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GP-2 GeoTest

Instruction Manual
GP-2 GeoTest 1. PRECAUTIONS AND SAFETY MEASURES...3
1.1. 1.2. 1.3. PRELIMINARY INSTRUCTIONS... 3 DURING USE..... 4 AFTER USE.... 4 INSTRUMENT DESCRIPTION... 6 INITIAL CHECK.... 8 POWER SUPPLY.... 8 CALIBRATION.... 8 STORAGE..... 8 EARTH 2 WIRES..... 9 EARTH 3 WIRES.... 12 "" MODE (Resistivity of the Earth)... 15
4.3.1. 4.3.2. SELECTING THE UNITS OF MEASURE...15 OPERATING INSTRUCTIONS...15

GENERAL DESCRIPTION....5

2.1. 3.1. 3.2. 3.3. 3.4.
PREPARING THE INSTRUMENT...8

SWITCH FUNCTIONS...9

4.1. 4.2. 4.3. 4.4.
ANOMALOUS SITUATIONS WHICH CAN OCCUR DURING TESTS.. 19 TO SAVE: "SAVE" KEY.... 22 TO RECALL: "RCL" KEY.... 23 TO CANCEL: "CLR" KEY... 24 HARD RESET.... 26 DEFAULT PARAMETERS... 26
6.2.1. D EFAULT PARAMETERS...26
HOW TO SAVE, RECALL AND CANCEL DATA...22

5.1. 5.2. 5.3.

RESET AND DEFAULT PARAMETERS...26

6.1. 6.2.

CONNECTING THE INSTRUMENT TO A PC...27 MAINTENANCE....28
8.1. 8.2. 8.3. GENERAL.... 28 BATTERY REPLACEMENT.... 28 CLEANING.... 28 TECHNICAL FEATURES... 29

GENERAL FEATURES....31

TECHNICAL SPECIFICATIONS....29
9.1. 9.2. 9.3. OPERATING CONDITIONS.... 32
9.2.1. 9.2.2. 9.3.1. ENVIRONMENTAL CONDITIONS...32 EMC....32 STANDARD AND OPTIONAL ACCESSORIES...32

ACCESSORIES..... 32

10. SERVICE....33
10.1. WARRANTY..... 33 10.2. SERVICE..... 33
11. PRACTICAL REPORTS FOR ELECTRICAL TESTS..35
11.1. EARTH RESISTANCE MEASUREMENT... 35 11.2. EARTH RESISTIVITY MEASUREMENT... 38

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1. PRECAUTIONS AND SAFETY MEASURES
This instrument complies with European Community (EC) standards: EN61557-1, EN61557-5 and EN 61010-1.

WARNING:

For your safety as well as that of the instrument you are recommended to follow the procedures described in this instruction manual and carefully read all the notes preceded by the symbol.
Before and during measurements please carefully heed the instructions below:

F F F F F

Do not measure in wet or dusty places; Do not measure in the presence of gas, explosive materials or combustibles; Do not touch the circuit under test if no measurement is being taken; Do not touch exposed metal parts, unused terminals, circuits, etc Do not use the instrument if it seems to be malfunctioning (i.e. if you notice deformations, breaks, leakage of substances, absence of messages on the display etc); F Be particularly careful when measuring voltages higher than 25V in potentially hazardous areas (such as building sites, swimming pools and so on) and higher than 50V in any area, in order to avoid the risk of electrical shocks. The following symbols are used in this manual: Warning: Should you fail to keep to the prescribed instructions you could damage the instrument and/or its components or endanger your personal safety. Switch

PRELIMINARY INSTRUCTIONS

F This instrument has been designed for use in environments with pollution rating 2.
It can be used for tests on installations of excess voltage category III max 250V (phase to earth). F Always keep to the usual safety standards intended to: - protect you against dangerous currents; - protect the instrument against incorrect operation. F Only the accessories supplied with the instrument guarantee compliance with the safety standards. Accordingly, they must be used only when in good condition and, if necessary, they must be replaced with identical factory parts. EN - 3
GP-2 GeoTest F Use the instrument in TT, TN, IT system and industrial, civil, medical electrical plants, in normal conditions (contact voltage limit 25V) and in particular condition (contact voltage limit 50V). F Do not take measurements on circuits exceeding the specified current and voltage limits. F Do not take measurements under conditions exceeding the limits stated in paragraphs 9.2.1 and 9.2.2. F Make sure that the batteries are properly installed. F BEFORE connecting the alligator clips to the circuit under test, make sure that the switch is in the correct position. F Make sure that the display shows the same function as that to which the switch is set.

