Frozen V Thawed

[captainwireman]

Will the resistance to ground change on a driven ground rod during seasonal changes that will either freeze the ground or thaw it?
Where does ice fall in the conductive scale?

 

[Dennis Alwon]

Will the resistance to ground change on a driven ground rod during seasonal changes that will either freeze the ground or thaw it?
Where does ice fall in the conductive scale?

A wet ground is definitely more conductive than a dry ground so I suspect with the ground frozen it will be better than a dry ground.

 

[GoldDigger]

A wet ground is definitely more conductive than a dry ground so I suspect with the ground frozen it will be better than a dry ground.

But if you are comparing moist unfrozen ground to the same ground when frozen, the conductivity will go down, although not dramatically (emphasis in below quote is mine).

Effect of freezing on conductivity of soils

Reducing temperature reduces electrolytic activity, and hence conductivity. The figure to the right shows this effect in terms of resistivity. Upon freezing, conductivity of water becomes that of ice, which is very low. However, freezing is rarely simple. Fresh water freezes at a higher temperature than saline water. Therefore solutes tend to become concentrated in a zone of unfrozen saline water adjacent to soil particles. Also, the electric field of cations adsorbed onto soil particles locally orients water molecules near the particle, preventing them from freezing.

The net effect is a slight and steady decrease in conductivity as temperatures approach freezing, then a levelling off through 0 degrees and a further decrease below freezing.

 

[captainwireman]

Is there a solution to the problem of having a code-compliant ground if the earth does not meet the 25 ohm requirement when frozen?

 

[GoldDigger]

Is there a solution to the problem of having a code-compliant ground if the earth does not meet the 25 ohm requirement when frozen?

Yes, just put in a second ground rod and then it will not matter.

 

[Dennis Alwon]

Is there a solution to the problem of having a code-compliant ground if the earth does not meet the 25 ohm requirement when frozen?

As far as the NEC is concern if you meet the 25 ohm rule when it is not frozen then it doesn't matter. If you have questions about the 25 ohms just drive another rod at least 6' away and you are good no matter what the reading may be.

 

[GoldDigger]

As far as the NEC is concern if you meet the 25 ohm rule when it is not frozen then it doesn't matter.

I would expect that it to depend on whether or not the ground was frozen at the time the rod was installed and inspected. I do not see an exception in the code for rating winter installations as if it were summer.

 

[captainwireman]

As far as the NEC is concern if you meet the 25 ohm rule when it is not frozen then it doesn't matter. If you have questions about the 25 ohms just drive another rod at least 6' away and you are good no matter what the reading may be.

Yes, but I was hoping to go to more of a safety/proper installation issue. For example, on the North Slope and other extreme, hazardous locations, what would be an appropriate and sensible installation? I have installed buried ground rings of #4AWG copper around skids but I was never involved in testing. Would this be good enough or just a band-aid because there is no better solution?

 

[GoldDigger]

Yes, but I was hoping to go to more of a safety/proper installation issue. For example, on the North Slope and other extreme, hazardous locations, what would be an appropriate and sensible installation? I have installed buried ground rings of #4AWG copper around skids but I was never involved in testing. Would this be good enough or just a band-aid because there is no better solution?

If you have permafrost below a seasonal thawing layer, going deeper will not help.
More area will always improve the electrode resistance up to a point where the resistivity distant from the rod becomes a limiting factor.
What are you actually seeing when doing ground resistance measurements? (Using fall of potential or other test?)

It seems to me that in terms of safety, static and lightning protection will not depend greatly on the electrode resistance, while fault current issues will be better handled through bonding and EGCs than through ground electrodes.

If the skids are getting external power, then a good EGC as part of the power feed will matter a lot more than grounding.
If the skids are self-powered, the ground should not be carrying fault current from that supply anyway.

It gets trickier when multiple skids are interconnected by power and control wiring, and especially if they are close enough for someone to touch both at the same time, but ground electrode resistance still does not play a major fault safety role, IMHO.

Answers would be different if you are using MV or HV circuits with earth return.

 

[iwire]

I would expect that it to depend on whether or not the ground was frozen at the time the rod was installed and inspected. I do not see an exception in the code for rating winter installations as if it were summer.

If it compliant when inspected the EC is done.

We don't worry if there is a drought either.

 

[Dennis Alwon]

I would expect that it to depend on whether or not the ground was frozen at the time the rod was installed and inspected. I do not see an exception in the code for rating winter installations as if it were summer.

I don't understand your reply to my post. I simply said if you got 25 ohms when it is not frozen-- assuming the summer or whatever, then the NEC doesn't care if there isn't 25 ohms during the freezing.

Anyway I think we are saying the same thing. Certainly it would be better if the 25 ohms is achieve during the worse case scenario however if you get 25 ohms at any time I suspect it will be better than most installs with 2 rods anyway. Around here I got 89 ohms with 2 rods in the moist season.

 

[roger]

Will the resistance to ground change on a driven ground rod during seasonal changes that will either freeze the ground or thaw it?
Where does ice fall in the conductive scale?

Actually the resistance will change daily. Test it today, it is x, test it tomorrow with the same test set it will be xxx.

Is there a solution to the problem of having a code-compliant ground if the earth does not meet the 25 ohm requirement when frozen?

Yes, the NEC should quit using 25 ohms as a requirement, I don't think one CMP member knows why it's there.

Yes, but I was hoping to go to more of a safety/proper installation issue.

Worry not, a driven rod won't really matter as far as safety at our voltage levels, it may help for surges and lightning strikes but we are talking much higher voltages. Bonding is much more of a safety guard

For example, on the North Slope and other extreme, hazardous locations, what would be an appropriate and sensible installation? I have installed buried ground rings of #4AWG copper around skids but I was never involved in testing. Would this be good enough or just a band-aid because there is no better solution?

It's good enough

 

[GoldDigger]

I don't understand your reply to my post. I simply said if you got 25 ohms when it is not frozen-- assuming the summer or whatever, then the NEC doesn't care if there isn't 25 ohms during the freezing.

I was just allowing for the possibility that the OP was doing the installation while the ground was frozen. Agreed that if it measures 25 ohms or less at whatever time it is initially tested, it is fine "forever".

 

[kwired]

Worry not, a driven rod won't really matter as far as safety at our voltage levels, it may help for surges and lightning strikes but we are talking much higher voltages. Bonding is much more of a safety guard It's good enough

A lot of truth in that statement. Some people get all worked up when they see no grounding electrode. I had an inspector look at a job one time and he seemed to have the opinion that this install was very dangerous because I had not yet installed a ground rod (I was going to but hadn't yet at the time). I had little concern over it, and if it were not for NEC requiring one maybe wouldn't even have installed one in this case. This was a poured concrete building for a waste water lift station. Though no CEE was readily available I kind of have a feeling between the concrete, the grounded equipment in contact with the concrete and the metallic piping both in the concrete and in the ground all had a lower resistance than driving several rods would ever have.