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TT earthing system - external earth loop impedance (Ze), Ze on test certificate and earth resistance Ra

Ding Wang

We have a TT earthing system for a 3-phase + N 400V system. Could anyone please help me with the questions below? 

We will need to measure the earth electrode resistance in order to use equation Ra x In < 50V. As per BS 7671 Guidance Note 3 method 2, we are allowed to use "a loop impedance tester" to "connect between the line conductor at the source of the TT installation and the earth electrode, and a test performed." Then impedance reading taken is treated as the electrode resistance. We use this value for "Resistance to earth" on the test certificate. 

Question 1: Is this measured value actually the external earth loop impedance  (Ze) for a TT earthing system? So does this mean that TT earthing system Ze includes Ra (earth resistance of the electrodes) i.e. Ra is part of Ze of a TT earthing system? Or Ze is still the external components (supply cable, transformer neutral etc.) without the Ra, and the tested value is Ze + Ra? This is not quite clear to me. All calculations refer to Zs = Ze + (R1+R2), in terms of TT earthing system, should Ze already include Ra? 

For the "Nature of Supply Parameters" section of a test certificate, we use the BS 7671 recommended method to measure the 3-ph prospective short circuit current. There is a Ze we need to fill in this section. 

Question 2: For the Ze in this "Nature of Supply parameters" section of the test certificate, should we use the reading from method 2 of of Guidance note 3 mentioned above? i.e. the reading which includes the Ra value as well. 

These two questions are related. Maybe the external component impedance are relatively small compared with the Ra and we assume Ra = Ze? 

Please help.

Many thanks 

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  • 0

    Without a second measurement electrode, you can only honestly say that the Ze is only 'less than' whatever the loop test reported...
    Of course the high resistance may not be where you think it is, and then it gets dangerous, though sometimes it is the neighbours in danger..- read this story ..

     RCD failure causes shock in neighbours house 

    Mike

  • 0 in reply to mapj1
    mapj1 said:
    Without a second measurement electrode, you can only honestly say that the Ze is only 'less than' whatever the loop test reported...

    Although, in retrospect, isn't this the case with the majority of earth electrode measurements, one way or another, because they contain some element of another electrode in the loop - except for fall of potential method, which if you can find the actual sweet spot, OK ... but in reality the measurement is affected by external influences of one sort or another.

    I think one of the best methods overall (considering safety as well as measurement efficacy) is the 2-clamp method ... but again the reading includes the parallel effective earth electrode resistances of the parallel paths back to MET.

Reply
  • 0 in reply to mapj1
    mapj1 said:
    Without a second measurement electrode, you can only honestly say that the Ze is only 'less than' whatever the loop test reported...

    Although, in retrospect, isn't this the case with the majority of earth electrode measurements, one way or another, because they contain some element of another electrode in the loop - except for fall of potential method, which if you can find the actual sweet spot, OK ... but in reality the measurement is affected by external influences of one sort or another.

    I think one of the best methods overall (considering safety as well as measurement efficacy) is the 2-clamp method ... but again the reading includes the parallel effective earth electrode resistances of the parallel paths back to MET.

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