Electrode resistance. From what to what?

[kwired]

If the charge on a metal object from a voltage source is steady, earthing the object will free one from receiving a shock on touching near or remote from it, provided the other terminal of the voltage source is not earthed. Metal objects with fluctuating charges are liable to give electrical shocks when there is considerable earth resistance.

I do not agree with that at all.

Even in ungrounded systems, there will be coupling with the earth and a huge potential difference between the hot conductors and the earth. Dropping one to ground will still establish possibly fatal step potentials.

Earthing one point of a voltage source simply brings that point to same potential as earth. There is no potential to anything else until you introduce a current flowing to another point of potential from the source, whether it be directly by contact with that point or indirectly through other loads.

 

[captainwireman]

"asymptotically"

I will have to throw that word out next time someone asks me why they need to drive two ground rods. Priceless! :lol:

 

[hurk27]

To understand the resistance around a driven ground rod or any electrode for that matter if your into reading take a look at this thread, the poster did some experiments that I did years ago while at the University of Florida in Gainesville, keep in mind that to wrap your mind around what is happening you must think in parallel paths in 3-dimensions not in two like you would with a wire conductor which is why most can't understand it.

In the below thread the poster was having a hard time in understanding why a ground rod would not prevent a touch potential, so he did an experiment using 50 volts from a controlled lab at his JROTC building, in it you will see that the current from his injecting electrode will flow in all direction from his injection rod equally 360? after you get out to the 4 foot mark, because closer the resistance can vary in a given direction.

Earth Shells Experiment = time to eat crow!

Here is also the video that Mike Holt produced that basically give the same info when he made a video of grounding myths:

NEC 2014 - Grounding Earthing Myths

You will notice that Mike as well as in the thread above it is found that the max sphere of influence is about 25 to 27 feet depending upon how conductive the soil is, the better the soil can conduct the smaller the SOI, once you get out of the SOI the Earth could be considered as 0 ohms resistance, this has been proved by engineers working on long 500 volt DC SWER (single wire Earth return) transmission lines as they found that all the resistance loss was the total resistance of the electrode system at each end of the run, the rest of the Earth between the two connection points did not contribute to any loss in the run.

In one of our test at UOF we did a current test from an isolated generator with electrodes about 75' apart, when we moved the electrodes to 1000 feet apart we got the same figures when we calculated in the extra wire resistance which showed that earth did not add any resistance to the longer run.

Like was said above it is the amount of soil around the electrode that has very few parallel paths that causes the higher resistance, as you get farther away the more parallel paths you have and the lower the resistance to the point of 26 feet to which it reaches the point that there is enough parallel paths that you can consider it to be 0 ohms.

Also the above info applies only to low frequency and low voltage systems, as once you get above a certain frequency or to the voltage that the electrons start to tunnel (arc) the science behind it changes.

 

[K8MHZ]

Great information, folks!

Thanks much!

I even learned a new word.

 

[hurk27]

Isn't this a bad idea? The current is attempting to return to the POCO transformer and will find a path through the earth. What if that path is through the neighbors nearby pool?

Well not really in the sense that a pool over 26 feet away from the current injection point would most likely not see any voltage rise, this distance would increase if the soil was of a poor conductive material such as sand, as the higher the soil resistance the larger the SOI.

One of the biggest myths is that stray voltage can travel far in the Earth, in fact this is not true, current can travel around the world but to measure any voltage would be like putting 1 amp on a 10' length of 2000 kcmil then placing a volt meter with the leads 1" apart somewhere in the middle, once we have reached the SOI of the current injection point which is about 25' the 3 dimensional parallel paths through the earth are so many the the resistance is so low that there is virtually no voltage drop.

Even current outside of the SOI is almost undetectable since you are only measuring a very extremely small section of all the parallel paths that the current has taken.

