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Earth Ground Resistance The Basics

- Diagnose intermittent electrical problems

- Avoid unnecessary downtime

- Learn earth ground testing principles

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Table of Contents

• Why Ground? Why Test?

• Grounding basics

• Methods of earth ground testing

• Measuring ground resistance

• Who to Target

• Size of the Opportunity

• Why we will win

• Sales Tools

Earth Ground - Foundation

Earth Ground – Sales Information

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Earth Ground – Foundation

What is ground?

NEC, Article 100, “A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of earth”

Beyond the actual definition of a ground, the important point is: - a connection to earth can be intentional or accidental.

Ground testing ensures that the intentional grounds are functioning

If the intentional path to ground fails and accidental path to ground is disturbed, systems can fail intermittently.

What is a ground? What does it do?

What is ground?

Catch the problem before it happens!

It’s estimated that at least 15% of power quality problems are related to grounding, ensuring good grounding by testing can make certain these problems don’t escalate (source - EPRI)

Lightning strikes on equipment with poorly maintained protection systems destroy millions of dollars of equipment and lost production every year (source – www.copper.org)

Earth Ground testing should be a part of a customer’s basic PdM program.

Why test grounding systems?

What is ground?

Ideally the ground resistance of a system is zero ohms.

But in reality, the goal is to achieve the lowest ground resistance possible that makes sense economically and physically.

• NFPA & IEEE: Recommends a ground resistance value of 5.0 ohms or less.

• Telecommunications Industry: Often uses 5.0 ohms or less as their value for grounding or bonding

• NEC: Make sure the system to ground is 25.0 ohms or less. In facilities with sensitive equipment, it should be 5.0 ohms or less. (source – NEC 250.56)

What is a good ground value?

Components of a ground electrode

Ground conductor

Connection between the ground conductor and ground electrode

Ground electrode

Spheres of Influence

There must be proper spacing between ground electrodes and earth stakes to reduce or eliminate their spheres of influence

The NEC code requires a minimum ground electrode length of 2.5 meters (8.0 feet) to be in contact with the soil. But, there are four variables that affect the ground resistance of a ground system:

• Length / Depth of the ground electrode – double the length, reduce ground resistance by up to 40%

• Diameter of the ground electrode – double the diameter, lower ground resistance by only 10%

• Number of ground electrodes – for increased effectiveness, space additional electrodes at least equal to the depth of the ground electrodes

• Ground system design – single ground rod to ground plate

What affects ground resistance?

Depending on the soil type and the level of earth ground resistance you are trying to achieve…..

You may choose to install any one of the following ground systems.

Types of Ground Systems

Single Ground Rod Multiple Ground Electrodes

Ground MeshGround Plate

• Soil Resistivity – uses four stakes

• Fall of Potential – uses two stakes

• Selective Testing – uses one clamp and two stakes

• Stakeless Testing – uses only two clamps

These are the only types of earth ground test methods available today.

Methods of Testing

The purpose of soil resistivity measurements is to quantify the effectiveness of the earth where a grounding system will be installed.

So, soil resistivity testing is most necessary when determining the design of the grounding system for new installations. Ideally, you would find a location with the lowest possible resistance.

The soil composition, moisture content and temperature of the soil all impact the soil resistivity.

Soil Resistivity

To test soil resistivity, connect the ground tester as shown.

The Fluke-1625 generates a known current through the two outer stakes and the drop in voltage potential is measured between the two inner ground stakes. Using Ohm’s Law (V=IR), the Fluke tester automatically calculates the soil resistance.

Soil Resistivity - Testing

Resistivity Measurement

From the indicated resistance value RE, the soil resistivity is calculated according to the equation :

E = 2 . a . RE

 E ...... mean value of soil resistivity (.m)RE ...... measured resistance ()a ...... probe distance (m)

Soil Resistivity - Calculation

First, the ground electrode of interest must be disconnected from its connection to the site.

Two earth stakes are placed in the soil in a direct line – away from the earth electrode. Normally, a spacing of 20 meters is sufficient.

A known current is generated by the Fluke 1625 between the outer stake and the ground electrode, while the voltage potential is measured between the inner earth stake and the earth electrode.

Fall of Potential Testing

To test the accuracy of the results and to ensure that the ground stakes are outside the sphere of influence, reposition the inner stake 1 meter and take a fresh measurement.