DURING USE

Please read carefully:
Should you fail to keep to the prescribed instructions you could damage the instrument and/or its components or endanger your personal safety.
F Before setting, or changing, the switch position, disconnect the alligator clips from the
circuit under test. F When the instrument is connected to the circuit under test, do not touch any unused terminal(s). F Do not take resistance measurements in the presence of external voltages; even if the instrument is protected, any excessive voltage may cause unit malfunction.
If, during use, the symbol appears, suspend the test and replace the batteries (see paragraph 8.2). The instrument is able to keep in memory any previously saved data for approximately 10 minutes.

AFTER USE

F After use, move the switch to the OFF position. F If you do not expect to use the instrument for more than 30 days, remove the batteries.

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2. GENERAL DESCRIPTION
Thank you for purchasing a quality AMPROBE Test Instrument. This instrument will provide you with accurate and reliable measurements provided that it is used according to this manuals instructions The GP-2 GeoTest is able to measure: F F F EARTH 2 WIRES: EARTH 3 WIRES: : 2 wires earth resistance 3 wires earth resistance 4 wires earth resistivity
The GP-2 GeoTest uses the "Fall of potential" method of measurement for all of the above tests.

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INSTRUMENT DESCRIPTION

DIST AVG

Rotary switch: Permits selection of the desired function

Figure 1: Front panel

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GP-2 GeoTest AVG

Displays the average value of the earth resistivity, calculated on the basis of all valid measurements taken.
Selects the distance "D" between the earth rods ( resistivity test).
Increases the value of the parameter D; Scrolls through the recorded test results; or Selects the unit m/ft.
Decreases the value of the parameter D; Scrolls through the recorded test results; or Selects the unit m/ft.

Starts a measurement.

Escape from (exit) the selected function or mode.

Saves test results.

Recall recorded test results.

DISP CLR

Displays recorded data from the selected memory location.
Deletes: - the average value of the measurements and the number of the tests included in the average value calculations - recorded test results from a specified memory location

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3. PREPARING THE INSTRUMENT

3.1. INITIAL CHECK

This instrument has been carefully checked for proper electrical and mechanical function prior to shipment. All possible precautions have been taken in order to deliver it in the best possible condition. Nevertheless, on receipt of the instrument we suggest that it be checked completely to make sure that no damage has occurred in transit. Should you find anomalies please contact the carrier immediately. Please also make sure that the package contains all the accessories and parts listed in paragraph 9.3.1. In case of discrepancies please contact your dealer. Should it be necessary to return the instrument, please refer to the instructions in paragraph 10.

POWER SUPPLY

The instrument is powered by 6 size 'AA' 1.5V batteries ( LR6 AM3 MN 1500) not included in the package. For correct installation of the batteries see paragraph 8.2. When the batteries are low the symbol appears. For replacement see paragraph 8.2.

CALIBRATION

The instrument complies with the standards mentioned in this manual. Its performance is guaranteed for one year from the date of purchase.

STORAGE

To guarantee accurate measurements, after extended storage in severe environmental conditions please allow the unit to normalize to proper ambient conditions.(see environmental conditions listed at paragraph 9.2.1).

