12 ms·

The 5 foot ground rod and its use in the National Electric Code (2018)

92 points by turtlegrids 7y ago

segfaultbuserr 7y ago
Note to readers: NEC is National Electric Code, which is the regulation of all electrical installations in the USA. The NEC here is not Numerical Electromagnetics Code, an electromagnetic simulation program, commonly used for antennas, which often have a ground connection as well. And it's obviously not the NEC Corporation, the Japanese electric company, whose semiconductor department is now a separate company known as Renesas. When I first saw the title, it took me a few seconds in my mind to solve the hash conflict here. I wonder how many people on HN know all of these entities simultaneously.
- namibj 7y ago
  You are not alone, at least.
robomartin 7y ago
L.A. County made us drive FOUR 8 foot ground rods into the ground, no less than 8 feet apart, for our ground-mounted 13 kW solar array. This is ridiculous beyond description and it is NOT in the N.E.C. at all. The problem you run into with places like L.A. County is that incompetent plan checkers just want to play "cover my ass" and make up rules. There is no practical way to challenge them. They are the absolute authority (unless you have millions of dollars and YEARS to fight them). I checked with contractors and the recommendation was "just do it and don't complain or they'll make your project a living nightmare". So, we did. They still made the project a nightmare to complete. They added a massive chunk to the overall cost and it took six months longer than it should have taken to complete it. Their interference made it such that this solar system will likely never recover the construction costs. Well, maybe in 30 years. It's a shame. Had I known the nightmare it is to deal with this bureaucracy I would have invested my money elsewhere. California is sucking more and more with time. L.A. County is even worse.
- ademup 7y ago
  A 5/8" x 8' copper grounding rod is $14 at Home Depot; 3\4 x 10 is $33. I'm very confused on how the extra $100 would meaningfully affect the project in either time or BOM.
  - RyJones 7y ago
    there's more to it than putting a copper rod in the ground.
  - mattrp 7y ago
    I’ve installed a bunch of these for myself. You always think about what you might be driving the stake through or if you’re going to hit rock... but you do have a point: after the cost of the rods, there’s the cost of the sledgehammer and the ground cable to wire back to the box. Kind of hard to see how this one issue caused a great deal of expense...
  - ggrrhh_ta 7y ago
    Giving the contractors a good excuse to rip off an employer they hold hostage...?
  - bradknowles 7y ago
    With respect, this is like saying “but the hard drive on Fry’s discount shelf only costs $100, why is your NVME-over-Fiber SSD storage system so expensive?” Hint: what the consumer can buy over the counter, and what gets used in a professional setting, are frequently two radically different things that may only be tangentially related.
    - mlyle 7y ago
      It's the same thing that gets used. Just one little detail-- it has to get driven into the earth before use.
    - tomku 7y ago
      With respect, what you're saying is absolute nonsense in the context of ground rods. They're hunks of metal clad in copper and driven into the ground with a sledgehammer. They're so simple that there's literally no part of a modern PC that's analogous. The ones you can buy from Home Depot for $15 are identical to the ones I buy from a professional electrical contracting supplier for $15 - they come off the same factory lines from the same manufacturer. There are other components used in contracting that have a bit more product segmentation, but the whole "frequently radically different things that may only be tangentially related" bit is wildly exaggerated. We use the appropriate components for the job, and often that means the same stuff you could buy at Home Depot. You're paying for that knowledge of what's appropriate/necessary and the labor to do the work, not for super-special premium contractor ground rods.
segfaultbuserr 7y ago
As we are talking about grounding... I think the greatest problem that troubles me personally is that the single-point grounding required by lightning protection is almost never implemented properly in the vast majority of installations. I realized the existence of this problem when I wanted to install a mesh network in a school campus, so I started reading documents about lightning protections, amateur radio publications have good resources on it, and all of them indicate a correct installation should eventually bound all grounds, including mains ground, telecommunication ground and lightning protection ground at a single-point via low-impedance connections (even if they have their own ground rods), also, all metallic objects and structures, such as gas pipes, fire escapes, etc, should be bounded to ground as well. This is to prevent a voltage gradient developing between different grounding points in case of a lightning strike, and it's usually required or implied in electrical or building code. However, who ever obey this rule in practice? I've seen various installations of outdoor/rooftop Wi-Fi networks, and all the lightning protection they have is a gas discharge tube and a ground connection of unknown quality, I don't think the installation is legal. Also, many buildings were in compliance of the code when they were built, but even adding an extra metal frame or a cable-TV box will invalidate the entire lightning protection, at least from a technical or legal perspective. I guess the reason behind the non-compliance is that the likelihood of a lightning strike is already low, even if it does strike, it's possible that even a non-compliant lightning protection system can work to an extent. And if it doesn't work, most people would just accept the damage. As a result, it's a concern for radio hobbyist only, whose antenna increases the likelihood significantly. But I was still shocked when I first understood the fact that most (low-rise) buildings don't have a technically sound lightning protection system and bet the safety of the building on luck. And its practical consequence is: if you want to put a rooftop antenna for amateur radio, with good lightning safety and in compliance with the code, in many cases, the answer is you can't - the building often doesn't even have a technically sound lightning protection system to begin with. I read many hardcore radio amateurs redo the entire electrical wiring at their homes to the proper standards. High-rise buildings are much better, lightning protection is serious business, but you are not usually allowed to put a mesh network antenna on top of them. But if you are allowed, I've heard that some people even got a chance to have a friendly conversation with the civil engineer who's in charge of the building's lightning protection system and work out a solution together.
