earth ground testing distributor training program -...

Distributor Training Program

Earth Ground Testing

Page Fluke Corporation FACT – Product Knowledge


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An estimated 58 people lose their life each week because of electric shock.

The most frequently cited OSHA electrical violation is improper grounding of equipment or circuits.

(Source: National Institute of Safety and Health [NIOSH] publication 98-3).

Improve up-time and reduce riskThe purpose of a ground besides the protec-tion of people, manufacturing plants and equipment is to provide a safe path for the dissipation of fault currents, lightning strikes, static discharges, EMI and RFI signals and interference.

During a ground fault, enough current must flow to open a breaker or fuse quickly to prevent shock, electrocution, or equipment damage. Even a few ohms in the grounding circuit will prevent, or greatly slow the open-ing of a breaker or fuse.

Without an effective grounding system, there is exposure to the risk of electric shock, not to mention instrumentation errors, harmonic distortion issues, power factor problems and a host of possible intermittent dilemmas. If there is a ground fault and fault currents have no path to the ground through a properly designed and maintained grounding system, electrical enclosures, motor frames, and other conductive structures such as handrails and walkways can become energized. A person

touching any of these may be electrocuted.Poor grounding not only contributes to

unnecessary downtime, but a lack of good grounding is also dangerous and increases the risk of equipment failure.

Back to the futureRetiring early from the printing industry, Benjamin Frank-lin was destined to achieve international fame through his experiments in electricity. Frank-lin, ever curious and inquisitive, became inter-ested in the natural phenom-enon known as electricity after witnessing demonstrations about static charge and the Leyden Jar.

Franklin was among the first to suggest that lightning was merely naturally-occurring electricity and that it could be drawn from the clouds. To prove this conjecture, he set up tall, pointed rods that provided an easy path for the electricity of lightning to follow. In France, these “Philadelphia experiments” were dupli-cated for the king and his court. Eventually, these pointed rods were modified to serve as lightning rods designed to protect people’s dwellings. By channeling the electricity of lightning strikes through a safe route to the ground, lightning rods eliminated the threat of fires.

Why test grounding systems?Over time, corrosive soils with high mois-ture content, high salt content, rust, and high temperatures can degrade ground rods and their connections. The resistance of the grounding system can increase if the ground rods are degraded in this way even if ground resistance values were low when initially installed. The purpose of testing is to determine the effectiveness of the grounding system with respect to earth ground.

To ensure the highest integrity of our

Product KnowledgeEarth Ground Testing

What you will learn in this module:

• Improve up-time and reduce risk

• Why test grounding systems?

• What is a ground and what does it do?

• What is a good ground resistance value?

• Grounding basics• What affects ground-

ing resistance?



Product Knowledge

electrical and control systems, it is highly recom-mended that all grounds and ground connections be checked at least annually

as a part of a normal Predictive Maintenance (PdM) plan. During these periodic checks, if an increase in resistance of more than 0% is measured, the technician should investi-gate the source of the problem and make the necessary correction to lower the resistance by replacing or adding ground rods or repairing defective connections in the ground system.

What is a ground and what does it do?When talking about grounding, it is actually two different subjects: earth grounding and equipment grounding.

Earth grounding is an intentional connec-tion from a circuit conductor, usually the neutral, to a ground electrode placed in the earth. Most electrical systems do not rely on the earth to carry load current but the earth may provide a path for fault currents, lightning strikes, and static discharges. Fault current uses the earth as a conductor to complete the circuit back to the source trans-former. The problem is that the earth, and connections to earth, are not good conductors.

Equipment grounding ensures that operat-ing equipment within a structure is properly grounded by means of a continuous copper conductor connected between the grounding electrode connection, at the source trans-former, and at each enclosure and equipment frame.

These two grounding systems are isolated from each other except for one common connection between the two systems. This prevents differences in voltage potential from a possible flashover from lightning strikes.

What is a good ground resistance value?The goal in ground resistance is to achieve the lowest ground resistance value possible, that makes sense economically and physi-cally, when contacting the earth, also known as the soil/ground rod interface. Ideally, a ground should be zero ohms of resistance.

