Sonel MRU-20 User manual

OPERATING MANUAL
EARTH RESISTANCE METER
MRU-20
SONEL SA
ul. Wokulskiego 11
58-100 Świdnica
Version 3.1 24/02/2017

OPERATING MANUAL MRU-20 version 3.1
2
The MRU-20 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.

OPERATING MANUAL MRU-20 version 3.1
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CONTENTS
1SAFETY......................................................................................................5
2MEASUREMENTS....................................................................................6
2.1 MEASUREMENT OF RESISTANCE USING THREE-POLE METHOD ....................... 6
2.2 MEASUREMENT OF RESISTANCE USING DOUBLE-POLE METHOD, .................... 9
2.3 MEASUREMENT OF THE RESISTANCE OF PROTECTIVE CONDUCTORS AND
EQUIPOTENTIAL BONDINGS .......................................................................... 11
2.4 CALIBRATION OF TEST LEADS...................................................................... 13
2.4.1 Activation of Autozeroing ........................................................................... 13
2.4.2 Switching-off Autozeroing ..........................................................................14
3POWER SUPPLY OF THE METER.....................................................16
3.1 MONITORING OF THE POWER SUPPLY VOLTAGE ...........................................16
3.2 REPLACEMENT OF BATTERIES (RECHARGEABLE BATTERIES ) ...................... 16
3.3 GENERAL PRINCIPLES REGARDING USING NI-MH RECHARGEABLE
BATTERIES...................................................................................................18
4CLEANING AND MAINTENANCE...................................................... 20
5STORAGE ................................................................................................20
6DISMANTLING AND UTILIZATION..................................................20
7TECHNICAL DATA................................................................................ 21
7.1 BASIC DATA................................................................................................. 21
7.2 SUPPLEMENTARY DATA............................................................................... 23
7.2.1 RE Measurement.......................................................................................... 23
7.2.2 Measurement of RCONT ................................................................................ 24
8EQUIPMENT ........................................................................................... 24
8.1 STANDARD EQUIPMENT............................................................................... 24
8.2 OPTIONAL EQUIPMENT................................................................................ 24
9MANUFACTURER...................................................................................26

OPERATING MANUAL MRU-20 version 3.1
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OPERATING MANUAL MRU-20 version 3.1
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1 Safety
MRU-20 meter is designed for measuring parameters important for safety of electrical installa-
tions. Therefore in order to provide conditions for correct operation and the correctness of the obta-
ined results, the following recommendations must be observed:
Before you proceed to operate the meter, acquaint yourself thoroughly with this manual and ob-
serve the safety regulations and specifications defined by the producer.
MRU-20 meter is designed to measure earth resistance and the resistance of protective conduc-
tors 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 re-
quired certificates for carrying measurements in electric installations. Unauthorized use of the me-
ter 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 regu-
lations 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 am-
bient 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 con-
nected 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, equi-
pment and technical data of the meter.
Note:
An attempt to install drivers in 64-bit Windows 8 may result in displaying "In-
stallation failed" message.
Cause: Windows 8 by default blocks drivers without a digital signature.
Solution: Disable the driver signature enforcement in Windows.

OPERATING MANUAL MRU-20 version 3.1
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2 Measurements
2.1 Measurement of resistance using three-pole method
The three-pole measuring method is the basic type of resistance-to-earth measurement.
Disconnect earth
electrode being
tested from the
installation of
the building.
Turn on the meter.
Set the rotational func-
tion selector
set at RE3p position.
Using Unbutton
set test voltage
on 25V or 50V.
After 5 sec. the
meter returns to
the measure-
ment mode.
You can also
confirm the
choice by
pressing the
START push-
button.
ESH
2,2m
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 earth electrode being tested should be connected to Esocket of the meter.
The earth electrode being tested and the current electrode and the voltage electrode should
be located one line

OPERATING MANUAL MRU-20 version 3.1
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The meter is ready for
measurement.
Value of interference
voltage is displayed
on the auxiliary di-
splay
Press START button.
Carry out the measurement.
Value of interference vol-
tage.
Read out main results:
earth electrode resistan-
ce RE.
Auxiliary results may be read
after pushing SEL push-button.
RH
resistance of current
electrode
RS
resistance of voltage
electrode
Measu-
rement
RE3p
Read the
results
Measu-
rement
RE3p
Read the
results

