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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

    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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  • 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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