2011-Main

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157993
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2011-Main
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2012-06-09 23:12:01
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  1. Describe the starting sequence of a glow-start single-tube fluorescent lightfitting.
    The glow starter bimetal strip bendsand closes the glow switch contacts.(½ mark)● Current then flows through the ballastand tube electrodes(1 mark)● Which heat up and cause ionisation ofthe gas in the tube at each end.(1 mark)● The glow switch contacts open (½ mark)● And a high-voltage surge fromcollapsing magnetic field in ballast,appears across tube and causes it tostrike.
  2. State the TWO purposes of the fluorescent powder coating found on theinside of a fluorescent tube.
    To protect the eyes against the harmfuleffects of UV rays.● Creates the light colour● Converts UV light into visible light
  3. In an MEN system the loop through the main neutral conductor is typicallyof lower impedance than the loop through the mass of earthIn an electrical installation the main neutral conductor and the mainearthing conductor are carrying similar currents. State the main reasonwhy this situation would occur.
    There is a high resistance joint in the mainneutral conductor.
  4. State TWO factors that determine the level of the prospective short-circuitcurrent in an electrical installation.
    TWO of:● Capacity (kA) of the supply source● The impedance of the earth fault loop.● Supply voltage.
  5. State the EIGHT components of an MEN system that are found in a MENlow voltage electrical installation.
    The main earth bar (½ mark)● The main earthing conductor. (½ mark)● The earth electrode (½ mark)● The MEN connection (link) (½ mark)● The main neutral bar (½ mark)● The main neutral conductor (½ mark)● Protective earthing conductors (½ mark)● Equipotential bonding conductors
  6. State the main function of the main earthing conductor and the earthelectrode in a low voltage, MEN electrical installation
    To connect the parts of an installationrequired to be earthed to the mass ofearth.● To keep the neutral potential to 0V● To keep the phase potential to 230V
  7. Explain why Class II (double-insulated) electrical equipment is notrequired to be connected to a protective earthing conductor.
    There is supplementary insulation betweenthe basic insulation and any metal that couldbecome live under fault conditions.
  8. Electronic starters are often used with three-phase a.c. induction motors.Explain how the operation of an a.c. variable speed controller is different tothe operation of an electronic soft starter.
    The variable speed controller variesfrequency and voltage to vary thespeed.(1 mark)● The electronic soft start varies voltageon starting to limit starting current.
  9. Explain the term ramp time as it relates to an a.c. variable speedcontroller
    The adjustment that controls the length oftime taken for a change in speed to takeplace.
  10. A 400V, three-phase motor is started with a DOL (direct-on-line) starter.When started, the motor draws 90A and produces 200% full-load torque.Calculate the starting current if the same motor is started with astar/delta starter.
    I in star = I DOL x 0.33 (½ mark)= 90 x 0.33 (½ mark)= 30A
  11. A 400V, three-phase motor is started with a DOL (direct-on-line) starter.When started, the motor draws 90A and produces 200% full-load torque.Calculate the percentage (%) of starting torque if the same motor is startedwith an auto-transformer starter with an 80% tap
    FLT DOL x 0.82 (½ mark)= 200 v 0.64 (½ mark)= 128%
  12. Explain why rewireable fuses are classed as course excess currentprotection.
    ● Rewireable fuses have a variableutilisation category (fusing factor).(1 mark)● The utilisation category is usually about2
  13. Explain why Gg type HRC fuses and MCBs are classed as close currentprotection.
    Gg HRC fuses and MCBs have a specificutilisation category (fusing factor).(1 mark)● The utilisation category (fusing factor)is not more than 1.5
  14. HRC fuses are used to provide short-circuit and overload protection to athree-phase motor final subcircuit.State the main reason why the HRC fuses do not provide overloadprotection for the motor.
    The HRC only provide short-circuitprotection for the circuit
  15. Apart from cost, state TWO technical advantages that HRC fuses havewhen compared to MCBs.
    Higher kA rating (½ mark)● Speed of operation
  16. Describe the term back-up protection in relation to the protection of finalsubcircuits.
    A protective device of adequaterupturing capacity is installed(1 mark)● In addition to a protective device ofinadequate rupturing capacity
  17. Describe the term discrimination in relation to electrical protection in anelectrical installation
    The protective device closest to thefault operates(1 mark)● Before any other protective device canoperate
  18. State TWO methods used to cool transformers.
    Air cooled.● Oil cooled.● Pumped or natural convection.● Forced or natural ventilation
  19. 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)= 26.45C (1 mark)P = V2R= 240 x 24026.45= 2177.6W
  20. Every low voltage electrical installation has a switchboard.(a) A low voltage, MEN electrical installation incorporates an MEN switchboardand a distribution switchboard.(i) Which switchboard would be connected electrically closest to thepoint of supply?
