Greenlee CMGRT-100 User manual

999 3791.3 © 2001 Greenlee Textron IM 1466 REV 1 12/01
CMGRT-100
CLAMP-ON GROUND
RESISTANCE TESTER
VERIFICADOR DE
RESISTENCIA DE
TIERRA CON PINZA
VERIFICATEUR DE
RESISTANCE DE
TERRE A PINCE
INSTRUCTION MANUAL
INSTRUCTION MANUAL
MANUAL DE INSTRUCCIONES
MANUEL D’INSTRUCTIONS
Read and understand all of the instructions and safety information in
this manual before operating or servicing this tool.
Lea y entienda todas las instrucciones y la información sobre seguridad
que aparecen en este manual, antes de manejar esta herramienta o darle
mantenimiento.
Lire attentivement et bien comprendre toutes les instructions et les
informations sur la sécurité de ce manuel avant d‘utiliser ou de procéder
à l’entretien de cet outil.

2
Description
The Greenlee CMGRT-100 Clamp-On Ground Resistance Tester is a hand-held testing device intended to
evaluate grounding systems. It has two modes:
• Active Mode for measuring resistance
• Passive Mode for measuring current
The Active Mode induces a voltage into a conductor and measures the resulting current. It then
calculates the resistance. The Passive Mode senses the electromagnetic field surrounding a conductor
to determine the current flow.
A test resistance loop is provided to perform a quick check of the unit.
Safety
Safety is essential in the use and maintenance of Greenlee tools and equipment. This instruction manual
and any markings on the tool provide information for avoiding hazards and unsafe practices related to
the use of this tool. Observe all of the safety information provided.
Purpose
This instruction manual is intended to familiarize all personnel with the safe operation and maintenance
procedures for the Greenlee CMGRT-100 Clamp-On Ground Resistance Tester.
The CMGRT-100 is protected by U.S. Patent Number D-362639.
Keep this manual available to all personnel.
Replacement manuals are available upon request at no charge.
Greenlee and are registered trademarks of Greenlee Textron.
KEEP THIS MANUAL

CMGRT-100
3
Important Safety Information
Read and understand this material before operating or servicing this
equipment. Failure to understand how to safely operate this tool can result
in an accident causing serious injury or death.
This symbol is used to call your attention to hazards or unsafe practices which could result in an
injury or property damage. The signal word, defined below, indicates the severity of the hazard.
The message after the signal word provides information for preventing or avoiding the hazard.
SAFETY ALERT SYMBOL
Immediate hazards which, if not avoided, WILL result in severe injury or death.
Hazards which, if not avoided, COULD result in severe injury or death.
Hazards or unsafe practices which, if not avoided, MAY result in injury or property damage.
Electric shock hazard:
Contact with live circuits can result in severe injury or death.

4
Important Safety Information
• Do not operate with the case open.
• Before opening the case, remove the jaw from the circuit and shut off the unit.
Failure to observe these warnings can result in severe injury or death.
• Do not attempt to repair this unit. It contains no user-serviceable parts.
• Do not expose the unit to extremes in temperature or high humidity. See Specifications.
Failure to observe these precautions can result in injury and can damage the unit.
Electric shock and fire hazard:
• Do not expose this unit to rain or moisture.
• Do not use the unit if it is wet or damaged.
• Use this unit for the manufacturer’s intended purpose only, as described in this manual. Any
other use can impair the protection provided by the unit.
Failure to observe these warnings can result in severe injury or death.
Using this unit near equipment that generates electromagnetic interference can result in unstable
or inaccurate readings.

