METREL MI 3250 User manual

MicroOhm 10A
MI 3250
Instruction manual
Version 1.1.2, Code no 20 751 893

2
Distributor:
Manufacturer:
METREL d.d.
Ljubljanska cesta 77
1354 Horjul
Slovenia
web site: http://www.metrel.si
e-mail: me[email protected]
Mark on your equipment certifies that this equipment meets the requirements of the EU
(European Union) concerning safety and electromagnetic compatibility regulations
© 2011 METREL
No part of this publication may be reproduced or utilized in any form or by any means
without permission in writing from METREL.

MI 3250 MicroOhm 10A Table of contents
3
Table of contents
1General description............................................................................................. 5
1.1 Features........................................................................................................ 5
2Safety and operational considerations .............................................................. 6
2.1 Warnings and notes....................................................................................... 6
2.2 Battery and charging ..................................................................................... 8
2.2.1 New battery cells or cells unused for a longer period.............................. 9
2.3 Standards applied........................................................................................ 10
3Instrument Description...................................................................................... 11
3.1 Operator’s Panel.......................................................................................... 11
3.2 Connectors and Battery cover ..................................................................... 12
3.2.1 Test leads connector ............................................................................ 12
3.2.2 Connector panel (right side) ................................................................. 13
3.2.3 Connector panel (left side).................................................................... 14
3.3 Accessories................................................................................................. 15
3.4 Display organization .................................................................................... 16
3.4.1 Measurement result window................................................................. 16
3.4.2 Measurement control window............................................................... 17
3.4.3 Message window.................................................................................. 17
3.4.4 Battery and time indication ................................................................... 18
3.4.5 Backlight operation.............................................................................. 18
4Main menu.......................................................................................................... 19
4.1 Instrument Main menu................................................................................. 19
4.2 Memory Menu.............................................................................................. 20
4.2.1 Saving results....................................................................................... 20
4.2.2 Recalling results................................................................................... 21
4.2.3 Deleting results..................................................................................... 22
4.2.4 Clearing complete memory content ...................................................... 22
4.3 Settings menu ............................................................................................. 23
4.3.1 Language selection .............................................................................. 23
4.3.2 Communication selection...................................................................... 23
4.3.3 Set Date and Time................................................................................ 24
4.3.4 Set Contrast ......................................................................................... 24
4.3.5 Temperature Compensation................................................................. 25
4.3.6 Set Limits.............................................................................................. 27
4.3.7 Instrument info...................................................................................... 27
4.4 Help Menu................................................................................................... 28
5Measurement...................................................................................................... 29
5.1 Four wire Kelvin method.............................................................................. 29
5.2 Resistance measurement............................................................................ 30
5.2.1 Single Mode ......................................................................................... 31
5.2.2 Continuous Mode ................................................................................. 31
5.2.3 Automatic Mode ................................................................................... 32
5.2.4 Inductive Mode..................................................................................... 33
6Communication.................................................................................................. 34
7Maintenance....................................................................................................... 35
7.1 Cleaning...................................................................................................... 35

MI 3250 MicroOhm 10A Table of contents
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7.2 Periodic calibration ...................................................................................... 35
7.3 Service ........................................................................................................ 35
8Technical specifications ................................................................................... 36
8.1 Resistance measurement............................................................................ 36
8.2 Measurement parameters............................................................................ 36
8.3 General data................................................................................................ 37

MI 3250 MicroOhm 10A General description
5
1 General description
1.1 Features
MicroOhm 10A (MI 3250) is a portable (2.8 kg) bidirectional low resistance ohmmeter
using four wire Kelvin method to measure low resistances of:
Switches
Relays
Connectors
Bus bars
Power distribution cable joints
Motor & generator winding
Power transformers
Power inductors
Rail track joints
Wire and cable resistance
Welding joints.
Instrument can be powered from mains or internal rechargeable batteries.
It is designed and produced with the extensive knowledge and experience acquired
through many years of of working in this field.
Available functions and features offered by the MicroOhm 10A meter:
Resistance measurement (Four wire Kelvin method);
High Resolution measurement (24-Bit Σ-Δ ADC);
Auto or Manual Range
Wide measuring range (0.1 μΩ ... 2 kΩ);
Adjustable test current (1 mA ...10 A);
Temperature compensation;
Hi/Lo limits;
Automatic thermal EMF elimination;
Four different measuring modes (single, continuous, inductive, automatic);
USB and RS232 communication;
High overvoltage category CAT IV / 300 V.
A 320x240 dot matrix LCD offers easy-to-read results and all associated parameters.
The operation is straightforward and clear to enable the user to operate the instrument
without the need for special training (except reading and understanding this Instruction
Manual).
Test results can be stored on the instrument. PC software HVLink PRO that is supplied
as a part of standard set enables transfer of measured results to PC where can be
analysed or printed.

