improving earth resistance in rock

I believe one of the substations built on solid rock down here had large diameter holes bored into the rock and filled with compacted coal and graphite. The earth rods (longer than normal) were then driven into the holes. Primarily the enlarged hole surface area was enough to reduce the earth resistance to an acceptable level, while the coal and graphite provided a good conductive path.

Interesting project you have there. I have a few Q4U´s , how do you measure the earth´s resistance? I am not used to worry much about lightning´s as they must be afraid of the cold or the dark here in Iceland :) But my first thought is that there must be some special method to measure the Re, having lightning power in mind, do you have extra high voltage output on the ohm-meter? How long is the between measuring points? The earth must have not only low ohm but also high capacitance for the power coming from the sparks of Thor´s Hammer.

Very common application. Google "measuring earth resistance" or http://wiki.myelectrical.com/index.php?title=Earth_Resistivity.

thanks for your responces

Earth resistance is being measured using a Kyoritsu 4105A digital earth resistance tester tester. Two earth spikes are driven at 7 or so meter intervils and the meter uses a frequency of aprox 840 hz and in the 20 ohm range it uses aprox 3 mA

I have found that wetting the spikes and or driving larger/deeper spikes dosent effect the resistance value so I beleive that spikes are making good contact.(the meter was calerbrated just before this job so hopefully no probs there)

the bentonite/gypsum mix is mixed to a paste and poured down the hole and sets once dry the drilled hole is about 80 mm in diameter(posably not enough surface area?)

I could do what i have seen done many times, get a water truck to wet every thing down the night before the tests but i have a little more pride than that lol

I have found that wetting the spikes and or driving larger/deeper spikes dosent effect the resistance value so I beleive that spikes are making good contact.(the meter was calerbrated just before this job so hopefully no probs there)

It will have an effect, just not a particularly big one. More earth rods should do the trick for this application (although I don't really know what your soil composition is).

I could do what i have seen done many times, get a water truck to wet every thing down the night before the tests but i have a little more pride than that lol

<Cringe>

"I say that magazine is sinking into the ground"

"Its a..ahh...self deploying bunker sir. Yes it is supposed to be at that angle sir."

Don't cringe too much. As God is my witness, I've known engineers to urinate on signal ground rods.

I've done it. Works great... for a while.

In this case It seem to be the best way to add more earth rods or perhaps a few cu-plates placed vertically in the ground would be a better choice they give more area than the rods. I guess it would be more expensive to completely change the ground material or add a few truckloads of ground with lower earth resistance? On a rough working area that could be an option.

I'm far from being an expert, but I have observed some similar situations. In one instance, a copper salt mixture (if memory serves, it was copper sulfate) was used to surround the rods. In another, sacrificial anodes (not the same exactly as grounding/earthing) were set in coke breeze. Suggest you investigate these at least. May be helpful.

I went to site this morning with a drill we drilled to 14.5 meters hoping to break through the rock in to some more conductive soil but no such luck, i had the site geo to come and give me some idea of what we are dealing with, for the first 3 or so meters we have wethered (poferite) granite then a small aquafer zone 100mm aprox and after that it is fine grain mica greisen all the way to china lol

i had heard of the urinating trick, tried that last time i tested for a laugh it didnt have much effect

since in this situation i am only a subby i a have passed on the info it will be interesting to see what they come up with

The grounding rod forms a spherical conductive path around the rod, so the longer the rod, the greater the conductivity.Longer rods will have more effect than adding more rods.Pouring salt around the rods is also an acceptable( by NEC) method to enhance conductivity, but the rods must be made of a material to resist the corrosive effect.You are sitting on top of a very excellent insulator(granite) and you need all of the enhancement you can get.Another important factor is to be certain that all of the rods are bonded together to prevent a difference in potential between individual rods.Connect the containers to the bonded rods at only one point.Same reason.Give the surge only one point to ground, or there will be possible arcing on surface due to potential differences.Keep all bends in the ground wire gradual and smooth.Lightning does not like to turn sharp corners, it had rather jump than turn sharply.Do not coil any of the excessive ground wire, as this will form a "choke" that resists high frequency current.

Good luck.