When the Navy tried to use ELF 76hz to send messages to their submarines they had to build a transmitter that had an antenna 32 miles long which was a fraction of the 2268.27 mile long wave length of 76hz, it took a tremendous amount of power just to send a a few watts of transmitted power, this was all started when it was discovered that we could detect the 50hz power systems from Europe with a highly amplified spectrum analyzer, but since this current was so spread out across the globe it was very weak, also the transmitter had to use very high resistance soil which they found at Republic, Michigan in the Upper Peninsula and at Clam Lake, Wisconsin, both were abandon around 2004, of course it was a one way communications since the subs had no way to supply that kind of power, but the problem wasn't that the current didn't go all around the earth, it was that by the time it took all the parallel paths around the globe and through it the signal was so weak it was too hard to receive it, the amount of power it took was never disclosed, a friend of mine who worked on the Clam Lake WI. one said they had a very large dedicated power plant to just supply power for the transmitter, so I can imagine the amount of amps they had to inject into the earth.

generally stray current is carried onto the property through a conductor such as the service neutral, it may be caused by a fault miles down the road but its not injected into the earth that far away, it comes down the road on the utility's MGN which is bonded to the service neutral that is raised above earth potential, earth is never raised above the neutral potential unless it is a very close event, it can also be brought in through water and gas lines or other conductive pathways, but earth has a greater ability to stay at it's reference zero volts then any conductor can raise it above it if there is any distances from the last point of electrode connection.

 

[GoldDigger]

Interesting.
I had thought that the system used air transmission and they were able to reach subs because the long wavelength penetrated the water to a greater depth.

PS: If high or medium voltage transmission is involved, the practical SOI can be much higher because the current (and therefore voltage) can be very high.

Also if the geology has lower conductivity below a certain depth, surface current density can fall off only as the inverse of the distance instead of the square of the distance.
One scenario is low conductivity rock with well watered suburban yards on top.

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[mivey]

also the transmitter had to use very high resistance soil which they found at Republic, Michigan in the Upper Peninsula and at Clam Lake, Wisconsin.

Didn't they have a long antenna in the Hawaii mountains to communicate with subs?

I had thought that the system used air transmission and they were able to reach subs because the long wavelength penetrated the water to a greater depth.

That's what I thought also.

 

[hurk27]

Interesting.
I had thought that the system used air transmission and they were able to reach subs because the long wavelength penetrated the water to a greater depth.

The feed lines ran in the air with the transmitter in the middle they fed two huge electrodes 32 miles apart buried deep in the earth, like I said the soil of this area had to be high resistance so the signal would go deep into the earth

PS: If high or medium voltage transmission is involved, the practical SOI can be much higher because the current (and therefore voltage) can be very high.

From what is known is once you get to the voltage that breaks down and the voltage starts arcing it will tunnel through the earth out in all directions till it dissipates, but once it gets to the SOI (25') of the soil it is pretty much shunted out, higher frequencies can travel on the surface of the soil a much greater distance which is why we see lightning strikes traveling some distance sometimes over soil.

Also if the geology has lower conductivity below a certain depth, surface current density can fall off only as the inverse of the distance instead of the square of the distance.
One scenario is low conductivity rock with well watered suburban yards on top.

It can have an effect if the higher conductivity rock or soil is large enough and within the SOI of the rod but once you are out of its influence the soil will return to what I said above, a lower resistance soil will have a smaller SOI then a higher resistance soil, the above would be like driving a ground rod next to a metal plate, the metal plate would make a lower resistance path through the rods SOI till it ends but then the SOI will take the shape of this plate into the its shape then the shells would follow after it leaves it, allot will depend upon how much of the higher conducting rock or soil is in the SOI of the rod versus out of its SOI as to how much effect it will have, take a rock that is mostly out of the SOI of the rod, it will favor the 0 volt reference of the surrounding soil if it is within the SOI of the rod depending upon how close it will favor the voltage of the rod it's all variables.

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[hurk27]

Didn't they have a long antenna in the Hawaii mountains to communicate with subs?

That's what I thought also.

I have never heard anything about one in Hawaii, for one the amount of land would have been expensive and the fact that the soil had to be of a high resistance type of soil between the two large electrodes they injected the modulated current into earth with which I would not think that the volcano ash or rock would have provided, the friend who worked on it was in the Navy and was the project manager as well as a radio engineer for the Navy, in 1986 he was moved to a base in Florida on Stock Island by Key West lucky dog, so I havent heard from him in a while.

I might have it wrong but he made it sound like they were injecting the signal into the earth using the earth as the antenna to propagate the signals through the earth, he said the amount of current was astronomical that was being sent into those electrodes.

