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High ZE compared to SSE figures on a TN-S

Gavin Standing

Hi, I'm carrying out a periodic EICR on a factory, can any inform me of the correct procedure for gaining a ze at  origin as I have always used the 2 wire method with main earth removed as stated in the on site guide, I have a reading of 0.77 ohms on a TN-S which is no where near the 0.22 ohms that SSE has recently recorded as they had to attend to the newly installed service head ( 5 years old ) due to some loose connections. The only way that I can get my readings that low is to use the 3 wire method.

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    I'm not  familiar with that tester but looking at the Meggar website 

    https://www.megger.com/sites/g/files/utfabz201/files/acquiadam_assets/2023-07/MFT1711-MFT1721-MFT1731-MFT1741_DS_en.pdf?changed=1739442054

    we see

    So on the hi  current 2 wire range, that reads 0.01 minimum, that could read between +/-0.03 ohms plus 5% of reading, so a reading of 0.77 could really be anything between 0.68 or 0.82, Had the other set of readings been within a few % of this band I'd have been muttering about 'identical within tolerances' but really this is not.

    Eliminate the obvious, dirty probes, poor contact between probe and circuit or probe leads and the meter itself etc, if you have not already one so.

    Is the supply really TN-s or TNC-s? given how new sounding it is, I'd expect it to be the latter in practice and if so there should be an N-E bond inside the company cut-out or near the meter but somewhere on the DNO side.

    If so, and there really is 0.7 or so  ohms in the neutral, I'd expect noticeable voltage drop issues - switching in a 30A load and dropping more than 20V will give very obvious lighting flicker and of course a low PSSC reading of 230/0.7 = ~ 350 amps L-E.

    I presume with 0.7 ohms, at that low PSSC,  there are issues about fuse clearing times for LE faults so it can't really be left.?

    Mike

  • 0 in reply to mapj1

    Is the supply really TN-s or TNC-s? given how new sounding it is, I'd expect it to be the latter in practice and if so there should be an N-E bond inside the company cut-out or near the meter but on the DNO side.

    Or even somewhere out in the street. Where there are multiple supplies into the same building (or other situations where diverted neutral currents can become a problem) some DNOs now place once common N-PE link outside and run separate N and PE conductors from there to each cut-out. It might look for all the world like TN-S at the intake, but it's still very much PME and has all the usual dangers associated with that, other than a reduced risk of diverted neutral currents flowing between neighbour's metalwork.

        - Andy.

  • 0 in reply to mapj1

    and of course a high PSSC reading of ~ 400 ohms L-E.

    I didn't quite follow that bit - did you mean 400 AMPS? (and low rather than high).

       - Andy.

  • 0 in reply to AJJewsbury

    yup I must have dropped a cog there - edited and corrected,

  • 0 in reply to mapj1

    SSE has been out & verified it as TN-S, I'm a bit suprised on how low their ze is though, I can't get no where near that, I'll have to record the 0.77 ohms ze, there will be a few mbc's that will not achieve their max loop times, many thanks for yours & everyone else's input Thumbsup

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  • 0 in reply to mapj1

    SSE has been out & verified it as TN-S, I'm a bit suprised on how low their ze is though, I can't get no where near that, I'll have to record the 0.77 ohms ze, there will be a few mbc's that will not achieve their max loop times, many thanks for yours & everyone else's input Thumbsup

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