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I was building a workshop last year and as I have few old 3-phase metalworking machines and the power comes through a 50m (150ft) buried cable I had to install

I was building a workshop last year and as I have few old 3-phase metalworking machines and the power comes through a 50m (150ft) buried cable I had to install proper ground for the building and another two independent ground connections for the lightning protection. I have no bedrock, but my soil is pretty much 100% clay plus a football sized stones here and there around the building. The code around here is that the ground connection should be no more than 4 ohms resistance. I was lucky with two lightning protection grounds. For one a single 2.4m rod was enough and for the other two rods (I had the type that screws into each other so you can make the rod longer and longer if necessary) were required. However, I wanted to locate the electrical ground few meters away to avoid the possibility of a lightning strike raising my ground potential. I ended up having to install 6 rods to get down to 4 ohms. The first two rods were 7 ohms, the next two 2 meters away were 15 and the final two were 25 if I remember correctly. The take away from that story is: always measure your ground resistance. Never assume 8ft is enough if you have a certain type of soil.

kube-system 7y ago
How exactly is that measured? If you’re trying to measure the resistance between the ground rod and actual ground, how do you get the second reference point?
- andromeduck 7y ago
  Constant current power supply + voltmeter or the reverse? Could also just measure between pairs of rods and guess a little.
- Roark66 7y ago
  There are two standard ways to measure. The meters that do the measurement usually support both. First method involves two mobile shallow ground electrodes you place 20m away from your ground rod (so electrodes are 40m apart and your ground rod in the middle). I'm not sure how exactly it is possible to measure this when the quality of ground those two extra electrodes is unknown, but it works. The meters cost few thousands $ so I assume it is not trivial. The second method involves an already known good ground - like for example one supplied by the electricity company. Here they usually bury galvanised steel thick tape along their ground cables. That tape is hundreds of meters long and underground so it is assumed to be a really good ground. It is only used to ground outside metal bits of the power delivery equipment that are likely to be touched by people and it is separate from the ground connection supplied in the cable itself although one is supposed to connect it to the delivered ground when connecting the house cable to the electricity delivery point. Then every house is supposed to have at least two lightning protection grounds and an extra "electricity supply" ground that is again connected with the ground in the supply cable. Please bear in mind I'm no electrician and all of the above is based on me observing electricians at work when they were connecting my house, my workshop and working on lightning protection for both. I may be mistaken on some details.

kube-system 7y ago

How exactly is that measured? If you’re trying to measure the resistance between the ground rod and actual ground, how do you get the second reference point?

andromeduck 7y ago
Constant current power supply + voltmeter or the reverse? Could also just measure between pairs of rods and guess a little.
Roark66 7y ago
There are two standard ways to measure. The meters that do the measurement usually support both. First method involves two mobile shallow ground electrodes you place 20m away from your ground rod (so electrodes are 40m apart and your ground rod in the middle). I'm not sure how exactly it is possible to measure this when the quality of ground those two extra electrodes is unknown, but it works. The meters cost few thousands $ so I assume it is not trivial. The second method involves an already known good ground - like for example one supplied by the electricity company. Here they usually bury galvanised steel thick tape along their ground cables. That tape is hundreds of meters long and underground so it is assumed to be a really good ground. It is only used to ground outside metal bits of the power delivery equipment that are likely to be touched by people and it is separate from the ground connection supplied in the cable itself although one is supposed to connect it to the delivered ground when connecting the house cable to the electricity delivery point. Then every house is supposed to have at least two lightning protection grounds and an extra "electricity supply" ground that is again connected with the ground in the supply cable. Please bear in mind I'm no electrician and all of the above is based on me observing electricians at work when they were connecting my house, my workshop and working on lightning protection for both. I may be mistaken on some details.