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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.

  • 0 in reply to Gavin Standing

    As I expected.

    And the distance to the transformer is?

  • 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

  • 0 in reply to Gavin Standing

    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 Thinking

  • 0 in reply to Billy Ray

    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.
     

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