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4. SWITCH FUNCTIONS

4.1. EARTH 2 WIRES

Whenever it is not possible to drive rods into the ground to take a 3-wire measurement, or in case of TT installations it is possible to use the simplified 2-wire method (Figure 2) which gives an excess (therefore safer) value. To carry out the test a suitable auxiliary rod is necessary; a rod is suitable when its earth resistance is negligible and it is independent of the earth equipment under test. In Figure 2 a water pipe has been used as auxiliary rod. However, any metal body driven into the ground can be used, provided that the above said requirements are met. Although this test is not provided for by the CEI 64.8 standard at present, it gives a value, which many 3-wire comparison tests have proved to be revealing for earth resistance.

Figure 2: 2 wire earth resistance measurement

Measuring procedure:

Insert the 4 connectors (black, red, blue and green) of the measuring cables into the corresponding input terminals of the instrument (E, S, H, ES). Connect the alligator clips as shown in Figure 2. Position the switch on EARTH 2 WIRES.

Display appearance:

Main display
Secondary display on the lefthand side: In this stage it displays the eventual interfering voltage present on the circuit

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Secondary display on the right-hand side
GP-2 GeoTest GO Press GO to take the measurement and read the result on the display. At the end of the test, a screen similar to the following will be displayed. If you keep pressing GO, the instrument takes more measurements consecutively. When a new value is acquired, the symbol blinks on the main display, the instrument emits a short sound and the counter shown on the secondary display on the left-hand side is updated. This counter indicates the number of measurements calculated on the basis of the average resistance value.
No. of earth resistance measurements included in the calculation of the average resistance

2 0.93

Value of the resistance
Average value of the resistance calculated as: (Val1+Val2++Valn)/(No. measurements)
When the message Measuring appears on the display, the instrument is measuring. Do not disconnect the alligator clips during the measurement.
if the operator takes three measurements consecutively, the instrument will display: - 1st measurement: main display = measured resistance value (Ex: 0.90) secondary display on the left-hand side = 001 (no. of measurements = 1 means that 1 earth measurement has been taken) secondary display on the right-hand side = average of the measurements taken (in case just one measurement has been taken the average value is equal to the measured value, in this case 0.90) - 2nd measurement: main display = measured resistance value (Ex: 0.96) secondary display on the left-hand side = 002 (no. of measurements = 2 means that 2 earth measurements have been taken consecutively) secondary display on the right-hand side = average of the measurements taken ((Val1+Val2)/no. of measurements = (0.90+0.96)/2 = 0.93) - 3rd measurement: main display = measured resistance value (Ex: 0.93) EN - 10

Last resistance value measured
The resistance measurement is taken according to a 4-wire voltmetric method. Therefore, it is not affected by the resistance value of the cable used: it is not necessary to calibrate the cables or their extension.
The test results can be stored in memory by pressing SAVE (see paragraph 5.1)

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"" MODE (Resistivity of the Earth)
The measurement is taken according to CEI 64.8, IEC 781, VDE 0413, EN61557-5 standards.
4.3.1. Selecting the units of measure
WARNING Whenever the units of measure are changed the instrument will perform a hard reset and all the data in the memory will be erased (see paragraph 6.1). SELECT THE UNIT press and hold the "RCL" key and turn the rotary switch. The instrument will display a screen that permits selection of the correct measurement unit: m or ft (used to determine distance between the ground rods) with the s or t keys. Press the SAVE or ESC key to save the measurement unit selected.
Regardless on the unit of distance D set up (either ft or m) the resistivity will be automatically calculated in OhmMeters!
4.3.2. Operating Instructions
Drive the 4 earth rods into the ground at the same distance D. The distance D between the rods is usually 3 to 30 ft (1 to 10 m). The distance D between the rods determines the depth at which the earth resistivity is measured. In order to identify the distance (and therefore depth) corresponding to the lowest resistivity value, the test must be repeated several times, positioning the rods at different distances. This depth will then have to be physically reached by the rods of the earth equipment. Insert the 4 connectors (black, red, blue and green) of the cables into the corresponding input terminals of the instrument (E, S, H, ES). Connect the alligator clips as shown in Figure 4.
Figure 4: 4 wire earth resistivity measurement EN - 15

Position the switch on .

