What is Earth Resistance

[abrahamJP]

I have two questions regarding earth resistance;

1.For single electrode R=(rho)/2(pi)[1/r] ->here why ressitivity of copper rod (material -copper) has not considered,but only resistivity of soil is considerd?

2.For concrete encased electrode,earth ressitance less than rod alone-How since concrete has a higher resistivity equivalent to dry soil

thanx in advance

Abraham

 

[7anoter4]

You are right Abraham, the total grounding resistance includes the rod resistance also.
However the resistance of the rod is negligible.
A stainless steel rod of ¾” diameter and 20’ length presents only 0.0015 ohm and the minimum grounding resistance in a very low resistivity soil it is more than 1 ohm.
The tolerance of soil resistivity measurement it will be more than the rod resistance.
According to IEEE 80/2013, indeed, the concrete specific resistance [resistivity] it is between 30 Ω-m to 200 Ω-m. There are other publications which consider this resistivity between 6-10 Ω-m. The soil resistivity could be between 10 to 100 [usually].
For a high resistivity soil a moisturized concrete will reduce the grounding resistance, of course.
By-the-way the grounding resistance of a rod directly buried is:
R=ro/(2*PI()*L)*(ln(4*L/a)-1) [in excel form]
ro=earth resistivity L =rod length a=rod radius
[IEEE 142/2007 Table 4-5—Formulas for the calculation of resistances to ground.
For concrete embedded electrode-see E.J.Fagan-and other- formula:
Res=k*(roe*(ln(8*length/2/conrad)-1)+roc*(ln(8*length/2/rodrad)-1)- roc*(ln(8*length/2/conrad)-1))
k=1/(2*pi()*length)
length-gounding rod length; conrad=concrete cylinder [around the rod] radius; rodrad=rod radius
roe=soil resistivity; roc=concrete resistivity.
See:
G-THE USE OF CONCRETE ENCLOSED REINFORCING RODS AS GROUNDING ELECTRODES by E.J.Fagan and others.

 

[rcw retired EE]

A small diameter rod has very little surface area in contact with the earth. A concrete foundation or encasement has a large surface area against the earth. Even though the reistivity is higher, the larger area compensates, making the total resistance to remote earth lower.

 

[abrahamJP]

Thank you very much for enlightning answers.

One question can I ask witout strating another thread;

Is there any thumb rule for distance between HV and LV Earth pits?for 11k,allowable GPR is around 300V as per IEEE ,I read.
But if there is any distance to be maintained by thumb rules something called "zone of influence.

thanx in advance

Abraham

 

[7anoter4]

IEEE 80- if this is the IEEE you referred-does not limit the GPR[ there is a mention this could be 5000 V], but the limit it is the touch and step potential. I don’t think you need a separate grounding grid for low and medium voltage. Usually low voltage does not leaves the substation area so no GPR will influence here. Only the cables leaving or entering-from outside substation-could rise the GPR.

 

[abrahamJP]

But following document mentions if HV & LV Separte keep 8mts distance from Earth Pits;

https://library.ukpowernetworks.co....istribution+Network+Earthing+Construction.pdf

that is my concern.

best regards

Abraham

 

[7anoter4]

This document is a property of UK Power Networks company, an utility distribution company in U.K.. This standard applies to the construction of earthing for secondary distribution substations, 11/6.6kV overhead networks and LV networks –operated by Company.
The low voltage cables supply the consumers and from the transformer-earthed on substation area-has to be insulated from low-voltage side and the secondary side has to be earthed far from substation limits in order to avoid transfer potential.
There is a number of possibility how to combine or separate the low-voltage and high-voltage electrodes according to BS 7430. See: 19.2 General earthing arrangement of substations.

 

[ScottyUK]

The old rule used to be that it was acceptable to combine the HV and LV earths if the electrode was less than one ohm to the mass of earth. The new rules require a calculation or modelling.

 

[7anoter4]

Thank you, Scotty. That means the BS7430/1998 it is the "old" one and BS7430/2011 this is the "new”. I have not this standard yet but I presume it is close to IEEE 80/2013 [which I have not with me still, also].