MI 3250 MicroOhm 10A Safety and operational considerations
6
2 Safety and operational considerations
2.1 Warnings and notes
In order to maintain the highest level of operator safety while carrying out various tests
and measurements Metrel recommends keeping your MicroOhm 10A instruments in
good condition and undamaged. When using the instrument, consider the following
general warnings:
The symbol on the instrument means »Read the Instruction manual with
special care for safe operation«. The symbol requires an action!
If the test equipment is used in a manner not specified in this user manual,
the protection provided by the equipment could be impaired!
Read this user manual carefully, otherwise the use of the instrument may
be dangerous for the operator, the instrument or for the equipment under
test!
Do not use the instrument or any of the accessories if any damage is
noticed!
Consider all generally known precautions in order to avoid risk of electric
shock while dealing with hazardous voltages!
Do not use the instrument in supply systems with voltages higher than
300 V!
Service intervention or adjustment is only allowed to be carried out by
competent authorized personnel!
Use only standard or optional test accessories supplied by your distributor!
Consider that older accessories and some of the new optional test
accessories compatible with this instrument only meet CAT II / 300 V over
voltage safety rating! This means that maximal allowed voltage between
test terminals and ground is 300 V!
The instrument comes supplied with rechargeable Ni-Cd or Ni-MH battery
cells. The cells should only be replaced with the same type as defined on
the battery compartment label or as described in this manual. Do not use
standard alkaline battery cells while the power supply adapter is connected,
otherwise they may explode!
Hazardous voltages exist inside the instrument. Disconnect all test leads,
remove the power supply cable and switch off the instrument before
opening the battery compartment.
All normal safety precautions must be taken in order to avoid risk of electric
shock while working on electrical installations!

MI 3250 MicroOhm 10A Safety and operational considerations
7
Warnings related to measurement functions:
Resistance and inductive measurement
Resistance measurement should only be performed on de-energized objects!
Do not touch the test object during the measurement or before it is fully
discharged! Risk of electric shock!
When a resistance measurement has been performed on an inductive object,
automatic discharge may not be done immediately!
High voltage can appear on test terminals when measuring inductive object!
Do not connect test terminals to external voltage higher than 300 V (AC or DC) in
order not to damage the test instrument!
General
Warning will appear on screen and the resistance test will not be performed
if voltages higher than 8 V (AC or DC) are detected between test terminals. There
will be no warning if all terminals are at the same potential.
PASS / FAIL indication is enabled when limit is set. Apply appropriate limit value
for evaluation of measurement results.

MI 3250 MicroOhm 10A Safety and operational considerations
8
2.2 Battery and charging
The instrument uses six alkaline or rechargeable Ni-Cd or Ni-MH battery cells. Nominal
operating time is declared for cells with nominal capacity of 3500 mAh.
Battery condition is always displayed in the upper right display part.
In case the battery is too weak the instrument indicates this as shown in figure 2.1.
Figure 2.1: Discharged battery icon
The battery is charged whenever the power supply is connected to the instrument. The
power supply socket is shown in figure 2.2.Internal circuit controls charging and assures
maximum battery lifetime.
Figure 2.2: Power supply socket
The instrument automatically recognizes the connected power supply and begins
charging.
Symbols:
Indication of battery charging
Figure 2.3: Charging indication
When connected to an installation, the instruments battery compartment can
contain hazardous voltage inside! When replacing battery cells or before opening
the battery compartment cover, disconnect any measuring accessory connected
to the instrument and turn off the instrument,
Ensure that the battery cells are inserted correctly otherwise the instrument will
not operate and the batteries could be discharged.
If the instrument is not to be used for a long period of time, remove all batteries
from the battery compartment.
Alkaline or rechargeable Ni-Cd or Ni-MH batteries can be used. Metrel
recommends only using rechargeable batteries with a capacity of 3500mAh or
above.
Do not recharge alkaline battery cells!

MI 3250 MicroOhm 10A Safety and operational considerations
9
2.2.1 New battery cells or cells unused for a longer period
Unpredictable chemical processes can occur during the charging of new battery cells or
cells that have been left unused for a longer period (more than 3 months). Ni-MH and Ni-
Cd cells can be subjected to these chemical effects (sometimes called the memory
effect). As a result the instrument operation time can be significantly reduced during the
initial charging/discharging cycles of the batteries.
In this situation, Metrel recommend the following procedure to improve the battery
lifetime:
Procedure
Notes
Completely charge the battery.
At least 6h (3500mAh) with in-built charger.
Completely discharge the battery.
This can be performed by using the
instrument normally until the instrument is
fully discharged.
Repeat the charge / discharge cycle at
least 2-4 times.
Four cycles are recommended in order to
restore the batteries to their normal
capacity.
Notes:
The charger in the instrument is a pack cell charger. This means that the battery
cells are connected in series during the charging. The battery cells have to be
equivalent (same charge condition, same type and age).
One different battery cell can cause an improper charging and incorrect
discharging during normal usage of the entire battery pack (it results in heating of
the battery pack, significantly decreased operation time, reversed polarity of
defective cell, …).
If no improvement is achieved after several charge / discharge cycles, then each
battery cell should be checked (by comparing battery voltages, testing them in a
cell charger, etc). It is very likely that only some of the battery cells are
deteriorated.
The effects described above should not be confused with the normal decrease of
battery capacity over time. Battery also loses some capacity when it is repeatedly
charged / discharged. Actual decreasing of capacity, versus number of charging
cycles, depends on battery type. This information is provided in the technical
specification from battery manufacturer.