    The MEN switchboard
  21. State THREE other features of MEN switchboard which are not foundwith a distribution switchboard.
    It has an MEN link between theneutral and earth busbars● It is connected to earth via anearth electrode● The MEN switchboard can supplyanother MEN switchboard● The main neutral can beconnected
  22. State TWO methods of connecting a low voltage, MEN electrical installationto the mass of earth.
    Earth rod· Buried electrode· Metallic reinforcing
  23. A low voltage, MEN electrical installation incorporates only one MENswitchboard. In the MEN switchboard the MEN link between the mainneutral bar and main earth bar is missing.State THREE factors that would occur if there was an earth fault in theinstallation when the MEN link was missing.
    The fault current path would flowthrough the mass of earth to the starpoint of the distribution transformer.● The impedance of the earth is generallymuch higher than the main neutral● The fault current may be limited to avalue insufficient to operate theprotective device.● If protection does not operate therecan be a shock hazard on metal-framedappliances.
  24. A low voltage, MEN domestic installation is required to have a mainearthing lead.(i) To which parts of the electrical installation are the ends of the mainearthing lead connected?
    The earth bar● The earth electrode
  25. What is required to be attached one end of the main earthing lead?
    A tag warning against disconnection
  26. IntroductionA three-phase, 4-pole, induction motor has the following characteristics:● Frequency 50 Hz● Line voltage 400V● Line current 8.5A● Efficiency 85%● Slip 4%● The line current lags the voltage by a phase angle of 350Use the information in the introduction to this question answer parts(a), (b) and (c).(a) Calculate the rotor speed of the motor

    (b) Calculate the power factor of the motor

    (c) Calculate the output power of the motor.
    (a) N = 60fP(½ mark)= 60 x 502(½ mark)= 1500 rpm (½ mark)Slip speed = N x slip (½ mark)= 1400 x 4% (½ mark)= 60 rpm (½ mark)Rotor speed = N – slip speed (½ mark)= 1500 - 60 (½ mark)= 1440 rpm (1 mark)(b) pf = cosФ (½ mark)= cos 350= 0.8191 lag (½ mark)(c) Input power = √3 VL IL cosФ (½ mark)= √3 x 400 x 8.5 x0.8191(½ mark)= 4823 W (1 mark)Output power = input power x efficiency (½ mark)= 4823 x 0.85 (½ mark)= 4099 W
  27. State THREE actions that need to be taken to ensure the electricalappliance complies with AS/NZS 3760 and is safe to return to service
    Repair faulty protective earthingconductor circuit(1 mark)● Conduct a protective earthingconductor test to ensure theresistance is 1 or less(1 mark)● Conduct an insulation resistancetest to ensure the insulationresistance is 1M or more
  28. AS/NZS 3000 requires that a test voltage of 500V d.c. is used for aninsulation resistance test of a low voltage electrical installation.(i) State ONE reason why a voltage of 500V is applied
    Any ONE of:● To stress the insulation at avoltage above 325V a.c. (or peaka.c. voltage.● To stress the insulation at avoltage of twice the r.m.s voltageof 250V.
  29. AS/NZS 3000 states that the minimum insulation resistance test results fora low voltage installation is 1M@. However, the figure can be reduced to10,000@ if there are MIMS elements in the installation.State the reason why the lower insulation resistance result is permitted.
    Any TWO of: (2 marks)● MIMS elements absorb moisture.● MIMS elements are electrically safeeven though the insulation resistance iswell below 1M C.● The insulation resistance of MIMSelements rises as the elements heat.
  30. State the main reason why a polarity test is carried out on the mains of asingle-phase, low voltage electrical installation.
    To ensure that no phase/neutraltransposition has occurred.
  31. An earth loop impedance test at the main switchboard of a low voltageelectrical installation shows a result of 50@State ONE reason why such a high test result would be obtained
    Any ONE of:● The main neutral is open-circuited● High resistance joint in the mainneutral
  32. The method of installing cables in electrical installations depends on theenvironment in which the wires and cables are to be installed.(a) TPS sheathed PVC cables are installed in an electrical installation, bothindoors and outdoors.(i) State TWO reasons why some TPS sheathed cables would be installedin PVC conduit.
    Any TWO of: (2 marks)● Because of corrosiveenvironments● To provide mechanical protection.● To allow cables to be installedunderground● To allow cables to be installed inconcrete● To provide environmentalprotection
  33. State TWO design requirements to be considered when PVC conduit isused outdoors
    Impact protection● Effect of UV rays● Use of expansion joints● Environmental conditions
  34. Extra-low voltage cables are installed in the same conduit as lowvoltage PVC cables.State the reason why the extra-low voltage cables are insulated to230V.
