Megger DET3T, DET4 User Manual

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TABLE OF CONTENTS

Contents

Introduction .......................................................................................................... 2

Clamp-On Testing versus Fall of Potential Testing ............................................. 3

Fall of Potential Testing ................................................................................. 3

Clamp-On Testing ........................................................................................... 4

Clamp-On Ground Testing Theory and Methodology ....................................... 6

Series Circuit ................................................................................................... 6

Parallel Circuit................................................................................................. 7

Parallel-Series Circuit ...................................................................................... 7

Clamp-On Test Methodology ........................................................................ 8

Summary ....................................................................................................... 11

Ground Leakage Current Measurement ........................................................... 11

Applications .................................................................................................. 12

Utility Poles/Service Entrance or Meter .................................................. 12

Street Lighting ......................................................................................... 14

Lightning Protection ................................................................................ 14

Street Cabinets ......................................................................................... 16

Telephone Pedestals ................................................................................ 16

Cell Towers (applications with buried ground ring) .............................. 17

Pad Mounted Transformer ...................................................................... 17

Pole Mounted Transformer ..................................................................... 18

Potential Sources of Error .................................................................................. 19

Factors in Selecting a Clamp-On Ground Tester ............................................... 20

Jaw Design .................................................................................................... 20

Clamp Head Size and Shape ........................................................................ 21

Instrument Size ........................................................................................... 23

Category (CAT) Rating ................................................................................. 24

Noise Filtering............................................................................................... 25

Backlight ....................................................................................................... 25

Data Hold ...................................................................................................... 25

Ergonomics ................................................................................................... 26

Alarm Limit Function.................................................................................... 26

Result Storage............................................................................................... 26

Clamp-on Ground Testers Available from Megger .......................................... 27

Models DET14C/24C...................................................................................... 27

Guide to Clamp-on Ground Testing 1

Introduction

Testing the quality of the grounding system has been a critical part of any electrical maintenance program for many years. Ground electrodes are used to provide a safe path to earth for the dissipation of fault currents, lightning strikes, static charges and EMF/RFI signals. Over time, ground systems deteriorate due to either environmental conditions or catastrophic events (like lightning strikes). Alternatively, facility expansion may change needs in the installed ground system.

The risks from ground system deterioration include potentially deadly electrical shocks, plant-wide equipment damage, disruption in the performance of sensitive electrical equipment, heat build-up and eventually fire on a single piece of electrical equipment and disruption in digital communication service. Grounding systems present a unique challenge because they are out of site, buried beneath the soil. The only way to ensure that the system remains capable of dissipating fault currents is to measure its resistance periodically.

Good grounding protects people and equipment and improves the performance of sensitive electronic equipment. The bonding to the ground system is also a critical part of the system. Testing the quality of grounds and bonding should be an active part of any electrical maintenance strategy. Ground (or earth) testing is done to determine the effectiveness of the ground system and connections to protect personnel and equipment and ensure optimal equipment performance. Fall of potential (and its variants) was the only method of testing ground system integrity until the 1980s. Clamp-on, or stake-less, ground testing first appeared in the 1980s and has gained in popularity and acceptance in the years since its introduction.

This booklet will focus on the clamp-on method of ground testing and is designed to give the reader a better understanding of the test method and where it can and cannot be used. Factors that a technician may want to consider when selecting a clamp-on ground tester will also be addressed. Please refer to Megger’s booklet “Getting Down to Earth” for further information on fall of potential tests and soil resistivity tests.

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Clamp-On Testing versus Fall of Potential Testing

Fall of Potential Testing

As mentioned, clamp-on, or stake-less, ground testing is a relatively new method of determining the quality of a ground system. The fall of potential method dates back to the 1930s and is based on the research of H.B. Dwight. It is the most accurate way of measuring and confirming ground rod resistance, but it has several major disadvantages. The basic methodology follows. This booklet will not go into the theory or math behind this method.

Proper fall of potential testing involves placing a current probe in the soil at a distance from the ground electrode being tested (please note that the ground electrode must be disconnected from the system). The actual distance is determined by the size of the ground electrode/system. The ground tester is then connected to the ground electrode under test, the current probe and a potential probe. The potential probe is placed in the soil at distances of 10%, 20%, 30%, up to 90% of the distance between the ground electrode and the current probe and a reading is taken at each location. The readings are then plotted against the distances and the point where the curve flattens is the approximate resistance of the ground electrode (see figure below).

