Hello all. Question regards any industry material that can support defining contractor Fall of Potential Testing after rainfall.
To summarize the issue, soil layers and method of construction at a switchyard by a turnkey contractor has raised concerns on the potential quality of soil layers installed and the effects on the FOP test after rainfall. There has been a comment that FOP testing directly after rainfall will not affect the outcome. Our past experience is that this is not true, especially if heavy rainfall. Therefore, we would like to constrain their ability to test to a not-to-exceed number of days after having had a number of inches of rainfall. We’re not as concerned about lack of rain as this test is likely to occur in Spring and we live in a humid environment with plenty of rain. We have agreement within our team to constrain the timing of their test but have a requirement to ensure this constraint is a known industry standard.
Honestly, I have not seen this before in the IEEE standards unless I’ve overlooked but have heard from many with experience testing that it is common to wait 2 days, some lean on 3 days and that it can take tenths of an inch to affect the results. The end goal is to ensure the tests are done as close to a normal scenario possible, not in a drought and not directly after rainfall so that moisture content is representative.
Is anyone aware of a whitepaper on this topic that may be of assistance? Any industry expert guideline? Or if it does indeed exist in a Geotechnical or Electrical industry standard?
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It is not mentioned in IEEE 81 to wait a certain length of time after rainfall. The best advice I’ve heard about it is that all testing should be done with the Earth in its “resting state,” so if you live in a wetter area, then its OK for the soil to be a bit more moist.
I was on a project in East Texas doing various soil resistivity tests including FOP. An engineer and our inter-company grounding expert told me that it wouldn’t affect results to test in the rain, but the client wanted us to wait 24-72 hours depending on the volume of rain.
Do with that what you will, but based on my experience, the ground being a little more or less moist than usual does not affect test results. If you are concerned about the results, have the technician perform a quick Wenner soil resistivity test and compare it to previous results (if you have them) to validate your FOP test. If it is soaking outside, 48 hours is probably a safe waiting period.
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Thanks for the reply! That’s interesting that someone else said it shouldn’t matter. In our case, if you look at IEEE 80 Table 7, DGA with fines, ranges from 140x10^6 or 4000 ohm-meters dry (depending on samples from NC or GA) to 1300 or 1200 ohm-meters wet. Then Table 8 for typical soil resistivity of 10 to 1000 for wet to dry soil. This is quite a difference in the two types and they have the ground laterals buried in a significant amount of DGA.
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You typically will not fine a definitive time period for preventing FOP testing after a substantive rainfall. Throughout the years I’ve seen the same caveat spelled out in customer specifications(especially federal/military) which has become our office requirement:
“Measure ground resistance no fewer than two full days after the last trace of precipitation and without soil being moistened by any means other than natural drainage or seepage and without chemical treatment or other artificial means of reducing natural ground resistance” -Grounding Gotchas: Common Testing Mistakes - Hood Patterson & Dewar
Leading to our office requiring at least 48hrs post substantive rainfall with best results after 72hrs to become our norm(just to prevent violation of specifications not reviewed by customers). That being said this does not have any backing by IEEE or other electrical standards that I have found.
There is reference in IEEE 80 talking about the effect of moisture, temperature, and chemical content:
IEEE 80 12.4 Effect of moisture, temperature, and chemical content
Electrical conduction in soils is essentially electrolytic. For this reason the resistivity of most soils rises abruptly whenever the moisture content accounts for less than 15% of the soil weight. The amount of moisture further depends upon the grain size, compactness, and variability of the grain sizes. However, as shown in curve 2 of Figure 17, the resistivity is little affected once the moisture content exceeds approximately 22%, as shown in IEEE Std 142™ [B86].
The effect of temperature on soil resistivity is nearly negligible for temperatures above the freezing point. At 0 °C, the water in the soil starts to freeze and the resistivity increases rapidly. Curve 3 shows this typical variation for a sandy loam soil containing 15.2% of moisture by weight.
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Thanks for the response! You’re hitting on exactly the tact I’ve been thinking of taking and I agree, all I can find are customer specifications, mostly UFGS, DOD, Army, and VA. Between those, tables showing the resistivity differences of clay (wet/dry) vs DGA, which is very drastic, and reference against effects of moisture on resistivity. That may be the best I can do.
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