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Exam question

Eclipse
Guys,

can anyone explain the how the following answer to this  question is derived ( its from an old test paper) which my son is doing, 

The maximum tabulated value of earth fault loop impedance for a circuit is: 1.65 ohms, if the resistance of the line to cpc of a new circuit is 1.13ohms  the maximum accepted external earth fault loop impedance is:

A. 0.52 OHMS

B. 0.45 OHMS

C. 0.35 OHMS

D.0.20 OHMS


The answer given to him is "D"  but no explanation as to how its worked out,

really appreciate any feedback!!
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    Chris Pearson:

    Part of it is in Appendix 3 now, but that old Appendix 14 addresses exactly the point that I was making.


    The appendix 3 (Informative) of B.S. 7671 has an equation for Zs(m). Can we use that here?


    Also the 0.8 is not identified as to its purpose as are the other equation components.


    Z.


Reply
  • 0
    Chris Pearson:

    Part of it is in Appendix 3 now, but that old Appendix 14 addresses exactly the point that I was making.


    The appendix 3 (Informative) of B.S. 7671 has an equation for Zs(m). Can we use that here?


    Also the 0.8 is not identified as to its purpose as are the other equation components.


    Z.


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