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AC Theory Practice exam with complete solutions 2024_2025 .
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AC Theory Practice exam with complete solutions 2024_2025 .
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AC Theory Practice exam with completesolutions 2024/2025
In New Zealand, the standard distribution system has the neutral conductor
connected to the earth at the main switchboard on all low voltage installations.
What name is given to this type of system? - ANSWER-Multiple Earthed Neutral or
MEN system.
State the test voltage used for carrying out an insulation resistance test on three-
phase, 400V, electrical equipment incorporating MIMS elements. - ANSWER-500 V
d.c.
State the minimum permitted test result for the insulation resistance test on
electrical equipment without MIMS elements - ANSWER-1MΩ
State the permitted test result for an insulation resistance test for the electrical
installation with all the permanently-connected electrical equipment connected. -
ANSWER-0.01MΩ
State the minimum permitted test result for the insulation resistance test on the
electrical equipment incorporating MIMS elements. - ANSWER-0.01MΩ
State ONE precaution that could be taken when using an insulation resistance
tester, set on the 500V d.c. range, to carry out an insulation resistance test on
electrical equipment containing semi-conductor devices. - ANSWER-Bridge out
between phase and neutral
why you would take this precaution on electrical equipment containing semi-
conductor devices. - ANSWER-The voltage of the test instrument may damage
semi-conductor 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. State ONE reason why the
voltage is a d.c. voltage. - ANSWER-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.
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. - ANSWER-To stress the insulation at a voltage
above 325V a.c. (or peak a.c. voltage).
Tests have been carried out on a three-phase 400V, Class I, plug-in electrical
appliance after it has been repaired. The test results are as follows:
Earth continuity test - out of range
Insulation resistance test - Infinity
What type of fault is indicated by the test results? - ANSWER-Any ONE of:
● A high-resistance joint in the
protective earthing circuit.
● An open-circuit in the protective
earthing conductor.
State THREE actions that need to be taken to ensure the electrical appliance is
safe to return to service. Include acceptable test results in the answer. -
ANSWER-● Repair faulty protective earthing conductor circuit
● Conduct an earth continuity test To ensure the resistance is 1Ω or less
● Any ONE of:
● Conduct an insulation
resistance test. To ensure the insulation
,resistance is 1MΩ or more.
● Conduct an earth leakage
test. To ensure that the leakage does not exceed 5 mA.
Define the term impedance as it applies to an a.c. circuit. - ANSWER-It is the total
opposition in Ω (ohms) to the current flow in an a.c. circuit.
Impedance is comprised of three components. State TWO of those components. -
ANSWER-Any TWO of:
● Resistance (R)
● Inductive reactance (Xl)
● Capacitive reactance (XC)
State one reason why the resistance of the protective earthing conductor of a
flexible cord for a Class I electrical appliance is set at a maximum of 1 Ω. -
ANSWER-• Under fault conditions, it prevents the touch voltages rising to
dangerous levels.
• To ensure sufficient fault current flows to operate the protective device
AS/NZS 3760 details two test methods of verifying that the insulation of a Class I
electrical appliance is in sound condition. For each method state
● The name of the test
● The permitted test result - ANSWER-Insulation resistance test
1 MΩ or 10,000Ω
Earth leakage test
5 mA
State ONE reason why an earth fault-loop impedance test is carried out on the
socket outlet furthest from the main switchboard in a low voltage electrical
installation. - ANSWER-Any ONE of:
● To confirm that the electrical protection for the final subcircuit will operate
within the required time.
, ● To confirm that the electrical protection all other final subcircuits supplied from
the main switchboard will operate with the required time.
describe the path of the earth fault loop being tested between the earth terminal
of the socket outlet and the phase terminal of the socket outlet. - ANSWER-● The
earth terminal of the socket outlet
● The final subcircuit earth conductor
● The main earth bar
● The MEN link
● The main neutral bar
● The consumers main neutral
● The distribution neutral
● The star point
● The transformer winding
● The phase conductor
● The main switch
● The switchboard cables
● The final subcircuit active conductor
● The phase terminal of the socket outlet
State ONE reason why having a protective earthing conductor resistance of less
than 1 Ω in a Class I electrical appliance effectively prevents electric shock
hazards under fault conditions. - ANSWER-Any ONE of:
● Under fault conditions, it prevents the touch voltages rising to dangerous
levels.
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