Document: Soil Resistivity Measurement Procedure

Doc. Code: D.200

SOIL RESISTVITY TEST PROCEDURE

1. Scope

This specification describes the procedure for the determination of soil resistivity

required for the design of earthing system for the National Irrigation system

2. Codes and Standards IEEE–80–2000 : Guide for Safety in

AC Substation Grounding

Theory

Wenner’s four electrode method is recommended for these types of field investigations. In

this method, four electrodes are driven into the earth along a straight line at equal intervals.

A current I is passed through the two outer electrodes and the earth as shown in fig. A and

the voltage difference V, observed between the two inner electrodes. The current I flowing

into the earth produces an electric field proportional to its density and to the resistance of

the soil. The voltage V measured between the inner electrodes is, therefore, proportional to

the field. Consequently, the resistivity will be proportional to the ratio of the voltage to

current.

Earth testers used for these tests comprise the current sources and meter in a single

instrument and directly read the resistance (R). The resistivity is evaluated from the

following equation:

� = 4 � R As per IEEE Std. 80-2000

1+ 2a/√(a2+4b

2) - a/(√a

2+b

2) Equation No. 44, Page No.53.

� = Resistivity of the soil in ohm-meters

R = Megger readings in ohms.

a = Distance between adjacent electrodes in mtrs.

b = Depth of electrode in mtrs. (0.6 to 0.8 meters)

Incase the depth of burial is less than 1/20th

of spacing (a), the Resistivity is then

calculated based on the formula as given below:

� = 2 � a R

Where, a = Distance between two consecutive electrodes. R =

Observed resistance in Ohms.

Test Procedure:

At the selected site, in the chosen direction, four electrodes are driven into the earth along

straight line at equal intervals, a. The megger is placed on a steady and approximately

level base, and the four electrodes shall be connected to the instrument terminals as shown in

enclosed drawing. A known current I shall be passed to the outer electrodes and the

potential difference V between the inner electrodes shall be measured. The resistivity of

shall be calculated from the above measured figures as mentioned below:

R = V / I Ohms.

Fig. A: Connection for a four Terminal meter

The same procedure shall be repeated on a particular point by changing the axis of

electrodes in various directions as mentioned below and shown in the enclosed

drawing:

1 . North-South

2 . North East- South East

3 . E a s t - W e s t

4 . South West-North West

Where, E1 & E2

P1 & P2

Notes:

– Current Probes

- Potential Probes

• Spike marking shall be in Straight line only.

• Spike distance between each of the electrodes, indicated as ‘a’ shall be equal and

shall be taken as 2 m.

• Potential Probes (P1 & P2) shall be placed close to each other in between Current

Probes (E1 & E2) as indicated above.

• Current Probes (E1 & E2) shall be placed outside the potential probes as indicated

above.

• The depth of burial of electrodes shall be selected suitably as per the site condition

E1 P1 P2 E2

a a a

The above mentioned procedure shall be repeated for the four (4) different locations on

the sites as indicated in the enclosed drawing and the same shall be recorded in the

enclosed format for results.

4. Format of Test Results

4.1 Details of Earth Tester:

1. Make :

2. Range :

3. Sl. No. :

4. Calibration Due Date* :

* - Copy of Calibration Certificates to be enclosed.

4.2 Spacing between Test Probes (a) :

4.3 Depth of Burial (b) :

4.4 Formula Used :

4.5 Condition of water system : salty, normal,

etc.