Sonel MRU-21 User manual

OPERATING MANUAL
Earth Resistance Meter
MRU-21
SONEL SA
ul. Wokulskiego 11
58-100 Świdnica, Poland
Version 1.04 28.04.2017

OPERATING MANUAL MRU-21 version 1.04
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The MRU-21 meter is a modern, easy and safe measuring device. Please acquaint yourself with the
present manual in order to avoid measuring errors and prevent possible problems related to operation
of the meter.

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CONTENTS
1SAFETY....................................................................................................................5
2SETTINGS................................................................................................................6
3MEASUREMENTS..................................................................................................7
3.1 EARTH RESISTANCE MEASUREMENT WITH THE THREE-LEAD METHOD...................7
3.2 EARTH RESISTANCE MEASUREMENT WITH THE DOUBLE-LEAD METHOD..............10
3.3 MEASUREMENT OF RESISTANCE OF PROTECTIVE CONDUCTORS AND
EQUIPOTENTIAL BONDING ...................................................................................12
3.4 CALIBRATION OF TEST LEADS..............................................................................14
3.4.1 Turning AUTOZERO on..............................................................................14
3.4.2 Turning AUTOZERO off..............................................................................15
4MEMORY...............................................................................................................16
4.1 STORING THE MEASUREMENT RESULT DATA IN THE MEMORY .............................16
4.2 VIEWING MEMORY DATA.....................................................................................18
4.3 DELETING MEMORY DATA...................................................................................18
4.3.1 Deleting bank data.......................................................................................18
4.3.2 Deleting the whole memory .........................................................................20
4.4COMMUNICATION WITH A COMPUTER .................................................................21
4.4.1 Computer connection accessories ...............................................................21
4.4.2 Data transmission........................................................................................21
5METER POWER SUPPLY...................................................................................22
5.1 MONITORING OF THE POWER SUPPLY VOLTAGE...................................................22
5.2 REPLACEMENT OF BATTERIES .............................................................................22
6CLEANING AND MAINTENANCE....................................................................25
7STORAGE ..............................................................................................................25
8DISMANTLING AND UTILIZATION................................................................25
9TECHNICAL SPECIFICATIONS .......................................................................26
9.1 BASIC DATA ........................................................................................................26
9.2 ADDITIONAL INFORMATION.................................................................................28
9.2.1 REmeasurement...........................................................................................28
9.2.2 RCONT measurement......................................................................................29
10 EQUIPMENT .........................................................................................................30
10.1 STANDARD EQUIPMENT.......................................................................................30

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10.2 OPTIONAL ACCESSORIES .....................................................................................30
11 MANUFACTURER ...............................................................................................31

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1 Safety
MRU-21 meter is designed for measuring parameters important for safety of electrical
installations. Therefore in order to provide conditions for correct operation and the correctness of the
obtained results, the following recommendations must be observed:
Before you proceed to operate the meter, acquaint yourself thoroughly with this manual and
observe the safety regulations and specifications defined by the producer.
MRU-21 meter is designed to measure earth resistance and the resistance of protective
conductors and equipotential bondings. Any application that differs from those specified in the
present manual may result in a damage to the device and constitute a source of danger for the
user.
The meter must be operated solely by appropriately qualified personnel members holding
required certificates for carrying measurements in electric installations. Unauthorized use of the
meter may result in its damage and may seriously endanger unauthorized user.
Using this manual does not exclude the need to comply with occupational health and safety
regulations and with other relevant fire regulations required during the performance of a particular
type of work. Before starting the work with the device in special environments, e.g. potentially fire-
risk/explosive environment, it is necessary to consult it with the person responsible for health and
safety.
It is unacceptable to operate the following:
a damaged meter which is completely or partially out of order,
a meter with damaged test leads insulation,
a meter stored for an excessive period of time in disadvantageous conditions (e.g. excessive
humidity).If the meter has been transferred from a cool to a warm environment of a high level
of relative humidity, do not carry out measurements until the meter is warmed up to the
ambient temperature (approximately30 minutes).
Before commencing measurements, make sure the test leads are connected to the appropriate
measurement sockets.
Do not operate a meter with an open or incorrectly closed battery compartment or power it from
sources other than those specified in this manual.
The inputs of the meter are protected electronically against overload e.g. due to having been
connected to a live circuit:
- for all combinations of inputs - up to 276V for 30 seconds.
Repairs may be carried out only by an authorized service point.
The device meets the requirements of standards EN 61010-1 and EN 61557-1, -4, -5.
Attention:
The manufacturer reserves the right to introduce changes in appearance,
equipment and technical data of the meter.
Note:
An attempt to install drivers in 64-bit Windows 8 may result in displaying
"Installation failed" message.
Cause: Windows 8 by default blocks drivers without a digital signature.
Solution: Disable the driver signature enforcement in Windows.

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2 Settings
Press the SET button to select the test voltage (Un) or power supply source (SuPP). After
replacing the batteries, always set the power supply type. The correct charge indication depends on
this setting (the discharge characteristics of disposable and rechargeable batteries are different).
Turn on the meter and press SET.
Use the and buttons to select the parameter: Un or
SuPP.
Use the and buttons to change the parameters: Un
= 25V or 50V, SuPP = bAt (disposable batteries) or Acc
(rechargeable batteries).
Press ENTER to validate the changes and exit the mode, or...
...press ESC to exit the setting mode without validating the changes.