CMGRT-100
5
Identification
1. Jaw
2. Lever
3. Memory Button
4. Alarm Button
5. Current and Decrement Button
6. Resistance and Increment Button
7. On/Off Button
8. Display
9. Hold Button
10. Test Resistance Loop
Display Icons
11. Speaker is enabled
12. HOLD Hold function is enabled
13. NOISE Excessive stray noise (greater than approximately 5 A or 50 V);
when this appears on the display, resistance measurements
are not valid
14. Open jaw
15. R<.1ΩMeasured resistance is < 0.1 Ω; when this icon appears on the display, measurements
may not be valid
16. mA, A Milliamperes or amperes
17. ΩOhms
18. 188 Battery power (up to 100%) and alarm set point (up to 199 Ω) stored in memory
19. AL Alarm
20. P Power-Off function is disabled
21. Battery low
22. 88 Memory location
23. MR Memory recall
24. MEM Memory function is enabled
25. 8.8.8.8 Measurement display; shows OL
(overload) when the measured
resistance is greater than 1200 Ω, or
the measured current is greater than
29.99 A RMS
Symbols on the Unit
Warning—Read the instruction manual
Double insulation
1
2
8
9
43 5 6 7
10
11
23 22 20
21 19
12 13 14 15
16
24
25 17
18

6
Using the Features
Battery Check
This feature checks the approximate amount of battery power, which appears as a number followed by a
percent sign in the lower right corner of the display.
Turn the unit on by pressing and holding the ON button for at least 2 seconds. The battery icon flashes
and the battery power appears in the lower right corner of the display while the ON button is held down.
Power-Off Function
To conserve battery power, the unit shuts itself off after approximately 5 minutes of inactivity. The unit
emits a short tone and the LCD flashes 15 seconds before the power turns off.
This feature is automatic. To disable it, press and hold the HOLD button while turning the power on.
The icon appears on the display.
Speaker
The speaker emits a tone to verify that the user has pressed a button.
To toggle this feature on or off, press and hold the Ωbutton while turning the power on.
Memory
The memory function stores up to 99 measurements—either resistance or current, or a mixture of both.
•To toggle this feature on or off, press and hold the A button while turning the power on.
•To store a measurement in memory, press MEM. The speaker emits one short, high-pitched tone
followed by a long, low-pitched tone and the two-digit number increments.
•When all of the memory locations are filled, MEM 99 flashes. You may continue to make measure-
ments when the memory is full, but you can no longer store them.
•To display the contents of the memory, press and hold the MEM button while turning the power on.
The MR icon and a two-digit number appear in the lower left corner of the display. Use the < and >
buttons to view each memory location. Press and hold the < or > button for rapid viewing.
Note: The contents of the memory are retained when the unit is turned off.
•To clear the memory, turn the unit on by pressing and holding the MEM and the ON button. After
three seconds, CLr appears on the display, followed by four short tones and one long tone.

CMGRT-100
7
Using the Features (cont’d)
Alarm
The alarm function provides a tone whenever the measured resistance is more than or less than the
alarm resistance value. Setting up the alarm is a two-step process: Use Step 1 to change the alarm
resistance value (the setpoint). Use Step 2 to determine whether the alarm will sound when the
measured resistance is more than or less than the setpoint.
Step 1. Changing the Setpoint
1. Press and hold the AL button while turning on the power; the rightmost digit of the alarm setpoint
flashes.
2. Use the < and > buttons to change the setpoint (0 Ωto 199 Ω).
3. To save the setting, turn the unit off.
Step 2. Changing the Alarm Trigger
1. Turn off and disconnect the unit from the
conductor.
2. Place the unit face down and remove the
screws from the back cover.
3. Remove the back cover by pulling down and
away from the unit.
4. To trigger the alarm when the measured
resistance is:
•More than the set point, set Switch 2 to the
lower position.
•Less than the set point, set Switch 2 to the
upper position.
5. To enable the alarm, press the AL button after
turning the unit on.
Hold
Press the HOLD button momentarily to hold the
present measurement on the display. Press again to
exit this mode.
Switch 1
Switch 2

