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Is earth rod impedance acceptable

Alan B

Hi I am trying to work out if I can pass and earth rod loop impedance measurement of 550 ohs, sorry this may be a bit of  a long question.

My customer has a property with a large garage, two detached house and various out buildings all owned by himself. It's a remote location with overhead wires, TT earth and private pumped water supply.

Power comes in to the garage with separate supplies for the two houses. My interest is in the main house, the feed for this supplies the house, garage and various outbuildings. There is a earth rod near the incoming connection in the garage and another rod near the actual house and as you would expect bonding to water pipes.

Using a normal earth loop impedance test yesterday the earth rod near the garage gave me 560 ohms, my understanding is that to be reliable the result should be less than 200 ohms. But on the positive side my Zs @DB with the rest if the earthing connected was 75 ohms. I haven't checked so far but my assumption is that most of this is coming from the private water pipes.

Also this is probably the driest point in the year and earth rod impedance is likely to go down rather than up.

My initial thought was that I need to put another earth rod in but then started thinking that as the water pipes are private it's acceptable to use them for earthing and also is it ok to rely on multiple dispersed earth rods even if they are at the other end of some armoured cables.

As some additional information all the circuits are protected by RCD's. Some by 100mA type S RCD's at the origin and where I have changed the CU i have installed 30mA RCBO's for each circuit. I have installed type 1,2,3 SPD's at the origin and type 2 spd's in the two houses.

Welcome your thoughts on weather I need to install an additional earth rod.

Also if earth rods are protecting two properties on different supplies is it permissible to connect them together. I am asking this because the two rods are about 6 inches apart and the other one is giving a much better result, or would connecting them cause confusion and risk?

Thanks

 

Alan

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    550 ohms seems quite high to me depending on the soil type and the length of the rod?

    It may very well meet the requirements for fault protection depending on the rating of the RCD.

    I would want to carry out a further investigation to verify why the earth resistance is so high. I am a big fan of earth mats buried 600mm plus in the ground to get a reliable low earth resistance and avoid the hazard of driving in an earth rod.

    Let us know what you find?

    JP

     

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    So we can use a private buried water pipe as an earth electrode. But just what precautions would we need to take to prevent its removal, and what does “and it has been considered for such use" mean? 

    542.2.6.

    Z.

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    “Also if earth rods are protecting two properties on different supplies is it permissible to connect them together. I am asking this because the two rods are about 6 inches apart and the other one is giving a much better result, or would connecting them cause confusion and risk?”

    I posed this question some years ago, where two earth rods for two different properties were close together but separated by a garden fence. OMS said to joint them together.

     

    Z.

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    If the rods are six inches apart they need to be joined together.

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    I don't think that you can “fail” it without doing a proper earth rod impedance measurement, so that would be FI from me; or arguably, such a measurement would be part of the EICR. Of course, it might be a lot less bother just to put in a new rod.

    From what you say, the private water supply would be an acceptable substitute for a rod.

    As a matter of interest, until yesterday my topsoil was bone dry down to about 18".

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    Have a read of Mike Peace's article in Wiring Matters on TT earthing a link is on my thread below.

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    Yes you can fail it, with 100 mA RCDs the upper limit is 500 ohms and you are measuring 550 ohms.

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    If you consider the rod to be the sole means of reliable Earthing then 560 Ohms is a definite fail for the 100mA RCDs as 411.5.3 (ii) requires Ra x Idn < 50V and even if that's a loop result and allowing 20 Ohms for Rb and 1 Ohm for the supplier's conductors, it's still definitely a fail (if only by 39 Ohms).

    The fact that another rod is only 6" away but giving quite different results would ring alarm bells for me (presuming you didn't have one rod connected while testing the other, but not vice versa - so in one case the other rod was providing a return path rather than the soil). I might wonder if there was a local reason why one was so much worse - e.g. rusted though 6" into the ground or a connection being loose.

       - Andy.

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    I have just remembered the advice from one of my tutors: if the earth rod impedance is a little high, p******* on it. ?

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    I have just remembered the advice from one of my tutors: if the e

     

    Only after verifying it is not live please or a Darwin award may await.…. 

    Mike

     

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