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3 Measurements
3.1 Earth resistance measurement with the three-lead method
The three-lead measuring method is the basic type of resistance-to-earth measurement.
Disconnect the tested
earth electrode from the
installation of the facility.
Turn on the meter.
Turn the rotary switch
to the RE3p position.
If necessary, adjust the voltage measurement according to section 2.
2,2m
CAT IV 300V
E S H
The current electrode (driven into earth) should be connected to Hsocket of the meter.
The voltage electrode (driven into earth) should be connected to Ssocket of the meter.
The tested earth electrode should be connected to Esocket of the meter.
The tested earth electrode and the current electrode and the voltage electrode should be
aligned.

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The meter is ready
for measurement.
Read the
interference voltage
on the auxiliary
display.
Press START.
Perform the measurement.
Interference voltage.
Read the main
measurement result.
Resistance of earth
electrode RE.
Read the auxiliary results (sequence as
below) by pressing << and >>.
RH
Resistance of current
electrode
RS
Resistance of voltage
electrode
Additional uncertainty
caused by resistance of
the electrodes.
Measur
ement
of RE
3p
Reading
the
results.
Measur
ement
of RE
3p
Reading
the
results.

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S
Repeat the measurements (steps 3-6)
after moving the voltage a few meters -
placing it farther and closer to the
measured earth electrode. If the RE
measurement results differ by more than
3%, the distance of the current electrode
from the tested earth electrode should be
considerably increased and the
measurements should be repeated.
Note:
The earth resistance measurement can be made when the interference voltage
does not exceed 24V. The voltage is measured up to 100V, but above 50V is
indicated as dangerous. The meter must not be connected to voltages
exceeding 100 V.
- Particular attention should be paid to quality of connection between the tested facility and the test
lead –the contact area must be free from paint, rust, etc.
- If resistance of the test probes is too high, REearth electrode measurement will include an additional
uncertainty. Particularly high measurement uncertainty occurs when a small value of resistance to
earth is measured with probes that have a weak contact with earth (such a situation occurs frequently
when the earth electrode is well made and the upper soil layer is dry and poorly conductive). In such a
case, the ratio of resistance of the probes to resistance of the tested earth electrode is very high and
consequently, uncertainty of measurement that depends on this ratio is also very high. Then, you can
make a calculation according to the formulas given in item 9 to estimate the influence of
measurement conditions, or you can use the graph also included in the appendix. This uncertainty is
also displayed in [%] as an additional result. It is calculated on the basis of measured valued. If such
additional uncertainty exceeds 30% the symbol is displayed You can improve the contact
between the probe and soil, for example by dampening with water the place where the probe is driven
into earth, driving the probe into earth in a different place, or using a 80 cm-long probe. Check also
the test leads for possible insulation damage and for corroded or loosened connection between the
banana plug and the test lead. In majority of cases the measurement accuracy achieved is
satisfactory. However, one should always be aware of the uncertainty included in the measurement.
- Factory calibration includes the resistance of the 2.2 m test lead (supplied).
Additional information displayed by the meter
and
Excessive interference voltage (> 24V). The measurement is
not possible.
Disconnect the source of interference or try another location of
the probes.
and
and continuous
audio signal 
Interference voltage exceeds 50V!
Disconnect the meter immediately!
Disconnect the voltage source before you reconnect the
meter.

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and
and continuous
audio signal 
Interference voltage exceeds 100V!
Disconnect the meter immediately!
Disconnect the voltage source before you reconnect the
meter.
with the
electrode
(electrodes) name
and
Interruption in measuring circuit or resistance of test probes
higher than 60 kΩ.
Check connections in the test circuit or reduce the probe
resistance by driving it into the soil again.
(in the field
below Cell) and
measurement result
and
Uncertainty of the REmeasurement caused by probes
resistance exceeds 30%. Reduce the probe resistance by
driving it into the soil again or by dampening the soil in its
immediate vicinity.
>1,99kΩ
The RE measuring range is exceeded.
>50kΩ
Test probes resistance above 50kΩ (but below 60kΩ).
Interference voltage above 10V, or unstable measurement
result, or the measured voltages or currents are too low in
relation to the noise.
and and
long audio signal

Measured voltages or currents are too low in relation to the
noise, or highly unstable measurement result. (The
symbol is displayed instead of the result).
and
Maximum allowed temperature inside the meter is exceeded.
3.2 Earth resistance measurement with the double-lead method
Turn on the meter.
Turn the rotary switch
to the RE2p position.
If necessary, adjust the voltage measurement according to section
2.
2,2m 1,2m
CAT IV 300V
E H
Connect the tested
facility to the Eand H
terminals.

