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Max ZS

Dave mcn

Sorry maybe a silly question but...

I have the house DB TT install -  Ze at 0.38ohms with a high current 2 lead test.

Sub-board 35 meters away, with it's own earth stake. Zdb with a 3 lead low current test is 20.0 ohms.

All circuits off the sub board are relatively short and the Zs tests are around 20.0 - 21.1 ohms. 

Should the max Zs on the test cert be 1667? As electraform Zs test results are in red as the readings are above max Zs levels for the RCBO's 

TIA

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  • 0 in reply to Chris Pearson

    "Sub-board 35 meters away, with it's own earth stake."

    I did wonder if the main installation was actually TN rather than TT though - 0.38Ω does seem implausibly low for a TT system (or the test had been done other than with consumer's end of the earthing conductor connected to nothing but the meter).

       - Andy.

  • 0 in reply to Chris Pearson

    Yeah i know what you mean. Definitely TT though

    Did the Ze with a high current 2 lead test and main earth removed at origin

    Tested board in the outbuilding with everything connected 3 lead low current and got 20 ohms.

    I just thought the Ze  was a better reading because of the high current test

  • 0 in reply to AJJewsbury

    More than wanting an explanation for the low Ze, I was curious to know why there was such a big difference when r1 + r2 of the distribution circuit must be of the order of one tenth of an Ohm.

  • 0 in reply to Chris Pearson

    As I read it, the remote DB has its own independent earthing arrangement ("it's own earth stake") - so the loop isn't going through the distribution circuit's R2 or indeed the main installation's earthing arrangement (whatever it turns out to be).

        - Andy.

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