--DIST

ft or m

0 V 010 ft/m
To select the distance D between the rods. This parameter can be chosen from the following values (expressed in ft/m):
3, 6, 9, 12, 15, 18, 21, 24, 27, 30 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
Press s or t to select the value of the parameter D:

Value of the parameter D

Press ESC to confirm the value previously set.
Press GO to take the measurement and read the value on the display. At the end of the test a screen similar to the one following will be displayed.
Measured earth resistivity value Measurement unit: m

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GP-2 GeoTest NOTE! Regardless on the unit of distance D, which is set up (either ft or m) the resistivity will be automatically calculated in OhmMeters!
If you keep pressing GO, the instrument takes more measurements consecutively. When a new value is acquired, the symbol m blinks on the main display, the instrument emits a short sound and, if the measured value is inserted in the calculation of the average resistivity value, the message Add appears on the secondary display on the right-hand side.

Interfering voltage measured.

- - 230 v - --

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5. HOW TO SAVE, RECALL AND CANCEL DATA
5.1. TO SAVE: "SAVE" KEY
If you want to save the test results: SAVE Press SAVE once. A screen similar to the one beside is displayed for 3 seconds, the instrument emits a sound, then the screen corresponding to the last measurement taken is displayed.
No. of memory location where the measure has been saved. MEM

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TO RECALL: "RCL" KEY
If you want to review the test results:
Press RCL. If the memory contains measurements, a screen similar to the one beside will be displayed.
No. of last memory location occupied. MEM
If the memory is empty, a screen similar to the one beside will be displayed.
Message rEc ord: the memory contains no values, therefore it is impossible to recall them.
t 2. s Press t , s to select the number of the memory location you wish to consult.
Press DISP to display the test result associated to the selected memory location.
t 4. s Press t , s to run over the saved results.
Press DISP again to review the memory locations again.
Press ESC at any time to leave the memory and return to the selected measuring function. EN - 23
TO CANCEL: "CLR" KEY
If you want to delete the test results:
Press RCL. A screen similar to the following will be displayed:
MEM No. of the last memory location occupied.
t 2. s Press t , s to select the number of the memory location. NOTE! when you cancel data the instrument cancels all the saved data from the SELECTED location up to the last memory location occupied.
Press DISP to display the test result associated with the selected memory location. Press DSP again to review the memory locations. Press CLR. The secondary displays are blinking.
MEM No. of last memory location which will be cancelled.
No. of the last memory location occupied. If you press CLR twice the instrument will cancel the memory locations from no. 2 to no. 8.
At this point you have two possibilities: CLR Press CLR again to cancel the test results from the selected location (main display) through the last location saved (secondary display on the left-hand side) Press ESC once to abort the cancellation and return to the number of the memory location. EN - 24

ESC Ex.:

Press ESC again to return to the selected measuring function.
97 test results have been saved. You want to clear from the 43rd to the 97th location. - Press RCL. - Select the 43rd memory location using t and s. - Press CLR. The symbol C will blink on the secondary display on the right-hand. The number 43 will appear on the main display and the number 97 will appear on the secondary display on the left-hand side. - Press CLR to erase saved data from the 43rd to the 97th location.

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6. RESET AND DEFAULT PARAMETERS

This paragraph describes the HARD RESET procedure and the default parameters set when a HARD RESET procedure is performed.

HARD RESET

BEFORE RESETTING, TRANSFER ALL SAVED DATA TO A PC.
1. Depress and hold the CLR key while turning on the instrument by rotating the switch. 2. This screen is displayed for 5 seconds. The reset has been performed. The screen corresponding to the selected function is displayed.

H E SE

As a consequence of the HARD RESET procedure all the saved is deleted and the parameter DST (distance between the rods, resistivity measurement) returns to the default value.

DEFAULT PARAMETERS

The default parameters automatically set when a HARD RESET procedure is performed are the following:
6.2.1. Default parameters
Parameter Parameter DST = distance between the rods, resistivity measurement Test results contained in the memory Default parameter set with the RESET procedure DST = 1

Memory empty

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7. CONNECTING THE INSTRUMENT TO A PC
Connect the instrument to a PC through the RS-232 port with the supplied cable. Refer to software manual for information how to set up software for download.
In order to transfer recorded data to a PC follow this procedure:
Position the switch on RS232.
Proceed as indicated in the manual of the management software.
the transmission speed of the GP-2 is 9600 baud (see software manual).
On RS232, the instrument will switch off automatically 2 minutes after the last selecting command from a PC.

