EWRB EXAM 49 QUESTIONS AND ANSWERS WITH
COMPLETE SOLUTIONS GRADED A++
State the main reason why a large industrial electrical installation has a higher prospective short
circuit current (PSSC) than a domestic electrical installation.
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
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
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.
, 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.
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.
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?
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
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.
COMPLETE SOLUTIONS GRADED A++
State the main reason why a large industrial electrical installation has a higher prospective short
circuit current (PSSC) than a domestic electrical installation.
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
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
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.
, 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.
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.
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?
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
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.
