When is site available? Wait until final substation grading? Soil moisture and temperature. TEST METHODS Schlumberger-Palmer test Vary potential inner pin separation, keeping distances between potential and current pins equal Can leave current pins in one place, moving only potential pins Could speed up measurement process move 2 pins instead of 4 pins Might better detect changes in soil resistivity vs.

Injects white noise current as high as several Amperes Automatically switches between the multiple potential probes Each measurement is actually several Schlumberger-Palmer measurements Software automatically displays 2-layer soil and parameter errors.

TEST METHODS Interferences Any conductive object in the vicinity that can divert the test current or distort the soil potentials Metal fences Buried pipes metal Grounding systems Transmission or distribution pole grounds, especially if connected to other pole grounds Distribution cables with bare concentric neutrals Any circuit that can induce voltages onto test leads Transmission or distribution lines Outside sources of current in the soil Lack of space to achieve desired maximum pin spacing.

As the actual resistivity increases or decreases with greater depth, the apparent resistivities also increase or decrease with greater probe spacings. The maximum change in apparent resistivity occurs at a spacing larger than the depth at which the corresponding change in actual resistivity occurs. Thus, the changes in apparent resistivity are always plotted to the right of the probe spacing corresponding to the change in actual resistivity.

The amplitude of the curve is always less than or equal to the amplitude of the actual resistivity vs.

In a multi-layer model, a change in the actual resistivity of a thick layer results in a similar change in the apparent resistivity curve. TEST METHODS Interpretation of results during testing If using software, input data in laptop while at site If using visual techniques, plot measurements by converting measured resistance to apparent resistivity Does apparent resistivity profile match expected based on soil type and environmental conditions?

Seasonal variations particularly for grounds buried in a permafrost or over a high resistivity stratum such as rock bed. Induce current in the loop made by the subject ground and multi grounded neutral or shield wire system.

Resistance is measured in series with a nearby low impedance grounding system such as power companys neutral system. Only Tagg method allows measuring distances from a convenient point on the perimeter. Not suitable for grounding system connected at more than one point such as substation ground grid. Resistance of subject ground must be significantly higher compared to multigrounded shield or neutral system.

CP and PP leads placed in proximity and parallel to metallic objects connected to the ground under test. Try for best estimate in other cases Non-uniform soil Large or irregular shaped ground grids Interconnected grounding systems. Reality has even more variables for which we can accurately account in our designs. What do we know? When we install a ground grid, what have we achieved? Green-Field Brown-Field. Can we measure the performance of the ground grid? How might we measure scalar potentials Transferred potentials?

Touch Potentials? Step Potentials? Inject current into the grid Measure the soil scalar potentials. Does this sound familiar? Similar concept to fall of potential testing.

Characteristics of the current circuit Current Generator Injection Collection point remote from ground grid How far is far enough? Variability in Design parameters Measure actual response of ground conductors Non-homogeneous soil Temperature at time of test Moisture at time of test Geology Actual grid! Nearby foundations, metallic structures, houses, industrial ground gridsall will be present under event conditions Many benefits to measuring actual Volts.

Measurements The actual grid response can be measured Measure, review and validate design compliance requirements Compare measurements with tolerable limits Measure open circuit conditions Measure loaded circuit conditions measure body current. Lay definition: Voltage across your feet spaced 1m apart.

Worst cases typically OUTSIDE substation where no insulating gravel is present Around sharp corners of ground conductors Significantly dependent on soil resistivity Around geological changes. Touch Voltages Lay Definition: Voltage from your hand to two feet typically 1m arm reach What can you touch in a substation or nearby which might have a voltage difference?

Metallic objects within the substation and the fence will be at the GPR of the site. What you are standing on will be a surface potential.

Also on gate while opening or unlocking Structure: pretty much everything else you can touch with a 1m reach. Remote Transferred Voltages Maximum voltage differential at 1m arm length: Water faucets Multi-grounded distribution neutral Telephone and cable boxes Fences Gas lines Cathodic Protection test points Light standards etc.

Construction Power feeds Source image courtesy of Dr. Transferred hazards to construction ground grid for 25kV fault in temporary substation! Specific Methods Staged Fault Actually fault the substation and measure touch and step voltages Almost impossible to perform without extensive resources and extremely high speed multi-channel data collection systems Some large utilities will perform these tests if the risks are sufficient enough.

Currents will split down any interconnected shield wires Voltages are measured Tuned volt meter frequency selective RMS voltages with and without signal Phase measurements can be significant. These can be commercially bought or made yourself.

RMS vs Switchmode. Injection: Generator A Return electrode: Transmission Tower and shield wires Measurement of phase angle is important. Measuring Voltages Tuned Volt-meter offfrequency Commercial gear Measuring phase for voltages less important. Measuring the soil potentials Small probe in contact with soil thin metallic probe Small plate in contact with the soil representing two feet.