8
Theory of Operation
The CMGRT-100 performs two measurements: resistance and current. It measures resistance by
applying a voltage at 1.689 kHz and indirectly measuring the resulting current. It measures current
indirectly by measuring the electromagnetic field around a conductor, which is proportional to the
current flowing through the conductor.
The following illustration and schematic diagram represent a typical multiple ground rod system.
The ground electrodes are represented by R (R1, R2, R3 …Rn), and the CMGRT-100 is represented
by V.
The CMGRT-100 induces a voltage (V) into the system and measures the resulting current (I). The
CMGRT-100 then divides the voltage by the current to derive the resistance (V/I = R). The equation for
the resistance of Rx is shown below.
where, usually
I
V
R1Rx R2 Rn-1 Rn
The equation shown above is the standard equation for finding the resistance of one leg of a parallel
circuit. However, the CMGRT-100 uses a simplified equation (V/I = Rx) since the rightmost section of
the formula (
11
Ri
n
i=1
Σ
) is a very small amount of resistance if “n”is sufficiently large. This is shown
mathematically in the example below.
Example: A grid with 101 ground electrodes, each with a resistance of 25 Ω.
The measured resistance, Rx, would equal the resistance of Rx in series with the parallel resistance
of the other 100 ground rods:
Rx = 25 Ω+ Rx = 25 Ω+ 0.25 ΩRx = 25.25 Ω
Note, however, that most grounded systems have more than 100 ground electrodes. Therefore, the
parallel resistance is negligible. It is practical to simplify the equation and the calculation to V/I = Rx.

CMGRT-100
9
Operation
1. See Using the Features.
2. Turn the unit on, selecting the appropriate features (turn the speaker on or off, change the alarm
setpoint, etc).
3. See Typical Applications for specific measurement instructions.
4. Test the unit on a known functioning circuit or the test resistance loop. To use the test resistance
loop, clamp the CMGRT-100 around the loop. It should read between 24.2 Ωand 25.8 Ω.
Note: This value is for ambient temperature between 20 ° C and 25 ° C (68 ° F and 78 ° F). It may
differ a few counts below or above this temperature.
•If the unit does not function as expected on a known functioning circuit or the test resistance
loop, replace the battery.
•If the unit still does not function as expected, send the unit to Greenlee for repair. See the
address shown under Warranty.
5. Take the reading(s) from the circuit or component to be tested.
•If the ground current exceeds 5 A or if the noise exceeds 50 V, the CMGRT-100 will not measure
the resistance accurately. Make a note of the location for maintenance and proceed to the next
test location.
•A reading of < 0.1 may indicate that the cable is part of a closed loop; for example; when two
bonding conductors are connected to the same enclosure and ground rod.
•A high resistance reading or OL (overload) may indicate any of the following:
The cable is not grounded at both ends (a lack of a ground, or a lack of a path back to the
system neutral).
The ground rod is in poor condition.
The ground circuit is broken, or open.
The bonds on the rod or splices are of poor quality. Check for buried split butts, clamps, and
hammered-on connections.
6. After taking the measurements, make a written record of the data, if necessary (date, location,
resistance measurement, and current measurement).

10
Typical Applications
General procedures for all applications:
1. Remove any molding from the ground conductor so that the jaws can close completely around the
conductor.
2. Center the conductor in the jaw for highest accuracy.
3. Take the reading.
4. Replace the molding.
Service Entrance or Meter
Note: Clamp onto the conductor between the
service neutral and both grounded points.
Water Pipe GroundMultiple Ground Rods
Service
Meter
Service
Box
Building
Wall
Ground
Rods
Service
Meter
Building
Wall
Water Pipe
Service
Box

CMGRT-100
11
Typical Applications (cont’d)
Central Office Locations
The main ground conductor from a ground window or a ground plane is usually the location to clamp
the unit. Due to the wiring practices within the central office, there are many locations at which you can
look at the water pipe or counterpoise from within the building. An effective location is usually at the
ground bus in the power room or near the backup generator.
By measuring resistance and current at several points and comparing the readings, you can identify
neutral loops, utility grounds, and central office grounds. The test is effective and accurate when the
ground window is connected to the utility ground at only one point.
Service Panel
At a service panel, several sources may be grounded at a single grounding point. As shown in the
following illustration, those sources could be electric neutral, telephone, CATV, and water.
The primary return path should be electric neutral. After taking the first measurement, disconnect one
source and take a new measurement. Continue in this manner until all other sources have been
disconnected. Any higher measurement may indicate that the electric neutral is defective or open.
WH
BK 120 VAC
Bare
240 VAC
PHONE
A B NEUTRAL
CATV
GROUND BUS
NEUTRAL BUS
Ground
Electrode
Electric Utility
Service Panel

