SPY 790 - Good Grounding Practices; Soil Moisture Content; Insulating Surfaces

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GOOD GROUNDING PRACTICES
Soil conditions must be evaluated prior to beginning the job
1. Soil Moisture Content
Since the Earth is being used as an electrical conductor the moisture content
of the soil must be taken into consideration. Soils with a high moisture
content offer a lower resistance to current flow and have a much lower
voltage loss. While soils of a low moisture content offer a higher resistance
to current flow, there is a greater voltage loss.
The voltage loss that is referred to here, exists between the ground rod
where the pipe is grounded to Earth and the ground rod or trailing wire where
the Holiday Detector is grounded to Earth. This distance could be 35 – 50
feet or even several 1000 feet depending on the job conditions.
As an example, if the job was located in an area of low soil moisture content,
called for an inspection voltage of 2.5KV and there happened to be a voltage
loss of 1.5KV between ground rods, the Holiday Detector would have to
be adjusted to 4KV to compensate for the 1.5KV loss in the Earth and yet
maintain the 2.5KV between the pipe and the coating. The 4KV voltage
setting would have been identified by performing a “Field Calibration”.
In this example, refer to illustration #3, avoid using the “Indirect grounding
using a trailing ground wire” method. This method allows the ground wire to
only come in contact with the surface of the soil. No ground rod is used to
locate an area of higher soil moisture content.
2. “Insulating” Surfaces
When the moisture content in a soil is so low it can’t provide the low resistance
return path for the low current and it basically becomes an “insulator” with
very high resistance and higher voltage loss.
For example, if the soil is very dry with low moisture content such as sand,
dry clay or a rocky surface, when the soil is frozen or there is ice on the
surface, these surfaces have become “insulators” and can’t provide a low
resistance return path. Other “insulating” surfaces would be concrete,
cement and asphalt. In each instance there would be a significant voltage
loss in the soil.