I= V/R replacing I for V/R gives
P=IV = V/R x V
P= V^2/R
As (Voltage) V=IR replacing V for (IR) gives
P=IV =I x (IR)
P=I^(2 )R
Substituting,
You can work out for resistance when you have the Power dissipated and Voltage supply using power formulae,
Since, (Power) P= V^2/R
Therefore, (Resistance) R= V^2/P
You can also work out for the resistance when you have the Power dissipated and the Current, using power formulae,
Since, (Power) P=I^(2 )R
Therefore, (Resistance) R=P/I^2
Saturday, 17 March 2012
Sunday, 22 November 2009
Health and Safety Revision Questions
1. A sign displaying a white pictogram on a circular blue background is a:
a. safety sign
b. mandatory sign *
c. warning sign
d. prohibition sign
2. A person who represents other workers on safety matters in an organisation is called a:
a. safety officer
b. safety representative *
c. Health care representative
d. health and safety officer
3. The Control Waste Regulations require that hazardous substances must be disposed of by
a. Refuse Collection Technicians
b. the local council
c. a licensed contractor
d. the occupational health officer
4. A penalty clause in a contract may be enforced if a
a. contract is completed before time
b. contract is completed below the original estimate
c. contractor fails to complete work on time
d. contractor employs JIB labour only
5. A permit to work is required
a. where apprentices are carrying out second fixing
b. when working on 230v systems
c. where tasks present a higher than normal risk
d. when extra work is required
6. On a worksite the location of qualified first aiders must be displayed
a. at the site entrance
b. in the first aid room
c. outside the canteen
d. at each scaffold lift
7. Accidents in the workplace can be reduced by providing:
a. more first aid centres
b. clearly marked exit signs
c. safety education and publicity *
d. more fire extinguishers
8. Carrying out a survey of the workplace and preparing a report on potential health hazards is called a:
a. risk survey procedure
b. risk assessment procedure *
c. health procedure assessment
d. health survey procedure
9. Development of new technology in the electro technical industry will require:
a. a need for continual retraining *
b. a need to employ younger staff
c. employees to move into less skilled jobs
d. employees to prepare for redundancy
10. The term that is used to describe the satisfactory performance of equipment is:
a. performs well at all times
b. fit and performs for the guarantee period
c. fit for the purpose intended *
d. performs without maintenance
11. A permit to work procedure must be issued by:
a. a qualified first aider
b. the Health and Safety Executive
c. a suitable responsible person *
d. the safety committee
12. When using portable equipment, the risk of electric shock can be reduced by using
a. low voltage supply
b. 230v d.c.
c. separated extra low voltage supply
d. 400v private generator
13. A build up of carbon monoxide in an enclosed space where a person is working islikely to cause
a. decreased vision and myopia
b. skin lesions
c. ulcers and blisters
d. unconsciousness and asphyxiation
14. Upon hearing a fire alarm, employees should leave a building. They should onlyre-enter
a. to reclaim valuables
b. when instructed by authorised personnel
c. to fight the fire
d. after 30 minutes
15. In buildings where fire doors are fitted, they should always be kept
a. closed and locked
b. open and locked
c. open
d. closed
16. Storming, forming, norming and performing are considered to be stages of
a. industrial processing
b. group problem solving
c. metal treatment sequences
d. lightning production sequence
17. Fire legislation requires public buildings, factories and hospitals to provide:
a. fire proof clothing and means of eye protection
b. fire proof lifts as an escape route
c. first aid posts on all escape routes
d. a fire warning system and fire fighting equipment *
18. Storming, forming, norming and performing are stages in the process of:
a. a training programme
b. re-organising a company
c. group problem solving *
d. a manufacturing process
19. The right to a minimum wage and claims for unfair dismissal are covered in the:
a. Employment Rights Act *
b. Data Protection Act
c. Human Rights Act
d. Race Relations Act
20. Protecting information stored about employees is guaranteed in the
a. Race Relations Act
b. Electricity at Work Regulations
c. Data Protection Act
d. Human Rights Act
21. Before and during work on an electrical installation an employee should ensure that
a. only one manufacturer’s accessories are used
b. a qualified supervisor is present
c. the work area is safe
d. only battery tools are used
22. The Health and Safety at Work Act sets out a framework stating how:
a. employers must safeguard the health and safety of their employees *
b. employees can claim compensation for accidents at work
c. to work in dangerous situations without using personal protective equipment
d. (b) employers can reduce their responsibilities for health and safety
23. When personal protective equipment (P.P.E.) is provided by an employer it is the responsibility of the employee to:
a. report damaged P.P.E. to the Health and Safety Executive
b. inspect P.P.E. before and after use *
c. wear damaged P.P.E. if replacements are not available
d. pay for repairs to damaged P.P.E.
