et36

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Author:
gregw
ID:
100827
Filename:
et36
Updated:
2011-09-10 05:32:43
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electrical theory exam
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electrical theory exam
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  1. MEN switchboards have characteristics that are not found on a distribution switchboard State TWO characteristics of a MEN switchboard that are not found on a distribution switchboard.
    • Any TWO of:
    • An MEN switchboard:/
    • Has an MEN link between the neutral and earth busbars
    • Has a connection to earth via an earthing lead to an earth electrode
    • Is electrically closest to the point of supply
    • Can supply to another MEN switchboard
  2. Why do three phase induction motors run smoother and quieter than single-phase induction motors?
    They have a constant rotating magnetic field
  3. State TWO reasons for using reduced voltage starting for a three-phase induction motor.
    • Any TWO of:
    • Reducing starting current
    • Reducing voltage surges
    • Limiting mechanical shock
  4. A Class I electrical appliance that incorporates a MIMS element has been repaired.Tests have been carried out on the appliance.
    (i) State the permitted insulation resistance test result for a test between live supply conductors and accessible metal parts.
    (ii) State the permitted protective earthing conductor test result for a test between the earth pin of the supply plug and the earthed accessible metal.
    • (i) 0.01 M ohm minimum
    • (ii) 1 ohm maximum
  5. State TWO factors that limit the prospective short-circuit current in the supply to an electrical installation.
    • Any TWO of:
    • Capacity of the supply source
    • Transformer impedance.
    • Circuit impedance
    • Length of cable route between the transformer and the main switchboard
  6. State the reason why the secondary terminal voltage of a transformer is less at full-load than at no-load.
    When current is flowing there is an internal volt drop due to the impedance of the windings.
  7. If the burden on a current transformer is increased, what practical effect, if any, does this have on the primary current?
    No effect
  8. What is the typical maximum secondary voltage of an instrument voltage transformer?
    • Any ONE of
    • 110 V
    • 63.5V
  9. An ohmmeter gives a reading of 24 ohms when used to measure the resistance 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, calculate the difference in the power consumed when the heater is operated on a 230V supply.
    • P = V2/R
    • = 240 x 24024
    • = 2400W

    • P = V2/R
    • = 230 x 23024
    • = 2204.17W
    • = 2400 – 2204.17
    • = 195.83
  10. Explain why earthing the metal frame of a Class I electrical appliance prevents electric shock hazards under fault conditions and also explain what such earthing ensures in relation to protective devices..
    • Under fault conditions the earthing of the metal frame keeps the frame of appliance at earth potential.
    • This ensures the operation of protective devices
  11. What is the minimum value of the permitted test result for the insulation resistance test of a three-phase, 400V, mains cable?
    1 M ohm
  12. What is the maximum value of the permitted test result for an earth leakage test of a 230V, Class I, plug-in electrical appliance?
    5mA
  13. State the TWO types of fault the MCB provides protection for.
    • Short-circuit
    • Overload.
  14. What type of fault does the RCD provide protection for?
    Any earth fault on the circuit.
  15. You have installed a single-phase socket outlet and circuit wiring.
    List the FOUR tests using test instruments – other than an insulation resistance test or a correct circuit connections test – which should be carried out in the socket outlet and circuit wiring.
    • Polarity
    • Earth Continuity
    • Earth Loop Impedance
    • RCD Test
  16. describe how to perform a polarity test
    • This test has to show
    • Use of voltage tester
    • Testing between active and neutral; active and earth; and neutral and earth on the socket outlet
    • Values of 230V between active and neutral and active and earth.0V between neutral and earth.
  17. describe how to perform a RCD test
    • This test has to show
    • Use of RCD tester
    • Plugging test into the socket outlet
    • RCD should operate within 300mS at the maximum residual current of 30 mA.
  18. describe how to perform a Earth loop test
    • This test has to show
    • Use of an earth loop impedance tester
    • Testing between the active conductor and the protective earthing conductor on the socket outlet
    • Impedance low enough to operate protection
  19. An electrician has wired a remote stop-start station to an existing DOL 230V starter circuit that controls a three-phase motor. The DOL starter was working normally before the work commenced. After the work had been completed the motor could be turned on but could not be turned off using either of the stop buttons.What is the cause of this fault?
    The stop buttons have been wired in parallel.
  20. An RCD test has been carried out on a portable residual current device.The device passed the residual current trip test.What additional check must be carried out on the portable residual current device to ensure it is electrically safe?
    It has to be tested to ensure that it is voltage-dependent (that is, it will not automatically reset if supply is restored.)
  21. 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 imbalance regardless of the polarity of the supply.
  22. State the full names for the following terms:
    PRCD
    RCBO
    SRCD
    • PRCD Portable Residual Current Device.
    • RCBO Residual Circuit Breaker with Overcurrent protection
    • SRCD Socket Residual Current Device
  23. Explain ONE practical method used to reduce iron losses when a transformer is being constructed.
    • Any ONE of:
    • The iron core is laminated.
    • Special types of alloys are used for the core

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