lutorm 7y ago
What do you do for grounding if your house sits on bedrock? We barely had enough for the "buried in trench, 30" deep" option at our house before hitting the solid lava everything here is built on. (And it's buried in mostly gravel, there is practically no dirt.)
- dangrossman 7y ago
  Apparently, according to an internet source I have not vetted, you lay the 8 foot rods in the bottom of the 30" deep trench.
  - supermatt 7y ago
    Thats what parent says they have already done.
- supermatt 7y ago
  Run a large gauge wire to another location where you can put a better earth. In reality, you are likely on a TN earthing system, so you already have a low impedance earth wire supplied from your local transformer and your local earthing is just a "backup".
- angry_octet 7y ago
  A ground driven into rock or buried in dry gravel isn't going to be of much use. Typically electrodes are buried in a mixture of bentonite and gypsum, which is pH neutral and provides low resistance. There are also chemical (salt) earth fills which work well but require thicker copper electrodes to deal with corrosion. Various conductor arrangements are used, but in shallow trenches there is always the problem of the wires being cut. A radial or cartesian mesh burial over a larger area would be typical. You can buy electrical mesh and ground enhancement fill for this purpose. Note that you still need it to be below the frost layer in your region -- which varies from 15cm (California) to 300cm (Canada) depending. An earth connected via a distant cable isn't going to be effective against lightning (transients are high frequency and hence conductivity is constrained by the skin effect, and long wires have high inductance even if they have low DC impedance), and additionally is poor practice as it is more lightly to be cut without someone knowing in the future. I doubt any EE would approve it when other methods are available. https://blog.nvent.com/erico/erico-what-is-soil-resistivity-and-how-does-it-affect-grounding/ https://blog.nvent.com/erico/erico-what-is-soil-resistivity-... Cf IEEE 80 & 81
myspy 7y ago
We're currently building a house in Germany and the grounding consists of a couple dozen meters of thick (maybe 10mm) wiring which is laid out in the concrete ground plate of the house.
- petre 7y ago
  A couple of dozen meters? Wiring? Encased in concrete? Five to ten meters of zinc coated 30x5mm flat bar steel bound to three to six 3/4" rods is usually enough depending on the soil characteristics. And it's usuall buried directly in the soil next to or around the bulding. I wonder what bulding codes you've got there.
  - brazzy 7y ago
    The relevant code is DIN 18014. And it distinguishes between different kinds of ground plate. Some are considered sufficiently "earth-feeling" to be considered ground themselves, others require separate corrosion-resistant grounding to be put in the earth around them. Zinc-coated steel is not considered sufficiently corrosion-resistant. Here's an article in German with some pictures of how it's done: https://www.elektro-plus.com/elektroplanung/fundamenterder/ausfuehrung-des-fundamenterders https://www.elektro-plus.com/elektroplanung/fundamenterder/a...
    - myspy 7y ago
      Interesting to read. I forgot that below the concrete plate is insulation and they put something in the foundation that touches the earth.
    - petre 7y ago
      Allright. This makes sense as it reduces the electro corrosion on the rebar and also prevents other unfortunate accidents such as the hot wire coming in contact an exposed metal part or a metal pipe which is itself in contact with the rebar. I have seen a scenario where ligtning struck a DHW solar panel installation, skiped from the plumbing to the rebar and also melted hole in the plumbing while doing so, draining all the antifreeze on the walls. In addition it destroyed all the electronic appliances in the house. If the panels along with the rebar would have been properly grounded, such as scenario would have been mitigated.
Roark66 7y ago
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.
vidanay 7y ago
For a great intro to grounding, check out this series of articles https://forums.qrz.com/index.php?threads/grounding-dos-donts-why-part-1.335582/ https://forums.qrz.com/index.php?threads/grounding-dos-donts... It's written from the perspective of a ham operator, but all the principles are the same no matter what.
FiatLuxDave 7y ago