 

[GoldDigger]

Again IMHO, the current injected into the earth electrodes was simply the return path for the current flowing in the antenna.
A low resistance earth would have been a effective ground plane under the antenna. Being that close to the wire, it would have canceled the generated signal at even short distances compared to the range they needed.
PS: I win:
https://www.google.com/url?sa=t&sou...Lo8Wxb1zZGXTZwfzw&sig2=HncSJNGUiK_6ocdWInGs9A

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[kwired]

Isn't this a bad idea? The current is attempting to return to the POCO transformer and will find a path through the earth. What if that path is through the neighbors nearby pool?

Well not really in the sense that a pool over 26 feet away from the current injection point would most likely not see any voltage rise, this distance would increase if the soil was of a poor conductive material such as sand, as the higher the soil resistance the larger the SOI.

And even if there is significant current in/around the pool, if it is constructed properly it should shunt that current around the users so they don't experience enough voltage potential to notice anything.

 

[ggunn]

In a related story...

When I was much younger and had to rely on mechanical conveyances parked in remote areas to spend time alone with... well, never mind about that. But anyway, at such times I used to listen to an AM radio station in Little Rock, Arkansas (KAAY - Beaker Street program - Clyde Clifford DJ), for mood music. In Baton Rouge, Louisiana, where I lived at the time, reception of that station was spotty, even though it was a Clear Channel station.

Someone snapped off my radio antenna in a parking lot as vandals were wont to do. Parked on the shore of one of the LSU lakes with no antenna mast, I could not get KAAY or much of anything else. I do not know what possessed me to try this, but I got the tire iron out of the trunk, drove it into the ground a few feet from my car, and ran a length of wire with the ends stripped from the stump of my antenna to the tire iron. KAAY came in like I was parked next to the transmitter. After that the hard part was remembering to retrieve my tire iron when it was time to leave.

 

[hurk27]

In a related story...

When I was much younger and had to rely on mechanical conveyances parked in remote areas to spend time alone with... well, never mind about that. But anyway, at such times I used to listen to an AM radio station in Little Rock, Arkansas (KAAY - Beaker Street program - Clyde Clifford DJ), for mood music. In Baton Rouge, Louisiana, where I lived at the time, reception of that station was spotty, even though it was a Clear Channel station.

Someone snapped off my radio antenna in a parking lot as vandals were wont to do. Parked on the shore of one of the LSU lakes with no antenna mast, I could not get KAAY or much of anything else. I do not know what possessed me to try this, but I got the tire iron out of the trunk, drove it into the ground a few feet from my car, and ran a length of wire with the ends stripped from the stump of my antenna to the tire iron. KAAY came in like I was parked next to the transmitter. After that the hard part was remembering to retrieve my tire iron when it was time to leave.

The thing about the AM broadcast band was it centered around the 1 Mhz mark in the radio frequency spectrum (.530 to 1400 Mhz) the wave lengths are quite long (600') so they tend to follow the ground as in what we call ground wave but at that frequency they would not have penetrated the earth very deep if at all, what probably happened was that you happened to use a length of wire that formed a divisional part of the wave length that made a good match for the frequency, as well as the earth around the tire iron, when I was a youngster I remember sitting out in my dads car just trying to see how far of a station I could pick up because at night the ionosphere would act as a mirror to these long wave lengths and you could sometimes here stations from all over the country, out in the everglades the high water table or standing water also helped as it reflected the signals up which directed more at the ionosphere, I can remember one time hearing stations from Alaska and Hawaii which had to be mutable hops, but the most common stations we heard were the big ones that ran 50kw ERP like WLS in Chicago, or WLW in Cincinnati, Ohio, or WSM in Nashville which we listened to the Grand Ole Opery every Sunday nite, but back in the 60's Cuba cranked up a multi megawatt station outside of Havana as a propaganda machine, it would make the AM broadcast band in South Florida about worthless as it splattered across the whole band as they would overdrive the modulation so it was sending out over 200khz wide signal or even more, back then the marine band 1.6 Mhz, which was also shared with some back woods law enforcement also was being wiped out till they got it shut down.