12
Typical Applications (cont’d)
Telephone Pedestal
Above Telephone Space
Note: Clamp onto the ground conductor
(as shown). The reading will indicate the
connection between the telephone circuit
ground and the power utility ground.
Note: Clamp onto the ground
conductor (as shown) or onto the
ground rod.
Phone
Pedestal
Ground Bar
Sheath Connection
Ground Level
Ground Rod
1. Remove ground bar connection
to pedestal rod.
2. Extend pedestal rod lead to the
ground bar using a temporary
jumper.
1
2
Phone
Pedestal
Ground Bar
Sheath Connection
Ground Level
Ground Rod
Ground wire doubled
under ground clamp

CMGRT-100
13
Frequently Asked Questions
Q: Does the ground electrode have to be disconnected and isolated as is required when using the
“Fall-Of-Potential”test with auxiliary electrodes?
A: No. Actually the electrode must be connected to the system to provide the path for test signal
injection as well as to provide the background impedance necessary as the reference. Additionally,
if the tested rod is poorly bonded to the ground connector, a high reading will be present.
Q: Does this mean I am not able to test an independent electrode?
A: No. However, as soon as the connection is made to a multiple electrode system, usually provided
by the connection to the system neutral, you can clamp on and make a measurement.
Q: Must the unit be clamped directly on the electrode?
A: No. The unit provides valid measurement results when clamped onto the electrode or the conductor
leading to the ground electrode. The reading through the conductor verifies not only the ground
electrode resistance, but also the connections of the ground wire to the rest of the system.
Q: Does the system under test have to be energized or de-energized to perform the test?
A: We are measuring the grounding network. The only requirements are that ground connections be
made to the system under test and that the voltage-to-ground at the tested point not exceed the
instrument rating.
Q: How does clamp-on ground testing compare with the standard “Fall-Of-Potential”test?
A: Empirical testing has validated that when performed correctly, both methods provide accurate and
repeatable readings for ground electrode resistance.
Q: How large a conductor can the jaw accommodate?
A: The inner diameter of the jaw is 32 mm (1.25") and can accommodate cables up to 1000 MCM.

14
Accuracy
Accuracy is specified as ±(a percentage of the reading + a fixed amount) within the following
parameters:
•Temperature: 23 °C ±3 °C (73.4 °F ±9 °F)
•Relative humidity: 40% to 60%
•Conductor location: Centered in jaw
•Battery charge: 8 V ±0.2 V
•External magnetic field: < 40 A/m
•External electrical field: < 1 V/m
•Loop resistance: Non-inductive
Ground Resistance
Ground or Leakage Current
Range Accuracy
1 mA to 299 mA ±(2.5 % + 2 mA)
0.300 A to 2.999 A ±(2.5 % + 2 mA)
3.00 A to 29.99 A ±(2.5 % + 20 mA)
Range Resolution Accuracy
0.10 to 1.00 Ω0.01 Ω±(2 % + 0.02 Ω)
1.0 to 50.0 Ω0.1 Ω±(1.5 % + 0.1 Ω)
50.0 to 100.0 Ω0.5 Ω±(2.0 % + 0.5 Ω)
100 to 200 Ω1 Ω±(3.0 % + 1 Ω)
200 to 400 Ω5 Ω±(6.0 % + 5 Ω)
400 to 600 Ω10 Ω±(10.0 % + 10 Ω)
600 to 1200 Ω50 Ω±25 % (approximately)

