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).

gkenyon · Answer

Ro said: Ok, if you are doing an EICR, these results are asked for also it asks for a r1r2 or2 reading (at least one column to be completed) so, if we are keeping all earths connected and doing a r1r2 or a live test Zs, I am asking why it is ok to add ZE/ZDB to r1r2 but not ZS from ZDB to attain r1r2 
 I disagree, read the Generic Schedule of Test Results form in Appendix 6 carefully . The form asks for '(R1+R2) or R2' 
 The reason one or the other, is it depends on the test method used for 'Continuity of Protective Conductors'. 
 
 For radial circuits: 
 
 If you use the (R1+R2) method for continuity of protective conductors, you enter the value measured for (R1+R2) in column 21 and N/A in column 22 
 If you use the 'wander lead' method, you enter N/A in column 21 and the measured value of R2 in column 22. 
 
 The practice for ring final circuits is: 
 
 Carry out end-to-end tests (step 1). Write the measured end-to-end values for r 1 , r n and r 2 in columns 18, 19 and 20 respectively. 
 Carry out the 'figure of 8' tests (steps 2 and 3). 
 Write down the greater of [(R1+R2)/4] or [largest measured value in the figure of 8 test step 3] in column 21, and 'N/A' in column 22. (NOTE: in theory, the [largest measured value in the figure of 8 test step 3] should not exceed [(R1+R2)/4] so usually it's [(R1+R2)/4] that goes in column 22.) 
 
 FINALLY, and this is VERY important: 
 (a) it's potentially dangerous to conduct a live test before dead tests are complete. The measurement of 'continuity of protective conductors' is an essential test for safety of the person carrying out live tests later. 
 (b) it doesn't conform to BS 7671. BS 7671 requires dead tests are carried out before live tests (see Regulation 643.1).

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

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