If there is a significant change in the reading (30%) you need to increase the distance between the ground electrode under test and the earth stakes.

Fall of Potential Testing (II)

The selective method is based on the Fall of Potential test, however it‘s not necessary to disconnect the ground electrode under test !

A current clamp is used to isolate the test current injected into the electrodes under test, the current will flow to earth by any path. By isolating the current, with use of the current clamp, the ground resistance of individual elements can be measured without disconnecting.

Selective Testing

If the total resistance of the ground system should be measured, then each earth electrode resistance must be measured by placing the clamp around each individual earth electrode.

Then the total resistance of the ground system can be determined by calculation.

Selective Testing (II)

This application example, at a central office, shows the benefit of the selective test in a typical installation.

First, position the ground spikes according to the requirements of the system under test.

Selective Testing - Application

Once the spikes are placed, individual elements of the system can be measured. No need to disconnect the ground system !

In this example, measurements are taken at the

• MGN (multi grounded neutral)• ground field,• water pipe,• structural or building steel

Selective Testing - Application

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The stakeless method eliminates the need for temporary ground stakes. This is useful in a wide range of situations. Examples include:

•Inside buildings•Airports•Urban locations•Chemical and industrial plants

The stakeless method is not available on all ground testers. However, it comes standard on the Fluke 1623 and 1625 earth ground testers.

The temporary ground stakes are replaced by two current clamps. The first clamp generates a voltage on the ground condutor, the second clamp measures the current flowing due to the generated voltage.

Stakeless Testing

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The Fluke 1623 and 1625 testers are able to measure earth ground loop resistances for multi grounded systems using only current clamps.

With this test method, two clamps are placed around the earth ground rod or connecting cable and each connected to the tester. Earth ground stakes aren‘t used at all.

Stakeless Testing

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The Fluke testers work on the principle that the parallel/multi-grounded sysems, the net resistance of all ground paths will be extremely low as compared to any single path (the one under test).

So, the net resistance of all the parallel return path resistances is effectively zero. If the ground system is not parallel to earth then you will either have an open circuit, or be measuring ground loop resistance.

Stakeless Testing

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The clamps are placed around the ground conductor

Stakeless Testing - Application

The stakeless method does not need a lot of parallel paths to be present to ensure good results.

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• If there is only one path to ground, like at many residential applications, the stakeless method will not provide an acceptable value and the Fall of Potential test method must be used.

• An abnormally high reading or an open circuit indication on the instrument points to a poor connection between two or more of the aforementioned critical components.

• An abnormally low reading could indicate the instrument is

measuring a loop of bonding conductors.

Stakeless Testing - Details

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Used where other methods are not available. Uses nearby metal structures as a temporary spike. Metal water pipes are typically used.

Drawbacks:

• The resistance of the metal pipe should be significantly less than the electrode under test.

• Metal pipes are being replaced with plastic.

• Some metal pipes use plastic couplings.

Two Pole ground resistance

Advantages Drawbacks

Fall-of-Potential • Widely accepted • You have to disconnect ground• The stakes may not be easy to drive• There may not be space around the ground electrode to drive the stakes

Selective Method

• Don’t have to disconnect electrode • The stakes may not be easy to drive• There may not be space around the ground electrode to drive the stakes

Stakeless Method

• Don’t have to disconnect electrode• Convenience

• Assumes a low-impedance parallel path• Possible to get very low readings by mistakenly measuring on a hard-wired loop

Two-pole Method

• Convenience • Impossible to judge the integrity of the “auxiliary electrode.”• Can’t be sure you are outside the area of influence

Selecting a Test Method

Introducing the new Fluke 1623 & 1625 Earth Ground Testers

Feature 1623 16253-pole earth measurement

4-pole earth measurement

Specific earth resistance (soil-resistivity according to Wenner)

2-pole resistance measurement DC

4-pole resistance measurement DC

2-pole resistance measurement AC

Selective earth measurement (1 clamp)

Stakeless earth measurement (2 clamps)

Earth impedance of high voltage pylons (55 Hz)

Measuring voltage 20/48 V

Measuring voltage <= 48 V

Automatic frequency control (AFC) (94 ... 128 Hz)

Measuring frequency 128 Hz

Programmable limits, settings

One button measurement concept

Protective rubber holster

Choosing the right instrument