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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!!
Parents
  • 0
    I agree that it is a poor question - it ought to say "measured resistance".


    There is an oddity with this 80% correction. Appendix 3 explains. However, the increase in resistance due to the heating effect of the load current is not uniform throughout the earth fault loop. For a typical domestic, part is R1 + R2 of the circuit; part is R1 of the tails and R2 of the earthing conductor; and the rest is Ze. A fully loaded circuit may well warm up considerably, but the tails and DNO's cables will still be only partially loaded and, therefore, not so hot. That is unless, of course, all the circuits are fully loaded.


    As ever, BS 7671 errs on the cautious side.
Reply
  • 0
    I agree that it is a poor question - it ought to say "measured resistance".


    There is an oddity with this 80% correction. Appendix 3 explains. However, the increase in resistance due to the heating effect of the load current is not uniform throughout the earth fault loop. For a typical domestic, part is R1 + R2 of the circuit; part is R1 of the tails and R2 of the earthing conductor; and the rest is Ze. A fully loaded circuit may well warm up considerably, but the tails and DNO's cables will still be only partially loaded and, therefore, not so hot. That is unless, of course, all the circuits are fully loaded.


    As ever, BS 7671 errs on the cautious side.
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