    To ensure that any fault on the lowvoltage system is not transferred to theextra-low voltage system
  35. Within the installation cables are installed in open cable ladders and opencable trays.(i) Some cables carry large currents.State the reason why is it necessary to securely fasten cables carryinghigh currents when installed in horizontal cable trays.
    To prevent cable movement if a highprospective short-circuit current faultoccurs.
  36. Telecommunication cables are installed in a separate compartment tolow voltage cables in the same cable tray.State the reason why the telecommunication cables are installed in aseparate compartment.
    To ensure that any fault on thelow voltage system is nottransferred to thetelecommunication system.● To voltage is not induced into thetelecommunications system.
  37. What provision is made where an open cable trays passes through thefire-rated wall of a switch room?
    The opening through which the cabletray passes is sealed with a fireresistantsealant.
  38. State the main effect on TPS sheathed cables that are installed throughwalls containing thermal insulation.
    The thermal insulation reduces the currentcarrying capacity of the cables.
  39. State TWO factors that reduce the life of an incandescent lamp
    TWO of:● Over voltage● Vibration● Ventilation● Frequent switching (in-rush current)
  40. State the main colour emitted by a high-pressure sodium lamp.
    Yellow
  41. State the main colour emitted by a high-pressure mercury vapourlamp.
    White
  42. Fluorescent lighting is installed mainly in commercial or industrialinstallations.(i) State the TWO main functions of a ballast in a fluorescent light fitting.
    To limit the current flowingthrough the electrode and tube.(1 mark)● To produce a high voltage whenthe starter opens to start the tubeglowing.
  43. A power-factor correction capacitor is connected across the supply to afluorescent fitting to improve the power factor of the lighting finalsubcircuit.State why the power factor needs correction?
    Because the fitting ballast is highlyinductive.
  44. An EMI suppressor (capacitor) is connected across the glow starter ofa fluorescent light fitting to prevent EMI.If the EMI suppressor (capacitor) short-circuits, state what the effectwould be on the operation of the fluorescent light fitting.
    Any ONE of:● The lamp does not start.● The lamp only glows at the ends.● The lamp does not strike.
  45. A machine shop includes rotating machinery and has incandescent andfluorescent lights installed.State the reason why incandescent lighting was installed together withfluorescent lighting in the machine shop.
    To reduce the stroboscopic effect of thefluorescent lighting.
  46. List the FOUR essential requirements for good lighting
    Sufficient light for the application● Minimum glare● Uniform light● Correct colour
  47. State TWO precautions that must be taken when handling and installinghalogen lamps
    Do not touch with naked finger● Install in horizontal position only● Do not cause excessive vibration
  48. A three-phase RCD protects a final subcircuit supplying a three-phase, 5pin socket outlet.Describe the internal operation of the three-phase RCD when there is anactive (phase) to earth fault in an electrical appliance that is plugged intothe 5 pin socket outlet
    There is an imbalance between theneutral current and the sum of thephase currents(1 mark)● A magnetic field is induced into theiron core.(1 mark)● The induced magnetic field inducesa voltage in the sensing coil(1 mark)● The tripping coil is energised,opening the RCD contacts
  49. An RCD trip test has been carried out on a PRCD (portable residual currentdevice). The device passed the trip test.What additional check must be carried out on the PRCD to ensure it iselectrically safe?
    It has to be tested to ensure that it isvoltage-dependent (that is, it will notautomatically reset if supply is restored.)
  50. An existing 230V MEN domestic electrical installation has no RCDprotection on the switchboard.A new socket outlet is to be installed in Zone 2 of a bathroom from anexisting final subcircuit that supplies three socket outlets.State TWO methods, either of which will provide RCD protection for to thenew 230V socket outlet in the bathroom
    Any TWO of:● Install RCD at the switchboard atthe origin of the final subcircuit.● Install an SRCD on the bathroomsocket outlet.● Install an SRCD on a socket outletupstream from the 1st new socketoutlet.
  51. An RCBO protects a single-phase final subcircuit. The RCBO providesprotection against earth leakage faults.State TWO other types of faults the RCBO provides protection from.
    Overcurrent● Short circuits
  52. The phase and neutral on the supply side of a single phase RCD have beentransposed.Would the RCD operate if it detected an earth leakage fault on the circuit itprotects? State a reason to support your answer.
    Yes (½ mark)The RCD detects the imbalanceregardless of the polarity of the supply.

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