EWRB Exam – Questions With Complete Solutions (A+)
State the main reason why a large industrial electrical installation has a higher
prospective short circuit current (PSSC) than a domestic electrical installation.
Right Ans - The fault loop impedance is lower
Because the distribution transformer is close to the installation or on-site
A CT metering circuit is measuring the consumption of an industrial electrical
installation at near to full load. The CT secondary must be shortcircuited
before disconnecting the meters from the CT. State TWO reasons why the CT
secondary must be short-circuited Right Ans - To prevent a high voltage
developing in the circuit.
To prevent damage to the CT winding insulation.
A result of fault loop impedance test at the main switchboard of a low voltage
electrical installation will show the prospective short circuit current level of
the installation or the impedance of the fault loop at the switchboard under
fault conditions. State the main purpose of the information gained from the
earth fault loop impedance test Right Ans - To confirm that the protective
devices can safely interrupt the installation PSSC.
Some protective devices in an electrical installation have a fault duty lower
than the prospective short circuit rating of the installation. State the main
hazard that could occur when a high prospective short-circuit fault current
occurs in the electrical installation. Right Ans - Flash over on switchboards
due to inadequate kA rating of protective devices
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. Right Ans - To stress the
insulation at a voltage above 325V a.c. (or peak a.c. voltage.
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 the voltage is a d.c. voltage. Right Ans - To ensure capacitive or
inductive reactance does not influence the test result.
A constant maximum voltage is a more stringent test than the momentary
peaks of an a.c. waveform.
, 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.
Define the term current rating as it applies to an HRC fuse? Right Ans - This
is the maximum current that the fuse is designed to carry continuously
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.
Define the term breaking (or rupturing) capacity as it applies to an HRC fuse
Right Ans - The maximum fault current the fuse can safely interrupt.
State TWO technical advantages that a soft starter has when compared to and
electromechanical type starter, such as auto-transformer. Right Ans -
Broad range of options for starting and stopping
Broad range of options for protection.
Visual feedback of key technical parameters.
Smoother starting
Reduced mechanical stress on equipment on starting. Reduced electrical
stress on equipment on starting
Explain the purpose of an interlocks in a star/delta starter Right Ans - To
prevent the star and delta contactors closing simultaneously
State the effect of the interlocks failing in a star/delta starter Right Ans - An
electrical interlock failure could prevent the motor starting. An electrical
interlock failure could cause a phase-to-phase short.
A mechanical interlock failure could prevent the motor starting.
A mechanical interlock failure could cause a phase-to-phase short.
Define the term pullout torque as it applies to an a.c. induction motor Right
Ans - The maximum torque the motor can develop.
State the synchronous speed of a three-phase, four-pole, 50 Hz motor.
Right Ans - 1500 rpm
State the Standard and section of that Standard that specifies the tests and
checks that need to be carried out on the socket outlet and final subcircuit
wiring. Right Ans - AS/NZS 3000
Section 8
State the main reason why a large industrial electrical installation has a higher
prospective short circuit current (PSSC) than a domestic electrical installation.
Right Ans - The fault loop impedance is lower
Because the distribution transformer is close to the installation or on-site
A CT metering circuit is measuring the consumption of an industrial electrical
installation at near to full load. The CT secondary must be shortcircuited
before disconnecting the meters from the CT. State TWO reasons why the CT
secondary must be short-circuited Right Ans - To prevent a high voltage
developing in the circuit.
To prevent damage to the CT winding insulation.
A result of fault loop impedance test at the main switchboard of a low voltage
electrical installation will show the prospective short circuit current level of
the installation or the impedance of the fault loop at the switchboard under
fault conditions. State the main purpose of the information gained from the
earth fault loop impedance test Right Ans - To confirm that the protective
devices can safely interrupt the installation PSSC.
Some protective devices in an electrical installation have a fault duty lower
than the prospective short circuit rating of the installation. State the main
hazard that could occur when a high prospective short-circuit fault current
occurs in the electrical installation. Right Ans - Flash over on switchboards
due to inadequate kA rating of protective devices
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. Right Ans - To stress the
insulation at a voltage above 325V a.c. (or peak a.c. voltage.
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 the voltage is a d.c. voltage. Right Ans - To ensure capacitive or
inductive reactance does not influence the test result.
A constant maximum voltage is a more stringent test than the momentary
peaks of an a.c. waveform.
, 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.
Define the term current rating as it applies to an HRC fuse? Right Ans - This
is the maximum current that the fuse is designed to carry continuously
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.
Define the term breaking (or rupturing) capacity as it applies to an HRC fuse
Right Ans - The maximum fault current the fuse can safely interrupt.
State TWO technical advantages that a soft starter has when compared to and
electromechanical type starter, such as auto-transformer. Right Ans -
Broad range of options for starting and stopping
Broad range of options for protection.
Visual feedback of key technical parameters.
Smoother starting
Reduced mechanical stress on equipment on starting. Reduced electrical
stress on equipment on starting
Explain the purpose of an interlocks in a star/delta starter Right Ans - To
prevent the star and delta contactors closing simultaneously
State the effect of the interlocks failing in a star/delta starter Right Ans - An
electrical interlock failure could prevent the motor starting. An electrical
interlock failure could cause a phase-to-phase short.
A mechanical interlock failure could prevent the motor starting.
A mechanical interlock failure could cause a phase-to-phase short.
Define the term pullout torque as it applies to an a.c. induction motor Right
Ans - The maximum torque the motor can develop.
State the synchronous speed of a three-phase, four-pole, 50 Hz motor.
Right Ans - 1500 rpm
State the Standard and section of that Standard that specifies the tests and
checks that need to be carried out on the socket outlet and final subcircuit
wiring. Right Ans - AS/NZS 3000
Section 8