CMGRT-100
15
Specifications
Display: 3-3/4–digit LCD (3000 counts)
Jaw Opening: 32 mm (1.25")
Resistance Measurement Frequency: 2403 Hz
Current Measurement Frequency: 47 Hz to 800 Hz
Automatic Power-Off: After 5 minutes of inactivity
Overvoltage Protection Categories:
Category III, 150 VAC, Pollution Degree 2
Category II, 300 VAC, Pollution Degree 2
Operating Conditions:
–10 °C to 40 °C (14 °F to 104 °F), 10% to 90% relative humidity
40 °C to 55 °C (104 °F to 131 °F), 10% to 75% relative humidity
Storage Conditions: –40 °C to 70 °C (–40 °F to 158 °F), 10 to 75% relative humidity
Remove battery
Elevation: 2000 m (6500') maximum
Battery: 9 V battery (NEDA 1604, JIS 006P or IEC 6F22)
Overvoltage Installation Categories
These definitions were derived from the international safety standard for insulation coordination as it
applies to measurement, control, and laboratory equipment. These overvoltage categories are explained
in more detail by the International Electrotechnical Commission; refer to either of their publications:
IEC 1010-1 or IEC 60664.
Overvoltage Category I
Signal level. Electronic and telecommunication equipment, or parts thereof. Some examples include
transient-protected electronic circuits inside photocopiers and modems.
Overvoltage Category II
Local level. Appliances, portable equipment, and the circuits they are plugged into. Some examples
include light fixtures, televisions, and long branch circuits.
Overvoltage Category III
Distribution level. Permanently installed machines and the circuits they are hard-wired to. Some
examples include conveyor systems and the main circuit breaker panels of a building’s electrical
system.
Overvoltage Category IV
Primary supply level. Overhead lines and other cable systems. Some examples include cables, meters,
transformers, and other exterior equipment owned by the power utility.

16
Maintenance
1. Disconnect the unit from the circuit. Turn the unit off.
2. Remove the screws from the back cover.
3. Remove the back cover.
4. Replace the battery (observe polarity).
5. Replace the cover and screws.
Cleaning
Periodically wipe the case with a damp cloth and mild detergent; do not use abrasives or solvents.
Battery Replacement
•Do not attempt to repair this unit. It contains no user-serviceable parts.
•Do not expose the unit to extremes in temperature or high humidity. See Specifications.
Failure to observe these precautions can result in injury and can damage the unit.
Before opening the case, remove the jaw from the circuit and shut off the unit.
Failure to observe these warnings can result in severe injury or death.

CMGRT-100
17
Descripción
El Verificador de resistencia de tierra con pinza CMGRT-100 de Greenlee es un dispositivo de bolsillo
que cabe perfectamente en la palma de la mano, diseñado para evaluar sistemas de puesta a tierra.
Este modelo incluye dos modos:
•Modo Activo para medir resistencia
•Modo Pasivo para medir corriente
El Modo Activo induce una tensión en un conductor y mide la corriente resultante. Luego, calcula la
resistencia. El Modo Pasivo detecta el campo electromagnético que rodea al conductor para determinar
el flujo de corriente.
Se proporciona un bucle de resistencia de prueba para realizar una verificación rápida de la unidad.
Acerca de la seguridad
Es fundamental observar métodos seguros al utilizar y dar mantenimiento a las herramientas y equipo
Greenlee. Este manual de instrucciones y todas las marcas que ostenta la herramienta le ofrecen la
información necesaria para evitar riesgos y hábitos poco seguros relacionados con su uso. Siga toda
la información sobre seguridad que se proporciona.
Propósito de este manual
Este manual de instrucciones tiene como propósito familiarizar a todo el personal con los procedi-
mientos de operación y mantenimiento seguros para el Verificador de resistencia de tierra con pinza
modelo CMGRT-100 de Greenlee.
El modelo CMGRT-100 estáprotegido por la patente estadounidense No. D-362639.
Manténgalo siempre al alcance de todo el personal.
Puede obtener copias adicionales de manera gratuita, previa solicitud.
Greenlee y son marcas registradas de Greenlee Textron.
CONSERVE ESTE MANUAL

