Electrical Theory exam fully solved &
updated
State the permissible test result for an insulation resistance test for the electrical
installation with ALL the permanently-connected electrical equipment
DISCONNECTED - ANSWER-1 Mega Ohm
State the permissible test result for an insulation resistance test for the electrical
installation with ALL the permanently-connected electrical equipment
CONNECTED - ANSWER-0.01 Mega Ohms
A three-phase induction motor normally runs on full-load. State ONE reason why
this motor should not be run LIGHTLY LOADED when it runs on full load. -
ANSWER-- The power factor of the motor will be poor
- The motor will be less efficient
- running costs rise
A three-phase induction motor has been OPERATING NORMALLY ON FULL-
LOAD. The motor starts to over-heat even though it is drawing the normal full
load current.
State ONE reason why this situation would occur - ANSWER-- The motor
ventilation is restricted in some manner
- The cooling fan has become detached from the motor shaft
A clip-on ammeter is being used to test the integrity of the insulation of a repaired
Class I electrical appliance, while the appliance is live. Where on the appliance
would the clip-on ammeter be used? Also state the maximum test result that
would allow the appliance to return to service. - ANSWER-Clamped around the
protective earthing conductor and 5mA
, Explain what is meant by the term INVERSE TIME-CURRENT CHARACTERISTIC
in relation to protective devices - ANSWER-as current through the protective
device increases, the time taken to operate decreases
State TWO components that are typically used to smooth the d.c output of a
rectifier - ANSWER-- Capacitors
- Inductors
- Resistors
The output circuit of an autotransformer MUST BE insulated for the highest input
or output voltage of the autotransformer. State ONE reason for this requirement -
ANSWER-An auto transformer can fail in such a way so the primary voltage
appears across the secondary
An isolator for 230V electrical equipment has been replaced. During the work the
phase (active) and neutral were accidentally transposed at the supply side of the
isolating switch.
What potential danger would this transposition create? - ANSWER-The internal
circuitry of the electrical equipment would be live when the isolating switch is in
the OFF position.
What to the initials BJT mean when applied to semiconductors - ANSWER-A bi-
polar junction transistor
State the function of a transducer - ANSWER-A device that changes energy from
one form to another
State TWO requirements for good illumination - ANSWER-Any TWO of:
● Provides clear vision without eye strain
● Sufficient light intensity
● Minimum glare
● Uniform light intensity throughout the illuminated area
● Suitable colour rendering
XC - ANSWER-Capacitive Reactance (ohms)
updated
State the permissible test result for an insulation resistance test for the electrical
installation with ALL the permanently-connected electrical equipment
DISCONNECTED - ANSWER-1 Mega Ohm
State the permissible test result for an insulation resistance test for the electrical
installation with ALL the permanently-connected electrical equipment
CONNECTED - ANSWER-0.01 Mega Ohms
A three-phase induction motor normally runs on full-load. State ONE reason why
this motor should not be run LIGHTLY LOADED when it runs on full load. -
ANSWER-- The power factor of the motor will be poor
- The motor will be less efficient
- running costs rise
A three-phase induction motor has been OPERATING NORMALLY ON FULL-
LOAD. The motor starts to over-heat even though it is drawing the normal full
load current.
State ONE reason why this situation would occur - ANSWER-- The motor
ventilation is restricted in some manner
- The cooling fan has become detached from the motor shaft
A clip-on ammeter is being used to test the integrity of the insulation of a repaired
Class I electrical appliance, while the appliance is live. Where on the appliance
would the clip-on ammeter be used? Also state the maximum test result that
would allow the appliance to return to service. - ANSWER-Clamped around the
protective earthing conductor and 5mA
, Explain what is meant by the term INVERSE TIME-CURRENT CHARACTERISTIC
in relation to protective devices - ANSWER-as current through the protective
device increases, the time taken to operate decreases
State TWO components that are typically used to smooth the d.c output of a
rectifier - ANSWER-- Capacitors
- Inductors
- Resistors
The output circuit of an autotransformer MUST BE insulated for the highest input
or output voltage of the autotransformer. State ONE reason for this requirement -
ANSWER-An auto transformer can fail in such a way so the primary voltage
appears across the secondary
An isolator for 230V electrical equipment has been replaced. During the work the
phase (active) and neutral were accidentally transposed at the supply side of the
isolating switch.
What potential danger would this transposition create? - ANSWER-The internal
circuitry of the electrical equipment would be live when the isolating switch is in
the OFF position.
What to the initials BJT mean when applied to semiconductors - ANSWER-A bi-
polar junction transistor
State the function of a transducer - ANSWER-A device that changes energy from
one form to another
State TWO requirements for good illumination - ANSWER-Any TWO of:
● Provides clear vision without eye strain
● Sufficient light intensity
● Minimum glare
● Uniform light intensity throughout the illuminated area
● Suitable colour rendering
XC - ANSWER-Capacitive Reactance (ohms)
