et 37

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Author:
gregw
ID:
100689
Filename:
et 37
Updated:
2011-09-10 04:14:22
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electrical theory
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et test
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  1. If the voltage and current are known, what TWO other factors are required to calculate the power consumed by a three-phase motor?.
    root3 Power factor.or 3Power factor
  2. What TWO factors are required to calculate the power consumed by asingle-phase electrical appliance?
    • Any ONE of:.
    • VoltageCurrent.
    • VoltageResistance.
    • CurrentResistance
  3. The direction of rotation of a 230V, single-phase motor has been reversed by:
    (i) Reversing the connections to the run winding. What type of single phase motor is being reversed?
    • Any ONE of:
    • Split-phase motor
    • Capacitor start motor
    • Cap-start, cap-run motor
  4. The direction of rotation of a 230V, single-phase motor has been reversed by:
    (ii) Reversing the connections to the armature brushes. What type ofsingle-phase motor is being reversed?
    • Any ONE of:
    • Universal motor
    • Series motor
  5. One reason why the metal frame of a Class I electrical appliance is earthed is to ensure the operation of a protective device under fault conditions.State ONE other reason why the metal frame of a Class I electrical appliance is earthed.
    Under fault conditions the earthing of the metal frame keeps the frame of appliance at earth potential.
  6. State TWO factors that determine the severity of electric shock upon the human body.
    • Voltage applied..
    • Shock current level..
    • Contact duration..
    • Skin dryness..
    • Current path.
  7. Protective devices have been installed in an electrical installation. The devices have a fault duty lower than the prospective short circuit current rating of the installation.State ONE reason why it is not desirable to have protective devices of a lower fault duty than the prospective short circuit current rating of the installation.
    • Any ONE of:.
    • A flash-over could occur on the switchboard due to inadequate kA rating of protective devices.
    • Damage to the protective devices
    • Cannot contain the level of faul tcurrent
  8. Some fittings (not protective devices) have been installed in an electrical installation. The fittings have a fault duty lower than the prospective short circuit current rating of the installation.
    State ONE reason why it is not desirable to install fittings of a lower fault duty than the prospective short circuit current rating of the installation.
    • Any ONE of:.
    • A fire hazard could occur due to fittings overheating under fault conditions.
    • Damage to equipment
  9. You have carried out a test on a low voltage final subcircuit in an electrical installation. The test result value was 20Mohms
    (i) What type of test has been carried out?
    Insulation resistance test
  10. The test was carried out with a multi-meter that has the required functions (scales).
    (A) What function (scale) was selected on the multi-meter?
    (B) What range was selected on the multi-meter?
    • (A) Any ONE of:
    • Insulation resistance testfunction
    • 500V d.c
    • (B) Any ONE of:
    • The Megohm range
    • 500V d.c.
  11. What minimum test result would you expect if the final subcircuit incorporated a fixed-wired appliance with MIMS elements?
    0.01 M ohm; or 10,000 ohm
  12. State the main reason why interlocks are used in a star/delta starter.
    . To prevent the star and delta contactors closing simultaneously. And causing a short-circuit.
  13. Calculate the power factor of a small three-phase printing factory with mainly motor loading. Metering instruments have recorded the following information:
    Power = 30000W
    Volts = 406V
    Amps = 57A per phase
    • pf = P/root3 x V x A
    • pf = 30000/root3 x 406 x 57
    • pf = 0.748
  14. In relation to HRC fuses:
    (i) Define the term “cut off time” as it applies to HRC fuses.
    • Any ONE of:
    • Pre-arcing time Time taken to melt the fuse elements
    • Arcing time The time taken between the initiation of the arc and the arc extinguishing
    • Cut off current The fault current that will cause the fuse to blow
    • Total clearing time The time it takes to interrupt the flow of current and extinguish the flame.
  15. In relation to HRC fuses
    (ii) Define the term “total clearing time” as it applies to HRC fuses
    This is the time it takes to interrupt the flow of current and extinguish the flame.
  16. State the purpose of the slip-rings on a wound rotor induction motor (or slip-ring motor).
    To connect the external resistance to the wound rotor
  17. A MEN, low voltage domestic installation is required to have a main earthing conductor.(i) To which parts of the electrical installation are the ends of the main earthing conductor connected?
    • The earth bar
    • The earth electrode
  18. What is required to be attached at each end of the main earthing conductor?
    Tags warning against disconnection
  19. The primary of a three phase, delta-star transformer is connected to a three phase supply.
    The secondary line voltage is 400V.
    The turns ratio is 2859:20.
    When the transformer is fully loaded, the full load primary line current is 4.37A.
    Assume that there are no losses in the transformer.

