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What are the advantages of perpendicular traverses when conducting a soil resistivity test?


Question: When conducting two soil resistivity traverses perpendicular to each other i.e. traverses at right angles AND intersecting this would cancel out interference effects from buried services. If the traverses do not intersect and are several meters away from each other but are still at right angles to one another, does this eliminate the interference from buried services or achieve anything?

Hi Mohammed,

Thank you for your question regarding soil resistivity testing and perpendicular traverses, it is our pleasure to help.

When conducting a soil resistivity test, it is often recommended that at least two (2) test traverses are conducted.  These traverses should be at either right angles to each other, or at diagonals crossing the site.  The general purpose of this is to minimize the effect of unknown buried objects on the test.  A single traverse may inadvertently be placed on top of a long forgotten buried pipe or other metallic object.  Sometimes, the field technician can tell that there is a problem with the data while conducting the test.  However, it is often only upon running the data through the computer that it becomes apparent that there is a significant problem with the data.  This can be an expensive error, especially if the test location is some distance away and will require additional travel costs to repeat the test.  This is why it is generally recommended to conduct several tests at right angles, and/or crossing each other at diagonals.  When this occurs, you will be happy to have the second set of numbers.

In answer to your question, the perpendicular and/or diagonal traverses are NOT intended to remove electrical interference issues.  The best and only real way to eliminate the influence of electrical noise on your soil resistivity test gear, is to use quality Direct-Current (DC) meters.  Currently, there are only two (2) manufacturers of test meters on the market that are approved for use in soil resistivity measurements: Advanced Geosciences Incorporated (USA) www.agiusa.com and Iris’s Syscal meters (France) http://www.iris-instruments.com/.

We also recommend using a true test lead cables (silicone, 18 gage, 65 strand wire) for conducting the tests.  These cables along with the 800 V p-p DC meters, tend to be immune to most electrical interference issues and conduct very accurate resistance measurements.

Meters such as the DET 2/2 are 2.5 watt 50 volt maximum Alternating-Current (AC) meters (105 to 160 Hz) and should not be used.  Many meters use terms such as “reversing DC” or “pulsed DC” as alternate ways of avoiding the admission that they use alternating current at varying frequencies to measure the soil.  “Pulsed DC” for example is simply another way of describing a  square-wave AC signal.  We can get into all the reasons why AC signals are bad, but all you really need to know is that this industry is not the only one to recommend DC meters.  The US Geological Survey (USGS) also uses DC meters for its testing.

The other basic way to insure accurate soil resistivity testing it to use good test procedures.  We base our soil resistivity testing on the specification from Safe Engineering Services ltd (SES) in Canada (www.sestech.com).  As you may know, they are the primary code writers (and researchers) for all the current standards and regulations governing our industry.

It is important to understand that there are two (2) types of resistivity data that are sometimes used.  The first one is called Apparent Resistivity, and the second one is Actual Resistivity.  Apparent Resistivity is a simple formula that only provides an average resistivity reading from the surface of the earth to the probe spacing distance.  It is NOT a real soil resistivity number.  What you really need are the actual resistivity values.  Unfortunately, there is really no way to properly hand-calculate actual soil resistivity, the formulas are simply to complex and numerous to do without the aid of a computer.  We use the RESAP module from the CDEGS engineering software program.

The computer algorithms require a lot of data, and the data must be collected so that there are not too many “gaps” between the spacing’s.  The maximum allowable interval between spacing’s is a 1.5 ratio, with a 1.33 ratio preferred.  So a measurement taken at a 20-ft spacing, would need to be followed up by a maximum 30-ft spacing, and preceded by at least a 14-ft spacing, in order to keep the 1.5 rule.  In other words, if you have data for a 40-ft spacing, and then jump to an 80-ft spacing, the distance between spacing’s is a factor of two (2) which is to great and will cause errors in the computers calculations (math).   You would need a 60-ft spacing between the 40-ft and 80-ft readings.

We recommend the following spacing’s for the typical 100-ft or less sized grounding system: 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 6, 7.5, 8, 9, 10, 12, 15, 20, 22.5, 30, 40, 45, 60, 80, 90, and 120 feet.  That’s a total of 24 measurements for a single (1) 360-ft long test traverse.  You should conduct several of these traverses depending on the local soil conditions and space available, preferably at crossing diagonals and/or perpendicular runs.

If your sites require Step & Touch calculations, please remember that the solutions for mitigating those hazards can be very expensive.  It is important that we collect good soil data so that we know that we are basing these critical life-saving grounding designs on valid data.  The shallow-depth readings (smaller spacing’s) that are listed above are critical for human safety as this is measuring the soil where people stand.  The deeper readings (larger spacing’s) are needed to understand how the electrical faults will propagate through the earth.

We hope you find this information useful.  If you should have any further questions, please do not hesitate to call us at 310-318-7151 California time, and we will be glad to speak with you about your project, free of charge.

Best regards,

The Engineering Team at E&S Grounding Solutions


Photo Credit: E&S Grounding Solutions

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