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Very low Ra reading disappears when connected back up.

aligarjon

Hi Guys

I had some very strange readings today when doing an EICR.   Property is a semi detached cottage in a very rural location. Over head supply. 

The first thing i noticed when i arrived was a piece of angle iron driven into the ground outside the front door with a corroded 10mm earth cable bolted to it exposed for all to see.

The consumer unit is located just inside the front door in a cupboard and i can see the earth cable going out through the door frame.  I disconnected it at the CU and got an Ra reading of just over 2 ohms so i am obviously amazed. I reconnect it to the CU to get a PFC reading etc and the Zdb is nearly 300ohms with the original earth reconnected. I disconnected it again and got just over 2 ohms again. Reconnected it and got around 300ohms.  

This is a new one on me, does anyone know what could be going on ?

Cheers  Gary

  • 0

    When you say you measured Ra, how exactly did you measure it?

  • 0

    Are you measuring RA like a loop test, live to electrode. ?  I wonder what is on the end of the angle iron, it is very low - there are some transformers that do not make 2 ohms, or indeed anything like so any consumer Zs on those runs are doomed to be more than that.

    Else give things a good clean - it may be the being moved around that it does not like ...

    Mike

  • 0

    Reply to both.  Yes i measured it like a loop test. Basically disconnected earth cable from board and put live and neutral prongs onto main switch terminals, or in this case an up front rcd.  I don't get how i can get 2 ohms then it disappears when i connect it back to the earth terminal in the consumer unit.

  • 0 in reply to aligarjon

    Live and Neutral prongs onto switch terminals? That is more likely to read P.S.C.C. rather than Ra of the angle iron. Or you are measuring L to N on a loop test setting? Live lead to L in house and N lead to earth electrode is required by my loop tester for the earth rod Ra.

    Z.

  • 0

    Rather odd then.

    Can you do a 2 wire Zs  test from  L to  electrode that does not need the neutral connection ?  That would depend on your meter, not all modern ones can.

    I can imagine a low current 3 wire test getting confused if  there is a neutral -earth offset that is the wrong way relative to the L-N phase, but it needs a neighbour on another phase to have significant earth leakage. (so when you add the L-E test current the NE voltage falls rather than rises.)

    (I'm sure you would have noticed if L-N had been reversed but I suppose I should ask that too for completeness .)

    And the really obvious - does that angle iron connect to something better earthed, or even neutral connected, elsewhere either above or below ground, like massive piling or maybe underground water pipes that share with the neighbours half a mile off who for some reason have PME or something.

    It is a puzzler, at least as presented,  it is not obvious.

    Mike.

  • 0 in reply to mapj1

    Earth Electrodes Part 2 - Installing, Loop Impedance Testing - Bing video

  • 0

    Thank you for the replies.  I didn't think to do a 2 wire test, I have a newish tester to me (KT66DL) which does have that function.   I have coded it as a C2 for now ( absence of a reliable and effective means of earthing)  along with a few others so no doubt will have to go back there at some point.

    Gary

  • 0 in reply to aligarjon

    An all singing all dancing tester.

    KT66DL Product Introduction - Kewtech (kewtechcorp.com)

    Z.

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