OPERATING MANUAL MRU-20 version 3.1
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Value of additional
uncertainty caused by
resistance of the
electrodes.
S
Repeat the measurements (points 3-6) shi-
fting the voltage electrode several meters:
the electrode should be shifted farther and
closer to the earth electrode being tested.
If REmeasurement results differ from each
other by more than 3%, the distance of the
current electrode from the earth electrode
being tested should be considerably in-
creased and the measurements should be
repeated
Note:
Measurement of resistance-to-earth may be carried out if voltage of interferen-
ces does not exceed 24 V. Voltage of interferences is measured up to the level
of 100 V but above 50 V it is signaled as dangerous The meter must not be
connected to voltages exceeding 100 V.
- Particular attention should be paid to quality of connection between the object being tested and the
test lead –the contact area must be free from paint, rust, etc.
- If resistance of test probes is too high, REearth electrode measurement will be burdened with addi-
tional uncertainty. Particularly high uncertainty of measurement occurs when a small value of resi-
stance-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 slightly conduc-
tive). In such a case, the ratio between resistance of the probes and resistance of the earth electrode
tested is very high and consequently, uncertainty of measurement that depends on this ratio is also
very high. Then according to guidelines defined in section 7 it is possible to calculate the effect of
measurement conditions - or to use the attached chart. This uncertainty is also displayed in [%] as an
additional result. It is calculated on the basis of measured valued. If such additional uncertainty exce-
eds 30% the following symbol is displayed with the measurement result . 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 be-
tween the banana plug and the test lead. In majority of cases the measurement accuracy achieved is
satisfactory. However, you should always be aware of the uncertainty included in the measurement.
- Calibration performed by the manufacturer takes into account the resistance of supplied measure-
ment lead of 2.2 m

OPERATING MANUAL MRU-20 version 3.1
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Additional informations displayed by the meter
and
Interference voltage is too high (> 24V) –the measure-
ment is impossible.
Turn off the source of interferences or try arrange probes
differently
and
and continuous audio
signal 
Interference voltage is above 50V!
Immediately disconnect the meter!
Before re-connecting, turn off the source of voltage.
and
and continuous audio
signal 
Interference voltage is above 100V!
Immediately disconnect the meter!
Before re-connecting, turn off the source of voltage.
with the name of
an electrode (or elec-
trodes) and
Interruption in measuring circuit or resistance of test pro-
bes is higher than 60kΩ.
Please check connections in the measuring circuit or re-
duce the resistance of the measuring probe by its reposi-
tioning.
and the measure-
ment result and
Uncertainty of REmeasurement introduced by the resi-
stance of test probes exceeds 30%. Reduce the resistan-
ce of the probe by its repositioning, or increase the damp-
ness of the ground in the direct vicinity of the probe.
>1,99kΩ
REmeasuring range is exceeded .
>50kΩ
Resistance of test probes is higher than 50kΩ (but lower
than 60kΩ).
Interference voltage is above 10V or unstable measure-
ment result or measured voltages or currents are low in re-
lation to noise.
and and
long audio signal

Measured voltages or currents are too low in relation to
noise or very unstable measurement result. (Symbol
is displayed instead of the result.)
and
Acceptable temperature inside the meter is exceeded.
2.2 Measurement of resistance using double-pole method,
Turn on the meter.
Set the rotational function se-
lector at RE2p position.

OPERATING MANUAL MRU-20 version 3.1
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UsingUnbutton
set test voltage
on 25V or 50V.
After 5 sec. the
meter returns
to the measu-
rement mode.
You can also
confirm the
choice by
pressing the
START push-
button.
E H 2,2m
1,2m
Connect the measured
object to terminalsE
and Hof the meter.
The meter is ready for
measurement.
Value of interference
voltage is displayed
on the auxiliary di-
splay.
Press START button.
Carry out the measurement.
Value of interference vol-
tage.
Read out test results:
The value of measured
resistance.
Note:
- Calibration performed by the manufacturer takes into account the resistance of supplied measure-
ment leads of 1.2 m and 2.2 m

OPERATING MANUAL MRU-20 version 3.1
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Additional informations displayed by the meter
and
Interference voltage is too high (> 24V) –the measure-
ment is impossible.
Turn off the source of interferences.
and
and continuous audio
signal 
Interference voltage is above 50V!
Immediately disconnect the meter!
Before re-connecting, turn off the source of voltage.
and
and continuous audio
signal 
Interference voltage is above 100V!
Immediately disconnect the meter. (symbol displayed in-
stead of voltage value).
Before re-connecting, turn off the source of voltage.
and
Interruption in measuring circuit.
>1,99kΩ
REmeasuring range is exceeded.
Interference voltage is above 10V or unstable measure-
ment result or measured voltages or currents are low in re-
lation to noise.
and and
long audio signal