One problem with the AM broad cast band was because the wave lengths were so long they would not work well inside of buildings and not at all inside of tunnels, I remember when we drove through a tunnel the radio would just go dead, but a very smart guy found a way to use this as a way to sell advertising in tunnels he built small low powered transmitters that he put in tunnels and it would play a set of advertisements that was set to the amount of time it took to get through the tunnel at the speed limit, long tunnels you might even hear local driving info or weather info, it didn't matter what part of the AM band you were tuned to as it covered the whole AM broadcast band, I remember thinking it was a kind of neat idea.

 

[mivey]

I have never heard anything about one in Hawaii

It was VLF in Lualualei, Hawaii, 21.4 kHz, 480 kW.

 

[mivey]

he said the amount of current was astronomical that was being sent into those electrodes.

2.6 MW between the two stations. The original design for the Wisconsin site was supposed to use 800 MW.

Clam Lake had one N-S antenna and one E-W antenna both at 300 amps each.

Republic had one N-S antenna and two E-W antennas each at 150 amps.

 

[kwired]

One problem with the AM broad cast band was because the wave lengths were so long they would not work well inside of buildings and not at all inside of tunnels, I remember when we drove through a tunnel the radio would just go dead, but a very smart guy found a way to use this as a way to sell advertising in tunnels he built small low powered transmitters that he put in tunnels and it would play a set of advertisements that was set to the amount of time it took to get through the tunnel at the speed limit, long tunnels you might even hear local driving info or weather info, it didn't matter what part of the AM band you were tuned to as it covered the whole AM broadcast band, I remember thinking it was a kind of neat idea.

Kind of sort of the first version of what we call "pop up" ads today

 

[ggunn]

The thing about the AM broadcast band was it centered around the 1 Mhz mark in the radio frequency spectrum (.530 to 1400 Mhz) the wave lengths are quite long (600') so they tend to follow the ground as in what we call ground wave but at that frequency they would not have penetrated the earth very deep if at all, what probably happened was that you happened to use a length of wire that formed a divisional part of the wave length that made a good match for the frequency, as well as the earth around the tire iron, when I was a youngster I remember sitting out in my dads car just trying to see how far of a station I could pick up because at night the ionosphere would act as a mirror to these long wave lengths and you could sometimes here stations from all over the country, out in the everglades the high water table or standing water also helped as it reflected the signals up which directed more at the ionosphere, I can remember one time hearing stations from Alaska and Hawaii which had to be mutable hops, but the most common stations we heard were the big ones that ran 50kw ERP like WLS in Chicago, or WLW in Cincinnati, Ohio, or WSM in Nashville which we listened to the Grand Ole Opery every Sunday nite...

I used to do the same thing with an old tube AM radio my dad gave me when I was a kid; late at night I could listen to AM stations all over the country and the dial was pretty full. Nowadays... not so much. What happened? Did radios get less sensitive, or did AM stations dial back their power, or did so much more RF come on line that everything is getting interfered with?

As to my experience with the tire iron - the connection to the earth (or at least to the tire iron) was what made the difference; the wire alone didn't do that much.

 

[GoldDigger]

I used to do the same thing with an old tube AM radio my dad gave me when I was a kid; late at night I could listen to AM stations all over the country and the dial was pretty full. Nowadays... not so much. What happened? Did radios get less sensitive, or did AM stations dial back their power, or did so much more RF come on line that everything is getting interfered with?

As to my experience with the tire iron - the connection to the earth (or at least to the tire iron) was what made the difference; the wire alone didn't do that much.

For long wavelengths like AM, the supplied antenna will be very short compared to the wavelength. That makes it inefficient in extracting energy from the wave.
Adding capacitance to the end of the antenna (top loading) may increase the signal strength considerably.

The result you got is not particularly surprising, and does not mean that you were picking up a signal traveling though the earth. You effectively reversed the normal situation at the input of the radio.
Instead of the (electrically isolated) metal of the car being one terminal ("ground") of the receiver input and the antenna wire the other, what you had was the antenna terminal being really grounded (earthed) by the rod, and the whole body of the car (high capacitance, larger dimensions) became the antenna.
It does not work very well when driving though.

And as for listening to AM radio at night, all of the things you mention factor in.
The radios are not as good for the most part, as AM is an afterthought and fewer tuned stages are used.
There are a lot more noise sources in our modern environment and the band is still cluttered with lower power local stations adjacent to the clear channel frequencies.
And the power output of the clear channel stations is probably lower.