Once I had a situation where the length of the ground rod mattered. We were building a fusion reactor prototype, and as far as I am aware the NEC code doesn't really have a section for that. I'm sure that much of what we did was probably against best practices. We had a number of high voltage power supplies running in one room of our lab, and on days when the humidity in the air was low, the ionization in the air would charge up all the metal surfaces in the room. So, even if something was not connected to a high voltage, you would get a nasty shock when you touched it. All these devices were connected to the normal, "third prong" wall socket ground, but apparently the route through the building to ground was too resistive to handle the large amount of ionization in the air. When working in a room with 180,000 volt power supplies, random shocks rapidly become more than annoying. Spence, our electrical engineer, had the idea that the quickest way to fix the problem was to run a grounding rod into the soil right outside the window. We pounded a rod into the ground with a sledgehammer as far as we could, which was about 5 feet, I guess. Then we ran a 1/4" copper cable from the rod, through the window to connect to the main reactor chamber. This worked great! At least for a couple of months. Our lab was located in a swamp, and the soil was basically a few meters of dried leaves. You could actually feel the soil move up and down when you walked on it. When August came around, it had not rained for quite a while, and the water level in the soil had dropped below the level of the grounding rod. The dried leaves in the soil insulated it quite well. We started getting shocks again, and it took us many hours of troubleshooting before we realized that the ground was not actually ground anymore. We went out and pounded the rod a few feet deeper, which was easier now that the soil was dry. This reached the water table, and fixed the problem.
- crtlaltdel 7y ago
  yup. had a control system installed where the rod was driven into a granite slab and was not sufficient
- Scoundreller 7y ago
  Also helps if you dump a large quantity of salty water. The salt eventually leaves too, but helps retain some of the water and makes it more conductive.
alkonaut 7y ago
Are all domestic buildings grounded this way in the US? Here (Sweden) I believe houses that aren’t far from transformers are only grounded at the nearest transformer (where power is transformed from 10kV 3-phase into 230/400V 3-phase+neutral for distribution to each house.
- serf 7y ago
  it's pretty common (pun unintended) , and usually preferred if you have any bigger electrical equipment like a garage shop.
- mindslight 7y ago
  Yes, the neutral is tied to the earth at the transformer and also at every house on the transformer. This means that some return current technically runs through the earth back to the transformer. shrug
- mturmon 7y ago
  Yes, all domestic buildings in the US have their own earth ground, and this is and has been required by the NEC (National Electrical Code). Often this is a thick, 8-foot long copper rod pounded into the ground not far from the electrical panel, connected to the panel by a ~1cm diameter copper cable. (There are other allowed methods, including using a similar copper electrode encased in concrete or in the building foundation.) It looks like this (which shows a galvanized-steel rod rather than the more canonical copper, and shows a further connection to a second ground rod, which is not required everywhere): http://www.wireyourownhouse.com/panel/images/grndrodexample.png http://www.wireyourownhouse.com/panel/images/grndrodexample.... Also, certain sub panels (e.g., in garages) will need their own separate earth ground, made the same way. This "grounding electrode" is in addition to a connection ("bond") to the domestic cold-water pipe, usually done where the pipe enters the building. (That pipe itself may be a hunk of copper buried in the ground.) The electrical bond to the water service is not a substitute for the dedicated grounding electrode mentioned above, it's to ensure that the plumbing does not become electrically charged.
  - mattrp 7y ago
erobbins 7y ago
Kinda like IBM, no one ever got fired for using an 8 foot rod in a situation where a 5 foot rod was to code.
watersb 7y ago
Grounding, Bonding, and All That I have been reviewing the Mike Holt NEC video seminars, just listening like background music while I build some workbenches on the weekends. https://www.youtube.com/playlist?list=PLRNS1x1jcKbEgNJx26RWB8O4Fm1Ny-bxK https://www.youtube.com/playlist?list=PLRNS1x1jcKbEgNJx26RWB...
- wmblaettler 7y ago
  ^ THIS - Go watch at least first video "Grounding - Safety Fundamentals" This was a very eye-opening video. I totally misunderstood grounding before watching this.
fyfy18 7y ago
I recently installed an IKEA kitchen hood, and was surprised that the power connector only had two pins (IEC C17/18). The hood has a aluminium frame (albeit painted) with metal pole switches. Can someone explain why this doesn't need to be grounded?
- lurquer 7y ago
  Perhaps the same reason toasters aren't. It is very likely that one will have two hands on the toaster when one is shocked. You have two choices: ground it, in which case the current races up one arm, through the torso, and down the other arm (frying your heart in the process). Or, keep it ungrounded and let the current race down your leg to the floor (sparing your torso and hopefully taking advantage of the fact that your shoes will provide a little more insulation that your two bare hands on metal in the first scenario.)
alextingle 7y ago
Why do the US regulations specify the method and not the result? That seems bizarre. Here in the UK, most earth connections are provided by the power company, but in rural areas you may be too far from the substation, so a ground rod is permitted - as long as it provides an earth loop impedance (Z) of less than 200 Ohm. So if you want to provide your own earth, you have to keep driving ground rods deeper, or adding new ones until the "Zeds" are good enough. It looks like the US regulations permit you to bang an "8 foot" rod into dry sand, and walk away as though that's job done??
walrus01 7y ago
I'm kind of surprised to read that entire article and not encounter the phrase "ohm meter" anywhere in a search. Rarely does anything serious have only one ground rod. Three legged telecom towers will have three rods, one attached to each leg, linked together by a ring of 6 gauge copper buried outside the perimeter of the concrete foundation. Multiple 5 foot ground rods in the correct configuration bonded together could offer much more conductivity to the earth than one 8 foot. The article seems to be obsessing about the length of the rod and not the scientifically measurable ohm resistance.

