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#2
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Question about earth rods and desired impedance
On 09/07/2019 11:56, John Rumm wrote:
On 08/07/2019 22:27, wrote: I'm new to earth rods so would like a little advice. First, some background. We bought the house a couple of years ago and at that time it was supplied via a 2-wire overhead line from a transformer some (unknown) distance away. The 2 wires terminated on a bracket at the back of the garage and then came through the wooden window frame to the main fuse. As soon as we moved-in I got the local electricity board to put the final leg underground and convert the supply to TN-C-S - it looks as if they have added a ground connection at the final pole but the existing connection to a local earth rod was left in place. I'm replacing the old CUs as part of a major refurb and decided to keep a local ground rod, but to use a different position. I've just put a 4ft rod in the ground and made some measurements (using 30V DC and checking with both polarities). The soil is clay and the rod went-in remarkably easily using the SDS hammer. New rod to MET: 60 ohms Old rod to MET: 150 ohms New rod to old rod (about 4m apart): 230 ohms 60 ohms seems rather high - what impedance should I be aiming for and is there any point (with clay soil) in driving the rod down another 4'? 60 does sound a bit higher than I would expect (I have found getting under 15 is usually easy with a single length rod in clay). It might be worth trying a different location. Having said that, if you have TN-C-S anyway, you have to question what you actually want to achieve. I'm doing it because I would only have one earth point if I lost the overhead neutral cable, so it seemed reasonable to have a second that is under my control. I accept that's not the strongest argument and I probably wouldn't have thought about it if there wasn't an existing rod (which, it turns-out, is virtually worthless at 150 ohms). Is there a recommended maximum impedance that I should be aiming for? Trying somewhere else is easy to say but getting the rod out might be not be so easy. I'll try a strap and a long piece of 2x4. |
#3
Posted to uk.d-i-y
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Question about earth rods and desired impedance
On Tuesday, 9 July 2019 13:37:48 UTC+1, wrote:
On 09/07/2019 11:56, John Rumm wrote: On 08/07/2019 22:27, wrote: I'm new to earth rods so would like a little advice. First, some background. We bought the house a couple of years ago and at that time it was supplied via a 2-wire overhead line from a transformer some (unknown) distance away. The 2 wires terminated on a bracket at the back of the garage and then came through the wooden window frame to the main fuse. As soon as we moved-in I got the local electricity board to put the final leg underground and convert the supply to TN-C-S - it looks as if they have added a ground connection at the final pole but the existing connection to a local earth rod was left in place. I'm replacing the old CUs as part of a major refurb and decided to keep a local ground rod, but to use a different position. I've just put a 4ft rod in the ground and made some measurements (using 30V DC and checking with both polarities). The soil is clay and the rod went-in remarkably easily using the SDS hammer. New rod to MET: 60 ohms Old rod to MET: 150 ohms New rod to old rod (about 4m apart): 230 ohms 60 ohms seems rather high - what impedance should I be aiming for and is there any point (with clay soil) in driving the rod down another 4'? 60 does sound a bit higher than I would expect (I have found getting under 15 is usually easy with a single length rod in clay). It might be worth trying a different location. Having said that, if you have TN-C-S anyway, you have to question what you actually want to achieve. I'm doing it because I would only have one earth point if I lost the overhead neutral cable, so it seemed reasonable to have a second that is under my control. I accept that's not the strongest argument and I probably wouldn't have thought about it if there wasn't an existing rod (which, it turns-out, is virtually worthless at 150 ohms). 150 is effective if everything is RCDed 60 is 4A at 240v. Is there a recommended maximum impedance that I should be aiming for? Trying somewhere else is easy to say but getting the rod out might be not be so easy. I'll try a strap and a long piece of 2x4. So leave it in. Options include: Screw another section onto it, hammer in connect to both rods NT |
#4
Posted to uk.d-i-y
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Question about earth rods and desired impedance
On 09/07/2019 13:37, wrote:
On 09/07/2019 11:56, John Rumm wrote: On 08/07/2019 22:27, wrote: I'm new to earth rods so would like a little advice. First, some background. We bought the house a couple of years ago and at that time it was supplied via a 2-wire overhead line from a transformer some (unknown) distance away. The 2 wires terminated on a bracket at the back of the garage and then came through the wooden window frame to the main fuse. As soon as we moved-in I got the local electricity board to put the final leg underground and convert the supply to TN-C-S - it looks as if they have added a ground connection at the final pole but the existing connection to a local earth rod was left in place. I'm replacing the old CUs as part of a major refurb and decided to keep a local ground rod, but to use a different position. I've just put a 4ft rod in the ground and made some measurements (using 30V DC and checking with both polarities). The soil is clay and the rod went-in remarkably easily using the SDS hammer. New rod to MET: 60 ohms Old rod to MET: 150 ohms New rod to old rod (about 4m apart): 230 ohms 60 ohms seems rather high - what impedance should I be aiming for and is there any point (with clay soil) in driving the rod down another 4'? 60 does sound a bit higher than I would expect (I have found getting under 15 is usually easy with a single length rod in clay). It might be worth trying a different location. Having said that, if you have TN-C-S anyway, you have to question what you actually want to achieve. I'm doing it because I would only have one earth point if I lost the overhead neutral cable, so it seemed reasonable to have a second that is under my control. I accept that's not the strongest argument and I probably wouldn't have thought about it if there wasn't an existing rod (which, it turns-out, is virtually worthless at 150 ohms). Is there a recommended maximum impedance that I should be aiming for? Trying somewhere else is easy to say but getting the rod out might be not be so easy. I'll try a strap and a long piece of 2x4. 200 ohms is the recommended maximum. Assuming the new CU is all RCD then you will be fine for 1666 ohms. If you are trying to create your own PME supply then the best of luck. -- Adam |
