ELECTRICAL THEORY EXAM
QUESTIONS AND ANSWERS 100%
CORRECT
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
Describe the characteristics of a thermistor - ANSWER-A thermistor is a device which
changes resistance with temperature
Where in a motor circuit would a thermistor be located? - ANSWER-in the windings of
the motor
New mains have been installed in a 230V, single-phase domestic installation. During
the work the phase and neutral were interchanged at the mains entry box
State TWO hazardous situations that could arise if the installation was livened -
ANSWER-Any TWO of:
- A touch voltage hazard between conductive parts and the mass of earth.
- Protective devices within the installation may not operate under fault conditions.
- The neutral is being switched throughout the installation.
- The protective device for the installation may not operate
, Some protective devices on a switchboard in an electrical installation have a fault duty
lower than the prospective short-circuit current rating of the installation.
State two hazards that could occur when a high prospective short-circuit fault current
occurs in the installation - ANSWER-- A flash over on switchboard between protective
devices
- A protective device could rupture and expose live parts
- A protective device could rupture and cause a fire
AS/NZS 3000 requires that a test voltage of 500V d.c. is used for an insulation
resistance test of a low voltage electrical installation.
Explain why a voltage of 500V is applied.
Explain why the voltage is a d.c voltage. - ANSWER-- To stress the insulation at a
voltage
above 325V a.c. (or peak a.c. voltage).
- To ensure capacitive or inductive reactance does not influence the test result OR A
constant maximum voltage is a more stringent test than the momentary peaks of an a.c
waveform
Define the term RAMP TIME as it applies to a variable speed drive controlling a motor -
ANSWER-The time taken for the motor to change from
one speed to another.
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
QUESTIONS AND ANSWERS 100%
CORRECT
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
Describe the characteristics of a thermistor - ANSWER-A thermistor is a device which
changes resistance with temperature
Where in a motor circuit would a thermistor be located? - ANSWER-in the windings of
the motor
New mains have been installed in a 230V, single-phase domestic installation. During
the work the phase and neutral were interchanged at the mains entry box
State TWO hazardous situations that could arise if the installation was livened -
ANSWER-Any TWO of:
- A touch voltage hazard between conductive parts and the mass of earth.
- Protective devices within the installation may not operate under fault conditions.
- The neutral is being switched throughout the installation.
- The protective device for the installation may not operate
, Some protective devices on a switchboard in an electrical installation have a fault duty
lower than the prospective short-circuit current rating of the installation.
State two hazards that could occur when a high prospective short-circuit fault current
occurs in the installation - ANSWER-- A flash over on switchboard between protective
devices
- A protective device could rupture and expose live parts
- A protective device could rupture and cause a fire
AS/NZS 3000 requires that a test voltage of 500V d.c. is used for an insulation
resistance test of a low voltage electrical installation.
Explain why a voltage of 500V is applied.
Explain why the voltage is a d.c voltage. - ANSWER-- To stress the insulation at a
voltage
above 325V a.c. (or peak a.c. voltage).
- To ensure capacitive or inductive reactance does not influence the test result OR A
constant maximum voltage is a more stringent test than the momentary peaks of an a.c
waveform
Define the term RAMP TIME as it applies to a variable speed drive controlling a motor -
ANSWER-The time taken for the motor to change from
one speed to another.
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