Touch Voltages Measure between the metallic objects using alligator clips or similar and the soil potentials Step Voltages Measure voltages between two points 1m apart Where? Issues with Probes: Does not represent a foot Provide scalar touch potentials as would be modeled in software Issues with Plates: Soil contact becomes significant Use a bit of water to achieve good contact with crushed rock or soil Provides realistic foot impedances in-situ Step Voltage.

Conventional Gear Four pin resistance meter Set up to measure a touch or step resistance In practice, touch and step resistance measurements are below the reliable range ie. Great noise rejection Cannot take loaded measurement. Bias in measurements If other circuits are energized: Imbalance zero sequence currents in the grid Induction on current circuit Stray DC currents Currents down unforeseen paths Conductive interference with return electrode ground grid.

Methods to overcome Noise Section 9. Standard Section 9. Follow equations. Model the test scenario to apply correction factors Advanced techniques required Can provide expected values. Loaded voltages with plates: Compare with body current tolerable current levels Voltage across Ohm resistor is a scaled version of the current through the body Source: IEEE Conventional Meter Touch and step resistances Multiply by expected earth-return current to get respective unloaded touch and step voltage values Compare with IEEE 80 tolerable threshold voltages Use of only probes will not easily represent loaded touch voltage values.

Only one slice of the pie You get an excellent picture of actual voltages. You have to decide whether seasonal variations are significant: Urban Rural Its only one slice of time. In Canada and US parts of the country must account for seasonal variations More engineering judgment is required.

Gain Experience Testing Who is doing these tests? It is highly recommended to go out in the field and perform this test. Get as much experience as you can in the field. The Ground Grid 2. Test Methods 3.

Test Result Interpretation 4. Safety Considerations. The Ground Grid Protects personnel by limiting step and touch voltages in the yard during normal and abnormal conditions Protects equipment by limiting transient voltages. Ground Grid Testing - General Verification that integrity of ground grid is intact No fully or partially corroded conductors or connections Can identify area of yard with relative high resistance. Measurement of Resistance between two risers Resistance Measurement test equipment calculates resistance from V, I, and.

Interpretation of Results What is a good resistance value? Some test equipment alternates the polarity of the test current to get both additive and subtractive currents and averages the resistance. Test Lead Resistance Where you measure affects results!

Measurement includes test lead impedance. Safety Considerations Generally Equipment safety - Voltage gradients across the ground grid conductors Personnel Safety - Touch and Step Voltages Specific Examples When using high current, ensure that appropriate rated equipment is used i. A potentially dangerous voltage can exist on the remote test lead at the reference location. Table 7 Typical surface material resistivities Sentences at end of third and fourth paragraph!

Thus, it is important that the resistivity of rock samples typical of the type being used in a given area be measured. Tests should be performed to determine the resistivity of the stone typically purchased by the utility. Problem no standardized test method currently exists but it still can be done. General Understanding the Circuit Like other tests comes down to the basics injecting a current and measure a voltage 4 - Pin.

General Safety Test doesnt require high current or voltage Field Standard field safety items! Traffic, system faults Box Considerations Large enough, non-conductive, easy to clean Sturdy! Able to withstand repeated compaction of material. How to perform - Lab versus field Same Two pin four pin Hard to replicate in-situ conditions.

Limitations Field testing Reproducibility Seasonal variations Quantifying parameters. Considerations for acceptance Conditions tested at vs those experienced in field Comparison to other testing results Historical testing performed.

Obtain representative sample and test! Evaluate ground grid using tested value. New Stations! Obtain sample from quarry and test! Design grid using tested value Presentation was based on the paper: Edlebeck, J.

PP, no. What difference could a standing voltage make? Open navigation menu. Close suggestions Search Search. User Settings. Skip carousel. Carousel Previous. Carousel Next. What is Scribd? Scope: The test methods and techniques used to measure the electrical characteristics of the grounding system include the following topics: a Establishing safe testing conditions b Measuring earth resistivity c Measuring the power system frequency resistance or impedance of the ground system to remote earth d Measuring the transient surge impedance of the ground system to remote earth e Measuring step and touch voltages f Verifying the integrity of the grounding system g Reviewing common methods and procedures for performing ground testing h Reviewing instrumentation characteristics and limitations i Reviewing various factors that can distort test measurements.

Purpose: The purpose of this guide is to present practical instrumentation methods that may be used for measuring soil resistivity, the impedance to remote earth, step and touch voltages, and current distributions in ground grids associated with electric utility facilities.

These grids typically consist of interconnected grounding systems ranging in complexity from a few ground rods to large grids with many ground rods or wells, buried conductors, and external ground connections. This guide is intended to assist the engineer or technician in obtaining and interpreting accurate, reliable data.

The factors that influence the choice of instruments are discussed along with a presentation of field techniques for various types of measurements. These factors include the purpose of the measurement, the accuracy required, the types of instruments available, the possible sources of error, and the nature of the ground or grounding system under test.

It also describes test procedures that promote the safety of personnel and property, and it seeks to minimize operating interferences with neighboring facilities.