Application Note

4
Grounding basics
Components of a
ground electrode
• Ground conductor
• Connection between the ground conductor
and the ground electrode
• Ground electrode
Locations of resistances
(a) The ground electrode and its connection
The resistance of the ground electrode and
its connection is generally very low. Ground
rods are generally made of highly conduc-
tive/low resistance material such as steel
or copper.
(b) The contact resistance of the surrounding
earth to the electrode
The National Institute of Standards (a gov-
ernmental agency within the US Dept. of
Commerce) has shown this resistance to be
almost negligible provided that the ground
electrode is free of paint, grease, etc. and
that the ground electrode is in firm contact
with the earth.
(c) The resistance of the surrounding
body of earth
The ground electrode is surrounded by earth
which conceptually is made up of concen-
tric shells all having the same thickness.
Those shells closest to the ground electrode
have the smallest amount of area resulting
in the greatest degree of resistance. Each
subsequent shell incorporates a greater area
resulting in lower resistance. This finally
reaches a point where the additional shells
offer little resistance to the ground surround-
ing the ground electrode.
So based on this information, we should focus
on ways to reduce the ground resistance when
installing grounding systems.
What affects the
grounding resistance?
First, the NEC code (1987, 250-83-3) requires a
minimum ground electrode length of 2.5 meters
(8.0 feet) to be in contact with soil. But, there are
four variables that affect the ground resistance of
a ground system:
1. Length/depth of the ground electrode
2. Diameter of the ground electrode
3. Number of ground electrodes
4. Ground system design
Length/depth of the ground electrode
One very effective way of lowering ground
resistance is to drive ground electrodes deeper.
Soil is not consistent in its resistivity and can
be highly unpredictable. It is critical when
installing the ground electrode that it is below
the frost line. This is done so that the resistance
to ground will not be greatly influenced by the
freezing of the surrounding soil.
Generally, by doubling the length of the
ground electrode you can reduce the resistance
level by an additional 40 %. There are occa-
sions where it is physically impossible to drive
ground rods deeper—areas that are composed of
rock, granite, etc. In these instances, alternative
methods including grounding cement are viable.
Diameter of the ground electrode
Increasing the diameter of the ground electrode
has very little effect in lowering the resistance.
For example, you could double the diameter of a
ground electrode and your resistance would only
decrease by 10 %.