HTRN

Hi,

I understood that the length of a grounding rod does not increase its effectivness linearly; as you go deeper, the added effect is minimized. As such, we are to use parrallel ground rods, or a horizontal rod; perhaps a group of rods driven into a relativly shallow gulley, tied together and covered in Bitumite. Have you ever used such an approach in rocky soil?

How do you envision the air terminal? individual vertical rods, or a ring around the top of the container connected to the ground ring at each corner?

Is adherence to an arbitrary value of ground resistance necessary? If the area on which you errect your installation is of the same substance-granite- of the same resistive value for kilometers around, is there not an element of relativity here?

lightning striking to ground anywhere will dissipate- eventually ;-)

from the explosives point of view, as long as you ensure that the potential of the ground raises equally throuhgout, and that you have no foreign grounds entering your installation (power or comm. lines with no surge arrestors), would it not be enough to build a series of shallow ground rods connected in parrallel or series at each corner, and then connecting these groups in a ring?

the lighting current would then dissipate away from the container, while the ring ensures that the whole installation raises in potential equally.

The NEC requires that all ground rods be a minmum of 8 feet long and driven down to the full length unless soil conditions (rock) make it impractical.The angled placement is not as effective as the vertical placement.The deeper a ground rod goes the larger the spherical area influenced by it.Check the NEC for furthur details.

In West Texas areas our substation/switchstations often require ground wells. Generally a 12" Dia 130-200ft deep. 3/4" x 20' ground rod thermowelded to 4/0 conductor placed to the bottom of the hole. Backfilled with either Bentonite or modified coke breeze. The intent is to hit water or a damp strata.

In extremely resitive areas (flint rock or sandstone) These can be placed as close as 20' apart. Target is <25ohms

Gentlepeople, greetings to all! Have any of you thought of using Ufer grounds (named after Herbert Ufer), who faced a very similar problem way back in WWII with ammo storage in the desert (sounds like you are doing the same thing today!). As there are many on line references to how well Ufer grounds work, and how they are installed, I'll not take up any more space here. You should be able to get below 5 ohms even in dry soil with Ufer grounding schemes. I am currently building a home in the Rocky Mtns where the soil is mainly granite, and used Ufer grounds for my grounding scheme, which the building department inspector quickly accepted. This is also spelled out in the NEC, Article 250.52(3).

Thank you for this incite, never heard of Ufer grounds until now.

Looks like the perfect grounding system for the rock scenerio.

To ausiandy,

With what you have started, your third action might give the desired result as long as the earth rods remain vertically bonded.

Cheers,

ethobil

We had to go through the same experience of adding lightning protection to a shipping container used to store fireworks. The health & safety people insisted we should have it but when I contacted a lightning protection company they said 'you don't need protection, it's an all metal cage'. The health & safety people are the ones who issue our storage license so they won the argument & we have earth bonding now. We also have to add 50²mm bonding straps from the doors to the frame.

We had no trouble getting the right ground resistance value but our store is in an old gravel works now used by sports anglers & we are only about 1-2m above the water table.

I was taught not to attempt to isolate lightning current, the voltages involved are too great and unpredictable, such that a flash over is always possible, particularily in wet conditions ;-)

Isolating down conductors set in ground out of the influence of the container/building ground is another story. Though air terminals (auldrey wires?) will have to be strung above the container such that the protection angles will reduce to a minimum the chances of strike into the container.

I still would not use rubber pads, but rather use a Faraday cage.

According to the methodology for lightning protection that I was taught, we are to build as good a faraday cage as possible. This allows the lightning current to run to ground along the outer faraday cage in as efficient a manner as possible. Sensitive equipment (I assume explosives are defined as such) are surrounded by yet another interior Faraday cage that is connected to the same potentional. Galvanic cables that enter one cage from the other are equipped with subsequently smaller SPDs.

When lightning stirkes the outer cage (exterior shell of the building, or the intended air teminals attached to down conductors and ground), the entire building is raised in potential equally. The ensuing electomagnetic feild propogates inward and is "caught" by the interior faraday cage, producing yet another current to ground, though much smaller. Assuming the container is metallic, it is one of the faraday cages.

In the case of explosives, I would consider yet a third Faraday cage.

In this application, the high earth resistance is a bit of a problem, since in such appllications, you must ensure that the down conductors enter the ground electrodes at a point where they are not under mutual ground influence of the ground on which the container is sitting. The distance I was taught to use is 2+ Re/5 in meters.