Soil Resistivity Measurement Procedure
Transcript of Soil Resistivity Measurement Procedure
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.