a. safety sign
b. mandatory sign *
c. warning sign
d. prohibition sign
2. A person who represents other workers on safety matters in an organisation is called a:
a. safety officer
b. safety representative *
c. Health care representative
d. health and safety officer
3. The Control Waste Regulations require that hazardous substances must be disposed of by
a. Refuse Collection Technicians
b. the local council
c. a licensed contractor
d. the occupational health officer
4. A penalty clause in a contract may be enforced if a
a. contract is completed before time
b. contract is completed below the original estimate
c. contractor fails to complete work on time
d. contractor employs JIB labour only
5. A permit to work is required
a. where apprentices are carrying out second fixing
b. when working on 230v systems
c. where tasks present a higher than normal risk
d. when extra work is required
6. On a worksite the location of qualified first aiders must be displayed
a. at the site entrance
b. in the first aid room
c. outside the canteen
d. at each scaffold lift
7. Accidents in the workplace can be reduced by providing:
a. more first aid centres
b. clearly marked exit signs
c. safety education and publicity *
d. more fire extinguishers
8. Carrying out a survey of the workplace and preparing a report on potential health hazards is called a:
a. risk survey procedure
b. risk assessment procedure *
c. health procedure assessment
d. health survey procedure
9. Development of new technology in the electro technical industry will require:
a. a need for continual retraining *
b. a need to employ younger staff
c. employees to move into less skilled jobs
d. employees to prepare for redundancy
10. The term that is used to describe the satisfactory performance of equipment is:
a. performs well at all times
b. fit and performs for the guarantee period
c. fit for the purpose intended *
d. performs without maintenance
11. A permit to work procedure must be issued by:
a. a qualified first aider
b. the Health and Safety Executive
c. a suitable responsible person *
d. the safety committee
12. When using portable equipment, the risk of electric shock can be reduced by using
a. low voltage supply
b. 230v d.c.
c. separated extra low voltage supply
d. 400v private generator
13. A build up of carbon monoxide in an enclosed space where a person is working islikely to cause
a. decreased vision and myopia
b. skin lesions
c. ulcers and blisters
d. unconsciousness and asphyxiation
14. Upon hearing a fire alarm, employees should leave a building. They should onlyre-enter
a. to reclaim valuables
b. when instructed by authorised personnel
c. to fight the fire
d. after 30 minutes
15. In buildings where fire doors are fitted, they should always be kept
a. closed and locked
b. open and locked
c. open
d. closed
16. Storming, forming, norming and performing are considered to be stages of
a. industrial processing
b. group problem solving
c. metal treatment sequences
d. lightning production sequence
17. Fire legislation requires public buildings, factories and hospitals to provide:
a. fire proof clothing and means of eye protection
b. fire proof lifts as an escape route
c. first aid posts on all escape routes
d. a fire warning system and fire fighting equipment *
18. Storming, forming, norming and performing are stages in the process of:
a. a training programme
b. re-organising a company
c. group problem solving *
d. a manufacturing process
19. The right to a minimum wage and claims for unfair dismissal are covered in the:
a. Employment Rights Act *
b. Data Protection Act
c. Human Rights Act
d. Race Relations Act
20. Protecting information stored about employees is guaranteed in the
a. Race Relations Act
b. Electricity at Work Regulations
c. Data Protection Act
d. Human Rights Act
21. Before and during work on an electrical installation an employee should ensure that
a. only one manufacturer’s accessories are used
b. a qualified supervisor is present
c. the work area is safe
d. only battery tools are used
22. The Health and Safety at Work Act sets out a framework stating how:
a. employers must safeguard the health and safety of their employees *
b. employees can claim compensation for accidents at work
c. to work in dangerous situations without using personal protective equipment
d. (b) employers can reduce their responsibilities for health and safety
23. When personal protective equipment (P.P.E.) is provided by an employer it is the responsibility of the employee to:
a. report damaged P.P.E. to the Health and Safety Executive
b. inspect P.P.E. before and after use *
c. wear damaged P.P.E. if replacements are not available
d. pay for repairs to damaged P.P.E.
Wednesday, 22 July 2009
301 - Science Questions with Answers
Electrician Course
Pass your electrical course 2330 level 3.
Revision Page to help you Pass.
