2014 Flashcards
State the main reason why a large industrial electrical installation has a
higher prospective short circuit current (PSSC) than a domestic electrical installation.
Any ONE of:
● 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
100 metres of three-phase mains cable gave an insulation test result of 75M. Calculate the insulation resistance of 375m of the same type of cable.
R2 = R1 x L1 / L2
= 75 x 100 / 375
= 20M
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.
Any ONE of:
● 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.
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
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.
Any TWO of:
● 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
State the effect of the interlocks failing in a star/delta starter.The maximum torque the motor can
develop.
Any ONE of:
● 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.
Explain the purpose of an interlocks in a star/delta starter
To prevent the star and delta contactors
closing simultaneously
An electrician has installed a low voltage, three-phase, five-pin, socket outlet and final subcircuit wiring in a bakery. The subcircuit wiring will be connected at the switchboard by another electrician at a later date.
State the document that must be completed once all the testing and
checking has been completed.
A certificate of compliance
Define the term pullout torque as it applies to an a.c. induction
motor.
The maximum torque the motor can
develop.
State the synchronous speed of a three-phase, four-pole, 50 Hz
motor.
1500 rpm
An electrician has installed a low voltage, three-phase, five-pin, socket outlet and final subcircuit wiring in a bakery. The subcircuit wiring will be connected at the switchboard by another electrician at a later date.
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.
● AS/NZS 3000
● Section 8
State the THREE tests, using test instruments that the electrician needs to carry out on the final subcircuit cable and socket outlet to ensure they are safe to connect to the electrically supply.
For each test:
● State the test instrument used.
● State the test voltage, is applicable
● Describe how you would carry it out.
● State the permitted test result (if applicable).
Test 1
Protective earthing conductor test
● Ohmmeter
● Test between the end of the protective earthing conductor of the cable
And the earth pin on the socket outlet.
● The resistance of the PEC is consistent with the characteristics of the cable.
Test 2
Insulation resistance test
● Insulation resistance tester
● 500V d.c.
● Test between each phase conductor
Test between each phase conductor
and the earth conductor
Test between each phase conductor
and the neutral conductor
Test between neutral conductor and
the earth conductor
● 1 MΩ minimum
Test 3
Correct circuit connections test (or polarity)
● Ohmmeter
● Each phase connected to the correct phase pin
● Neutral connected to neutral pin
● Earth connected to the earth pin
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 potentially hazardous situations that could arise when the
installation was livened.
● The main fuse will not operate due to
the high impedance of the earth fault
loop.
● Any metal not normally live could be
live at up to 230V.
● The main switch does not isolate the
installation.
● Internal components of equipment are
live with control switch in the off
position.
● Fire hazard at a high-resistance joint
New mains have been installed in a 230V, single-phase domestic
installation. The installation has been livened.
The main switch and MEN system are contained within a metal meter box.
(i) State the equipment you would use to determine whether or not a
phase/neutral transposition has occurred on the mains.
A voltmeter
Remote (independent) earth
Trailing lead
New mains have been installed in a 230V, single-phase domestic
installation. The installation has been livened.
The main switch and MEN system are contained within a metal meter box.
The equipment you would use to determine whether or not a
phase/neutral transposition has occurred on the mains is a:
- A voltmeter
- Remote (independent) earth
- Trailing lead
Describe how you would use the equipment stated above to test for
a phase/neutral transposition on the mains. Include in your description:
- The test results if there is no transposition
- The test results if there is a transposition
● Use the voltmeter to test at the
supply side of the main switch.
● Use the voltmeter to test at the
earth/neutral bar.
No transposition
● About 230V at the supply side of
the main switch.
● About 0V if at the earth/neutral
bar.
With transposition
● About 0V at the supply side of the
main switch.
● About 230V if at the earth/neutral
bar.
New mains have been installed in a 230V, single-phase domestic
installation. The installation has been livened.
The main switch and MEN system are contained within a metal meter box.
The equipment you would use to determine whether or not a
phase/neutral transposition has occurred on the mains is a:
- A voltmeter
- Remote (independent) earth
- Trailing lead
Describe how you would use the equipment stated above to test for
your own safety before testing for a phase/neutral transposition on the
mains.
● Insert the remote earth stake in
the ground a reasonable distance
away from the installation earth
● Use the trailing lead to connect
the remote earth to the voltmeter.
● Use the voltmeter whether the
frame of the meter box is live.