Measured voltages or currents are too low in relation to no-
ise or very unstable measurement result. (Symbol is
displayed instead of the result.)
and
Acceptable temperature inside the meter is exceeded.
2.3 Measurement of the resistance of protective conductors and
equipotential bondings
Attention:
When carrying out measurements of very low resistance, or when using leads other
than original (1.2 m and 2.2 m) the calibration of test leads must be performed.
Turn on the meter.
Set the rotational function se-
lector at RCONT 200mA posi-
tion.

OPERATING MANUAL MRU-20 version 3.1
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E S
2,2m
1,2m
Connect the
measured object
to terminals Sand
Eof the meter.
The meter is ready
for measurement.
Press START button.
Carry out the measurement.
Read out the result.
Note:
- test current flows unidirectionally. To obtain results of a bidirectional current flow, the measurement
should be repeated with replaced test leads and then respective arithmetic mean should be calculated
from both measurements.

OPERATING MANUAL MRU-20 version 3.1
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Additional informations displayed by the meter
and
Interference voltage is too high (> 3Vrms) –the measure-
ment is impossible.
Turn off the source of interferences.
and
and continuous audio
signal 
Interference voltage is above 50V!
Immediately disconnect the meter!
Before re-connecting, turn off the source of voltage.
> 199Ω
RCONT measuring range is exceeded.
Interference voltage with value of 1..3Vrms during RCONT
measurement. Obtained test results may be burdened with
additional uncertainty.
and and
long audio signal

Very unstable measurement result.
2.4 Calibration of test leads
In order to eliminate the impact of the resistance of test leads on measurement result, the com-
pensation (autozeroing) of resistance may be performed. Therefore measurement function RCONT has
its sub-function called AUTOZERO.
2.4.1 Activation of Autozeroing
Turn on the meter.
Set the rotational function se-
lector at RCONT ZERO posi-
tion.
E S
Short the test leads by
putting crocodile clips
on exposed ends of the
test leads.
Press START button.
Perform Autozeroing.

OPERATING MANUAL MRU-20 version 3.1
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Autozeroing
performed.
Note:
- remember that the resistance of crocodile clips and crocodile-banana connections is added to the
resistance of the test leads.
2.4.2 Switching-off Autozeroing
Turn on the meter.
Set the rotational function se-
lector at RCONT ZERO posi-
tion.
ES
Open out the test leads.
Press START button.
Autozeroing was turned
off by the user.
During measurements
the meter will compensa-
te the resistance of origi-
nal test leads (1.2m and
2,2m.)

OPERATING MANUAL MRU-20 version 3.1
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Attention:
It is sufficient to perform single compensation for the test leads. It is stored in
memory even after turning off the meter.

OPERATING MANUAL MRU-20 version 3.1
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3 Power supply of the meter
3.1 Monitoring of the power supply voltage
The level of the charge of the batteries or rechargeable batteries is currently indicated by the
symbol in the right upper corner of the display:
Batteries (or rechargeable batteries) charged
Batteries (or rechargeable batteries) low.
Batteries (or rechargeable batteries) need charging!
Note:
The symbol displayed on the display indicates insufficient power supply voltage and the need
to charge or replace batteries.
Measurements carried out with insufficient meter power supply voltage are distorted with additio-
nal errors which are impossible to ascertain by the user and thus they cannot constitute a basis
for a conclusion of correctness of the tested earthing.
3.2 Replacement of batteries (rechargeable batteries)
MRU-20 Meter is powered by eight batteries or rechargeable batteries R6 (it is recommended to
use alkaline batteries). Batteries (rechargeable batteries) are inserted into a container in the bottom
part of the casing.
WARNING:
Before replacing the batteries (or rechargeable batteries), disconnect the test
leads from the meter.
In order to replace batteries or the set of rechargeable batteries it is necessary to do the following:
1. Remove all the test leads from the measurement circuit and turn the meter off,
2. Remove the battery compartment (in the bottom of the case) after loosening three screws,
3. Replace all batteries (rechargeable batteries). New batteries or rechargeable batteries must be
inserted observing the correct polarity ("-" on the springy part of the contact plate). Incorrect posi-
tion of batteries will not damage the meter or batteries, but the meter with incorrectly inserted bat-
teries will not work.
4. Insert the battery compartment and fix it with screws.