Successfully Answers at the end scroll down
1. The current in a coil changes from 5A to 2A in 0.05seconds and induces a voltage of 30v in the coil the inductance of the coil is?
a. 5H
b. 0.5H
c. 1.5H
d. 2.0H
2. A coil has an inductance of 0.1 H and is connected across a 50Hz a.c. supply, the value of inductive reactance of the coil will be?
a. 0.03142
b. 0.3142
c. 3.142
d. 31.42
3. A Capacitor of Capacitive Reactance 50ohm, an inductor of 80ohm Inductive Reactance and a Resistor of 40ohm are connected in series to a 200v 50 Hz supply. The impedance of the circuit will be?
a. 40
b. 50
c. 80
d. 170
4. A capacitor of 100BF is connected in parallel with a 20 ohm pure resistor. The group connected to a 200v 50Hz supply. Calculate the power dissipated in the combination
a. 2000W
b. 2400W
c. 3000W
d. 3600W
5. The property of a capacitor to store charge when its plates are at different potentials is defined as its?
a. capacitance
b. volume
c. charge level
d. charge value
6. Inductive loads connected to a. c. supplies cause?
a. increased power dissipation
b. lagging power factors
c. leading power factors
d. reduced power dissipation
7. The property of a capacitor to store charge when its plates are at different potentials is defined as its?
a. capacitance
b. volume
c. charge level
d. charge value
8. To cause a Thyristor (SCR) to conduct it is necessary apply a trigger voltage to the?
a. anode
b. cathode
c. gate
d. collector
9. A silicon chip on which a number of discrete components are formed is known as a?
a. composite circuit
b. multiple component device
c. discrete circuit
d. integrated circuit
10. An electronic device used to detect changes in levels of light is?
a. a photo diode
b. an opto-coupler
c. a fibre optic link
d. a capacitor
11. To correct the on load power factor of a single phase induction motor, a capacitor would be connected how?
a. in series with the start winding
b. in parallel with the start winding
c. across the motor terminals
d. across the centrifugal switch terminals.
12. Find the resistance of a 230v 3 kW immersion heater element.
a. 13.04 ohm
b. 17.63 ohm
c. 19.2 ohm
d. 76.66 ohm
You can work out for resistance when you have the Power dissipated and Voltage supply using power formulae.
Power = ( Current) I x Voltage (V)
Power Formulae
1. (Power) P=IxV
2. (Power) P= V^2/R
3. (Power) P= I^(2 )R
As (Current) I= V/R replacing I for V/R gives
V/R x V P= V^2/R
As (Voltage) V=IR replacing V for (IR) gives
I x (IR) P=I^(2 )R
For the question, the rating of the immersion Heater element is 3kW and the supply is 230V.
Power=3000Watts
Voltage=230Volts
Resistance=????
Since, P= V^2/R
Therefore,
(Resistance) R= V^(2)/P
(230 x 230)/3000 Ohms
13. If a purely inductive load having an inductive reactance of 10 is connected across a 100V 50Hz supply, the power dissipated in the circuit will be?
(a) 1000W (b) 100W (c) 10W (d) 0W
14. The voltage supplied to the primary winding of an ideal step down transformer is 1000V and the output voltage is 200V. If the primary current is 5A the value of the secondary current will be?
(a) 5A (b) 10A (c) 25A (d) 100A
15. The force on a conductor 750mm long carrying a current of 6A placed at right angles in a field of 1.2 Tesla will be?
a. 3.75 N
b. 5.40 N
c. 9.60 N
d. 9.81 N
16. A four-pole 50Hz three-phase induction motor is connected to a 400v supply. If the motor has 4% slip the rotor speed will be?
a. 12.5 rev/sec
b. 12 rev/sec
c. 25 rev/sec
d. 24 rev/sec
17. The synchronous speed of the rotating magnetic field in a four pole 50HZ synchronous motor is?
a. 12.5 r/s
b. 16.6 r/s
c. 25 r/s
d. 50 r/s
18. Electrical connections are made to the rotor of a three phase wound rotor induction motor via?
a. the rotor bars
b. the commutator and brushes
c. the slip rings and brushes
d. a direct connection
19. A three phase cage rotor induction motor connected to a constant load will operate at?
a. above synchronous speed
b. at synchronous speed
c. at varying speeds
d. at constant speed
20. Capacitor start capacitor run induction motors are?
a. three phase machines
b. single phase machines
c. synchronous machines
d. wound rotor machines
21. The direction of rotation of a single phase capacitor start induction motor can be reversed by?
a. reversing the connections to the start winding
b. reversing the supply connections
c. removing and reversing the rotor
d. reversing the capacitor connections
22. The type of motor starter which could be used on all cage motor ratings from fractional kW to 1000 kW would be?
a. rotor resistance
b. soft start
c. capacitor start
d. star – delta
23. In a four wire star connected balanced three phase system the current in each phase is 40A, the current in the neutral conductor is?