Unfortunately, there is not one standard ground resistance threshold recognized by all certifying agencies. The NFPA and IEEE recommend a ground resistance value of 5 ohms or less while the NEC has stated to “Make sure that system impedance to ground is less than 5 ohms specified in NEC 50.56. In facilities with sensitive equipment it should be 5ohms or less.” The telecommunications industry has often used 5 ohms or less as their value for grounding and bonding while electric utilities construct their ground systems so that the resistance at a large station will be no more than a few tenths of one ohm. In general, the lower the ground resistance, the safer the system is considered to be.

Grounding basicsThe ground test must include multiple sources of resistance, the ground electrode and its connection, the contact resistance of the surrounding earth to the electrode and the resistance of the surrounding body of earth.

. The ground electrode and its connectionThe resistance of the ground electrode and its connection is generally very low. Ground rods are generally made of highly conductive/low resistance material such as steel or copper.

. The contact resistance of the surrounding earth to the electrode

The National Institute of Standards (a governmental agency within the US Dept. of Commerce) has shown this resistance to be almost negligible if the ground electrode is free of paint, grease, etc. and that the ground electrode is in firm contact with the earth.

3. The resistance of the surrounding body of earth

The ground electrode is surrounded by earth which conceptually is made up of concentric shells all having the same thickness. Those shells closest to the ground electrode have the smallest amount of area resulting in the great-est degree of resistance. Each subsequent shell incorporates a greater area resulting in lower resistance. This finally reaches a point where the additional shells offer little resis-tance to the ground surrounding the ground electrode.

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Product Knowledge

Based on this information, we should focus on ways to reduce ground resistance when installing grounding systems.

What affects grounding resistance?First, the NEC code (987, 50-83-3) requires a minimum ground electrode length of .5 meters (8.0 feet) to be in contact with soil. Soil composition, moisture content, and temperature all influence the soil resistivity, so it is recommended that the ground rods be placed as deep as possible into the earth to be most effective.

Nevertheless, four variables affect the ground resistance of a ground system:

. Length/depth of the ground electrodeOne very effective way of lowering ground resistance is to drive ground electrodes deeper. Soil is not consistent in its resistivity and can be highly unpredictable. It is critical when installing the ground electrode that it is below the frost line. This is done so that the resistance to ground will not be greatly influenced by the freezing of the surround-ing soil. Generally, by doubling the length of the ground electrode you can reduce the resistance level by an additional 40%. There are occasions where it is physically impos-sible to drive ground rods deeper—areas that are composed of rock, granite, etc. In these instances, alternative methods including grounding cement are viable.

. Diameter of the ground electrodeIncreasing the diameter of the ground elec-trode has very little effect in lowering the resistance. For example, you could double the diameter of a ground electrode and your resistance would only decrease by 0%.

3. Number of ground electrodesAnother way to lower ground resistance is to use multiple ground electrodes. In this design, more than one electrode is driven into the ground and connected in parallel to lower the resistance. For additional electrodes to be effective, the spacing of additional rods needs to be at least equal to the depth of the driven rod. Without proper spacing of the ground electrodes, their spheres of influence will inter-sect and the resistance will not be lowered.

4. Ground system designSimple grounding systems consist of a single ground electrode driven into the ground. The use of a single ground electrode is the most common form of grounding and can be found outside your home or place of busi-ness. Complex grounding systems consist of

multiple ground rods, connected, mesh or grid networks, ground plates, and ground loops. These systems are typically installed at power generating substations, central offices, and cell tower sites. Complex networks dramati-cally increase the amount of contact with the surrounding earth and lower ground resis-tances.



Product Knowledge

Fluke.Keeping your world up and running.

Chapter Summary

Improve up-time and reduce risk

• During a ground fault, enough current must flow to open a breaker or fuse quickly to prevent shock, electrocution, or equipment damage.

• Poor grounding not only contributes to unnecessary downtime, but a lack of good grounding is also dangerous and increases the risk of equipment failure.

Why test grounding systems?

• Over time, corrosive soils with high moisture content, high salt content, rust, and high temperatures can degrade ground rods and their connections.

What is a ground and what does it do?

• Earth grounding is an intentional connection from a circuit conductor, usually the neutral, to a ground electrode placed in the earth.

• Equipment grounding ensures that operating equipment within a structure is properly grounded by means of a continuous copper conductor connected between the grounding electrode connection, at the source transformer, and at each enclosure and equipment frame.

