Electrical Grounding Help Information
for
LORESCO Horizontal Resistance Calculator

Data Input
NOTE: Do not use commas in the input values. If the decimal is not entered, it is assumed to be just to the right of the right-most digit entered.

Use English or Use Metric: You may choose English or Metric input units by use of the input unit selection button prior to entering variables. For example, if you wish to enter variables in English units and English units (ft & in) appear next on the input variable entry slots, simply enter the proper numbers and continue. However, if you wish to enter the variables in English units and Metric units (m & cm) appear next to the input variable entry slots, left click the mouse on the "Use English" button. Note: the required units for Resistivity are ohm-cm regardless of the input units selection.

Resistivity, (ohm-cm): Enter the average resistivity of the environment (earth) around the grounding electrode in ohm-cm. This is the soil resistivity value determined though field testing. Soil resistivities may vary from a low of 100 to a high of 1000000 ohm-cm. If the resistivity is in ohm-m multiply by 100 to convert to ohm-cm. Regardless of the input units selected, the units for resistivity must be in ohm-cm.

Length of electrode, (m or ft): Enter the length of the grounding electrode in meters or feet. If LORESCO grounding backfill is used, this is the total grounding backfill length. If grounding backfill is not used, this is the length of the electrode itself.

Depth to center, (m or ft): Enter the depth from the surface of the earth to the center of the horizontal grounding electrode in meters or feet.

Diameter/Width, (cm or in): If the electrode shape is cylindrical, enter the diameter of the electrode or electrode backfill column in centimeters or inches. This is the diameter of the LORESCO carbon backfill column, if used. If no backfill is used, this is the diameter of the electrode itself. If the electrode shape is rectangular, enter the width of the electrode or electrode backfill column in centimeters or inches. This is the width of the LORESCO carbon backfill column, if used. If no backfill is used, this is the width of the electrode itself.

Thickness (Rect. only), (cm or in): If the electrode shape is rectangular, enter the thickness of the electrode or electrode backfill column in centimeters or inches. This is the thickness of the LORESCO carbon backfill column, if used. If no backfill is used, this is the thickness of the electrode itself.

If the electrode shape is cylindrical, no thickness should be entered. If a thickness is entered, it is ignored by the calculations.

Rectangular or Cylindrical: Choose one of the two horizontal electrode shapes from the drop-down menu. This refers to the cross-sectional shape of the LORESCO grounding backfill column, if used. If grounding backfill is not used, this is the cross- sectional shape of the grounding electrode itself.

Result: This is the resistance-to-earth in ohms of the electrode system described by the input data. If the estimated resistance is higher than the required value, one or more of the design variables may be changed in order to determine its effect on the expected resistance. The greatest decrease in resistance is achieved by increasing the length up to the point where attenuation along the single electrode becomes significant. Once attenuation becomes significant, additional horizontal electrodes are desirable.

The most recent result along with the input data is displayed in output column 1. You may recalculate by reentering the required variables while changing any one or all of the input data values for the next calculation. As additional calculations are undertaken, the output results automatically scroll to the right. In other words, at any time you may compare the three most recent calculation results with the most recent calculation results.

General Information

This calculator employs horizontal electrode equations developed by Erling D. Sunde and found in his book Earth Conduction Effects in Transmission Systems. In the case where the electrode shape is chosen as rectangular, Sunde's equation for a horizontal strip electrode is used if the width is greater than eight (8) times the thickness. If the width is less than eight (8) times the thickness, an equivalent diameter is calculated for the electrode by determining the diameter necessary to equate the surface area for the rectangular electrode and a cylindrical electrode. Then, Sunde's equation for a horizontal cylindrical electrode is used assuming this equivalent diameter. Of course, if the electrode shape is chosen as cylindrical, Sunde's equation for a horizontal cylindrical electrode is used with the diameter entered. These equations apply to a single, continuous horizontal electrode.

Development of the equations assumed a homogeneous electrolyte with a single resistivity. The equations ignore any attenuation effects along the grounding electrode system. Although the equations were derived for direct current flow, they may be used for low frequency alternating current systems and even in estimating lightning surge resistance if the length of interconnecting wire is relatively short (less than 100 meters). As the length of the grounding electrode or interconnecting wires become greater in AC systems, the impedance (inductance) becomes more significant and should be taken into account.

If the horizontal electrode system consist of multiple horizontal electrodes either placed in individual LORESCO grounding backfill columns rather than a single continuous backfill column or without grounding backfill, the calculator cannot determine the exact resistance-to-earth. However, the resistance may be estimated as follows. Calculate the resistance assuming the Length of the electrode is the length from the beginning of the first electrode or electrode backfill column to the end of the last electrode or electrode backfill column (In other words, the total end-to-end length of the system). Next, calculate the resistance assuming the Length of the electrode is the sum of the individual electrodes or electrode backfill columns (The space between the electrodes or electrode backfill columns is subtracted from the total end-to-end length). The actual resistance will be between these two calculated values.

These calculations do not take into account the contact resistance between each electrode and the LORESCO grounding backfill. However, the contact resistance for each electrode is small, being on the order of a few tenths of an ohm or less. Since all of the electrodes in a grounding system are in parallel, the total contact resistance is the contact resistance for one electrode divided by the total number of electrodes. Therefore, for practical cases this contact resistance is negligible and may be ignored.