a. 120A
b. 40A
c. 20A
d. 0A
24. A single-phase motor consists of two stator windings, two capacitors, a centrifugal switch and a rotor. This type of motor is known as?
a. induction start and run
b. shaded pole
c. capacitor start, capacitor run
d. permanent capacitor
25. The current in the rotor bars of a cage motor are produced by?
a. self induction
b. mutual induction
c. primary induction
d. mutual detrimental influence
26. To correct the on load power factor of a single phase induction motor, a capacitor would be connected how?
a. in series with the start winding
b. in parallel with the start winding
c. across the motor terminals
d. across the centrifugal switch terminals
27. A single conductor is placed at right angles to a permanent magnetic field. When a d.c supply is connected to the conductor it is seen to move. This is the basic principle of the:
a. d.c generator
b. alternator
c. d.c. motor
d. rectifier
28. The rotor of a cage induction motor is laminated to?
a. reduce the weight of the motor
b. reduce the Copper losses
c. reduce the eddy current losses
d. reduce the volume of the motor
29. Infinitely variable speed control of a cage motor can be achieved by using a?
a. direct on line starter
b. inverter control
c. auto transformer
d. pole changing
30. A short circuit current will flow when a fault occurs between?
a. Neutral and Earth
b. Phase and Earth
c. Phase and Neutral
d. Exposed and extraneous parts
31. An overcurrent device may have a lower breaking capacity than the value where it is installed provided?
a. it is protected by an rcd
b. it is to BS EN 60898
c. it is supplied via a device with adequate breaking capacity
d. the fault current is cleared in five seconds
32. A residual current device will operate when a?
a. phase neutral fault occurs
b. earthing conductor to CPC fault occurs
c. phase to earth fault occurs
d. phase to phase fault occurs
33. In a TN-S system, when a fault current flows, the path it takes is known as?
a. bonding impedance path
b. earthing circuit
c. earth loop impedance path
d. supplementary loop path
34. The primary function of a fuse is to protect?
a. circuit conductors from damage
b. against electric shock
c. appliances connected to the circuit
d. against motor starting currents
35. A residual current device will operate if?
a. a phase to neutral fault occurs
b. a phase to earth fault occurs
c. a short circuit occurs between phase and neutral
d. an open circuit fault occurs
36. In a TT earthing system the connection to earth is achieved by connecting to?
a. a metal rod driven into the ground
b. the sheath of the supply cable
c. the neutral conductor of the system
d. the gas and water supplies
37. The most common earthing system used in the U.K. is?
a. TN-CS system
b. TN-C system
c. TT system
d. TN -S system
38. Isolators are designed to disconnect the supply under?
a. short circuit conditions
b. no load conditions
c. full load conditions
d. overload conditions
39. The most common type of cable used for the underground distribution of electricity is?
a. M I M S cable
b. P I L C S W A cable
c. L S F cable
d. F P 200 cable
40. Typical voltages used for the transmission and distribution are respectively:
a. 33 kV and 25 kV
b. 175 kV and 11 kV
c. 400 kV and 11 kV
d. 25 kV and 400v
41. Underground distribution systems within an industrial estate will most likely use?
a. M.I.C.C
b. XLPE SWA
c. FP 200 Gold
d. Busbar trunking
42. A three-phase four-wire TT system will require at the intake position the following switching arrangement:
a. four individual single pole switches
b. a triple pole switch with solid neutral
c. a four-pole linked switch
d. a triple pole switch with a dis-connectable neutral
43. Identify which of the following groups indicates the correct ascending order of efficacy of the light sources
a. tungsten, low pressure sodium, fluorescent, high pressure sodium
b. low pressure sodium, high pressure sodium, fluorescent, tungsten
c. tungsten, fluorescent, high pressure sodium, low pressure sodium
d. fluorescent, tungsten, low pressure sodium, high pressure sodium
44. What is the function of a semi-condutor diode
a. To shart out power strikes
b. To produce half-wave d.c.
c. To smooth a.c.
d. To produce full wave d.c.
45. The resistor has a value of 1.8Ω. Using standard value marking code the resistor would be marked as:
a. R18
b. R1.8
c. 1R8
d. 1.8R
46. Single Phase installation short-circuit current flows when Z between phase and neutral is:
a. 1MΩ
b. 10MΩ
c. 100MΩ
d. Negligible
47. A 100Ω is connected in series to a 10 d.c. supply. The power dissipated in will be
a. 1 W
b. 10 W
c. 0.1 W
d 100 W
Pass your electrical course 2330 level 3.