92 points by turtlegrids 7y ago

Note to readers: NEC is National Electric Code, which is the regulation of all electrical installations in the USA. The NEC here is not Numerical Electromagnetics Code, an electromagnetic simulation program, commonly used for antennas, which often have a ground connection as well. And it's obviously not the NEC Corporation, the Japanese electric company, whose semiconductor department is now a separate company known as Renesas. When I first saw the title, it took me a few seconds in my mind to solve the hash conflict here. I wonder how many people on HN know all of these entities simultaneously.

namibj 7y ago
You are not alone, at least.

robomartin 7y ago

L.A. County made us drive FOUR 8 foot ground rods into the ground, no less than 8 feet apart, for our ground-mounted 13 kW solar array. This is ridiculous beyond description and it is NOT in the N.E.C. at all. The problem you run into with places like L.A. County is that incompetent plan checkers just want to play "cover my ass" and make up rules. There is no practical way to challenge them. They are the absolute authority (unless you have millions of dollars and YEARS to fight them). I checked with contractors and the recommendation was "just do it and don't complain or they'll make your project a living nightmare". So, we did. They still made the project a nightmare to complete. They added a massive chunk to the overall cost and it took six months longer than it should have taken to complete it. Their interference made it such that this solar system will likely never recover the construction costs. Well, maybe in 30 years. It's a shame. Had I known the nightmare it is to deal with this bureaucracy I would have invested my money elsewhere. California is sucking more and more with time. L.A. County is even worse.

