Zs taken by live test and r1 r2 calculated by deducting Ze or Zdb

Question

Is the only reason you do not calculate r1 r2 by deducting Ze/Zdb from Zs is that it is down to parallel paths. The reason I am asking is, when the apprentices are doing their trade test, they are encouraged to short the circuit they are working on to the earth bar via a crocodile clip to get r1 r2. Surely this is picking up parallel paths also? Or is there is another reason?

gkenyon · Answer

Ro said: CAN ANYONE OFFER A DEFINITIVE ANSWER TO THIS QUESTION. PLEASE READ AND ANSWER ACCORDINGLY...................... 
 Yes, I can. 
 Loop impedance tests are not very accurate, so it wouldn't be recommended. 
 Ro said: The reason I am asking is, when the apprentices are doing their trade test, they are encouraged to short the circuit they are working on to the earth bar via a crocodile clip to get r1 r2. Surely this is picking up parallel paths also? 
 There are two schools of thought on parallel paths and testing: 
 1. Eliminate parallel paths by removing the cpc (at least for initial verification). 
 2. Don't remove the cpc to prevent damage and avoid dismantling which can lead to errors in reconnection (more valid for periodic inspection and testing, or when extending a new circuit). 
 
 With respect to Ze and Zs, BS 7671 does NOT require an accurate measurement of either Ze or Zs. Both can be determined with no measurements (e.g. by calculation) or verification according to Part 6. 
 What is needed to be tested in terms of loop impedance/continuity, as a minimum are: 
 
 Test to demonstrate there is an earth fault path (rather than an accurate reading of Ze). 
 Test for continuity of protective conductors . (and live conductors of a ring final circuit only).

Zs taken by live test and r1 r2 calculated by deducting Ze or Zdb

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