Revision Page to help you Pass.
Successfully Answers at the end scroll down
1. The current in a coil changes from 5A to 2A in 0.05seconds and induces a voltage of 30v in the coil the inductance of the coil is?
a. 5H
b. 0.5H
c. 1.5H
d. 2.0H
2. A coil has an inductance of 0.1 H and is connected across a 50Hz a.c. supply, the value of inductive reactance of the coil will be?
a. 0.03142
b. 0.3142
c. 3.142
d. 31.42
3. A Capacitor of Capacitive Reactance 50ohm, an inductor of 80ohm Inductive Reactance and a Resistor of 40ohm are connected in series to a 200v 50 Hz supply. The impedance of the circuit will be?
a. 40
b. 50
c. 80
d. 170
4. A capacitor of 100BF is connected in parallel with a 20 ohm pure resistor. The group connected to a 200v 50Hz supply. Calculate the power dissipated in the combination
a. 2000W
b. 2400W
c. 3000W
d. 3600W
5. The property of a capacitor to store charge when its plates are at different potentials is defined as its?
a. capacitance
b. volume
c. charge level
d. charge value
6. Inductive loads connected to a. c. supplies cause?
a. increased power dissipation
b. lagging power factors
c. leading power factors
d. reduced power dissipation
7. The property of a capacitor to store charge when its plates are at different potentials is defined as its?
a. capacitance
b. volume
c. charge level
d. charge value
8. To cause a Thyristor (SCR) to conduct it is necessary apply a trigger voltage to the?
a. anode
b. cathode
c. gate
d. collector
9. A silicon chip on which a number of discrete components are formed is known as a?
a. composite circuit
b. multiple component device
c. discrete circuit
d. integrated circuit
10. An electronic device used to detect changes in levels of light is?
a. a photo diode
b. an opto-coupler
c. a fibre optic link
d. a capacitor
11. To correct the on load power factor of a single phase induction motor, a capacitor would be connected how?
a. in series with the start winding
b. in parallel with the start winding
c. across the motor terminals
d. across the centrifugal switch terminals.
12. Find the resistance of a 230v 3 kW immersion heater element.
a. 13.04 ohm
b. 17.63 ohm
c. 19.2 ohm
d. 76.66 ohm
You can work out for resistance when you have the Power dissipated and Voltage supply using power formulae.
Power = ( Current) I x Voltage (V)
Power Formulae
1. (Power) P=IxV
2. (Power) P= V^2/R
3. (Power) P= I^(2 )R
As (Current) I= V/R replacing I for V/R gives
V/R x V P= V^2/R
As (Voltage) V=IR replacing V for (IR) gives
I x (IR) P=I^(2 )R
For the question, the rating of the immersion Heater element is 3kW and the supply is 230V.
Power=3000Watts
Voltage=230Volts
Resistance=????
Since, P= V^2/R
Therefore,
(Resistance) R= V^(2)/P
(230 x 230)/3000 Ohms
13. If a purely inductive load having an inductive reactance of 10 is connected across a 100V 50Hz supply, the power dissipated in the circuit will be?
(a) 1000W (b) 100W (c) 10W (d) 0W
14. The voltage supplied to the primary winding of an ideal step down transformer is 1000V and the output voltage is 200V. If the primary current is 5A the value of the secondary current will be?
(a) 5A (b) 10A (c) 25A (d) 100A
15. The force on a conductor 750mm long carrying a current of 6A placed at right angles in a field of 1.2 Tesla will be?
a. 3.75 N
b. 5.40 N
c. 9.60 N
d. 9.81 N
16. A four-pole 50Hz three-phase induction motor is connected to a 400v supply. If the motor has 4% slip the rotor speed will be?
a. 12.5 rev/sec
b. 12 rev/sec
c. 25 rev/sec
d. 24 rev/sec
17. The synchronous speed of the rotating magnetic field in a four pole 50HZ synchronous motor is?
a. 12.5 r/s
b. 16.6 r/s
c. 25 r/s
d. 50 r/s
18. Electrical connections are made to the rotor of a three phase wound rotor induction motor via?
a. the rotor bars
b. the commutator and brushes
c. the slip rings and brushes
d. a direct connection
19. A three phase cage rotor induction motor connected to a constant load will operate at?
a. above synchronous speed
b. at synchronous speed
c. at varying speeds
d. at constant speed
20. Capacitor start capacitor run induction motors are?
a. three phase machines
b. single phase machines
c. synchronous machines
d. wound rotor machines
21. The direction of rotation of a single phase capacitor start induction motor can be reversed by?
a. reversing the connections to the start winding
b. reversing the supply connections
c. removing and reversing the rotor
d. reversing the capacitor connections
22. The type of motor starter which could be used on all cage motor ratings from fractional kW to 1000 kW would be?
a. rotor resistance
b. soft start
c. capacitor start
d. star – delta
23. In a four wire star connected balanced three phase system the current in each phase is 40A, the current in the neutral conductor is?