18
Lea y entienda este documento antes de manejar esta herramienta o darle
mantenimiento. Utilizarla sin comprender cómo manejarla de manera
segura podría ocasionar un accidente y, como resultado de éste, graves
lesiones o incluso la muerte.
Este símbolo se utiliza para indicar un riesgo o práctica poco segura que podría ocasionar
lesiones o daños materiales. Cada uno de los siguientes términos denota la gravedad del riesgo.
El mensaje que sigue a dichos términos le indica cómo puede evitar o prevenir ese riesgo.
SÍMBOLO DE ALERTA SOBRE SEGURIDAD
Peligros inmediatos que, de no evitarse, OCASIONARÁN graves lesiones o incluso la muerte.
Peligros que, de no evitarse, PODRÍAN OCASIONAR graves lesiones o incluso la muerte.
Peligro o prácticas peligrosas que, de no evitarse, PUEDEN OCASIONAR lesiones o daños
materiales.
Peligro de electrocución:
El contacto con circuitos activados puede ocasionar graves lesiones o
incluso la muerte.
Importante Información sobre Seguridad

CMGRT-100
19
Peligro de electrocución e incendio:
•No exponga esta unidad ni a la lluvia ni a la humedad.
•No utilice esta unidad si se encuentra mojada o dañada.
•Utilícela únicamente para el propósito para el que ha sido diseñada por el fabricante, tal como
se describe en este manual. Cualquier otro uso puede menoscabar la protección
proporcionada por la unidad.
De no observarse estas advertencias pueden sufrirse graves lesiones o incluso la muerte.
•No haga funcionar esta unidad con la caja abierta.
•Antes de abrir la caja, retire del circuito la pinza, y apague la unidad.
De no observarse estas advertencias pueden sufrirse graves lesiones o incluso la muerte.
•No intente reparar esta unidad, ya que contiene partes que deben recibir mantenimiento por
parte de un profesional.
•No exponga la unidad a ambientes de temperatura extrema o altos niveles de humedad.
Consulte las “Especificaciones”.
De no observarse estas precauciones podrían sufrirse lesiones o daños a la unidad.
Al utilizar esta unidad cerca de equipo que genere interferencia electromagnética quizáse
obtenga una lectura inexacta e inestable.
Importante Información sobre Seguridad

20
Identificación
1. Pinza
2. Palanca
3. Botón de Memoria
4. Botón de Alarma
5. Botón de Corriente y Disminución
6. Botón de Resistencia e Incremento
7. Botón On/Off (de encendido/apagado)
8. Pantalla
9. Botón “Hold”
(Retención de datos en pantalla)
10. Bucle de Resistencia de Prueba
Iconos de la pantalla
11. Se activa el altavoz
12. HOLD Se activa la función “Hold”(Retención de datos en pantalla)
13. NOISE Ruido parásito excesivo (mayor que 5 A o 50V aproximadamente);
cuando este icono aparece en la pantalla, las mediciones de
resistencia no son válidas
14. Pinza abierta
15. R<.1ΩLa resistencia medida es < 0,1 Ω; cuando este icono aparece en la pantalla,
las mediciones podrían no ser válidas
16. mA, A Miliamperios o amperios
17. ΩOhmios
18. 188 Potencia de pila (hasta 100%) y punto de
establecimiento de la alarma (hasta 199 Ω)
almacenados en la memoria
19. AL Alarma
20. P Se desactiva la función de Apagado
21. Pila baja
22. 88 Ubicación de la memoria
23. MR Recuperación de memoria
24. MEM Se activa la función Memoria
25. 8.8.8.8 Pantalla de medición; muestra OL (sobrecarga) cuando la resistencia medida
es mayor que 1200 Ω, o la corriente medida es mayor que 29,99 A eficaces
Símbolos en la unidad
Advertencia—Lea el manual de instrucciones
Doble forro aislante
1
2
8
9
43 5 6 7
10
11
23 22 20
21 19
12 13 14 15
16
24
25 17
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