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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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    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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    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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