a. 120A
b. 40A
c. 20A
d. 0A
24. A single-phase motor consists of two stator windings, two capacitors, a centrifugal switch and a rotor. This type of motor is known as?
a. induction start and run
b. shaded pole
c. capacitor start, capacitor run
d. permanent capacitor
25. The current in the rotor bars of a cage motor are produced by?
a. self induction
b. mutual induction
c. primary induction
d. mutual detrimental influence
26. To correct the on load power factor of a single phase induction motor, a capacitor would be connected how?
a. in series with the start winding
b. in parallel with the start winding
c. across the motor terminals
d. across the centrifugal switch terminals
27. A single conductor is placed at right angles to a permanent magnetic field. When a d.c supply is connected to the conductor it is seen to move. This is the basic principle of the:
a. d.c generator
b. alternator
c. d.c. motor
d. rectifier
28. The rotor of a cage induction motor is laminated to?
a. reduce the weight of the motor
b. reduce the Copper losses
c. reduce the eddy current losses
d. reduce the volume of the motor
29. Infinitely variable speed control of a cage motor can be achieved by using a?
a. direct on line starter
b. inverter control
c. auto transformer
d. pole changing
30. A short circuit current will flow when a fault occurs between?
a. Neutral and Earth
b. Phase and Earth
c. Phase and Neutral
d. Exposed and extraneous parts
31. An overcurrent device may have a lower breaking capacity than the value where it is installed provided?
a. it is protected by an rcd
b. it is to BS EN 60898
c. it is supplied via a device with adequate breaking capacity
d. the fault current is cleared in five seconds
32. A residual current device will operate when a?
a. phase neutral fault occurs
b. earthing conductor to CPC fault occurs
c. phase to earth fault occurs
d. phase to phase fault occurs
33. In a TN-S system, when a fault current flows, the path it takes is known as?
a. bonding impedance path
b. earthing circuit
c. earth loop impedance path
d. supplementary loop path
34. The primary function of a fuse is to protect?
a. circuit conductors from damage
b. against electric shock
c. appliances connected to the circuit
d. against motor starting currents
35. A residual current device will operate if?
a. a phase to neutral fault occurs
b. a phase to earth fault occurs
c. a short circuit occurs between phase and neutral
d. an open circuit fault occurs
36. In a TT earthing system the connection to earth is achieved by connecting to?
a. a metal rod driven into the ground
b. the sheath of the supply cable
c. the neutral conductor of the system
d. the gas and water supplies
37. The most common earthing system used in the U.K. is?
a. TN-CS system
b. TN-C system
c. TT system
d. TN -S system
38. Isolators are designed to disconnect the supply under?
a. short circuit conditions
b. no load conditions
c. full load conditions
d. overload conditions
39. The most common type of cable used for the underground distribution of electricity is?
a. M I M S cable
b. P I L C S W A cable
c. L S F cable
d. F P 200 cable
40. Typical voltages used for the transmission and distribution are respectively:
a. 33 kV and 25 kV
b. 175 kV and 11 kV
c. 400 kV and 11 kV
d. 25 kV and 400v
41. Underground distribution systems within an industrial estate will most likely use?
a. M.I.C.C
b. XLPE SWA
c. FP 200 Gold
d. Busbar trunking
42. A three-phase four-wire TT system will require at the intake position the following switching arrangement:
a. four individual single pole switches
b. a triple pole switch with solid neutral
c. a four-pole linked switch
d. a triple pole switch with a dis-connectable neutral
43. Identify which of the following groups indicates the correct ascending order of efficacy of the light sources
a. tungsten, low pressure sodium, fluorescent, high pressure sodium
b. low pressure sodium, high pressure sodium, fluorescent, tungsten
c. tungsten, fluorescent, high pressure sodium, low pressure sodium
d. fluorescent, tungsten, low pressure sodium, high pressure sodium
44. What is the function of a semi-condutor diode
a. To shart out power strikes
b. To produce half-wave d.c.
c. To smooth a.c.
d. To produce full wave d.c.
