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GROUND MAT RESISTANCE TESTING PROCEDURES
POWER
SYSTEM
MAINTENANCE
MANUAL
AREA POWER
ADMINISTRATION
T OF ENERGY
DE
PA
RTM
EN
U
E
NITEDSTAT SOFA ERICAM
February 1990
CHAPTER 8
GROUND MAT RESISTANCE
TESTING PROCEDURES GROUND MAT RESISTANCE
TESTING PROCEDURES
FEBRUARY 1990
WESTERN AREA POWER ADMINISTRATION
POWER SYSTEM MAINTENANCE MANUAL
CHAPTER 8
Approved for Publication and Distribution
__________________________________
M.F. Groves
Director, Division of
Operation and Maintenance
__________________________________
Date Disclaimer
The information contained in this manual regarding commercial products or firms may not be used for advertising
or promotional purposes and is not to be construed as an endorsement of any product or firm by the Western
Area Power Administration (Western). The information contained in this manual was developed for Western; no
warranty as to the accuracy, usefulness, or completeness is expressed or implied. Preface
This guide is issued by the Western Area Power Administration (Western) and is designed to provide specific
guidelines, instructions, procedures, and criteria for performing ground mat resistance tests in Westerns electri-
cal facilities. Procedures and guidelines are in accordance with established industry standards and current
industry practices. Any corrections or comments concerning this guide may be addressed to the Western Area
Power Administration, A6200, Golden, Colorado.
-i- Table of Contents
Page
1.
INTRODUCTION ................................................................................................1
2.
GENERAL PHILOSOPHY..................................................................................2
3.
TEST OBJECTIVES...........................................................................................3
3.1 General Requirements ................................................................................3
3.2 Specific Requirements ................................................................................3
4.
TEST PROCEDURE ..........................................................................................4
4.1
Job Hazard Analysis (JHA).......................................................................4
4.2
Equipment .................................................................................................4
4.3
Preliminary Information .............................................................................4
4.4
Location of the Tester................................................................................4
4.5
Routing of the Test Leads .........................................................................4
4.6
Initial Test Setup........................................................................................4
4.7
Preliminary Checks ...................................................................................6
4.8
Determining Background Voltages............................................................6
4.9
Measurement of Resistance to Remote Earth .........................................8
4.10 Verification of Results ...............................................................................9
4.11 Test Confirmation ......................................................................................9
5.
GLOSSARY ......................................................................................................10
6.
REFERENCES .................................................................................................11
Appendix A
Job Hazard Analysis.........................................................................................12
Appendix B
Equipment Requirements for Ground Mat Resistance
Tests Performed by Fall-of-Potential Method ...................................................18
Appendix C
Ground Mat Resistance Test Report Form.......................................................21
Appendix D
Data Treatment Using the Tagg Slope Method Tables......................................23
Appendix E
Data Treatment Using the Intersecting Curve Method .....................................27
Appendix F
Tagg Slope Method Tables ...............................................................................31
-ii- List of Figures
Page
Figure 1
Ground Mat Resistance Testing Setup
(Fall-of-Potential Method)....................................................................................5
Figure 2
Flow Chart of Instrument Operation ...................................................................7
Figure 3
Measurement of Background Voltages ...............................................................8
Figure D-1
Ground Resistance: Sample Fall-of-Potential Plot ..........................................25
Figure E-1
Ground Mat Connection ...................................................................................28
Figure E-2
P2 vs. R ............................................................................................................29
Figure E-3 vs. R...............................................................................................................29
List of Tables
Page
Table A-1
Potential Hazards and Countermeasures.........................................................15
Table A-2
Sample of Written Procedures Approval Sheet ................................................16
Table A-3
List of Emergency Telephone Numbers............................................................17
-iii- 1. Introduction
Personnel safety and many aspects of an electrical installations stable operation are directly related to the
adequacy and continued performance of the electrical facilitys ground mat. Therefore, Western Area Power
Administration (Western) has implemented a ground mat resistance testing program using the fall-of-potential
method to determine the adequacy and continued performance of the ground mats that are connected to its
electrical facilities.
This document provides the necessary information and criteria needed to establish a safe and effective ground
mat testing program in Western. The purpose of this document is twofold: to briefly review the function of ground
mats, and to outline, in detail, the recommended ground mat testing procedures. The information contained in
this document, when used in conjunction with other applicable safety standards and codes, will provide the nec-
essary information and procedures to perform ground mat tests safely and effectively.
Western will revise and expand this document as needed. Any suggestions that would improve the contents of
this document should be sent to Western Headquarters, Division of Operation and Maintenance (O&M), Attn:
A6210. 2. General Philosophy
The principal factor in determining the adequacy of a ground mat is its impedance to earth. Designing and
achieving a low impedance ground still includes a good deal of both art and science. Impedance is certainly a
major consideration in the design of all electrical facilities. However, there are no legislated standards for accept-
able ground mat impedance levels at large electrical installations. Instead, the general philosophy is the lower
the better, with some consideration of economics.
Western specifications consider acceptable impedance levels as less than 1 ohm for large electrical facilities.
Although there can certainly be exceptions in a specific case, it is generally true that a ground mat impedance of
1 ohm will be adequate to:
Protect personnel and property from injury or damage by high voltage surges resulting from lightning, switching, or
other causes.
Handle discharge currents from lightning arrestors, overvoltage gaps, and so on.
Provide a ground return path for grounded wye generators and transformers.
Provide stable ground conditions for protective relays.
Improve the reliability of electric process controls, computers, and communication circuits by providing low-resis-
tance ground connections.
2 3
3. Test Objectives
3.1 General Requirements. From the discussion in section 2, we understand that ground mats are engineered
to achieve impedance levels that provide adequate protection and stability for a facility, and once installed they
remain physically and electrically stable. Many workers would then ask So why test an existing ground system?
The following are valid reasons for considering such tests:
To obtain realistic resistance data for use in calculating actual ground potential rise (GPR) values for the mat, and
for determining the step and touch potentials which may occur within a station during fault or surge conditions.
To help to resolve the instability of equipment relays or communications, which might be caused by inadequate
grounding.
To determine the continued adequacy of the ground mat wherever system changes are implemented which increase
the available fault current at the station.
To determine the effect on the integrity and performance of the ground mat caused by known physical changes
(planned or unplanned) in the grounding system.
To establish ongoing documentation of the stability and continued performance of the groundi