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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 

  • 0 in reply to Ding Wang

      , RA is the sum of (REE+R2) in this case -  If you wanted to measure RA, you'd have to include R2 to the exposed-conductive-parts (because  for 411.5.3, RA includes that resistance).

    and therefore you would be:

    • measuring (Ze+REE) at the origin (and recording the value for Ze includes REE) - see Note to 643.7.1 b) (1); and
    • measuring or determining Zs by other means at the extremity of each circuit to comply with either:
      • Table 41.5 where RCDs are used for ADS (see Regulation 411.5.3 and NOTE to 643.7.1); or
      • the formula Zs ≤ U0×Cmin÷Ia  where OCPDs are used (see Regulation 411.5.4, Ia being the value of current causing operation of an OCPD according to the disconnection time of Regulation 411.3.2.2 or 411.3.2.4, usually 0.2 s or 1 s depending on circuit rating/function for 230/400 V TT supplies, although for installations within buildings where protective bonding is in place, disconnection times for TN systems may be used for circuits covered by Regulation 411.3.2.2)

    Interesting, though, that, having discussed the designer's obligations, where RCDs are used for ADS, the person carrying out initial/periodic verification also needs to verify the effectiveness of protection by testing the RCD as well as verifying that the requirements of Chapter 41 are met (see Regulation 643.7.1 b) (2) and related NOTE). Having said that, this is the minimum standard.

    Note: we ought to be careful, Ra is defined differently to RA as well - see Page 47 of BS 7671:2018+A2:2022

  • 0 in reply to gkenyon

    Graham, thank you for your help. 

  • 0

    I asked what was basically the same question many years ago and highly trusted member of the forum said if all I was doing was testing to ensure there would be adequate protection when using an RCD in a TT earthed installation, I should stop worrying and trying to overthink things.

  • 0 in reply to Sparkingchip

    Well,. sort of ... it's all covered in the relevant Regulations (411.5.3 and 643.7.1) ... and of course, for RCDs it's generally covered where Zs meets the criteria in Table 41.5 ?

    Of course, as earlier post, that's fault protection (for ADS) put to bed ... but overcurrent protection is another thing entirely.

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