et34

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Author:
gregw
ID:
101007
Filename:
et34
Updated:
2011-09-11 06:41:14
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electrical theory
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electrical
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  1. In an MEN system the neutral is earthed at the star point of a transformer and at the main switchboard of an installation.
    State TWO technical reasons why the neutral is earthed in an MEN system
    • Any TWO of:
    • It ensures that the voltage between P/E and P/N never rises above 230 v under fault conditions..
    • It ensures that between the general mass of earth and any earthed metal there will always be 0 V potential.
    • Provides a parallel path in the event of the loss of the main neutral
    • Provides a low impedance path for the fast operation of the protection
  2. A three-phase induction motor normally runs on full-load. It has been serviced 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 when it 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 following readings:
    L1 – L2 20 ohms
    L1 – L3 open circuit
    L2 – L3 open circuit
    State 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 or open-circuited.
  4. AS/NZS 3760 states that testing the integrity of the insulation of a Class I electrical 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 integrity of the insulation of a Class I electrical appliance
    (ii) Describe how you would carry out an earth leakage test on a Class I electrical appliance.
    Note: A PAT tester is not to be used
    • (i) Any ONE of:
    • Earth continuity (protective earthing conductor) test Visual test
    • (ii)Any ONE of:
    • In-line ammeter
    • Clamp-on ammeter.
    • Turn on the appliance and measure the current in the protective earthing conductor.
  5. State TWO reasons why it is preferable to operate an electrical installation at 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 from the supply.
    • Reduce system losses.
    • Reduce voltage drop in lines and cables
  6. State the reason why the voltage on the secondary terminals of a transformer is less at full load than it is with no load.
    • When current is flowing there is an internal volt drop
    • Due to the impedance of the windings.
  7. Calculate the copper losses of a 100kVA distribution transformer operating at 75% full load when the full-load copper losses are 1200W.
    (75 ÷100)2 x 1200 = 675W
  8. Most appliances sold in New Zealand are designed to operate at 230V or 240V. An electric heater is rated at 2kW when operated at 230V. Calculate the power output when supplied at 240V
    • R = V2/P
    • = 230 x 230 / 2000
    • = 26.45 ohms

    • P = V2/R
    • = 240 x 240 / 26.45
    • = 2180 W

    • Alternative solution
    • ( V2 / V1) 2 x W
    • = ( 240 / 230 ) 2 x 2000
    • = 2177.6 W
  9. An earth fault loop impedance tester was used at the main switchboard in a domestic electrical installation. The following information was recorded:
    233V
    0.175 ohms
    From this information, calculate the prospective short circuit current at the switchboard.
    I PSC = 233 / 0.175 = 1331 A
  10. (i)What is the minimum value of the permitted test result for the insulation resistance test of a three-phase, 400V, mains cable?
    (ii) What is the minimum value of the permitted test result for the insulation resistance test of a 230V, Class I, plug-in electrical appliance?
    • (i) 1M ohm
    • (ii) Any ONE of:
    • 1M ohm
    • 0.01M ohm
  11. Cables carrying large currents are installed in horizontal cable trays.State why is it necessary to securely fasten the cables in the cable trays?
    To prevent movement if a high prospective short-circuit current fault occurs.
  12. A 400 metre cable supplies a sawmill. Apart from length, state TWO other factors that will determine the voltage drop of the cable.
    • Any TWO of:
    • Load current
    • Resistivity of the conductors
    • Cross-sectional area
    • Ambient temperature
  13. TPS flat cable is required to have maximum temperature rating not exceeding 75ºC when operated on full load.
    Explain why it is dangerous to continuously overload the cable so that it exceeds its maximum temperature rating?
    • The insulation deteriorates attemperatures over 75ºC.
    • Any ONE of:
    • The cable may become a fire hazard
    • The cable may become a shock hazard.
  14. A 25mm² single-core XLPE insulated copper conductor has a current rating of 165 amps.
    A 25mm² single core PVC insulated copper cable has a current rating of 143 amps.
    Explain the reason why the XLPE insulated conductor has a higher current rating than the PVC insulated conductor.
    The XLPE cable insulation is able to withstand a much higher continuous temperature
  15. A 35 mm² cable is run 600 metres to a pump station. The insulation resistance of the cable is 140M ohm.Calculate the change in the insulation resistance value if the cable was shortened to 450m when the pump station was relocated.
    • R2 = (R1 x L1) / L2
    • = (600 x 140) / 450
    • = 186.67 M ohm
    • = 186.67 – 140
    • = 46.67 M ohm
  16. When carrying out testing on a low voltage electrical installation, which document details the tests and checks required to be carried out?
    AS/NZS 3000
  17. A new three-phase water heater has been installed in an industrial installation to replace an existing water heater. It has three elements rated at 400V, each with a resistance of 30 ohms. The elements can be connected either in star or delta.No changes will be made to the circuit supplying the new water heater.
    (i) Calculate the line current drawn when the elements are connected in star.
    (ii) Calculate the line current drawn when the elements are connected in delta.
    • (i)
    • IPH = VPH/RPH
    • = 230/30
    • IPH (IL) = 7.66A

    • (ii)
    • IPH = VPH/RPH
    • = 400/30
    • = 13.33A
    • IL = IPH x root3
    • = 13.33 x root3
    • = 23.1A
  18. In a squirrel cage induction motor the rotor bars are skewed at an angle across the face of the rotor.
    State TWO reasons why the rotor bars are skewed rather than being straight.
    • Any TWO of:.
    • Smoother running.
    • More even torque.
    • Less noise.
    • To reduce vibration.
    • Increased torque
  19. List THREE types of reduced-voltage starters (other than electronic) that can be used with a three-phase induction motor.
    • Star/delta.
    • Auto transformer.
    • Primary resistance
  20. Describe the operation of an RCD when there is an active (phase) to earth fault that occurs on the circuit the RCD protects.
    • There is an imbalance between the neutral current and the sum of the phase currents
    • A magnetic field is induced into the iron core
    • The induced magnetic field induces a current in the sensing coil.
    • The tripping coil is energised,opening the RCD contacts
  21. State TWO types of electrical protection provided by a RCBO.
    • Any TWO of:.
    • Earth leakage.
    • Overload.
    • Short circuit
  22. A 230V/100V step-down transformer with 2000 turns on the primary supplies 15A on the secondary when fully loaded.
    (i) Calculate the number of secondary turns.
    (ii) Calculate the primary current when fully loaded.
    (iii) What would be the most common form of cooling for this transformer?
    • (i) NS = NP x VS/VP
    • = 2000 x 100/230
    • = 870 turns

    • (ii) IP = IS x VS/VP
    • = 15 x 100/230
    • = 6.52A

    (iii) Natural air convection
  23. For a double-wound transformer, explain why an increase in the secondary current causes an increase in the primary current.
    • When the load is increased, the secondary current (by Lenz’s law) produces a greater demagnetising force.
    • This reduces the primary flux, reducing the back emf induced in the primary
    • When the back emf in the primary reduces, the supply voltage is increased.
    • More current flows in the primary

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