45. The resistor has a value of 1.8Ω. Using standard value marking code the resistor would be marked as:
a. R18
b. R1.8
c. 1R8
d. 1.8R
46. Single Phase installation short-circuit current flows when Z between phase and neutral is:
a. 1MΩ
b. 10MΩ
c. 100MΩ
d. Negligible
47. A 100Ω is connected in series to a 10 d.c. supply. The power dissipated in will be
a. 1 W
b. 10 W
c. 0.1 W
d 100 W
p=v.i
v=i/r
i=v/r
i=10/100
i=0.1A
therefore:- p= 10.0.1
p= 1w
48. Two resistors, of value 7.3 ohm and 4.2 ohm are connected in series with a 24 volts battery. What is the total current flow in this circuit?
a. 0.11 A
b. 9 A
c. 2.086 A
d. 0.479 A
i=v/r,
v=24v, r=7.3ohms and 4.2 ohms therefore current wiil be equal through out the circuit
i=24/11.7
i=2.08A
49. A copper conductor has a resistivity of 17.8×10ˉ⁹ and a csa of 2.5 mm². What will be the resistance of a 30m length of this conductor
a. 0.21 Ω
b. 2.5 Ω
c. 0.12 Ω
d. 1.2 Ω
R = ρl/α
Where
ρ= the resistivity constant for the material(Ωm)
l= length of the material(m)
α=cros-section area of the material (m²)
Convert mm² to m² →
1000mm=1mm therefore: 2.5mm²= (2.5/1000²) m²
→2.5×10⁻⁶m²
R=17.8× 10⁻⁹.30/(2.5mm²×10⁻⁶)
R=0.2136 ohms
50. A coil of 0.159 H is connected with a 100Ω resistor across a 230V 50 Hz supply. calculate the circuit impedance
a. 120 Ω
b. 107.1 Ω
c. 111.8 Ω
d. 90 Ω
Xι=Inductive reactance
Xι=2πfL
Xι=2X3.142×50Hz×0.159H
Xι=49.9578Ω
:- Z=√(R²+Xι²)
:- Z=√(100Ω²+49.9578²)
:- Z=111.7845Ω
51. A 230V supply to 12V bell transfomer is constructe with 800 primary turns winding. calculate the primary current when the transformer supplies the12V 12W alarm bell.
a. 42 A
b. 0.052 A
c. 12 A
d. 1 A
Vp=230V
Vs=12V
Np=800
Vp/Vs=Np/Ns=Is/Ip
Secondary turns,
Therefore, Ns=NpVs/VP
Ns= (800×12V)÷230V
Ns=42 turns
Secondary Current,
Is=power/Vs
:- Is =12W/12V
Is=1A
Primary turn,
Ip=(Is×Vs)÷Vp
Ip= (1A×12V)÷230V
Ip=0.052A
52. A single-phase trransfomer with 3000 primary turns and 600 secndary turns is fed from a 230 volts a.c supply. What will the volts per turn in secondary be?
a. 2.6 V
b. 0.077 V
c. 13 V
d. 0.38 V
Vp/Np=Vs/Ns
Therefore
Vs=(Vp/Np)×Ns
Vs =VpNs/Np
Vs=(230÷3000)×600
Vs =42V
Volts per turns
42v/600turns
=o.0766666
*b* voltage supplies to a primary winding of an ideal step down transformer is 1200V and the output voltage is 230 V. if the primary current is 6 A, what will be the value of the secondary current??
a. 5.2 A
b. 31.3 A
c. 44.14 A
d. 1.15 A
53. An electric motor supplies 5 kW of power when operating at an efficient of 75%. What must the minimum power rating be of the motor
a. 8.75 kW
b. 3.75 kW
c. 6.66 kW
d. 1.33 kW
54. A 200 ohm resistor is connected in series to 15 V d.c supply. The power dissipated in it will be
a. 1.125 W
b 0.075 W
c. 13.33 W
d. 3000 W
P=vi P=V²/R
I=V/R OR P= 15²/200
V=15V P=1.125W
I=15/200=0.075A
Therefore,
P=15V×0.075A
P=1.125W
55. Mutual inductance in a two coils is 1 henry and 1 volt induced. What is the chnging rate of current
a. 2 A/s
b. 10 A/s
c. 1 A/s
d. 1000A/s
℮=−M(di/dt)
1V=−1H(i/s)
(i/s)=1V/1H
i/s=1 A/s
56. A coil has a self inductance of 4 H. if the current through it changes from 0.6 A to 0.1 A in 0.02s, the induced back emf will be:
a. 6.25 V
b. 100 V
c. 12 V
d. 20 V
℮=-L(di/dt)
℮=-4H((0.6-0.1)/0.02s)
℮=-100V
57. A current of 5A through a certain coil is reversed in 0.1s, and the induced e.m.f. is 15V. calculate the self-inductance of the coil.??