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8. MAINTENANCE

8.1. GENERAL

GP-2 GeoTest is a precision instrument. During its use and for its storage follow the recommendations and instructions contained in this manual in order to avoid possible damage or danger. Never use the instrument in environments with a high humidity or temperature. Do not expose the instrument to direct sunlight. Always turn off the instrument after use. If you expect to not use it for extended periods, remove the batteries.

BATTERY REPLACEMENT

appears, the batteries must be replaced.

When the symbol

Only qualified technicians can perform this operation. Before replacing the fuse/fuses disconnect the alligator clips from circuit under voltage in order to avoid electrical shocks. The tester is capable of keeping the values stored for approx. 10 minutes.

1. 2. 3. 4.

Disconnect the cables from the input terminals. Set the switch to the OFF position. Remove the battery cover screws and then remove the cover. Replace the batteries with 6 new ones of the same type: 1.5V AA ( LR6 AM3 MN 1500). 5. Reposition the cover and replace the screws.

CLEANING

Use a soft dry cloth to clean the instrument. Do not use wet clothes, solvents, water etc.

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9. TECHNICAL SPECIFICATIONS

9.1. TECHNICAL FEATURES

Resistance measurement
Range (**) ( ) 0.01 19.99 20.0 199.1999 Resolution ( ) 0.01 0.Accuracy (*) (2% reading + 3 digits)
(*) If RP > 100R E and/or RC > 100RE , RP > 50k and/or RC > 50k, if the instrument carries out the test the accuracy of the instrument is (10%Reading) RP = resistance of the voltage circuit RC = resistance of the current circuit RE = earth resistance (**) Automatic selection of the range.
Measuring frequency 125Hz / 75Hz/ 41.66Hz Measuring current 10mA Open-terminal measuring voltage 25Vrms Waveform of measuring voltage: sine wave Interfering voltage: - amperometric circuit: the measurement is taken with the stated accuracy if the interfering voltage is 3V, while for interfering voltages between 3 and 30V inclusive, the accuracy decreases progressively; with an interfering voltage of about 30V the instrument does not perform the test. - voltmetric circuit: the measurement is taken if the interfering voltage is 3V; in case of higher voltages the instrument does not perform the test.

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Resistivity measurement
Range (**) 0.6-125.6 m 0.125 1.256 km 1.25-19.99 km 20.0 199.9 km Resolution 0.1 m 0.001 km 0.01 km 0.1 km Accuracy (*) (2% reading + 3 digits)
(*) If RP > 100RE and/or RC > 100RE , RP > 50k and/or RC > 50k, if the instrument carries out the test the accuracy of the instrument is (10%Reading) RP = resistance of the voltage circuit RC = resistance of the current circuit RE = earth resistance = 2DRE = calculated resistivity (**) Automatic selection of the range

Measuring frequency 125Hz / 75Hz/ 41.66Hz Measuring current 10mA Open-terminal measuring voltage 25Vrms Waveform of measuring voltage: sine wave Interfering voltage: - amperometric circuit: the measurement is taken with the stated accuracy if the interfering voltage is 3V, while for interfering voltages between 3 and 30V inclusive, the accuracy decreases progressively; with an interfering voltage of about 30V the instrument does not perform the test. voltmetric circuit: the measurement is taken if the interfering voltage is 3V; in case of higher voltages the instrument does not perform the test.
Interfering voltage measurement
Range (V) 500 Resolution (V) 1 Accuracy (2% reading + 2 digits)
Safety standards This instrument complies with EN 61010, EN 61557-1, EN 61557-5 standards. Insulation Pollution Maximum altitude Surge voltage category class 2, double insulation 2 2000m CAT III 250V (phase to earth)

