Measuring Earth Resistance

What affects grounding resistance?

First, the NEC code requires a minimum ground electrode length of 2.5 meters (8.0 feet) to be in contact with soil. Soil composition, moisture content, and temperature all influence the soil resistivity, so it is recommended that the ground rods be placed as deep as possible into the earth to be most effective.

Nevertheless, four variables affect the ground resistance of a ground system:

1. 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 occasions 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.

2. 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 0%.

3. 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 needs 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.

4. 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, connected, mesh or grid networks, ground plates, and ground loops.

Complex networks dramatically increase the amount of contact with the surrounding earth and lower ground resistances.