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Reliability of earth rod extension

Alan B

I had my first experience today of having to connect earth rods together to get deep enough to hit wet ground and get a decent reading.

It was at the top of a hill, 30cm wet soil, then 1m + of bone dry something and then presumably at some point some wet ground as the second rod length gave a decent result. Previous attempt with 2 rods  separated gave me about 350 ohms.

I was wondering how reliable the screw joint between the rods will be over time. Also should I have protected the joint with petroleum jelly or something, would it make any difference 1.4m underground and hopefully at least damp.

It probably doesn't make any difference but going by the effort requited and geology of the area I suspect I was driving the rod through Portland stone, a form of lime stone.

  • 0

    We regularly hit similar geological problems around here too - mainly solid lime and gritstone, so to save the grief and cost I get the DNO in to PME the supply instead.

  • 0

    My first reaction would not to be too worried - even if the steel threads corrode somewhat, rust isn't exactly an insulator so even if it adds a few Ohms onto the overall loop reading if you're starting off at below 200 Ohms and have a limit of say 1667 Ohms (30mA RCDs) (or 500 Ohms, 100mA RCD), there's quite a margin to play with.

       - Andy.

  • 0 in reply to whjohnson

    If it was the main house i would have tried fro pme but it was for a garage with existing supply. Too expensive and difficult to get a 10mm earth to the garage. Main house had a tns connection but at 0.15 ohme earth loop, in reality was probably PME.

    You re going to have fun if IET change from recommending an earth rod for each installation to requiring it.

  • 0 in reply to Alan B
    Alan B said:
    if IET change

    'Joint IET/BSI Committee' rather than 'IET'?

    Whilst the IET jointly publishes BS 7671, and provides the Secretariat services for the Committee, changes are by consensus of the Joint Technical Committee JPEL/64. The IET has some representatives ... there are far more representatives from other bodies in the industry, as shown on pages 13 & 14 in Amendment 2.

  • 0
    Alan B said:
    I was wondering how reliable the screw joint between the rods will be over time.

    That's somebody else's problem, assuming of course that an EICR is commissioned in 10 years' time.

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