Ahdab Electrode Resistance

8/13/2019 Ahdab Electrode Resistance

1/19

2/10/20

MEASUREMENT OF GROUND

ELECTRODE RESISTANCE

Professor Ahdab Elmorshedy


When an electrode system has been designedand installed, it is necessary to measure the

ground resistance between the electrode and

true Earth.

The most commonly used method of

measuring the ground resistance of a ground

electrode is the 3-point measuring technique

shown in Figure 1.

This method is derived from the 4-point

method, which is used for soil resistivity

measurements.


2/19

2/10/20


1. Arrangement of the Electrodes for the Fall of Potential Method


The 3-point method, called the fall of

potential method, comprises the Ground

Electrode to be measured and two other

electrically independent test electrodes, usually

labeled P (Potential) and C (Current).

These test electrodes can be of lesser quality

(higher ground resistance) but must be

electrically independent of the electrode to bemeasured.


3/19

2/10/20


Analternating current (I)is passed through

the outer electrode C and the voltage is

measured, by means of an inner electrode P,

at some intermediate point between them.

The Ground Resistance is simply calculated

using Ohms Law; Rg= V/I.


The measurement of ground resistance is as

much an art as it is a science, and resistance

measurements can be affected by many

parameters, some of which may be difficult to

quantify.

It is best to take a number of separate readings

and average them, rather than relay on the resultsof a single measurement.


4/19

2/10/20


The auxiliary test electrode C must be

positioned far enough from the ground electrodeunder test so that the auxiliary test electrode P

will lie outside the effective resistance areas of

both the ground system and the other test

electrode.

If the current test electrode, C, is too close, the

resistance areas will overlap and there will be a

steep variation in the measured resistance as the

voltage test electrode is moved.


If the current test electrode is correctly

positioned, there will be a flat resistance area

somewhere in between it and the ground system,

and variations in the position of the voltage test

electrode should only produce very minor

changes in the resistance figure.


5/19

2/10/20


The instrument is connected to the groundsystem under test via a short length of test

cable, and a measurement is taken.

Measurement accuracy can be affected by the

proximity of other buried metal objects to the

auxiliary test electrodes.


Objects such as fences and buildingstructures, buried metal pipes or even othergrounding systems can interfere with themeasurement and introduce errors.

Often it is difficult to judge, from visualinspection of the site, a suitable location for

the tests stakes and so it is always advisableto perform more than one measurement toensure the accuracy of the test.


6/19

2/10/20


1. Fall of Potential MethodThis is one of the most common methods

employed for the measurement of ground

resistance and is best suited to small systems that

dont cover a wide area.

It is simple to carry out and requires a minimal

amount of calculation to obtain a result.


This method is generallynot suited to large

grounding installations, as the stake

separations needed to ensure an accurate

measurement can be excessive, requiring

the use of very long test leads (refer to

Table 1).


7/19

2/10/20


Normally, the outer test electrode, or current test

stake, is driven into the ground30 to 50 metersaway from the ground system, (although this

distance will depend on the size of the system

being tested - refer to Table 1) and

the inner electrode, or voltage test stake, is then

driven into the ground mid-way between the

ground electrode and the current test stake, and in a

direct line between them.

Table 1: Variation of current and voltage electrode separation

with maximum ground system dimensions, in meters.


8/19

2/10/20


Let E be the electrode whose resistance toground is required to be measured and let P

and Cbe the auxiliary rods driven into the

ground.

A known value of current I is circulated

between C and E, and the voltage drop V

betweenEandPis measured.

The resistance of the electrodeEto the ground

is V/I.


The optimum location for the potential

electrodePis0.62 of the distance fromE

to Cwhen the distance D is at least 30

times the depth of the electrodeE.

Let the base of the electrode E be a

hemisphere of (equivalent) radius r and

the other two electrodes designated asshown in the figure.


9/19


10/19

2/10/20

The potential atEdue to the entering current

isI/2rand due to the current leaving at Cis -I/2D.

The total potential atEcan be given by

D

I

r

IV

22=

Similarly the total potential at Pdue to the

current entering at E and that leaving at C

can be given by

)(22 HD

I

H

IV

=


11/19

2/10/20

The net potential difference betweenEandPwill be given byV = VE -VP

=

)(2222 HD

I

H

I

D

I

r

IV

+=

)(

1111

2 HDHDr

IV


If the resistance curve (Figure 2) between Eand Cis observed, it flattens beyond P (if located

optimally between Eand C) and the resistance of

the ground electrode between E and P and that

betweenEandCare nearly same.