ademup 7y ago
A 5/8" x 8' copper grounding rod is $14 at Home Depot; 3\4 x 10 is $33. I'm very confused on how the extra $100 would meaningfully affect the project in either time or BOM.
- RyJones 7y ago
  there's more to it than putting a copper rod in the ground.
- mattrp 7y ago
  I’ve installed a bunch of these for myself. You always think about what you might be driving the stake through or if you’re going to hit rock... but you do have a point: after the cost of the rods, there’s the cost of the sledgehammer and the ground cable to wire back to the box. Kind of hard to see how this one issue caused a great deal of expense...
- ggrrhh_ta 7y ago
  Giving the contractors a good excuse to rip off an employer they hold hostage...?
- bradknowles 7y ago
  With respect, this is like saying “but the hard drive on Fry’s discount shelf only costs $100, why is your NVME-over-Fiber SSD storage system so expensive?” Hint: what the consumer can buy over the counter, and what gets used in a professional setting, are frequently two radically different things that may only be tangentially related.
  - mlyle 7y ago
    It's the same thing that gets used. Just one little detail-- it has to get driven into the earth before use.
  - tomku 7y ago
    With respect, what you're saying is absolute nonsense in the context of ground rods. They're hunks of metal clad in copper and driven into the ground with a sledgehammer. They're so simple that there's literally no part of a modern PC that's analogous. The ones you can buy from Home Depot for $15 are identical to the ones I buy from a professional electrical contracting supplier for $15 - they come off the same factory lines from the same manufacturer. There are other components used in contracting that have a bit more product segmentation, but the whole "frequently radically different things that may only be tangentially related" bit is wildly exaggerated. We use the appropriate components for the job, and often that means the same stuff you could buy at Home Depot. You're paying for that knowledge of what's appropriate/necessary and the labor to do the work, not for super-special premium contractor ground rods.

segfaultbuserr 7y ago

As we are talking about grounding... I think the greatest problem that troubles me personally is that the single-point grounding required by lightning protection is almost never implemented properly in the vast majority of installations. I realized the existence of this problem when I wanted to install a mesh network in a school campus, so I started reading documents about lightning protections, amateur radio publications have good resources on it, and all of them indicate a correct installation should eventually bound all grounds, including mains ground, telecommunication ground and lightning protection ground at a single-point via low-impedance connections (even if they have their own ground rods), also, all metallic objects and structures, such as gas pipes, fire escapes, etc, should be bounded to ground as well. This is to prevent a voltage gradient developing between different grounding points in case of a lightning strike, and it's usually required or implied in electrical or building code. However, who ever obey this rule in practice? I've seen various installations of outdoor/rooftop Wi-Fi networks, and all the lightning protection they have is a gas discharge tube and a ground connection of unknown quality, I don't think the installation is legal. Also, many buildings were in compliance of the code when they were built, but even adding an extra metal frame or a cable-TV box will invalidate the entire lightning protection, at least from a technical or legal perspective. I guess the reason behind the non-compliance is that the likelihood of a lightning strike is already low, even if it does strike, it's possible that even a non-compliant lightning protection system can work to an extent. And if it doesn't work, most people would just accept the damage. As a result, it's a concern for radio hobbyist only, whose antenna increases the likelihood significantly. But I was still shocked when I first understood the fact that most (low-rise) buildings don't have a technically sound lightning protection system and bet the safety of the building on luck. And its practical consequence is: if you want to put a rooftop antenna for amateur radio, with good lightning safety and in compliance with the code, in many cases, the answer is you can't - the building often doesn't even have a technically sound lightning protection system to begin with. I read many hardcore radio amateurs redo the entire electrical wiring at their homes to the proper standards. High-rise buildings are much better, lightning protection is serious business, but you are not usually allowed to put a mesh network antenna on top of them. But if you are allowed, I've heard that some people even got a chance to have a friendly conversation with the civil engineer who's in charge of the building's lightning protection system and work out a solution together.

lutorm 7y ago

What do you do for grounding if your house sits on bedrock? We barely had enough for the "buried in trench, 30" deep" option at our house before hitting the solid lava everything here is built on. (And it's buried in mostly gravel, there is practically no dirt.)