MI 3250 MicroOhm 10A Safety and operational considerations
10
2.3 Standards applied
The MicroOhm 10A instrument is manufactured and tested in accordance with the
following regulations:
Electromagnetic compatibility (EMC)
EN 61326
Electrical equipment for measurement, control and laboratory
use –EMC requirements
Class A
Safety (LVD)
EN 61010-1
Safety requirements for electrical equipment for measurement,
control and laboratory use –Part 1: General requirements
EN 61010-031
Safety requirements for hand-held probe assemblies for
electrical measurement and test
Note:
Immunity to radiated RF fields (Field strength: 10V/m, Modulation: AM, 80%, 1 kHz)
Current
Range
Operational
conditions
Disturbance > 0,25 %
Disturbance < 0,25 %
1mA
Range 2kΩ
100 MHz ÷ 500 MHz
500 MHz ÷ 1 GHz
Note about EN and IEC standards:
Text of this manual contains references to European standards. All standards of
EN 6XXXX (e.g. EN 61010) series are equivalent to IEC standards with the same
number (e.g. IEC 61010) and differ only in amended parts required by European
harmonization procedure.

MI 3250 MicroOhm 10A Safety and operational considerations
11
3 Instrument Description
3.1 Operator’s Panel
The operator’s panel is shown in Figure 3.1 below.
Figure 3.1: Front panel
Legend:
1
START / STOP
Start or stop measurement.
2
ON / OFF
Switches the instrument power on or off.
The instrument automatically turns off 15 minutes after the last
key was pressed.
3
MEM
Store / recall / clear tests in memory of instrument.
4
SELECT
To enter set-up mode for the selected function or to select the active
parameter to be set.
5, 6

Select an option upward, downward.
7, 8

Decrease, increase the selected parameter.
9
ESC
Exit the selected mode.
10
LIGHT
Turn the display backlight ON or OFF.
Instrument’s RESET (hold key for 3 s or more).

MI 3250 MicroOhm 10A Instrument Description
12
3.2 Connectors and Battery cover
The MicroOhm 10A tester contains the following terminals:
Four banana safety sockets, for connection of test leads (Figure 3.2),
Mains socket, for connection of mains supply cable (Figure 3.3),
Communication terminals (USB and RS232) (Figure 3.3),
Battery cover (Figure 3.4).
3.2.1 Test leads connector
Figure 3.2: Test leads connector
Legend:
1
C1
Current terminals
Measuring inputs / outputs
2
C2
3
P1
Voltage terminals
4
P2
Warnings!
Maximum allowed voltage between any test terminal and ground is 300 V!
Maximum allowed voltage between test terminals is 300 V!
Use original test accessories only!

MI 3250 MicroOhm 10A Instrument description
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3.2.2 Connector panel (right side)
Figure 3.3: Communication and mains connectors
Legend:
1
RS232 connector
Communication with PC RS232 port.
Communication with printer.
2
USB connector
Communication with PC USB (1.1) port.
3
Mains connector
Mains supply of the instrument and battery charger supply
Warnings!
Maximum allowed voltage between L-N (Mains connector) is 300 V (CAT II)!
Use original test accessories only!

MI 3250 MicroOhm 10A Instrument description
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3.2.3 Connector panel (left side)
Figure 3.4: Measuring inputs/outputs and battery compartment
Legend:
1
Measuring inputs / outputs.
2
Battery compartment.
Figure 3.5: Correct inserted batteries
Warnings!
When connected to an installation, the instruments battery compartment
can contain hazardous voltage inside!
Disconnect all test leads, remove the power supply cable and switch off
the instrument before opening the battery compartment.
Ensure batteries are used and disposed of in accordance with
Manufacturers guidelines and in accordance with Local and National
Authority guidelines.