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General features
Mechanical features Dimensions: Weight (batteries included): Power supply Batteries: 6 batteries 1.5 V size AA ( LR6 AM3 MN 1500) The symbol appears on the display when the battery voltage is low. about 300 measurements F 100 mA (not accessible to the operator) 222 (L) x 162 (W) x 57 (H) mm about 1000g

Low battery indication:

Battery life: Fusible Link:

Auto Power Off:

the instrument will automatically switch off 2 minutes after last selecting a function or PC command.
Display Features: Memory: Interfaces: standard LCD 65mm x 65mm. 999 memory locations opto-insulated serial output RS232 to transfer data to a PC.

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OPERATING CONDITIONS
F F <80% -F <70% (23 5C) (-10C 50 C) (-C)
9.2.1. Environmental conditions
Reference temperature: Operating temperature: Relative humidity: Storage temperature: Storage humidity:

9.2.2. EMC

This instrument has been designed in compliance with the EMS standards in force and its compatibility has been tested for: Irradiated emissions: Immunity: Electrostatic discharges: R.F. range: Fast transient: EN55011 EN50140, EN 61000 EN61000-4-2 EN50140 EN61000-4-4

ACCESSORIES

9.3.1. Standard and optional accessories
Standard accessories * - 1 carrying case containing: - 4 earth rods - 4 cables banana-crocodile Carrying case Optical serial cable Software and manual Code GP-2CON GP-2CC C2000 www.amprobe.com

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10. SERVICE

10.1. WARRANTY

This instrument is guaranteed against any defects in material and manufacturing for one year, in compliance with the general sale terms and conditions. During the warranty period all defective parts may be replaced, but the manufacturer reserves the right to repair or replace the product. If the instrument is to be returned, its transport expenses must at the customer's expense. Shipping instructions may be obtained by contacting the factory. A Return Material Authorization (RMA) number must be obtained from the factory prior to return. A report should accompany the returned product, stating the reason(s) for return. The unit should be packed in its original carton or equivalent; any damage that may be due to non-original packing shall be charged to the customer. The manufacturer disclaims any responsibility for damages caused to people and/or objects. The Warranty is not applicable in the following cases: Any repair and/or replacement of accessories and the battery (not covered by the guarantee). Any repair that might be necessary as a consequence of a misuse of the instrument or of its use with non-compatible devices. Any repair that might be necessary as a consequence of improper packaging. Any repair that might be necessary as a consequence of attempted service carried out by unauthorized personnel. Any change to the instrument carried out without the express authorization of the manufacturer. Use not intended for as per the instrument specifications or in the instruction manual.
The content of this manual cannot be reproduced in any form whatsoever without express authorization of the manufacturer. This product is patented and its trade mark registered. The manufacturer reserves the right to modify the product specifications without prior notification.

10.2. SERVICE

If the instrument does not work properly, before returning, check the cables and replace them, if necessary. Should the instrument still operate improperly, check that the operation procedure is correct and corresponds to the instructions given in this manual. If the instrument is to be returned, its transportation is at the customers expense. Shipping instructions may be obtained by contacting the factory. A Return Material Authorization (RMA) number must be obtained from the factory prior to return. EN - 33
A report should accompany the returned product, stating the reason(s) for return. The unit should be packed in its original carton or equivalent; any damage that may be due to non-original packing shall be charged to the customer. The manufacturer disclaims any responsibility for damages caused to people and/or objects.

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11. PRACTICAL REPORTS FOR ELECTRICAL TESTS
11.1. EARTH RESISTANCE MEASUREMENT
PURPOSE OF THE TEST This test is carried out to make sure that the protective device is coordinated with the earth resistance value. You cannot take off hand a limit earth resistance value as a reference for the test result: occasionally you must make sure that the prescribed coordination is respected. EQUIPMENT PARTS TO BE TESTED The earth equipment and its working conditions. The test must be carried out without disconnecting the earth references. ADMISSIBLE VALUES The earth resistance value, however measured, must meet the following requirement:

RE < 50 / Ia

where: RE= measured resistance of the earth equipment. The value can be determined with the following tests: earth resistance with 3-wire voltmetric method loop impedance (see (*)) 2-wire earth resistance (see (**)) 2-wire earth resistance in the socket (see (**)) earth resistance given by the contact voltage Ut (see (**)). earth resistance given by tripping time of RCDs (A, AC), RCD S (A, AC) (see (**)).