dangrossman 7y ago
Apparently, according to an internet source I have not vetted, you lay the 8 foot rods in the bottom of the 30" deep trench.
- supermatt 7y ago
  Thats what parent says they have already done.
supermatt 7y ago
Run a large gauge wire to another location where you can put a better earth. In reality, you are likely on a TN earthing system, so you already have a low impedance earth wire supplied from your local transformer and your local earthing is just a "backup".
angry_octet 7y ago
A ground driven into rock or buried in dry gravel isn't going to be of much use. Typically electrodes are buried in a mixture of bentonite and gypsum, which is pH neutral and provides low resistance. There are also chemical (salt) earth fills which work well but require thicker copper electrodes to deal with corrosion. Various conductor arrangements are used, but in shallow trenches there is always the problem of the wires being cut. A radial or cartesian mesh burial over a larger area would be typical. You can buy electrical mesh and ground enhancement fill for this purpose. Note that you still need it to be below the frost layer in your region -- which varies from 15cm (California) to 300cm (Canada) depending. An earth connected via a distant cable isn't going to be effective against lightning (transients are high frequency and hence conductivity is constrained by the skin effect, and long wires have high inductance even if they have low DC impedance), and additionally is poor practice as it is more lightly to be cut without someone knowing in the future. I doubt any EE would approve it when other methods are available. https://blog.nvent.com/erico/erico-what-is-soil-resistivity-and-how-does-it-affect-grounding/ https://blog.nvent.com/erico/erico-what-is-soil-resistivity-... Cf IEEE 80 & 81

myspy 7y ago

We're currently building a house in Germany and the grounding consists of a couple dozen meters of thick (maybe 10mm) wiring which is laid out in the concrete ground plate of the house.

petre 7y ago
A couple of dozen meters? Wiring? Encased in concrete? Five to ten meters of zinc coated 30x5mm flat bar steel bound to three to six 3/4" rods is usually enough depending on the soil characteristics. And it's usuall buried directly in the soil next to or around the bulding. I wonder what bulding codes you've got there.
- brazzy 7y ago
  The relevant code is DIN 18014. And it distinguishes between different kinds of ground plate. Some are considered sufficiently "earth-feeling" to be considered ground themselves, others require separate corrosion-resistant grounding to be put in the earth around them. Zinc-coated steel is not considered sufficiently corrosion-resistant. Here's an article in German with some pictures of how it's done: https://www.elektro-plus.com/elektroplanung/fundamenterder/ausfuehrung-des-fundamenterders https://www.elektro-plus.com/elektroplanung/fundamenterder/a...
  - myspy 7y ago
    Interesting to read. I forgot that below the concrete plate is insulation and they put something in the foundation that touches the earth.
  - petre 7y ago
    Allright. This makes sense as it reduces the electro corrosion on the rebar and also prevents other unfortunate accidents such as the hot wire coming in contact an exposed metal part or a metal pipe which is itself in contact with the rebar. I have seen a scenario where ligtning struck a DHW solar panel installation, skiped from the plumbing to the rebar and also melted hole in the plumbing while doing so, draining all the antifreeze on the walls. In addition it destroyed all the electronic appliances in the house. If the panels along with the rebar would have been properly grounded, such as scenario would have been mitigated.

Roark66 7y ago

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.

vidanay 7y ago

For a great intro to grounding, check out this series of articles https://forums.qrz.com/index.php?threads/grounding-dos-donts-why-part-1.335582/ https://forums.qrz.com/index.php?threads/grounding-dos-donts... It's written from the perspective of a ham operator, but all the principles are the same no matter what.

FiatLuxDave 7y ago

Once I had a situation where the length of the ground rod mattered. We were building a fusion reactor prototype, and as far as I am aware the NEC code doesn't really have a section for that. I'm sure that much of what we did was probably against best practices. We had a number of high voltage power supplies running in one room of our lab, and on days when the humidity in the air was low, the ionization in the air would charge up all the metal surfaces in the room. So, even if something was not connected to a high voltage, you would get a nasty shock when you touched it. All these devices were connected to the normal, "third prong" wall socket ground, but apparently the route through the building to ground was too resistive to handle the large amount of ionization in the air. When working in a room with 180,000 volt power supplies, random shocks rapidly become more than annoying. Spence, our electrical engineer, had the idea that the quickest way to fix the problem was to run a grounding rod into the soil right outside the window. We pounded a rod into the ground with a sledgehammer as far as we could, which was about 5 feet, I guess. Then we ran a 1/4" copper cable from the rod, through the window to connect to the main reactor chamber. This worked great! At least for a couple of months. Our lab was located in a swamp, and the soil was basically a few meters of dried leaves. You could actually feel the soil move up and down when you walked on it. When August came around, it had not rained for quite a while, and the water level in the soil had dropped below the level of the grounding rod. The dried leaves in the soil insulated it quite well. We started getting shocks again, and it took us many hours of troubleshooting before we realized that the ground was not actually ground anymore. We went out and pounded the rod a few feet deeper, which was easier now that the soil was dry. This reached the water table, and fixed the problem.

