Ground Impedance Calculator Guide
Bill Hooper, Siemens Residential Products Division
Electrical System Grounding. Electrical power systems, such as the secondary winding of a transformer are grounded (connected to the earth) to limit the voltage caused by lightning, line surges, or unintentional contact by higher-voltage lines.
FPN: An important consideration for limiting the imposed voltage is the routing of bonding and grounding conductors so that they are not any longer than necessary to complete the connection without disturbing the permanent parts of the installation and so that unnecessary bends and loops are avoided.
Equipment Grounding. Metal parts of electrical equipment are grounded (connected to the earth) to reduce induced voltage on metal parts from exterior lightning so as to prevent fires from an arc within the building or structure.
DANGER: Failure to ground the metal parts can result in high-voltage on metal parts from an indirect lightning strike to seek a path to the earth within the building—possibly resulting in a fire and or electric shock.
Danger: Because the contact resistance of an electrode to the earth is so high, very little fault current returns to the power supply if the earth is the only fault current return path. Result—the circuit overcurrent device will not open and clear the ground fault, and all metal parts associated with the electrical installation, metal piping, and structural building steel will become and remain energized.
Measuring the Ground Resistance
A ground resistance clamp meter, or a three-point fall of potential ground resistance meter, can be used to measure the contact resistance of a grounding electrode to the earth.
Ground Clamp Meter. The ground resistance clamp meter measures the contact resistance of the grounding system to the earth by injecting a high-frequency signal via the service neutral conductor to the utility ground, and then measuring the strength of the return signal through the earth to the grounding electrode being measured.
Fall of Potential Ground Resistance Meter. The three-point fall of potential ground resistance meter determines the contact resistance of a single grounding electrode to the earth by using Ohm’s Law: R=E/I.
This meter divides the voltage difference between the electrode to be measured and a driven potential test stake (P) by the current flowing between the electrode to be measured and a driven current test stake (C). The test stakes are typically made of 1⁄4 in. diameter steel rods, 24 in. long, driven two-thirds of their length into earth.
Soil Resistivity
The earth’s ground resistance is directly impacted by soil resistivity, which varies throughout the world. Soil resistivity is influenced by electrolytes, which consist of moisture, minerals, and dissolved salts. Because soil resistivity changes with moisture content, the resistance of any grounding system varies with the seasons of the year. Since moisture is stable at greater distances below the surface of the earth, grounding systems are generally more effective if the grounding electrode can reach the water table. In addition, having the grounding electrode below the frost line helps to ensure less deviation in the system’s contact resistance to the earth year round.
The contact resistance to the earth can be lowered by chemically treating the earth around the grounding electrodes with electrolytes designed for this purpose.
Determining Actual Ground Path Impedance
While low ground impedance levels are desirable for personnel and equipment protection, their realization in the real residential and commercial environment is problematical at the high frequency component of lightning events. These high energy surges exhibit very fast leading edges with the equivalent frequency domain range of 0.25MHz to 0.50MHz. At these high frequencies, the conductor inductive properties can easily dominate the effective ground impedance path.
An interactive ground impedance calculator is offered as a resource in determining the actual ground path as a function of wire gauge, temperature, length, frequency, and ground electrode resistance. Based on these user entries, the calculator will compute reactance (XL), DC resistance, AC resistance, wire impedance, and, finally, the total run impedance.
The wire inductance calculation is based on Grover’s well-accepted equation for single wire inductance (“Inductance Calculations”, Frederick Grover, Dover Publ., 2004 edition, page 35).
The DC resistance is based on the actual installation temperature and is based on copper temperature coefficients. The AC resistance calculation is based on the high frequency “skin effect” characteristics of round conductors. The results correlate very well with published wire resistance as a function of frequency.
The wire impedance calculation combines the wire resistance and wire reactance. The total ground run impedance combines the ground electrode resistance and the wire impedance, thus effectively combining the effects of AWG, temperature, length, frequency, and electrode resistance.
An illustration of the calculator is offered below:
Click here to access this calculator in an excel spreadsheet file.
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