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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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    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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  • 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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