#5
Posted to uk.d-i-y
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Question about earth rods and desired impedance
On 09/07/2019 18:47, ARW wrote:
On 09/07/2019 13:37, wrote: On 09/07/2019 11:56, John Rumm wrote: On 08/07/2019 22:27, wrote: I'm new to earth rods so would like a little advice. First, some background. We bought the house a couple of years ago and at that time it was supplied via a 2-wire overhead line from a transformer some (unknown) distance away. The 2 wires terminated on a bracket at the back of the garage and then came through the wooden window frame to the main fuse. As soon as we moved-in I got the local electricity board to put the final leg underground and convert the supply to TN-C-S - it looks as if they have added a ground connection at the final pole but the existing connection to a local earth rod was left in place. I'm replacing the old CUs as part of a major refurb and decided to keep a local ground rod, but to use a different position. I've just put a 4ft rod in the ground and made some measurements (using 30V DC and checking with both polarities). The soil is clay and the rod went-in remarkably easily using the SDS hammer. New rod to MET: 60 ohms Old rod to MET: 150 ohms New rod to old rod (about 4m apart): 230 ohms 60 ohms seems rather high - what impedance should I be aiming for and is there any point (with clay soil) in driving the rod down another 4'? 60 does sound a bit higher than I would expect (I have found getting under 15 is usually easy with a single length rod in clay). It might be worth trying a different location. Having said that, if you have TN-C-S anyway, you have to question what you actually want to achieve. I'm doing it because I would only have one earth point if I lost the overhead neutral cable, so it seemed reasonable to have a second that is under my control. I accept that's not the strongest argument and I probably wouldn't have thought about it if there wasn't an existing rod (which, it turns-out, is virtually worthless at 150 ohms). Is there a recommended maximum impedance that I should be aiming for? Trying somewhere else is easy to say but getting the rod out might be not be so easy. I'll try a strap and a long piece of 2x4. 200 ohms is the recommended maximum. Assuming the new CU is all RCD then you will be fine for 1666 ohms. Thanks, I hadn't managed to find the recommendation before (and hadn't woken-up to the fact that 30mA is what sets the max z - doh!) but searching backwards it seems that there's some debate about whether the recommended max is 200 or 100 ohms. Given what I'm trying to do I'll leave the rod in place, whack-in the last few inches and accept the 60 ohms. If you are trying to create your own PME supply then the best of luck. Belt, braces and a piece of string ;-) |
#6
Posted to uk.d-i-y
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Question about earth rods and desired impedance
If you are worried about losing the overhead neutral, you could go for an all RCBO CU.
When selecting your RCBOs, choose ones with functional earth wires. These are thin and white. Should a loss of neutral occur, the RCBOs will all trip. S. |
#7
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Update - Question about earth rods and desired impedance
On 09/07/2019 17:02, wrote:
On Tuesday, 9 July 2019 13:37:48 UTC+1, wrote: On 09/07/2019 11:56, John Rumm wrote: On 08/07/2019 22:27, wrote: I'm new to earth rods so would like a little advice. First, some background. We bought the house a couple of years ago and at that time it was supplied via a 2-wire overhead line from a transformer some (unknown) distance away. The 2 wires terminated on a bracket at the back of the garage and then came through the wooden window frame to the main fuse. As soon as we moved-in I got the local electricity board to put the final leg underground and convert the supply to TN-C-S - it looks as if they have added a ground connection at the final pole but the existing connection to a local earth rod was left in place. I'm replacing the old CUs as part of a major refurb and decided to keep a local ground rod, but to use a different position. I've just put a 4ft rod in the ground and made some measurements (using 30V DC and checking with both polarities). The soil is clay and the rod went-in remarkably easily using the SDS hammer. New rod to MET: 60 ohms Old rod to MET: 150 ohms New rod to old rod (about 4m apart): 230 ohms 60 ohms seems rather high - what impedance should I be aiming for and is there any point (with clay soil) in driving the rod down another 4'? 60 does sound a bit higher than I would expect (I have found getting under 15 is usually easy with a single length rod in clay). It might be worth trying a different location. Having said that, if you have TN-C-S anyway, you have to question what you actually want to achieve. I'm doing it because I would only have one earth point if I lost the overhead neutral cable, so it seemed reasonable to have a second that is under my control. I accept that's not the strongest argument and I probably wouldn't have thought about it if there wasn't an existing rod (which, it turns-out, is virtually worthless at 150 ohms). 150 is effective if everything is RCDed 60 is 4A at 240v. Is there a recommended maximum impedance that I should be aiming for? Trying somewhere else is easy to say but getting the rod out might be not be so easy. I'll try a strap and a long piece of 2x4. So leave it in. Options include: Screw another section onto it, hammer in connect to both rods NT I had another rod so screwed it on and drove down another 4 feet (parallel rods need to be a long way apart so that wasn't an option). Initially it made little difference, down to about 50 ohms, but after a few days the resistance between the MET and the new rod had dropped to less than an ohm (measured using PSU and ammeter and also by DMM). I don't understand why it changed, but the new figure has been stable for a week or so - it's possible that I had a bad contact before, but I'm confident that I didn't. |
#8
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Update - Question about earth rods and desired impedance
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