How To Perform ASTM G57 & IEEE FOP Testing Methods

IEEE Fall Of Potential (FOP)

 

The Institute of Electrical and Electronics Engineers (IEEE) Standard 81-2012 “Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System” suggests that the fall-of-potential (FOP) grounding test be used to evaluate the capacity of an electrical grounding system.

 

An FOP test is often used by power engineers who need a way to bring a neutral to a building in order to prevent accidents of electrocution or damage to equipment caused by lightning. To do this, they bury conductive grounding rods or grounding mesh next to a substation, power pole, cell tower, or any other installation that needs to be protected. By conducting an FOP grounding test, they can see:

  • How effective the grounding mechanism is.

  • What the grounding grid resistance value is.

  • Whether the ground has been implemented and installed correctly (and meets the National Electric Code).

In other words, if energy was synonymous with water, a power engineer would want to evaluate how much water a bucket could hold before pouring over the edges.

 

These are the steps of an IEEE FOP test:

 

  1. Disconnect the grounding system from the substation equipment. This is important so you get an accurate measurement (and don’t damage or hurt any machinery or personnel during the survey). Note that high voltages can occur and possibly harm personnel and/or damage equipment if neutrals are disconnected from energized equipment. Appropriate safety rules must be followed.

  2. For a fall-of-potential grounding test, you’ll use four electrodes—two current electrodes and two potential electrodes. In this test the grounding system will act as two electrodes, one current and one potential electrode.

  3. Place the other current electrode away from the grounding system—at least at a distance which is 5 times the size of the grounding system, 10 times is recommended if possible.

  4. Connect the SuperSting instrument to the grounding system for the fall-of-potential grounding test. Take the final potential electrode and measure along the line between your grounding point and the remote current injection electrode.

  5. Gather the data and plot it into a curve graph that plots resistance versus distance from the grounding point. The horizontal section of the curve will tell you the resistance value of the grounding system.

ASTM G57 Standard

Also known as the four-pin Wenner method, the ASTM G57 standard is performed by geotechnical engineers who want to find out the resistivity of the ground and soil. This is often done for cathodic protection design, or to find a conductive subsurface for grounding grids at electrical substations. ASTM G57 is similar to vertical electrical sounding (VES), but uses the Wenner electrode array instead of the Schlumberger array.

 

These are the steps to perform an ASTM G57 test:

 

  1. Pick a center point and expand your four electrodes around it. The current electrodes are called A and B, and the potential electrodes are M and N. The electrode sequence will be A, M, N, and B, and the center point will be midway between M and N.

  2. The distance between A, M, N, and B should be equal between each other. The distance between the four electrodes is referred to as value “a.” The survey is performed for a preset number of a-values. For each new measurement all four electrodes has to be moved so that the new “a” value is satisfied. This sets the ASTM G57 standard apart from VES (vertical electrical sounding), which uses the Schlumberger array which only requires the current electrodes to move apart for each measurement..

  3. Each time you stop, take a measurement and graph it using a logarithmic X-Y plot (where the X axis is A/B distance divided by two, and the Y axis is apparent resistivity).

  4. As the electrodes expand, you’ll be exploring deeper into the formation. The larger both dipoles become, the deeper in the ground you’ll be able to measure.

  5. The data can be inverted from apparent resistivity into “true” resistivity using the EarthImager 1D inversion software.

  6. The soil resistivity is used to determine the corrosivity of the soil. The lower resistivity value of the soil, the more corrosive the ground is and this determines how to design the cathodic protection.