Page 22 - Issue 2 2023

TECHNICAL

c) The resistance of the surrounding body of earth Number of ground electrodes
The ground electrode is surrounded by earth which conceptually is made up Another way to lower ground resistance is
of concentric shells all having the same thickness. Those shells closest to the to use multiple ground electrodes. In this
ground electrode have the smallest amount of area resulting in the greatest design, more than one electrode is driven
degree of resistance. Each subsequent shell incorporates a greater area into the ground and connected in parallel
resulting in lower resistance. This finally reaches a point where the additional to lower the resistance. For additional
shells offer little resistance to the ground surrounding the ground electrode. electrodes to be effective, the spacing of
additional rods needs to be at least equal
So based on this information, we should focus on ways to reduce the ground to the depth of the driven rod. Without
resistance when installing grounding systems. proper spacing of the ground electrodes,
their spheres of influence will intersect
What affects the grounding resistance? and the resistance will not be lowered.
First, the NEC code (1987, 250-83-3) requires a minimum ground electrode length To assist you in installing a ground rod
of 2.5 meters (8.0 feet) to be in contact with soil. The four variables that affect the that will meet your specific resistance
resistance of a ground system: requirements, you can use the table of
1. Length/depth of the ground electrode ground resistances, below. Remember,
2. Diameter of the ground electrode this is only to be used as a rule of thumb
3. Number of ground electrodes because the soil is in layers and is rarely
4. Ground system design homogenous. The resistance values will
vary greatly.
Length/depth of the ground electrode
One very effective way of lowering ground resistance is to drive ground electrodes Ground system design
deeper. Soil is not consistent in its resistivity and can be highly unpredictable. It Simple grounding systems consist
is critical when installing the ground electrode, that it is below the frost line. This of a single electrode driven into the
is done so that the resistance to the ground will not be greatly influenced by the ground. This is the most common form
freezing of the surrounding soil. of grounding and can be found outside
Generally, by doubling the length of the ground electrode you can reduce the your home or place of business. Complex
resistance level by an additional 40%. There are occasions where it is physically grounding systems consist of multiple
impossible to drive ground rods deeper – areas that are composed of rock, ground rods, connected, mesh or grid
granite, etc. In these instances, alternative methods including grounding cement networks, ground plates, and ground
are viable. loops. These systems are typically
installed at power-generating substations,
Diameter of the ground electrode central offices, and cell tower sites.
Increasing the diameter of the ground electrode has very little effect in lowering Complex networks dramatically
the resistance. For example, you could double the diameter of a ground electrode increase the amount of contact with the
and your resistance would only decrease by 10 %. surrounding earth and lower ground
resistances.

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Figure 2: Ground resistances table #Installation

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