58. Calcualte the force on a conductor 150mm long situated at right angles to a magnetic field of flux density 0.85T and carrying a current of 15A
a. 1.913 N
b. 1912.5 N
c. 19.13 N
d. 191.25 N
F=BIL
F=0.85T×15A×150×10⁻³
F=1.913N
59. Determine the flux density in a magntic field in which a conductor 0.3m long situated at a right angle and carrying a current of 15A experience a force of 3.5N
a. 15.75 T
b. 1.575 T
c. 77.78 T
d. 0.78 T
F=BIL
B=F/IL
B=3.5/15x0.3
B=0.78T
60. The magnetic flux linking 1600 turns of an electromagnet changes from 0.9 mWb to 0.6 mWb on 40 ms. The value of induced back e.m.f will be:
a. 12 V
b. 120 V
c. 1.2 V
d. 1200 V
emf=−[(φ₂-φ₁)/t]xN (V)
emf=−[(0.9mWb-0.6mWb)/40ms]x1600
emf=−[(0.3x10⁻³/40x10⁻³)x1600
emf=−120V
61. The magnetic flux linking 1500 turns of an electromagnet changes from 0.8 mWb to 0.7mWb in 50ms. The value of induced back e.m.f. will be???
a. 0.03 V
b. 3 V
c. 30 V
d. 300V
62. a coil of 0.16 H is connected across a 100 V 50 Hz supply. The inductive reactance of the coil will be.
a. 0.0198 ohm
b. 16 0hm
c. 50.26 ohm
d. 8 ohm
Xι=2πfL (Ω)
π=3.142 a constant
f=the frequency of supply (Hz)
L=the inductance of the circuit (H)
Therefore,
Xι=2x3.142x50Hzx0.16H
Xι=50.26Ω
63. A capacitor of 17.9 µF is connected across a 230 V 50 Hz supply. The capacity reactance will be:
a. 0.17 Ω
b. 5.62 Ω
c. 177.8 Ω
d. 562.3 Ω
Xc=1/2πfC
Xc=1/(2x3.142x50Hzx17.9μF)
Xc=1/(2x3.142x50x17.9x10⁻⁶
Xc=177.82Ω
64. Three identical loads of 30Ω resistance are connected to a 400V 3 phase supply. if the loads are delta connected, the phase current will be?
a. 13.33A
b. 7.66A
c. 23.06A
d. 133.6A
65. Three identical loads of 40Ω resistance are connected to a 400V 3 phase supply. if the load are delta connected, the phase current will be
a. 0.1 A
b. 5.77 A
c. 17.32 A
d. 10 A
66. Three coils of resistance 80Ω and inductive reactance 60Ω are connected in delta to a 400V 50Hz three phase supply. The total power will be
a. 2217 W
b. 3840 W
c. 6000 W
d. 1979 W
67. A coil has a resistance of 15Ω and an inductive reactance of 30Ω. What is the Power Factor of this circuit arrangement:
a. 0.45 lagging
b 0.5 lagging
c 0.5 leading
d. 0.45 leading
68. A transformer having a turns ratio of 3:55 is connected to a 230 V supply. The output voltage will be:
a. 138 V
b. 383 V
c. 460 V
d. 60 V
69. Three line four wire system supplies a balanced load of 10 kVA with a 0.85 Power Factor. What is the power consumption of this system?
a. 0.85 W
b. 11.76 W
c. 8.5 W
d. 14.72 W
70. A 30Ω resistor is connected in series with a capacitor with a reactance of 40Ω. What is the Power Factor of this circuit arrangement?
a. 0.6 leading
b. 0.6 lagging
c. 0.8 lagging
d. 0.8 leading
Answers
1B, 2D, 3B, 4A, 5A, 6B,7A,8C, 9 D,10 A,11 C,12 B,13 D,14 C,15B,16 D, 17 C, 18C, 19D, 20B, 21A, 22B, 23D, 24C, 25B, 26C, 27C, 28C, 29B, 30C, 31C, 32C, 33C, 34A, 35B, 36A, 37D, 38B, 39B, 40C, 41B, 42C, 43 C 44B, 45C, 46D,47A, 48C, 49A, 50C, 51B, 52B, 53D, 54A, 55C, 56B, 57, 58A, 59D, 60A, 61B, 62C, 63C, 64A, 65D, 66, 67A, 68B, 69C, 70A
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