What is a good ground resistance value?

• The goal in ground resistance is to achieve the lowest ground resistance value possible, that makes sense economically and physically, when contacting the earth, also known as the soil/ground rod inter-face.

Multiple sources of resistance:

. The ground electrode and its connection . The contact resistance of the surrounding earth to the electrode 3. The resistance of the surrounding body of earth

Test your knowledge

FlukeCorporation PO Box 9090, Everett, WA USA 9806

Formoreinformationcall: In the U.S.A. (800) 443-5853 or Fax (45) 446-56 In Canada (800)-36-FLUKE or Fax (905) 890-6866 Web access: http://www.fluke.com

©009 Fluke Corporation. All rights reserved. Printed in U.S.A. Phase , Module 0, Rev

1.Thepurposeofagroundsystemisto_________________________________.

A.provideasafepathforfaultcurrents

B.dissipatelightningstrikes

C.protectpeople,plantsandequipment

D.Alloftheabove

2.DuringanannualPdMinspection,ifanincreaseofmorethan______ismeasured,thetechnicianshouldinvestigatethesourceandeffectrepairs.

A.5% C.15%

B.12% D.20%

3.Agroundtestmustincludemultiplesourcesofresistancethatinclude_____________________.

A.thegroundelectrodeanditsconnection

B.thecontactresistanceofthesurroundingearthtotheelectrode

C.theresistanceofthesurroundingbodyofearth

D.Alloftheabove

4.Ideally,agroundshouldbe_______ohmsofresistance.

A.-10

B.0

C.5

D.10

1.D2.D3.D4.B


Earth Ground Testers

Product Knowledge

There are four types of earth ground testing methods available:

• Soil Resistivity (using stakes) • Fall-of-Potential (using stakes) • Selective (using clamp and stakes) • Stakeless (using 2 clamps only)

Soil resistivity measurement

Why determine the soil resistivity?Resistivity, necessary during the design phase of a grounding system for new installations (green field applications), is a physical property of the soil and can vary by several orders of magnitude from area to area. Ideally, a location with the lowest possible resistance is best, however soil composition, moisture content, and temperature all impact the soil resistivity, so multiple tests with the Fluke 623 and 625 may be neces-sary to find the best location. After finding this location, it’s recommended that the ground rods be placed as deep as possible into the earth, and installed where there is a stable temperature, for instance below the frost line.

The measuring procedure used by the Fluke 623 and 625 is the universally accepted Wenner method developed by Dr. Frank Wenner of the US Bureau of Standards in 95. The Wenner method involves placing 4 probes in the earth at equal spacing. The probes are connected with wires to the ground resistance test set. The test set passes a known amount of current through the outer two probes and measures the voltage drop between the inner two probes. Using ohms law it will output a resistance value, which can then be converted to a resistivity value using a simple mathematical equation.

The four earth ground stakes are positioned in the soil in a straight line, equidistant from one another. The distance between earth ground stakes should be at least three times greater than the stake depth. For example, if the depth of each ground stake is one foot make sure the distance between stakes is greater than three feet. The Fluke 623 and 625 automatically calculates the soil resistance. Because soil resistivity measure-ments are often corrupted by the existence of existing ground currents and their harmonics, the 625 uses an Automatic Frequency Control (AFC) System, designed to automatically select the testing frequency with the least amount of noise enabling a clear reading. Once the data

are collected, the next step is the design of the grounding system. For the engineer, doing the design upfront ensures the grounding system will achieve the target resistance value. (Without wasting time and money through experimenting by driving ground rods, taking measurements and ‘hoping’ the target ground resistance value is met).

The Fall-of-Potential test method is used to measure the ability of an earth ground system or an individual electrode to dissipate energy from a site.

How does the Fall-of-Potential test work?First, the earth electrode of interest must bedisconnected from its connection to the site. Second, the tester is connected to the earth elec-trode. Then, for the 3-pole Fall-of-Potential test, two earth stakes are placed in the soil in a direct line—away from the earth electrode. Normally, spacing of 65 feet is sufficient. A known current is generated by the Fluke 62, 623 or 625 between the outer stake (auxiliary earth stake) and the earth electrode, while the drop in volt-age potential is measured between the inner earth stake and the earth electrode. Using Ohm’s Law (V = IR), the tester automatically calculates the ground resistance of the earth electrode.

