2010 Questions

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2010 Questions
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  1. In an MEN system the neutral is earthed at the star point of atransformer and at the main switchboard of an installation.State TWO technical reasons why the neutral is earthed in an MENsystem
    Any TWO of: (2 marks). It ensures that the voltage betweenP/E and P/N never rises above 230vunder fault conditions.. It ensures that between the generalmass of earth and any earthedmetal there will always be 0Vpotential. Provides a parallel path in the eventof the loss of the main neutral Provides a low impedance path forthe fast operation of the protection
  2. A three-phase induction motor normally runs on full-load. It has beenserviced and you are re-commissioning it. When you load test the motor you find that it is now required to run lightly loaded.State why it is not desirable to allow a motor to run lightly loaded whenit normally runs on full load.
    The power factor of the motor will be poor
  3. The tests on a star-connected squirrel cage motor show the followingreadings:L1 – L2 206L1 – L3 open circuitL2 – L3 open circuitState the most likely fault indicated by these readings.
    Any ONE of:. An open circuit in the L3 winding.. One of the links could be missing oropen-circuited.
  4. AS/NZS 3760 states that testing the integrity of the insulation of a Class Ielectrical appliance can be carried out either by:< An insulation resistance test< An earth leakage test.(i) State the type of test you would carry out before testing the integrityof the insulation of a Class I electrical appliance
    Any ONE of: (½ mark) Earth continuity (protectiveearthing conductor) test Visual test
  5. Describe how you would carry out an earth leakage test on a Class Ielectrical appliance.Note: A PAT tester is not to be used
    Any ONE of:- In-line ammeter- Clamp-on ammeter