Guide to Clamp-on Ground Testing 3

The fall of potential method is extremely reliable, as the results can be checked by testing at different current probe distances. This built-in proof capability means that results do not have to be accepted on faith. The operator has complete control of the test set-up. This method can be used on any size ground system as long as the current probe can be placed far enough from the ground system under test. It conforms to IEEE 81 and is IEEE approved. In an ideal world, fall of potential would be the only method used. Unfortunately, nothing is ideal and this method has three important disadvantages:

1. It is exceedingly time consuming and labor intensive. Temporary probes must be placed and moved. Cables must be run. Readings must be taken and plotted.

2. The operator must disconnect the ground electrode to make the test. As a result, the system is not protected during the test. The ground electrode must then be reconnected after the test, which, in addition to being time consuming, leaves the possibility for error if it is poorly bonded.

3. In real-world situations, space constraints can make it difficult to place the remote probes.

Clamp-On Testing

The clamp-on ground tester is an effective and time-saving method when used correctly because the user does not have to disconnect the ground system to make a measurement or place probes in the ground. The theory behind this method and the methodology itself will be covered in more detail later in this booklet. The method is based on Ohm’s Law, where R (resistance) = V (voltage) / I (current). The clamp includes a transmit coil, which applies the voltage and a receive coil, which measures the current. The instrument applies a known voltage to a complete circuit, measures the resulting current flow and calculates the resistance (see figure on next page).

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The clamp-on method requires a complete electrical circuit to measure. The operator has no probes and therefore cannot set up the desired test circuit. The operator must be certain that earth is included in the return loop. The clamp-on tester measures the complete resistance of the path (loop) that the signal is taking. All elements of the loop are measured in series. The method assumes that only the resistance of the ground electrode under test contributes significantly. Based on the math behind the method (to be reviewed later), the more returns, the smaller the contribution of extraneous elements to the reading and, therefore, the greater the accuracy.

The major advantage of the clamp-on method is that it is quick and easy. The ground electrode does not have to be disconnected from the system to take the measurement and no probes need to be driven and no cables connected. In addition, it includes the bonding and overall connection resistance. Good grounding must be complemented by “bonding”, having a continuous low-impedance path to ground. Fall of potential measures only the ground electrode, not the bonding (leads must be shifted to make a bonding test). Because the clamp-on uses the grounding conductor as part of the return, an “open” or high resistance bond will show up in the reading. The clamp-on ground tester also allows

Guide to Clamp-on Ground Testing 5

the operator to measure the leakage current flowing through the system. If an electrode has to be disconnected, the instrument will show whether current is flowing to indicate whether it is safe to proceed.

Unfortunately, the clamp-on ground tester is often misused in applications where it will not give an effective reading. The clamp-on method is effective only in situations where there are multiple grounds in parallel. It cannot be used on isolated grounds as there is no return path. Therefore, it cannot be used for installation checks or commissioning new sites. It also cannot be used if an alternate lower resistance return exists not involving the soil (such as with cell towers). Unlike with fall of potential testing, there is no way of proofing the result, meaning the results must be taken on “faith.” The clamp-on ground tester does fill a role as one tool that the technician could have in his “bag”, but not the only tool.

Clamp-On Ground Testing Theory and Methodology

Understanding how and why the clamp-on method works helps in understanding where it will and will not operate, and how to optimize its use. As mentioned, the clamp-on test method is based on Ohm’s Law (R = V/I). Understanding Ohm’s law and how it applies to series and parallel circuits is the first step to understanding how and why a clamp-on ground tester works. The following graphics will show a series circuit, a parallel circuit and a series-parallel circuit, and the math used to determine the total current and resistance.

Series Circuit

In a series circuit, total current and total resistance are calculated as follows: It = I1 = I2 = I

Rt = R1 + R2 + R

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Parallel Circuit

In a parallel circuit, total current and total resistance are calculated as follows:

It = I1 + I2 + I

Rt = 1/ (1/R1 + 1/R2 + 1/R3)

Parallel-Series Circuit

In a parallel-series circuit, total current and total resistance are calculated as follows:

It = I1 + I2 = I3 = I4 + I

Rt = 1/ (1/R1 + 1/R2) + 1/ (1/R3 + 1/R4)

Guide to Clamp-on Ground Testing 7

Clamp-On Test Methodology

The head of a clamp-on ground tester includes two cores (see figure at right). One core induces a test current and the other measures how much was induced. The input or primary voltage of the test current inducing core is kept constant, so the current actually induced into the test circuit is directly proportional to the loop resistance.

The important thing to remember with clamp-on testing is that clamp-on ground testers effectively make loop resistance measurements. Clamp-on measurements are loop measurements. For the clamp-on method to work there must be a seriesparallel resistance path (and the lower the better). The more electrodes or ground paths in the system the nearer the measurement gets to the actual electrode under test’s true resistance. The following figure shows a pole ground configuration, one of the most effective applications of the clampon ground tester.

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