How do you place the stakes?To achieve the highest degree of accuracy when performing a 3–pole ground resistance test, it’s essential that the probe is placed outside the sphere of influence of the ground electrode under test and the auxiliary earth.

Product KnowledgeEarth Ground Testers

What you will learn in this module:

• Methods of earth ground testing

• The product family• Features and benefits• Key functions• The Fluke advantage

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Product Knowledge

This is true for ground rods connected together as well. For example, an 8-foot ground rod should be driven at least 8 feet away from an installed ground rod. If you do not get outside the sphere of influence, the effective areas of resistance will overlap and invalidate any measurements taken.

623 and 625 models can also perform 4- pole fall of potential ground resistance tests These are essentially the same as the 3-pole except the 4th connection is an additional link made from the instrument back to the earth electrode. This cancels out any additional resistance added by the test leads of the instruments. This is important for low resistance earth electrode readings to improve the accuracy of the test.

Selective measurementSelective testing provides the same measure-ments as the Fall-of-Potential test but in a much safer and easier way. This is because with Selective testing, the earth electrode of interest is not disconnected from its connection to the site! In this way, the technician doesn’t need to endanger himself by disconnecting the ground, nor endanger other personnel or electrical equipment inside a non-grounded structure.

Just as with the Fall-of-Potential test, twoearth stakes are placed in the soil in a directline away from the earth electrode at a spacing of 65 feet. The tester is then connected to the earth electrode of interest, but in this case a special clamp is placed around the earth electrode.

Just as before, a known current is generated by the Fluke 625 between the outer stake(auxiliary earth stake) and the earth electrode,while the drop in voltage potential is measured between the inner earth stake and the earth electrode. Only the current flowing through the earth electrode of interest is measured using the clamp. This current along with the known voltages is used to calculate the resistance of the earth electrode under test.

Testing individual ground electrode resistances of high voltage transmission towers requires that the overhead ground or static wire be dis-connected. If a tower has more than one ground at its base, these must also be disconnected one by one and tested.

However, the Fluke 1625 has an optional accessory, a 12.7 inch diameter clamp-on current transformer, which can measure the individual resistances of each leg, without disconnecting any ground leads or overhead static/ground wires.

Stakeless measurementThe Fluke 623 and 625 earth ground

testers are able to measure earth ground loop resistances for multigrounded systems using only current clamps. One clamp induces a volt-age onto the conductor (using a special signal) and the other clamp senses the current flowing as a result of the first clamp. The combina-tion of the two known quantities is used to determine the resistance of the earth electrode under test. This test technique provides a convenient alternative to the dangerous, and time consuming activity of disconnecting paral-lel grounds, as well as the process of finding suitable locations for auxiliary ground stakes. Your customer can also perform earth ground tests in places they havn’t considered before: inside buildings, on power pylons or anywhere they don’t have access to soil.

With this test method, two clamps are placed around the earth ground rod or the connecting cable and then connected to the tester. The tester automatically determines the ground

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Product Knowledge

loop resistance. Your customer will like the fact that earth ground stakes are not used at all, making this test fast and easy.

In addition to 623 and 625 having this capability, a dedicated stakeless earth ground tester is available, the 630. The 630 combines both clamps into one simple hand-held unit for convenience and portability.

Ground impedance measurementsPower utilities technicians, testing high voltage transmission lines, are interested in two things. The ground resistance in case of a lightning strike, and the impedance of the entire system in case of a short circuit on a specific point in the line. Short circuit in this case, means an active wire breaks loose and touches the metal grid of a tower. The outcome isn’t pretty.

When attempting to calculate possible shortcircuit currents in power plants and other high voltage/current situations, determining the complex grounding impedance is very important.

The impedance is frequency dependent, for this reason, the Fluke 625 can utilize a 55 Hz signal to ensure that the measurement result is close to the value at the true operating frequency. Using this feature of the Fluke 625, accurate direct measurement of grounding impedance is possible.