Ia= 50= (*) (**)

tripping time in 5s increments of the automatic switch; nominal tripping current of the RCD (in case of RCD S 2 In). Limit safety voltage (reduced to 25V for special places). In presence of an RCD the test must be carried out on the RCD upstream or downstream, bypassing it to avoid its intervention. These methods, even if not yet prescribed for by the CEI 64.8 standard, give values which many 3-wire comparison tests proved to be revealing for earth resistance.

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GP-2 GeoTest EXAMPLE OF EARTH RESISTANCE MEASUREMENT You must test equipment protected by a RCD at 30 mA. Measure the earth resistance according to one of the above-described methods. In order to check whether the resistance complies to the standards in force, multiply the value for 0.03A (30 mA). If the result is lower than 50V (or 25V for special places) the equipment is to be considered as coordinated as it meets the above stated requirement. In case of RCDs at 30 mA (used for nearly the totality of civil equipment) the maximum earth resistance admitted is 50/0.03=1666###. This also permits use of the simplified methods described, which give an approximate value (although not extremely precise) for the calculation of the coordination.
VOLTAMPEROMETRIC METHOD For small earth plant Let a current circulate between the earth rod and a current probe positioned at a distance from the earth equipment outline corresponding to five-times the diagonal of the area delimiting the earth equipment (see Figure 5). Position the voltage probe halfway between the earth rod and the current probe, then measure the voltage between the two.

Use several rods in parallel and moisten the surrounding ground if the instrument is not able to supply the current necessary to perform the test because of an high earth resistance.

Green Black

Figure 5:
Earth resistance measurement (voltamperometric method for small earth plant)

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GP-2 GeoTest For large earth plants This procedure is also based on the voltamperometric method, but it is mainly used when it is difficult to position the auxiliary current rod at a distance corresponding to five-times the diagonal of the area of the earth equipment. Position the current probe at a distance equal to the diagonal of the area of the earth equipment (see Figure 6). To make sure that the voltage probe is positioned outside the area affected by the rod under test, take more measurements, first positioning the voltage probe halfway between the rod and the current probe, then moving the probe both towards the earth rod and towards the current probe.

R ed Green Black Blue

Figure 6:
earth resistance measurement (voltamperometric method for big earth plant)

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11.2. EARTH RESISTIVITY MEASUREMENT
PURPOSE OF THE TEST This test is intended to analyze the resistivity value of the ground in order to define the type of rods to be used. EQUIPMENT PARTS TO BE TESTED For the resistivity test admissible values do not exist. The various values measured by positioning the rods at growing distances a must be quoted in a graph. According to the resulting curve, suitable rods will be chosen. As the test result can be affected by metal parts buried (such as pipes, cables or other rods), in case of doubt take a second measurement positioning the rods at an equal distance "a", but rotating their axis by 90.
2nd measurement: compared to the previous measurement the rods are rotated by 90.
1st measurement: the rods are positioned at a distance a

a a a a

B lu e
The resistivity value is calculated with the following formula: =2aR where: = specific resistivity of the ground (m) a= distance between the rods (ft/m) R= resistance measured by the instrument () EN - 38

en G re

GP-2 GeoTest The measuring method allows definition of the specific resistance up to the depth approximately corresponding to the distance a between the rods. If you increase the distance a you can reach deeper ground layers and check the ground homogeneity. After several measurements, at growing distances a, you can trace a profile like the following ones, according to the most suitable rod chosen: Curve1: as decreases only in depth, it is possible to use only a rod in depth. Curve2: as decreases only until the depth A, it is not useful to increase the depth of the rod beyond A. Curve3: even at a superior depth, does not decrease, therefore a ring rod must be used.