Application Note

Number of ground electrodes
Another way to lower ground resistance is to use
multiple ground electrodes. In this design, more
than one electrode is driven into the ground and
connected in parallel to lower the resistance. For
additional electrodes to be effective, the spacing
of additional rods need to be at least equal to
the depth of the driven rod. Without proper
spacing of the ground electrodes, their spheres
of influence will intersect and the resistance will
not be lowered.
To assist you in installing a ground rod that
will meet your specific resistance requirements,
you can use the table of ground resistances,
below. Remember, this is to only be used as a
rule of thumb, because soil is in layers and is
rarely homogenous. The resistance values
will vary greatly.
Ground system design
Simple grounding systems consist of a single
ground electrode driven into the ground. The use
of a single ground electrode is the most common
form of grounding and can be found outside your
home or place of business. Complex grounding
systems consist of multiple ground rods, con-
nected, mesh or grid networks, ground plates,
and ground loops. These systems are typically
installed at power generating substations, cen-
tral offices, and cell tower sites.
Complex networks dramatically increase the
amount of contact with the surrounding earth
and lower ground resistances.
Each ground electrode has its own
‘sphere of influence’.
Ground
systems
Single ground electrode
Multiple ground electrodes
connected
Mesh network
Ground plate
Type
of soil
Soil
resistivity
R
E
Earthing resistance
Ground electrode depth
(meters)
Earthing strip
(meters)
ΩM 3 6 10 5 10 20
Very moist soil,
swamplike
30 10 5 3 12 6 3
Farming soil, loamy
and clay soils
100 33 17 10 40 20 10
Sandy clay soil 150 50 25 15 60 30 15
Moist sandy soil 300 66 33 20 80 40 20
Concrete 1:5 400 - - - 160 80 40
Moist gravel 500 160 80 48 200 100 50
Dry sandy soil 1000 330 165 100 400 200 100
Dry gravel 1000 330 165 100 400 200 100
Stoney soil 30,000 1000 500 300 1200 600 300
Rock 10
7
- - - - - -
5