    Turn on the appliance andmeasure the current in theprotective earthing conductor.
  6. State TWO reasons why it is preferable to operate an electrical installationat near unity power factor rather than at 0.85 power factor.Note: Cost is not a reason.
    Any TWO of:. Lower the line current drawn fromthe supply. Reduce system losses. Reduce voltage drop in lines andcables
  7. State the reason why the voltage on the secondary terminals of atransformer is less at full load than it is with no load.
     When current is flowing there is aninternal volt drop(1 mark) Due to the impedance of thewindings
  8. Calculate the copper losses of a 100kVA distribution transformer operatingat 75% full load when the full-load copper losses are 1200W.
    = (75 ÷100)2 x 1200 (1 mark)= 675W
  9. Most appliances sold in New Zealand are designed to operate at 230V or240V. An electric heater is rated at 2kW when operated at 230V. Calculatethe power output when supplied at 240V
    R = V2P(½ mark)= 230 x 2302000(½ mark)= 26.45< (½ mark)P = V2R= 240 x 24026.45= 2180W (½ mark)Alternative solution( V2 / V1) 2 x W= ( 240 / 230 ) 2 x 2000= ( 240 / 230 ) 2 x 2000= 2177.6W
  10. An earth fault loop impedance tester was used at the main switchboard in adomestic electrical installation. The following information was recorded:< 233V< 0.175WFrom this information, calculate the prospective short circuit current at theswitchboard.
    IPSC = 2330.175W(1 mark)= 1331 A
  11. What is the minimum value of the permitted test result for theinsulation resistance test of a three-phase, 400V, mains cable?
    (i) 1M< (1 mark)
  12. What is the minimum value of the permitted test result for theinsulation resistance test of a 230V, Class I, plug-in electricalappliance?
    Any ONE of: (1 mark) 1M< 0.01M<
  13. MEN switchboards have characteristics that are not found on a distributionswitchboardState TWO characteristics of a MEN switchboard that are not found on adistribution switchboard.
    Any TWO of:An MEN switchboard:/ Has an MEN link between the neutraland earth busbars/ Has a connection to earth via anearthing lead to an earth electrode/ Is electrically closest to the point ofsupply/ Can supply to another MEN switchboard
  14. Why do three phase induction motors run smoother and quieter thansingle-phase induction motors?
    They have a constant rotating magnetic field
  15. State TWO reasons for using reduced voltage starting for a three-phaseinduction motor.
    Any TWO of:/ Reducing starting current/ Reducing voltage surges/ Limiting mechanical shock
  16. A Class I electrical appliance that incorporates a MIMS element has beenrepaired.Tests have been carried out on the appliance.(i) State the permitted insulation resistance test result for a test betweenlive supply conductors and accessible metal parts.
    0.01 M6 minimum
  17. State the permitted protective earthing conductor test result for a testbetween the earth pin of the supply plug and the earthed accessiblemetal.
    16 maximum
  18. State TWO factors that limit the prospective short-circuit current in thesupply to an electrical installation.
    Any TWO of:/ Capacity of the supply source/ Transformer impedance./ Circuit impedance/ Length of cable route between thetransformer and the main switchboard
  19. State the reason why the secondary terminal voltage of a transformer isless at full-load than at no-load.
    When current is flowing there is an internalvolt drop due to the impedance of thewindings.
  20. If the burden on a current transformer is increased, what practicaleffect, if any, does this have on the primary current?
    No effect
  21. What is the typical maximum secondary voltage of an instrumentvoltage transformer?
    Any ONE of 110 V 63.5V
  22. An ohmmeter gives a reading of 24 ohms when used to measure theresistance of a plug-in heater designed for use on 230V/240V supply.If the heater draws 10 amps when plugged into a 240 V supply, calculatethe difference in the power consumed when the heater is operated on a230V supply.
    P = V2R= 240 x 24024= 2400WP = V2R(½ mark)= 230 x 23024(½ mark)= 2204.17W (½ mark)= 2400 – 2204.17 = 195.83
  23. Explain why earthing the metal frame of a Class I electrical applianceprevents electric shock hazards under fault conditions and also explain whatsuch earthing ensures in relation to protective devices..
    Under fault conditions the earthing ofthe metal frame keeps the frame ofappliance at earth potential.(1 mark)/ This ensures the operation of protectivedevices
  24. What is the minimum value of the permitted test result for theinsulation resistance test of a three-phase, 400V, mains cable?
    1M6
  25. What is the maximum value of the permitted test result for an earthleakage test of a 230V, Class I, plug-in electrical appliance?
    5mA
  26. If the voltage and current are known, what TWO other factors arerequired to calculate the power consumed by a three-phase motor?
    Any ONE of: (1 mark). /3Power factor. 3Power factor
  27. What TWO factors are required to calculate the power consumed by asingle-phase electrical appliance?
    Any ONE of:. VoltageCurrent. VoltageResistance. CurrentResistance
  28. The direction of rotation of a 230V, single-phase motor has been reversedby:(i) Reversing the connections to the run winding. What type of singlephasemotor is being reversed?
    Any ONE of: (1 mark) Split-phase motor Capacitor start motor Cap-start, cap-run motor
  29. Reversing the connections to the armature brushes. What type ofsingle-phase motor is being reversed?
    Any ONE of: Universal motor Series motor
  30. One reason why the metal frame of a Class I electrical appliance is earthedis to ensure the operation of a protective device under fault conditions.State ONE other reason why the metal frame of a Class I electricalappliance is earthed.
    Under fault conditions the earthing of themetal frame keeps the frame of appliance atearth potential.
  31. State TWO factors that determine the severity of electric shock upon thehuman body.
    . Voltage applied.. Shock current level.. Contact duration.. Skin dryness.. Current path.
  32. Protective devices have been installed in an electrical installation. Thedevices have a fault duty lower than the prospective short circuitcurrent rating of the installation.State ONE reason why it is not desirable to have protective devicesof a lower fault duty than the prospective short circuit current rating ofthe installation.
    Any ONE of:. A flash-over could occur on theswitchboard due to inadequate kArating of protective devices. Damage to the protective devices Cannot contain the level of faultcurrent
  33. Some fittings (not protective devices) have been installed in anelectrical installation. The fittings have a fault duty lower than theprospective short circuit current rating of the installation.State ONE reason why it is not desirable to install fittings of a lowerfault duty than the prospective short circuit current rating of theinstallation.
    (ii) Any ONE of:. A fire hazard could occur due tofittings overheating under faultconditions(1 mark). Damage to equipment
  34. You have carried out a test on a low voltage final subcircuit in an electricalinstallation. The test result value was 20M.(i) What type of test has been carried out?
    Insulation resistance test
  35. The test was carried out with a multi-meter that has the requiredfunctions (scales).(A) What function (scale) was selected on the multi-meter?(½ mark)____________________________________________________(B) What range was selected on the multi-meter?
    Any ONE of: Insulation resistance testfunction 500V d.c(½ mark)(B) Any ONE of: The Megohm range 500V d.c.
  36. What minimum test result would you expect if the final subcircuitincorporated a fixed-wired appliance with MIMS elements?
    0.01 M; (or 10,000<)
  37. State the main reason why interlocks are used in a star/delta starter
    To prevent the star and deltacontactors closing simultaneously(1 mark). And causing a short-circuit.
  38. Calculate the power factor of a small three-phase printing factory withmainly motor loading. Metering instruments have recorded the followinginformation:= Power = 30000W= Volts = 406V= Amps = 57A per phase
    pf = P/3 x V x A(½ mark)= 30000/3 x 406 x 57(½ mark)= 0.748
  39. In relation to HRC fuses:(i) Define the term “cut off time” as it applies to HRC fuses.
    Any ONE of: Pre-arcing timeTime taken to melt the fuseelements Arcing timeThe time taken between theinitiation of the arc and the arcextinguishing Cut off currentThe fault current that will causethe fuse to blow Total clearing timeThe time it takes to interrupt theflow of current and extinguish theflame.
  40. Define the term “total clearing time” as it applies to HRC fuses
    This is the time it takes to interrupt theflow of current and extinguish theflame.
  41. State the purpose of the slip-rings on a wound rotor induction motor (orslip-ring motor).
    To connect the external resistance to thewound rotor.

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