Fluke 1621 User Manual

1621 Earth Ground Tester

Addendum to Users Manual

Earth Ground Resistance

Earth ground resistance consists of:

• resistance of the connecting lead to the earth ground electrode

• resistance of the earth ground electrode, earthing rod, earthing plate,

earthing strip, mesh earth electrode, and similar

• dissipation resistance (the resistance between the earth ground

electrode and soil potential)

The resistances of the connecting lead and earth ground electrode are
negligible (after correct dimensioning), so the earth ground resistance consists
primarily of the dissipation resistance.

To determine the exact earth ground conditions, an accurate measurement of
the dissipation resistance is required. Because dissipation resistance is
dependent on soil resistivity and the shape of the earth ground electrode, a
metrological check must be made even if the position of the earth ground
electrode and the condition of the soil are known.

When redesigning an earth ground system (for example, for lightning
protection), the resistance can be calculated using Table 1. As a basis for this
calculation, the soil resistivity of the location where the earth ground electrode
is to be installed must be known. See “Soil Resistivity.”

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1621

Users Manual Addendum

Table 1. Earth Ground Resistance Calculation

Soil

Resistivity

(ρE)

Earth Ground Resistance e

Soil Type

Moist humus

Earth Ground Rod

Depth in meters

em

3 m

(9 ft)

6 m

(20 ft)

10 m

(33 ft)

30 10 5 3 12 6 3

Earth Ground Strip

[1]

Length in meters

5 m

(16 ft)

10 m

(33 ft)

[1]

20 m

(66 ft)

soil, moor
soil, swamp

Farming soil,

100 33 17 10 40 20 10
loamy and
clay soils

Sandy clay

150 50 25 15 60 30 15
soil

Moist sandy

300 66 33 20 80 40 20
soil

Dry sandy

1000 330 165 100 400 200 100

soil

Concrete

[2]

1 : 5

400 160 80 40

Moist gravel 500 160 80 48 200 100 50

Dry gravel 1000 330 165 100 400 200 100

Stony gravel 30000 1000 500 300 1200 600 300

Rock 107 - - - - - -

[1]

All values in the table are in meters except where specifically noted

[2]

For 1 : 7 concrete mixtures, increase value 24 %

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Earth Ground Tester
Addendum to Users Manual

Soil Resistivity

Soil resistivity (ρE) is the resistance measured between two opposing surfaces
of a cube of soil, with a lateral length of 1 meter. Soil resistivity is measured in
ohms-meters (em). See Figure 1.

2

A = 1m

=

E

L

L = 1m

Figure 1. Soil Resistivity

evp007.eps

Soil resistivity primarily depends on soil type (like farming soil, dry sand,
moist sand, concrete, gravel), although seasonal changes can also influence
resistivity. Dry soil has a higher resistivity than moist soil, and frozen ground
has a higher resistivity than dry, warm sand. See Figure 2 for examples of how
resistivity can change over the course of a year.

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