crtlaltdel 7y ago
yup. had a control system installed where the rod was driven into a granite slab and was not sufficient
Scoundreller 7y ago
Also helps if you dump a large quantity of salty water. The salt eventually leaves too, but helps retain some of the water and makes it more conductive.

alkonaut 7y ago

Are all domestic buildings grounded this way in the US? Here (Sweden) I believe houses that aren’t far from transformers are only grounded at the nearest transformer (where power is transformed from 10kV 3-phase into 230/400V 3-phase+neutral for distribution to each house.

serf 7y ago
it's pretty common (pun unintended) , and usually preferred if you have any bigger electrical equipment like a garage shop.
mindslight 7y ago
Yes, the neutral is tied to the earth at the transformer and also at every house on the transformer. This means that some return current technically runs through the earth back to the transformer. shrug
mturmon 7y ago
Yes, all domestic buildings in the US have their own earth ground, and this is and has been required by the NEC (National Electrical Code). Often this is a thick, 8-foot long copper rod pounded into the ground not far from the electrical panel, connected to the panel by a ~1cm diameter copper cable. (There are other allowed methods, including using a similar copper electrode encased in concrete or in the building foundation.) It looks like this (which shows a galvanized-steel rod rather than the more canonical copper, and shows a further connection to a second ground rod, which is not required everywhere): http://www.wireyourownhouse.com/panel/images/grndrodexample.png http://www.wireyourownhouse.com/panel/images/grndrodexample.... Also, certain sub panels (e.g., in garages) will need their own separate earth ground, made the same way. This "grounding electrode" is in addition to a connection ("bond") to the domestic cold-water pipe, usually done where the pipe enters the building. (That pipe itself may be a hunk of copper buried in the ground.) The electrical bond to the water service is not a substitute for the dedicated grounding electrode mentioned above, it's to ensure that the plumbing does not become electrically charged.
- mattrp 7y ago

erobbins 7y ago

Kinda like IBM, no one ever got fired for using an 8 foot rod in a situation where a 5 foot rod was to code.

watersb 7y ago

Grounding, Bonding, and All That I have been reviewing the Mike Holt NEC video seminars, just listening like background music while I build some workbenches on the weekends. https://www.youtube.com/playlist?list=PLRNS1x1jcKbEgNJx26RWB8O4Fm1Ny-bxK https://www.youtube.com/playlist?list=PLRNS1x1jcKbEgNJx26RWB...

wmblaettler 7y ago
^ THIS - Go watch at least first video "Grounding - Safety Fundamentals" This was a very eye-opening video. I totally misunderstood grounding before watching this.

fyfy18 7y ago

I recently installed an IKEA kitchen hood, and was surprised that the power connector only had two pins (IEC C17/18). The hood has a aluminium frame (albeit painted) with metal pole switches. Can someone explain why this doesn't need to be grounded?

lurquer 7y ago
Perhaps the same reason toasters aren't. It is very likely that one will have two hands on the toaster when one is shocked. You have two choices: ground it, in which case the current races up one arm, through the torso, and down the other arm (frying your heart in the process). Or, keep it ungrounded and let the current race down your leg to the floor (sparing your torso and hopefully taking advantage of the fact that your shoes will provide a little more insulation that your two bare hands on metal in the first scenario.)

alextingle 7y ago

Why do the US regulations specify the method and not the result? That seems bizarre. Here in the UK, most earth connections are provided by the power company, but in rural areas you may be too far from the substation, so a ground rod is permitted - as long as it provides an earth loop impedance (Z) of less than 200 Ohm. So if you want to provide your own earth, you have to keep driving ground rods deeper, or adding new ones until the "Zeds" are good enough. It looks like the US regulations permit you to bang an "8 foot" rod into dry sand, and walk away as though that's job done??

walrus01 7y ago

I'm kind of surprised to read that entire article and not encounter the phrase "ohm meter" anywhere in a search. Rarely does anything serious have only one ground rod. Three legged telecom towers will have three rods, one attached to each leg, linked together by a ring of 6 gauge copper buried outside the perimeter of the concrete foundation. Multiple 5 foot ground rods in the correct configuration bonded together could offer much more conductivity to the earth than one 8 foot. The article seems to be obsessing about the length of the rod and not the scientifically measurable ohm resistance.