Calculating Earth Strip Size

Your state or country authority is the source for this info.

Unless you are experienced in the field of grounding, which I believe you are not based on your questions, it is best left to someone who is.

Generally a soil conductivity test is performed first for the area intended to be used to place the ground mat. If the conductivity proves to be too low for the amount of available area for the mat, it may require amending the soil to increase its conductivity.

Once soil conductivity is determined, ground mat configuration can be calculated based on fault current available.

If you know the answer of the above question. Then give a correct answer with complete formula of earthing.beacuse soil resistivity is first thing and after soil resistivity what we do.

you can measure this soil resistance with a high resistance meter. the way of the test can be found in the cr4. I remember I posted some of it.

Hello Raj : You can try to solve/calculate this by yourself using IEEE 80 ( substation grounding ) or I could try to help in some of the steps .

1st : what do you mean by 'earth strip size' ? do you mean area ( square meters ) of earth grid ?

2nd : what is 50 x 8 GI ? 3rd : is this a transformer substation ? and can you describe the single line diagram ?

If you use GI strips for earth grid then the Cross sectional area of the conductor A

A=I*sq root of time/K

k=80 FOR GI strips

I= The fault current

t= Duration of fault current

So if system fault level is 65 ka for 1 sec

Then cross sectional area will be 65000*1/80 =812 sq mm

So the cross sctional area of the earth strip will primarily depend on the system fault level whereas the soil resistivity decides the total length of the strips required,

Again when we get 812 sq mm strips from the formula we have to add suitable corrosion allowance to get the actual cross sectional area of the earth strip.Here soil resistivity is important as lower the resistivity more will be the corrosion allowance.

Hi Raj : Your calcs for size of "earthing conductor" is fine but it has nothing to do with sizing the "earth grid" and its conductors( all of which are presumably direct buried in good low resistivity soil).What you have calculated is okay for above-ground ( earth soil) conductors that are used to connect electrical equipment to earth. Also, you haved assumed all of the fault current, 65kA, to be flowing thru this earth conductor 'A'.

1st: this sounds like a 3 phase max fault current at some point? Since I don't have your single line diagram, I will try to guess one : you mention 500kV transformer : I'll assume 500kV primary delta/25kV secondary wye, resistance neutral ground(NGR)/200MVA/Z=12%. This gives a 3phase max fault, on secondary side of 38kA ! So, your transformer is somewhere in this category or magnitude.

2nd: Now this 38kA ( or your 65kA) is NOT the fault current that one uses for the earth grid sizing and its underground conductors! You have to use the available " earth fault current ( line-to-ground)" on the primary side, coming from the utility or power company towards your substation transformer! Usually, this would be around 3kA to 8kA ( much less).

3rd: Maybe you are the utility? Okay, in that case, you could use the fault current on the secondary side but it would not be the 3phase fault; it would be the line-to-ground fault current(which is the only current that could flow into your earth grid) and in your case the NGR is limiting it to some small value like 300A ? So, even if you are the utility, you would NOT size the earth grid based on the secondary side line-to-ground fault current( too small), but rather based on the primary side line-to-ground fault current. What is this current? You have to calculate it! It certainly will not be 65kA or 38kA but will be more like 3kA or 6kA ? ( you must find this out).

4th: Now, assuming that the earth fault current, on the primary side, that can possibly flow into the earth grid is, say, 5.8kA, you can proceed with the earth grid sizing based upon IEEE80 ! You need to know the resistivity of the soil. Let's assume that it is average of 200 ohm-meters.

5th: Next, typically, all underground conductors in North America are copper-stranded #4/0 awg ( 120 mm sq).

6th: Next, typically, all substation earth grids have a 150 mm thick top layer of crushed stone and/or concrete.

7th: Typically, our ( Canada) electrical safety code requires the following to be met :(for 0.5 sec fault) E(step) < 3105 volts; E(touch) < 885 volts; and GPR < 5000 volts.

8th: A typical earth grid will have a grid pattern of conductors, as well as some earth rods ( 19 mm dia copper-clad steel, 3 meters long, buried vertically).

9th: The most influential parameters in a low-resistance and safe ( to humans) grid are the resistivity and the area covered by the grid.

10th: Let's assume : fault duration = 0.5 sec; grid depth = 0.75 meters; grid conductor spacing = 2.4 meters ( both directions); Area of grid = 100m x 100m; # of ground rods = 60 .

11th: Based on IEEE80 equations : Rg( grid) = 0.645 ohms ; GPR = Ig x Rg = 5768 A x 0.645 ohms = 3722 volts ; Es = 3014 volts; and Et = 148 volts

These all meet the code safety requirements; I had to go thru several parametric iterations to get these optimum results.

Next time, with your permission, we'll look at the NGR conductor sizing.

Cheers, Ovide.

Dear sir thanx for your vital information will u plz confirm me what is the value of K for copper strip. thanx raja sen

material constant k for copper is 205 for current rating 1 sec..

and k for copper is 118 for current rating 3 sec..

these values are for bare conductors..

Regards..

Shridhar Patel

(India)

Really good info, can u plz provide the K value for Aluminum & Galvanized iron. Plz provide the info. Best Regards Mayank

Email Removed - CR4 Admin

k factor of aluminium is:

for 1 sec current ratinginA/mm2= 126

for 3 sec current ratinginA/mm2= 73

k factor for steel

for 1 sec current ratinginA/mm2= 80

for 3 sec current ratinginA/mm2= 46

thanks for your answers. please help how the no of electrodes and length of GI strip can be evaluated?

Does it go BANG?

That would be one way to <...evaluate...> it.

Declined.

Use the protocols in British Standard 7671 if the installation is in the UK.