MEGABRAS MTD-20KWe Manual de usuario
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MTD-20KWe
Digital earth tester
User’s Guide
GU-1192
© 2006 MEGABRAS. All rights reserved.
Printed in Brazil
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This page was left blank intentionally.
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Index
1. Description______________________________________________________6
2. Operating instructions _____________________________________________7
2.1. Panel controls________________________________________________7
2.2. Output Terminals ~___________________________________________8
2.3. Internal battery _______________________________________________8
2.4. Battery status check ~_________________________________________8
2.5. Power supply source ~________________________________________8
2.6. Battery charge________________________________________________9
2.7. Measurement ranges _________________________________________10
2.8. Grounding resistance measurement______________________________10
2.9. Spurious voltages measurement ________________________________11
2.10. Soil resistivity measurement –Wenner Method ____________________11
2.11. Audible Alarm ______________________________________________14
2.12. Influence of grounding resistance of the auxiliary rods_______________15
2.13. Special considerations about grounding resistance measurements_____15
3. Equipment operation checking______________________________________17
4. Technical Specifications __________________________________________19
5. Warranty ______________________________________________________21
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1. Description
The MTD-20KWe digital earth tester allows for the measurement of Earth
Resistances and Soil Specific Resistivity, and also the spurious voltages
caused by parasitic voltages present in the soil.
This equipment is suitable for fast and easy measurement of the grounding
resistance in house and industrial buildings, hospital installations, lightning
rods, antennas, substations, etc. Soil resistivity measurement allows for soil
stratification in order to optimize the most complex grounding systems
engineering. Its state-of-the-art system of active and passive filters provides
it with high immunity to electric interferences, making it possible to obtain
reliable measurements even in the presence of spurious voltages, such as
the ones that can be found in some urban areas and near primary
substations.
It has an audible signal which advises the operator when the generated
current is not enough to carry out reliable measurements. Due to the fact
that it may not be noticed, this alarm also prevents further testing.
Because of its wide range of measurement (from 0.01 Ωup to 20 kΩ), this
equipment allows for reliable testing in all kinds of soils, including those that
offer very high resistivity. The use of this instrument is very simple, it has a
high-visibility 3½ -digit display with direct readings, even under sunlight.
This earth tester is supplied with a rechargeable internal battery. The smart
charger is microprocessor-controlled, and can be powered from a 12 V car
battery (or a similar one).
It has a sturdy, easy and safe to carry cabinet, with IP54 protection level. It
is suitable to work under adverse geographical and environmental
conditions, with extreme temperatures in cold or tropical regions and in high
mountain areas, showing a reliable performance in the field.
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2. Operating instructions
2.1. Panel controls
!
"
#
$
%
&
'
(
)
*
+
!-3½ digits display
"-Battery Charger Status LED
#-Output Terminal H (EC)
$-Output Terminal S (ET)
%-Output Terminal ES (EXT)
&-Output Terminal E (EXC)
'-START (Pushbutton)
(-FUNCTION SELECTOR (Rotary Switch)
R (3 poles) = Grounding Resistance
V = AC Voltmeter
ρ(4 poles) = Resistivity (4 rods)
)-DC Power Input
*-RANGE SELECTOR (Rotary Switch)
+-BATTERY TEST (Pushbutton)
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2.2. Output Terminals ~
Green E (EXC) Terminal –Earth Electrode
Black ES (EXT) Terminal –Voltage Electrode
Blue S (ET) Terminal –Voltage Electrode
Red H (EC) Terminal –Current Injection
E -ES -S -H: Standard nomenclature currently recommended by the IEC.
EXC -EXT -ET-EC:Nomenclature traditionally used by Megabras earth
meters. It is maintained to make migration easy.
Having the function selector switch in the R position (grounding resistance
measurement) the E and ES terminals are internally shorted. The ES
terminal is not used for this measurement.
2.3. Internal battery
The MTD-20KWe has a built-in rechargeable battery. Furthermore, it can be
powered by means of an external 12 V battery.
2.4. Battery status check ~
Before starting each test, it is recommended to check if the internal battery
has enough charge. This can be done by pressing the Battery Test
Pushbutton+. The value shown on the display must be higher than
1000. If it is not, the battery needs to be charged. This checking can be
repeated during measurements, under the equipment highest consumption
conditions.
2.5. Power supply source ~
The power supply source provided can be connected to 220-240 V~ mains.
It is used to charge the battery.
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2.6. Battery charge
This equipment has an internal battery charger with a smart,
microprocessor-controlled circuit which adjusts the battery charge to the
optimized parameters to ensure the maximum service life. It is supplied by
an external source for 220-240 V∼(supplied with the equipment) or by a
12 V car battery.
In order to recharge the battery, connect the equipment to the mains supply
or to the external battery. After a few seconds, the BATTERY CHARGER
STATUS LED"will flash during one second in green and red alternately.
