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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

  • 0

    A resounding FI needed for me.

    In almost any soil except perhaps dry gravel ,500  ohms is higher than is really acceptable for a full length rod.  I've even managed 200-300  ohms in fairly dry sand. As  others have said, not safe for a 100mA RCD.

    Even 75 ohms for plumbing of any length is quite high - is it just a few metres of pipe ?

     Have you tried pulling it up, or perhaps first  checking how corroded is the clamp and any joints ? 

    Paralleling up electrodes is fine, as is having them spread around so you have one on each corner of the site. To get the best from multiple electrodes they need to be a rod length or 2 (or 3 ) apart or they are connecting to the same carrot shape of earth.

      To have tops of 2 rods 6 inches apart and not being more connected to each other than to the plate at the end of the universe also seems quite wrong, is one of them in a large flower pot or similar ? 

    Let us know what you find.

    Mike.

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    mapj1: 
     

    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

     

     

    Of course, it's not clear from the article whether the person came into contact with the rail accidentally through direct bodily contact … or the stream (would that actually cause a fatal shock with DC … perhaps, I'm open-minded about that, and not volunteering to try it out)

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    Chris Pearson: 
     

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

    But just a passing fix.

    Jaymack

  • 0

    Hi All

    Thanks for your time.e and answers

    Answering some of the questions

    I inspected connection to top of the rod and it looked OK. Impedance from top of rod to the met was under one ohm.

    To be honest I don't know much about the water pipes once they leave the houses but will investigate further.

    It could be that most of my earth connections are coming from rods. 

    It sounds like I can connect the earth rods from both properties together which would give a significant improvement. I may get a written confirmation of this fro. Napit.

    I think I will also try getting another rod I as its going to be quicker than fault finding. My tester can do various earth rod tests but I don't have the test probes and not sure how often I would use them.

  • 0

    you do not need the fancy probes, just some lengths of jumper flex or something and some metal tent pegs or garden forks or indeed just about any domestic object presenting a similar bare metal area to the soil - we do not care about the resistance of the test spikes or their wires too much as it gets factored out of the calc. , just needs to be good enough to get  the test current to flow, an  ohm or ten makes  no odds here.

    Mike

  • 0

    I wouldn’t view the situation as unsafe. An appropriate notice at intake to indicate that the private water pipes were acting as installation earth would likely serve as an acceptable precaution.

    There appears to be an obvious issue with the connection to the 560 ohm electrode. However, if  this was the only electrode and you were confident that connections were sound and the resistance was stable, then the only issue would be the slight increase in voltage drop across Ra at currents up to IdeltaN as disconnection parameters will be met.

    I live in the foothills of the Mourne Mountains where some buildings are effectively on rock with just a shallow surround of peat-type soil. Trenches with bare conductors or mats often get values to comply with Table 41.5 for a 100mA RCD but the DNO will not connect unless Ra is below 200 ohms (although  they have been known to connect at values just less than that when the upfront RCD was 500mA!). 
    Several years ago we were beat at a new National Trust collection of cottages where even with mats and other measures we couldn’t get Ra below the 200 ohm mark to meet DNO requirements (even though the RCDs were 30mA). A device called Chem Rod came to the rescue And got us down below the magic value. Acts much like the advice given by that tutor fella!

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