a substation ground grid

I would suggest that as a primer on the topic you study Chapter 14 of the IEEE Brown Book "IEEE Recommend Practices for Power Systems Analysis"

Are you a practicing engineer? If so your first lesson is to study IEEE 80 to know grounding system design. If not, forget it! take help of a consultant.

Mr B.Das don't be so much harsh in replying to questions and giving advice. All engineers do not have access to read IEEE 80 . It is a costly affair to purchase a copy even a copy may not be available in their library .More over do not expect that all those who post questions in this column may be practicing engineers .They may be students or those doing certain projects for their masters course .They are not expected to know everything . How many of our own practicing engineers in India have even read Indian electricity Act and IS standards.I have worked with hundreds of Electrical Engineers( BE , B Tech, M Tech) for various projects ,many of them were even ignorant of certain essential safety requirements of Electrical Installations. Learning is a continuous process and if any one ask for help do give them . If you have a copy of relevant IEEE 80 please try to give relevant quotes from it . I do not know how much is the length of your experience or expertise . Please take this as an elderly advice please help young Engineers to clear their doubts if possible .Do not discourage them from asking questions .Sorry I am out of country away from my office on a long trip and expected to return to India only in October otherwise I would have given all the required information . Sorry if I have told too much and please do not mind .Thanks and Regards

I recommend to you to hire a certified electrician; this will be safer.

I endorse the views of Mr Abraham.

Mr Das already answered harshly for some other post also in this forum.

I think you are refering to the design of Earth mat in a substation. It is too long to be presented here. Basically it is done to keep the step and Touch potential within safe limits.There are good text books which will give u or Google search will give some papers on this.There is an Indian author's book substation design which u can refer.

IEEE80 Clause 4.1: "To provide means to carry electric currents into the earth under normal and fault conditions without exceeding any operating and equipment limits or adversely affecting continuity of service," and "To assure that a person in the vicinity of grounded facilities is not exposed to the danger of critical electric shock" under normal and fault conditions. " Grounded facilities are only as safe as the quality of the ground. Facilities are only safe when they are effectively grounded. Effectively grounded, as defined by the NESC on page 6, is "Intentionally connected to earth through a ground connection or connections of sufficiently low impedance and having sufficient current-carrying capacity to limit the buildup of voltages to levels below that which may result in undue hazard to persons or to connected equipment." In other words, the second objective of Standard 80 is to make sure that all metallic non-current-carrying substation parts are effectively grounded.
The voltages that Standard 80 is concerned with are touch voltage, step voltage, mesh voltage, metal-to-metal touch voltage and transferred voltage. Touch voltage is the potential difference between the earth surface on which a person may stand and the surface of a facility the person is touching. Step voltage is "The difference in surface potential experienced by a person bridging the distance of 1 m with the feet without contacting any grounded object." Mesh voltage is "The maximum touch voltage within a mesh of a ground grid." Metal-to-metal touch voltage is "The difference in potential between metallic objects or structures within a substation site that may be bridged by direct hand-to-hand or hand-to-feet contact." Transferred voltage is "A special case of touch voltage where a voltage is transferred into or out of the substation from or to a remote point external to the substation site." The people that Standard 80 is trying to protect are the people who work within substations and the general public who may approach a substation. Though we can partially reduce the hazard to workers within a substation by requiring them to wear personal protective equipment like insulated footwear, the hazard to the public, someone outside the fence, can only be reduced by proper design of the grounding system.

A typical substation grounding system designed for installation in low resistance soil per Standard 80 might look something like the following:

A network of 4/0 AWG bare copper conductors buried 12 to 18 inches below grade in a grid pattern spaced 10 to 20 feet apart and extending outside the perimeter fence.

For Substations we in India design earth pits ( filled with charcoal and salt ) and earthing ( Earthing Plates, Earth roads and Funnel as per IS standards )separately for Body earthing , Neutral earthing and Lightening Protection systems .

Once an earthing system is made you must check the earth Resistance as per standards.
IEEE Standard 81-1983
IEEE Standard 81 is the IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System. The most accurate method for measuring soil resistance is what they call the Wenner four-point method. This method is discussed in detail in section 7, page 12 of Standard 81. The four point method consists of driving four short ground rods into the soil, placed in a straight line, separated by a distance (a). The rods are only driven into the ground to a depth (b) = a /10. The test instrument induces a current (I) into the soil between the two outer rods and then measures the voltage (V) on the two inner rods. The resistance is equal to the voltage divided by the current (V/I). The instrument does the calculation. The instrument reading then has to be multiplied by 2pa to get the average resistance.

The resistance we measure with a rod separation of 2 meters is the average resistivity of the soil to the depth of 2 meters. The resistance we measure with a rod separation of 4 meters is the average resistivity of the soil to the depth of 4 meters. If the 2-meter resistance and the 4-meter resistance are different, then the soil type, moisture content, and/or temperature of the top two meters of soil is different from the bottom two meters of soil. One instrument, which can be used to measure the soil resistance.

The following questions need to be answered before I can provide appropriate information.........

a) What country are you building the substation in (this is VERY important as it will help dictate which of the many, many standards you should be working to)?

b) What is your level of electrical and engineering experience (I am NOT going to, in good conscience, offer advice to someone who is not qualified to do the job as the result could easily result in damage to, or destruction of the substation, death, etc)?

c) What is your actual roll in the substation project (or is this a theoretical or homework discussion)?

d) Please explain clearly what you mean by "how can i put the rods"

Your spelling and use of lowercase "i" and "plz" have me worried.

Please advise.

Of the help to all:

I downloaded free IEEE 80 from www.scribd.com

Hello, I am supervising an earth network installation where even after modifying the soil the contractor didn´t reach the designed resistance to earth of 5 Ohm and they asked to accept joining the new network to an old one where after connecting both they reached the 5 Ohm and eve below, I can´t find an answer to this in IEEE80?

Sounds like the contractor and/or designer/consultant doesn't want to spend the money to get it up to the designed spec.

It is likely in another relevant standard (perhaps the general electrical codes and practice), I don't know which in your part of the country however so cannot say if they have to legally meet the 5 ohm value or not.

Have you tried talking with, say the network utility?