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The meter is ready
for measurement.
Read the interference
voltage on the
auxiliary display.
Press START.
Perform the measurement.
Interference voltage.
Read the measurement
result.
Measured resistance.
Note:
- Factory calibration includes the resistance of the 1.2 m and 2.2 m test leads (supplied).
Additional information displayed by the meter
and
Excessive interference voltage (> 24V). The measurement is
not possible.
Disconnect the source of interference.
and
and continuous
audio signal 
Interference voltage exceeds 50V!
Disconnect the meter immediately!
Disconnect the voltage source before you reconnect the
meter.
and
and continuous
audio signal 
Interference voltage exceeds 100V!
Disconnect the meter immediately! (The symbol is
displayed instead of the interference voltage).
Disconnect the voltage source before you reconnect the
meter.

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and
Interruption in the test circuit.
>1,99kΩ
The RE measuring range is exceeded.
Interference voltage above 10V, or unstable measurement
result, or the measured voltages or currents are too low in
relation to the noise.
and and
long audio signal

Measured voltages or currents are too low in relation to the
noise, or highly unstable measurement result. (The
symbol is displayed instead of the result).
and
Maximum allowed temperature inside the meter is exceeded.
3.3 Measurement of resistance of protective conductors and
equipotential bonding
NOTE
Calibrate the test leads when measuring very small resistance values or when using
test leads different than the supplied 1.2 m and 2.2 m.
Turn on the meter.
Turn the rotary switch
to the RCONT position.
2,2m
1,2m
CAT IV3 00V
ES
Connect
the tested
facility
to the S
and E
terminals.
The meter is ready
for measurement.

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Press START.
Perform the measurement.
Read the measurement
result.
Note:
- The test current flows in one direction. To obtain the result for both directions, switch the test leads
and perform the measurement again, then calculate the arithmetic mean of both results.
Additional information displayed by the meter
and
Excessive interference voltage (> 3Vrms). The measurement
is not possible.
Disconnect the source of interference.
and
and continuous
audio signal 
Interference voltage exceeds 50V!
Disconnect the meter immediately!
Disconnect the voltage source before you reconnect the
meter.
> 199Ω
The RCONT measuring range is exceeded.
1..3Vrms interference voltage during the RCONT measurement.
Measurement is slightly unstable. The results may include an
additional uncertainty.
and and
long audio signal

Measurement is highly unstable.
and
Maximum allowed temperature inside the meter is exceeded.

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3.4 Calibration of test leads
In order to eliminate the impact of the resistance of test leads on the measurement result, the
compensation (autozeroing) of resistance may be performed. This is made with the AUTOZERO
function in the Rcont measurement.
3.4.1 Turning AUTOZERO on
Turn on the meter.
Turn the rotary switch
to the the RCONT ZERO
position.
CAT IV 300V
E S
Short the test leads by
putting crocodile clips
on exposed ends of the
test leads.
Press START.
Perform autozeroing.
Autozeroing
completed.
Note:
- Remember that the resistance of crocodile clips and crocodile-banana connections is added to the
resistance of the test leads.

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3.4.2 Turning AUTOZERO off
Turn on the meter.
Turn the rotary switch
to the the RCONT ZERO
position.
CAT IV 300V
E S
Disconnect the test
leads.
Press START.
Autozeroing is off.
During the
measurements, the
meter will compensate
the resistance of the
supplied 1.2 m and 2.2
m leads.
NOTE
It suffices when the compensation for given test leads is performed only once.
It is remembered when the meter is turned off.

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4 Memory
The MRU-21 meters feature memory that can store 990 single measurement results The whole
memory is divided into 10 memory banks, with 99 cells in each bank. Each measurement result can
be stored in a memory cell marked with a selected number and in a selected memory bank. Thanks to
this, the user of the meter can, at his/her option, assign memory cell numbers to individual
measurement points and the memory bank numbers to individual facilities. The user can also perform
measurements in any sequence and repeat them without losing other data.
Memory of measurement result data is not deleted when the meter is switched off. Thanks to
this, the data can be later read or sent to a computer. The number of a current memory cell or
memory bank is not changed either.
Note:
- One cell can contain the results of a single measurement.
- After each entry of the measurement result to the cell, its number is automatically incremented.
- It is recommended to delete the memory after reading the data or before performing a new series of
measurements that may be stored into the same memory cells as the previous ones.
4.1 Storing the measurement result data in the memory
Press ENTER after completion of
the measurement.
The meter is in the memory
storing mode.
Cell is empty.
Use the and symbols to select the
bank...
or use the and symbols to select the
cell.

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Press ENTER again. The screen (shown below)
appears for a moment, accompanied by three
short beeps, and then the meter returns to
display the last result of the measurement.
An attempt to overwrite a result causes
the warning symbol to appear.
lub
Press ENTER to overwrite the
result or ESC to abort.
Note:
- Stored in the memory is a complete set of results (main result and supplementary results), as well
as the test voltage for RE.

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4.2 Viewing memory data
Turn on the meter.
Turn the rotary switch
to the MEM position.
The content of the last saved
cell appears.
Use the and symbols to select the
bank...
or use the and symbols to select the
cell.
Press << or >> to view additional results.
4.3 Deleting memory data
4.3.1 Deleting bank data
Turn on the meter.
Turn the rotary switch
to the the MEM
position.