The product family

The Fluke 1625 offers these advanced features:

• Automatic Frequency Control (AFC) Designed to identify existing interference and choose a measurement frequency that minimizes its effect, providing a more accu-rate earth ground value

• R* measurement To calculate earth ground impedance at 55 Hz to more accurately reflect the earth ground resistance that a fault-to-earth ground would see

• Adjustable limits For quick test result verification

• 3- and 4-Pole Fall of Potential testing methods – (Also available on 623) standard earth ground testing using two ground stakes

• 4-Pole Soil Resistivity testing – (Also available on 623) determines soil resistivity using four ground stakes

• Selective testing – (Also available on 623) without disconnecting ground rods, a technician can measure ground resistance using a combination of stakes and a clamp

• Stakeless testing – (Also available on 623 and 630) innovative solu-tion using only clamps, instead of ground stakes, to measure earth ground loop resistance

Complete kits are available for the Fluke 1623 or 1625 tester. Kits include a set of two leads, 4 earth ground stakes, 3 cable reels with wire, 2 clamps, all appropriate connectors and cables, batteries and manual - all inside a rugged Fluke carrying case.

1623

16251621

1630

Fluke 1625 Kit



Product Knowledge

Fluke.Keeping your world up and running.

Chapter Summary

Four types of earth ground testing methods available

• Soil Resistivity (using stakes) - Primarily used for commissioning new installations - Four stakes are placed in a direct line equidistant from each other (3X greater than stake depth) • Fall-of-Potential (using stakes) - Measures the ability of an earth ground system to dissipate energy from a site - Earth electrode must be disconnected to perform test - Two stakes are placed in a direct line typically at 65 foot spacing • Selective (using clamp and stakes) - Same results as test above (Fall-of-Potential) - Earth electrode is NOT disconnected to perform test (Much safer than Fall-of-Potential test) - Two stakes are placed in a direct line typically at 65 foot spacing - A special current sensing clamp is placed around the earth electrode under test • Stakeless (using 2 clamps ONLY) - Eliminates the dangerous disconnection of parallel grounds - Now able to perform earth ground tests in places not considered before - Connect the special current clamps just 4-inches apart on the ground under test - Available as a separate stand alone tester with clamps integrated into one handheld unit

Test your knowledge

FlukeCorporation PO Box 9090, Everett, WA USA 98206

Formoreinformationcall: In the U.S.A. (800) 443-5853 or Fax (425) 446-56 In Canada (800)-36-FLUKE or Fax (905) 890-6866 Web access: http://www.fluke.com

©2009 Fluke Corporation. All rights reserved. Printed in U.S.A. Phase 3, Module 3, Rev 2

1.Theearthgroundtestthatuse1clampandstakesisthe_______________________________test.

A.SoilResistivity

B.Fall-of-Potential

C.Selective

D.Stakeless

2.TheFall-of-Potentialtestisprimarilyusedto:

A.measureresistivityduringnewinstallations.

B.testhighvoltagetransmissionlines.

C.measuretheabilityofanearthgroundsystemtodissipateenergyfromasite.

D.noneoftheabove

3.The_________________________testdoesnotrequiretheearthelectrodetobedisconnectedfromtheconnectiontothesitebutprovidessimilarresultsasthe________________________test.

A.Fall-of-Potential,Selectivemeasurement

B.Selectivemeasurement,Fall-of-Potential

C.SoilResistivity,Fall-of-Potential

D.Fall-of-Potential,Stakeless

4.TheFluke______________earthgroundtestercanperformallfourtypesofmeasurements.

A.1623

B.1625

C.bothAandB

D.neitherAnorB

1.C2.C3.B4.C

Fluke Corporation PO Box 9090, Everett, WA USA 98206

Fluke Europe B.V. PO Box 1186, 5602 BD Eindhoven, The Netherlands

For more information call: In the U.S.A. (800) 443-5853 or Fax (425) 446-5116 In Europe/M-East/Africa +31 (0) 40 2675 200 or Fax +31 (0) 40 2675 222 In Canada (800)-36-FLUKE or Fax (905) 890-6866 From other countries +1 (425) 446-5500 or Fax +1 (425) 446-5116 Web access: http://www.fluke.com

©2009 Fluke Corporation. Specifications subject to change without notice. Printed in U.S.A 1/2009 FACT Distributor Training Program C-EN-N Rev A

Fluke. Keeping your world up and running.®

earth ground testing distributor training program -...

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