An upward bend in the resistance curve above,

near the point C is due to the resistance of the

auxiliary rod C itself and does not affect

measurement of the resistance of the main

electrode E.


12/19

2/10/20

Therefore the potential difference

between E and Cand between E and Pwill also be the same.

Hence, the measured resistance of the

electrodeEwill be

I

VR =

+=

)(

1111

2 HDHDrR

wherec = D/randp = H/r.

But the resistance of the ground electrode is

R= /2r.

If the measured value Ris to be equal to R,

the condition to be satisfied will be

+=

pcpcrR 1111

2

0111

=

+pcpc


13/19

2/10/20


ccccc

p

cpcp

pcpc

618.02

1-5

2

)4(

0

0111

22

22

=

=

+=

=++

=

+


From this, it follows thatH = 0.618D will

satisfy this condition.

This indicates that for any separation of

the current electrodes (E and C), the true

resistance of one of them is obtainable

when the potential electrode (P) is 61.8%

of the distance toward the other.


14/19

2/10/20


The Fall of Potential method incorporates a

check to ensure that the test electrodes arepositioned far enough away for a correct readingto be obtained.

To perform a check on the resistance figure, twoadditional measurements should be made; thefirst with the voltage test electrode (P) moved10% further away from the ground under test,and the second measurement with the (P)electrode moved 10% closer to the ground undertest.



If these two additional measurements are in

agreement with the original measurement,

within the required level of accuracy, then the

test stakes have been correctly positioned and

the DC resistance figure can be obtained by

averaging the three results.


15/19

2/10/20


If there is disagreement amongst any of

these results, then it is likely that the stakes

have been incorrectly positioned, either by

being too close to the ground system being

tested, too close to one another or too close

to other structures that are interfering with

the results.


The stakes should be repositioned at a larger

separation distance or in a different direction

and the three measurements repeated until a

satisfactory result is achieved.


16/19

2/10/20


2. The 62% Method

3. Other Test Methods

(a) The Slope Method

(b) The Star-Delta Method

(c) The Four Potential Method


2. The 62% Method

The Fall of Potential method can be adapted slightly foruse with medium sized grounding systems.

This adaptation is often referred to as the 62% Method,as it involves positioning the inner test stake at 62% ofthe ground electrode-to-outer stake separation (recallthat in the Fall-of-Potential method, this figure was50%).

All the other requirements of test stake location - thatthey be in a straight line and be positioned away from

other structures - remain valid.When using this method, it is also advisable to repeat

the measurements with the inner test stake moved 10%of the ground electrode-inner test stake separationdistance, as before.


17/19

2/10/20


The main disadvantage with this method is that

the theory on which it is based relies on theassumption that the underlying soil is

homogeneous, which in practice is rarely the

case.

Care should be taken in its use and a soil

resistivity survey should always be carried out.

Alternatively, one of the other methods should

be employed.


3. Other Test MethodsMany other methods exist for taking ground

resistance measurements.

Many of these methods have been designed in

an attempt to alleviate the necessity for

excessive electrode separations, when

measuring large ground systems, or the

requirement of having to know the electrical

center of the system.


18/19

2/10/20


(a) The Slope Method

This method is suitable for use with large grounding

systems, such as substation grounds.

It involves taking a number of resistance measurements atvarious ground system to voltage electrode separationsand then plotting a curve of the resistance variationbetween the ground and the current.

From this graph, and from data obtained from tables, it ispossible to calculate the theoretical optimum location for

the voltage electrode and thus, from the resistance curve,calculate the true resistance.


(b) The Star-Delta MethodThis technique is well suited to use with large

systems in built up areas or on rocky terrain,where it may be difficult to find suitable locationsfor the test electrodes, particularly over longdistances in a straight line.

Three test electrodes are used, set up at thecorners of anequilateral trianglewith theground

system in the middleand measurements are madeof the total resistance between adjacentelectrodes, and also between each electrode andthe grounding system.


19/19

Documents

Ahdab Electrode Resistance