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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 think its just a very poor exam question. If its objective is (at least partially) to test the candidate's awareness of ambient versus max temp cable resistance, it just fails. What is a "tabulated" value? A value taken from a table in BS 7671? From the OSG? From a manufacturer's data sheet? Does that table already factor in Cmin and/or the 20% derating? Who knows?
Reply
  • 0
    I think its just a very poor exam question. If its objective is (at least partially) to test the candidate's awareness of ambient versus max temp cable resistance, it just fails. What is a "tabulated" value? A value taken from a table in BS 7671? From the OSG? From a manufacturer's data sheet? Does that table already factor in Cmin and/or the 20% derating? Who knows?
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