    (a) Calculate the primary voltage.
    • Vph = V/ root 3 (½ mark)
    • = 400/ root 3 (½ mark)
    • = 230.9V (1 mark)
    • Ns = Vs/Np Vp/Vp = Np x Vs/Ns (½ mark)
    • = 142.95 X 230.91 (½ mark)
    • = 33007 V (1 mark)
  20. State the maximum permitted rated residual current of an RCD installed for:
    (i) Personal protection in a domestic installation.
    (ii) Personal protection in a medical-electrical installation
    (iii) The protection of property
    • (i) 30 milliamps.
    • (ii) 10 milliamps
    • (iii) Between 100 milliamps and 300milliamps.
  21. A person is using a Class I electrical appliance supplied from an RCD protected final subcircuit. An earth leakage fault occurs and earth leakage current flows through the person’s body.Why doesn’t the person receive a severe electric shock when the earth leakage current flows through their body?
    Because the RCD trips in milliseconds
  22. A Residual Current Circuit Breaker (RCCB) is installed in a single phase circuit.Explain why the RCCB does not operate when there is a short circuit between active (phase) and neutral in the single phase circuit?
    • Any ONE of:
    • The short circuit does not produce an imbalance between the phase an neutral currents in the RCCB
    • The RCCB does not incorporate an overload function.
  23. In New Zealand, what type of RCD is required to be voltage dependent.
    Portable Residual Current Device
  24. Briefly explain why the type of RCD (Portable Residual Current Device) is required to be voltage dependent.
    • To ensure that the PRCD opens when supply is lost.
    • And that it must be manually reset after supply has been restored.
  25. The phase and neutral on the supply side of a single phase RCD have been transposed.Would the RCD operate if it detected an earth leakage fault on the circuit it protects?
    State a reason to support your answer.
    • Yes
    • The RCD detects the im balance regardless ofthe polarity of the supply.
  26. State the full names for the following terms:
    RCBO
    SRCD
    • RCBO Residual Circuit Breaker with Overcurrent protection
    • SRCD Socket Residual Current Device
  27. You are going to carry out testing using a multi-meter that has the required functions (scales) to verify whether a mains phase/neutral transposition has taken place.
    (i) State the function (scale) on the multi-meter you would select.
    (ii) State the range on the multi-meter you would select.
    (iii) State the additional equipment that should be used with the multimeter.
    • (i) a.c. voltage
    • (ii) Any ONE:
    • A range that will read 230V
    • Auto range
    • (iii) A temporary earth stake
    • An earth lead
  28. The low voltage installation is protected on the distribution supply system by a 60A rewireable pole fuse.State the main reason why a phase and neutral transposition on the mains will not operate (“blow”) the pole fuse.
    • Any ONE of:
    • The fault current is too low to blow the fuse.
    • Because the impedance of the fault loop is too high.
  29. Metering in a small 400V, three-phase, 4-wire commercial installation has produced the following results:
    Red 60kW
    White 173.91A
    Blue 82kW
    Assume a unity power factor
    (a) Calculate the total power of the installation
    (b) Determine the neutral current
    • (a) White phase P = V x I=230 x173.91=39999.3W
    • Total powerP = R+W+B = 60+39.99+82=181.99kW

    • (b) Blue phase I = PV= 82000 230= 356.62A
    • Red phase I = PV= 60000 230= 260.89A

    I = 150A (accept any answer between145 and 160A)
  30. a motor is to be started direct on-line and protected by a thermal overload unit and HRC fuses. The thermal overload unit is incorporated in the DOL starter and protects the motor from overloads.Briefly explain the term overload as it relates to the thermal overload.
    A current larger that the current setting of the thermal overload.That is sustained long enough to operate the overload
  31. What would be the typical connection for a three-phase, 400V, 20kW motor?
    • Any ONE of:
    • Delta connection
    • DOL starter
    • Star/delta starter
  32. What does the term IP mean?
    • Any ONE of:
    • International protection
    • Ingress protection
  33. An insulation resistance test is carried out with a test voltage of 500V d.c
    (i) Briefly explain why a voltage of 500V is applied.
    (ii) Briefly explain why the voltage is a d.c. voltage.
    • (i) To stress the insulation at levels above the nominal operating (or peak) voltage.
    • (ii) 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.
  34. You carried out the insulation resistance test and the result for the entire installation was 0.2 M ohms.State the most likely reason why the test result is lower than the permitted value of 1 M ohm.
    Either the range or the water heater (or both) have insulation resistance much lower than 1 M ohm;
  35. You proceeded to certify an installation on a Certificate of Compliance with a insulation resistance test result of 0.2 M ohm.Describe how you carried out the insulation resistance test so that you could verify that this installation complied with AS/NZS 3000.Include in your description the permitted minimum value of any testresults.
    • The test method described has to show:.
    • The MEN link and main neutral disconnected. Disconnection of the range and water heater.
    • All switches and circuit breakers in the “ON” position
    • Testing between live parts and earth.
    • A minimum of 1 M ohm; test value for the installation. Testing between phase/neutral and earth of the water heater. A minimum value of 10,000;..
    • Testing between phase/neutral and earth of the range. A minimum value of 10,000;.

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