High ZE compared to SSE figures on a TN-S

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

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.

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.

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

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

I do EICRs and often the Ze is above the limits, so I call SPEN,but when the lads from SPEN have been and I do a retest, they claim they never did anything, but the Ze is then perfect

If they said they did anything they would be admitting that there had been a fault to correct.
What they probably do open and close a few covers and check the likely screws for tightness, and in the process the oxide layers that build up on conductors get graunched through to make a metal to metal contact again.
The other thing that can fix a high resistance joint is a high current test - as the applied voltage and available energy are enough to punch through the oxide  and then create microscopic spot welds (*) . And if instead the high current causes the connection to blow completely open circuit, at least you have a definite fault to find.

Mike.

* Something similar occurs with twisted wire joints, that microscopically only really make contact on the high spots - if you roughly twist two bits of wire where the copper is a bit dark looking and measure the resistance with a meter that passes less than a mA can often be surprisingly high - maybe more than an ohm. If you then use that joint in  circuit with a higher current, perhaps  to say light a car headlamp there may be a noticeable flicker, but then the voltage drop on the joint falls and is much lower than you might have expected from the initial measurement.
 

If they said they did anything they would be admitting that there had been a fault to correct.
What they probably do open and close a few covers and check the likely screws for tightness, and in the process the oxide layers that build up on conductors get graunched through to make a metal to metal contact again.
The other thing that can fix a high resistance joint is a high current test - as the applied voltage and available energy are enough to punch through the oxide  and then create microscopic spot welds (*) . And if instead the high current causes the connection to blow completely open circuit, at least you have a definite fault to find.

Mike.

* Something similar occurs with twisted wire joints, that microscopically only really make contact on the high spots - if you roughly twist two bits of wire where the copper is a bit dark looking and measure the resistance with a meter that passes less than a mA can often be surprisingly high - maybe more than an ohm. If you then use that joint in  circuit with a higher current, perhaps  to say light a car headlamp there may be a noticeable flicker, but then the voltage drop on the joint falls and is much lower than you might have expected from the initial measurement.