Dear, Bryan, Pierre;
As you requested :
1. What are the calculations used to determine the GES will perform its function properly?
Answer.
In terms of NEC, the calculation to get a ?Resistance to Ground? with a value less than the ?magical 25ohms? (borrowing Bryan?s term), This condition is the basis if the GES will function properly.
For the calculations I refer to (greenbook)IEEE?s preference in using ? E.B.Dwight?s Equation :
(1) R = (r/2pL)[ln(4L/a)-1] For simple installations & good soils
where: R = resistance of rod in ohms (W), r = soil resistivity in ohm-centimeters (W-cm)
L = length of rod in centimeters (cm), ln = natural logarithm, a = radius of ground rod in centimeters (cm)
= 0.5 x rod diameter (dia) in centimeters (cm). For N, multiple rods, it is considered as parallel resistance. A = land area occupied by ground grid with rod
(2) Rod & Wire Grid Electrodes from Std 80-2000-Eq 53,54, 56 ( industrial , bad soils)
Example : Resistance of ground rod bed R2= (r/(2pi*N*L))*(ln(4L/a)-1+((2kL/A^0.5)*(N^0.5-1)^2
An equivalent equation :
(3) Rg = (r /4)*(pi/A)^0.5 + r / Lt,
Lt = total lenght of (grid conductors + rod lengths)
Min qty. of rods can also be approximated by fault current over the derived fault capacity of rod. Also from IEEEStd 80.
I?m using a 3 Ohm criteria for commercial GES, & less than 1 ohm for Industrial or substation Installations.
All the formulas above is B_l S_t, if the soil is high ?r? and small soil area to drive these rods on.
That?s why soil is the most important part of a GES, for the unlucky guys, rockdrill then bentonite or carbonacious back fill is done before sinking the pipe type electrode. Or direct drill of the pipe electode for deeper depths. For surface soils, chemical treatment is used like common salt for cheap solution.
A US military based we did overseas, we are made to put a 2 meter layer of low resistance soil over a rockbed. Electrodes are separate CEEs , not foundation footing.
For horizontal projects we take soil samples prior to selecting the type of GES and GES area. For initial design, values of 100,000 -10,000 ohm*cm are selected.
For vertical structures usually with basements 100 ft below normal ground, ( where the building sits on the whole lot, GES is a uniform rod & wire 3 to 6 meter grid is driven on the whole area and tested before a lean concrete is cast.( a Ufer!) Then mat foundation steel work will be laid on top of this leveled slab.
2. How do you know the variables will remain (not degrade) after the installation?
Answer.
We don?t know and you won?t either.( in cased of concrete casts.)
That?s why I?m always ready to get down with anybody telling me to finish it, just like ordering hotdogs. Especially those civil works guys, yes, even my own boss. Workmanship,Testing and corrective actions is all we can do. But technically Specs should take care of that. Deeper soils behave the same way ( I observed this during dismantling works copper grounding wires and its crimp connections survived after 50yrs) over the years except for reclaimed areas or ones near water. Galvanised or stainless steel rods are recommended for salty soils,wont be too many coz of low resistivity the soil. Expected life usually considered is 20yrs. Additional surface electrodes are used if possible,Test wells are placed to monitor and also maintain as redundant solutions.
3. Can you describe this corrosive property of copper in more detail?
Answer : ( I?m not chemist so this is not really accurate )
The corrosion process is much like the action happening on the ordinary battery dry cell or car lead acid type, Metals have different ability to lose electrons some easily(high potential ? anodic), some not( low potential-cathodic) In this order (high-Mgnesium, zinc, aluminum, steel, lead, copper, carbon - low ). In an electrolyte ( acids or salt solutions) parent metal atoms are forced to combined with the oxygen atom as they release electrons to form water or hydrogen. ( this force is our voltage, comes from with in the metal atom with respect to another atom. Combine the three is the ingredient of the our battery -anode metal + (magnesium connected to carbon rod)) in salt paste(electrolyte) inside a cathode metal -zinc can(insulated from the paste). Connecting a conductor between + and ? will start the corrosion process . ( for us electrical guys ?to power our lamps?). Over time the paste becomes like rust, in other words the magnesium is corroded. Any combination of high and low metal will result in a battery(corrosion) process. Aluminum&copper, Steel & copper, Mg & steel and the higher will always corrode.
Burrying steel and copper together on a soil with moist and dry portions will start the corrosion process of steel. Soil as well as water or moist acting as both electrolye and conductor. Complete envelopment of concrete & copper material will not trigger the corrosion, after the water from the fresh concrete has been dried up by cement. Unless a crack develops and moisture comes in then its ?disaster in progess?.
Which is not impossibe due to the tendency of buildings settle itself as it weighs.
Thus in a foundation or footing used as ufer ground, it important that the same copper in contact with the steel rebar should not be in contact with the soil as the case of interconnecting to other electrodes outside the footing. And penetrations to the concrete where moisture can enter should be avoided. All necessary inter-electrode bonding and should be done above ground or permanently cast in concrete along with the rebars.
That?s why in structures built near the beaches we don?t use the foundation footing as the main ground electrode, still it is bonded to the rebars but at the columns inside the building, making sure of continuity to rebar below by tying extra jumpers or welding a short piece rebar jumper, if it?s the allowed procedure.
But on other sites whenever theres a ?safe? chance, I tell my boys to put connectors to that ?good spot? and make sure the steel work has continuity
As for my GES design, I consider the use of structural parts like footings for GE,CEE as ?unreliable?, due different sizes of steel and integrity of rebar connections. (?And this is not part of the electrical system? as far as contract is concerned) Thus considered as ?Not Available? , and use the same statement in NEC 250.50 ? Where none of these electrodes are available, one or more of the electrodes ? shall be installed or used. ? going back to the plain o?l rods.
If by chance the building collapses due to some freak of nature or somebody?s sloppy work, we don?t want them pointing to our ?stupid? CEE as the culprit wouln?t we, Never mind the CEE and NEC, This is what me and the AHJ / Electrical Inspector would always drink to at the end of the day.
I only bond below grade metal pipes when they are close to a grounding conductor or electrode. Otherwhise I leave them alone, when opportunity is present bond it (especially pipes riser to roof) to rebars.
I hope my reply is not that long.
DavidV
ES
