Ground book

URNUD D R E SSSTA C E DEIS T I NG E R I TA N N T N G G O N
Current supply

E I
Ammeter (I)

Voltimeter (E)

X

Auxiliary potential electrode

Y

Auxiliary current electrode

Z

Rx

R1

R2

Rn-1

Rn

Resistance in %

Ground rod and clamp

100 75 50 25 0 1/2 5/8 3/4 1 1 1/4 Rod diameter (inches) 1 1/2 1 3/4

Contact resistance between rod and soil

• SoilResistivity
Concentric shells of earth

• Ground Resistance • 3-Point Measurements • 4-Point Measurements • Clamp-on Measurements

TA B L E O F C O N T E N T S
Soil Resistivity ................................................................................................. 2 Soil Resistivity Measurements (4-Point Measurement)............................. 4 GroundElectrodes........................................................................................... 6 Ground Resistance Values.............................................................................. 9 Ground Resistance Testing Principle (Fall of Potential – 3-Point Measurement) ................................................. 11 Multiple Electrode System........................................................................... 16 Two-Point Measurement (Simplified Method) ......................................... 17 Continuity Measurement ............................................................................. 17 Tech Tips ......................................................................................................... 18 Touch Potential Measurements................................................................... 21 Clamp-on Ground Resistance Measurement (Models 3711 and 3731) ................................................................................ 23 Telecommunications ..................................................................................... 28 Summary Quiz............................................................................................... 32 References....................................................................................................... 34 Grounding Nomograph ............................................................................... 35 Fall of Potential Plot...................................................................................... 36

Models 3711/3731 have replaced Models 3710/3730.

Workbook Edition 7.0950.WKBK-GROUND 08/00

NOTES

SOIL RESISTIVITY
Why Measure Soil Resistivity?
Soil resistivity measurements have a threefold purpose. First, such data are used to make sub-surface geophysical surveys as an aid in identifying ore locations, depth to bedrock and other geological phenomena. Second, resistivity has a direct impact on the degree of corrosion in underground pipelines. A decrease inresistivity relates to an increase in corrosion activity and therefore dictates the protective treatment to be used. Third, soil resistivity directly affects the design of a grounding system, and it is to that task that this discussion is directed. When designing an extensive grounding system, it is advisable to locate the area of lowest soil resistivity in order to achieve the most economicalgrounding installation.

Effects of Soil Resistivity on Ground Electrode Resistance
Soil resistivity is the key factor that determines what the resistance of a grounding electrode will be, and to what depth it must be driven to obtain low ground resistance. The resistivity of the soil varies widely throughout the world and changes seasonally. Soil resistivity is determined largely by its content ofelectrolytes, which consist of moisture, minerals and dissolved salts. A dry soil has high resistivity if it contains no soluble salts (Figure 1).

Soil Ashes, cinders, brine,waste Clay, shale, gumbo, loam Same, with varying proportions of sand and gravel Gravel, sand, stones with little clay or loam

Resistivity (approx), Ω-cm Min. Average Max. 590 340 1,020 59,000 2,370 4,060 15,800 94,000...