During this period, the charger checks the battery initial status, selecting in
this way the charge optimized parameters. After this, the BATTERY
CHARGER STATUS LED"will remain lit in red until the charge is
complete, then the led will turn into green and will remain this way until the
equipment is disconnected from supply.
If, during the battery charge with 12 V car battery, the equipment is used to
perform measurements, the charge will be momentarily interrupted,
returning to the charge process once measurements are finished. In the
following table we summarised the meaning of the luminous indications of
the LED"
Alternate green and red lights
Evaluation of the battery initial status when the
source is connected, during one second.
Steady red light Battery under charge.
Flashing red light The battery was not successfully charged. It de
notes
some trouble in the battery charging process.
Steady green light The charge has successfully finished. The battery is
OK.
Note: Under storage, the battery loses part of its charge. Thus, before using the
equipment for the first time, or after some time of being out of use, the battery must
be recharged. As the kind of rechargeable battery that this equipment uses does not
show the “memory effect” and due to the charger intelligent characteristics, there are
no restrictions to start charging it as many times as is needed. As soon as the
charger detects that the battery is completely charged, it will automatically turn to the
Float state (Green indication), protecting it from overcharging.
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2.7. Measurement ranges
The equipment has 4 resistance measurement ranges:
Range Unit Reading resolution
0 –20.00 Ω0,01
0 –200.0 Ω0,1
0 –2000 Ω1
0 –20 kΩ0,01
2.8. Grounding resistance measurement
1. Insert in the soil the two auxiliary rods: the E3 current rod and the E2
voltage rod. Connect them by means of provided cables to H(EC)#
and S(ET)$output terminals, respectively. The E(EXC)&terminal must
be connected to the grounding system under test (E1) using the 5
meters cable (see fig. 01).
E3 E2
E1
fig. 01
D1
D2
40m cable
20m cable 5m cable
Current
rod
Voltage
rod Ground
to be
measured
2. Select the R(3 pole) position in the FUNCTION Selector Switch(.
Select the 20 kΩ range with the RANGE Selector Switch*and press
the START Pushbutton'. If the resistance value is lower than 2kΩ,
select the most adequate, electing the scale from the highest to the
lower. If the acoustic alarm bips, this indicates some abnormality in the
rods wiring, the reason may be a misconnection or an excessively high
diffusion resistance in the current rod. Check the installation to fix this
problem. (See item 5).
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If a number 1 appears in the most significant digit position, this means
that the measured value is higher than the highest one of the selected
range (Overrange). In this case, the immediately higher range must be
selected until finding a valid reading value (this will not be possible if
the measured resistance is higher than 20 kΩ).
2.9. Spurious voltages measurement
1. In order to check the existence and to measure the spurious voltages
present in the soil, the inserted rods must be kept in place, connected
to the equipment as shown in figure 01.
2. Select the V position with the FUNCTION Selector Switch(and
press the START Push-button'. The display will show the AC
voltage between the grounding E1 and the E2 probe, up to 200 V.
2.10. Soil resistivity measurement -Wenner Method
1. Insert four rods (spikes) in the soil, well aligned and with a constant D
separation, as shown in fig. 02. When performing this measurement,
the distance between the rods is critical as it takes part in the
resistance calculation.
2. Select the position ρ(4 pole) in the FUNCTION Selector Switch (
and connect the rods, as it is shown in figure 02.
fig. 02
D
H(E)
C
S(E)
T
ES(E)
XT
E(E)
XC
D
D
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3. Find the most appropriate range and press the START Pushbutton'.
4. To obtain the soil resistivity value, Wenner’s equation should be
applied. In its simplified form, this equation is as follows:
ρ= 2πR D
Where:
•ρ= soil resistivity value, expressed in [Ωm]
•π= 3.14159
•R= value shown on the display.
•D= distance between the rods, expressed in meters.
In this way, mean soil resistivity is obtained from the surface up to a
depth equal to the D distance between the rods. The information
required to determine the soil stratification by means of a graphic
method or by the use of an appropriate software is obtained by
performing several measurements with different distances between the
rods. The distances which are generally adopted are: 1m, 2m, 4m, 8m,
16 m. In order to carry out measurements with D=16m, cables can be
distributed as shown in figure 03, with the instrument near the ES rod.
D
H(E)
C
S(E)
T
ES(E)
XT
E(E)
XC
D
D
p
40m cable
5m cable
fig. 03
20m cable
20m cable
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