MI 3250 MicroOhm 10A Instrument description
15
3.3 Accessories
The accessories consist of standard and optional accessories. Optional accessories can
be delivered upon request. See attached list for standard configuration and options or
contact your distributor or see the METREL home page: http://www.metrel.si.
Figure 3.6: Standard set of the instrument
Instrument MI 3250 MicroOhm 10A
Current test leads with crocodile clip, 2,5 m, 2,5 mm2, 2 pcs (Kelvin)
Current test leads 2,5 m, 2,5 mm2, 2 pcs (red)
Potencial test leads 2,5 m, 1,5 mm2, 2 pcs (black)
Crocodile clips, 4 pcs (black, red)
Test probes, 2 pcs (black)
Mains cable
NiMH rechargeable batteries, 3500mAh, 6 pcs
RS232 serial cable
USB cable
Bag for accessories
PC SW HVLink PRO
Instruction manual
Calibration certificate

MI 3250 MicroOhm 10A Instrument description
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3.4 Display organization
Figure 3.7: Typical function display
1
Measurement result window
2
Measurement control window
3
Messages window
4
Battery and time indication
3.4.1 Measurement result window
Measurement window shows all relevant data during measurement campaign.
Figure 3.8: Measurement window
Measured resistance is shown in the center of a screen with the largest font. During
measurement campaign this result is refreshed every few seconds. After finishing
measurement result is hold on screen, until new measurement is started.
Bar graph graphically represents measured resistance in respect to the measurement
range.
R+ shows resistance in the positive direction. During measurement campaign this result
is refreshed every few seconds. After finishing measurement result is hold on screen,
until new measurement is started.
R- shows resistance in the negative direction. During measurement campaign this result
is refreshed every few seconds. After finishing measurement result is hold on screen,
until new measurement is started.
Ix shows current flow through the measured resistance. During measurement campaign
this result is refreshed every few seconds. After finishing measurement result is hold on
screen, until new measurement is started.
Amb shows the value of the ambient temperature that was measured with the
temperature probe or was entered manually (appears when the temperature
compensation is turned on).
Ref@ shows the value of the reference temperature that was entered manually
(appears when the temperature compensation is turned on).
Dis: shows the discharging time (appears only in inductive mode).

MI 3250 MicroOhm 10A Instrument description
17
3.4.2 Measurement control window
Control window permit user to modify control measurement parameters.
Figure 3.9: Control window
Mode allows user to select desired measuring mode. It is possible to select one of
following modes: Single, Automatic, Continuous and Inductive. See chapter 5.2 for more
details.
Auto Range allows user to select or deselect automatic range selection.
Range allows user to select desired measuring range. It is possible to select one of
following ranges: 2 mΩ, 20 mΩ, 200 mΩ, 2 Ω, 20 Ω, 200 Ω, 2 kΩ.
Current allows user to select proper current for resistance measurement. It is possible
to select one of following currents: 1 mA, 10 mA, 100 mA, 1 A, 10 A.
Notes:
Auto Range is always deselected in Inductive mode.
Resistance range depends on selected current.
Example: with selected 1 A current, only 20 mΩ, 200 mΩ and 2 Ω ranges can be
selected. See table 8.1 for more details.
3.4.3 Message window
In the message field warnings and messages are displayed.
No batteries or wrong polarity of inserted batteries.
Battery charging in progress.
High voltage is present on measuring terminals.
Measurement in progress.

MI 3250 MicroOhm 10A Instrument Description
18
Instrument is overheated. Measurement process is disabled.
Battery supply voltage is low.
P1, P2, C1 or C2 terminal is not connected to the instrument or
too high resistance is detected.
Measurement current and result is within defined limits.
Measurement current is out of defined limits.
Measurement result is out of defined high limit.
Measurement result is out of defined low limit.
3.4.4 Battery and time indication
The indication indicates the charge condition of battery and charger connection.
Battery capacity indication.
Low battery.
Battery is too weak to guarantee correct result. Replace or
recharge the battery cells.
Recharging in progress (if mains supply cable is connected).
Time indication (hh:mm).
Note:
Date and time is attached to each stored result.
3.4.5 Backlight operation
After turning the instrument ON the LCD backlight is automatically turned ON. It can be
turned OFF and ON by simply clicking the LIGHT (☼) key.
Note:
If you press and hold the Light (☼) key for approximately 5 s the instrument will
RESET!

MI 3250 MicroOhm 10A Main menu
19
4 Main menu
4.1 Instrument Main menu
From the main menu of the instrument there are four options available: Measurement,
Memory Menu, Settings Menu and Help Menu.
Figure 4.1: Instrument Main menu
Keys:

Select one of the following menu items:
<Measurement> See chapter 5.2;
<Memory Menu> Memory management, see chapter 4.2;
<Settings Menu> Setup of the instrument, see chapter 4.3;
<Help Menu> Help screens, see chapter 4.4;
SELECT
Confirms selection.

MI 3250 MicroOhm 10A Main menu
20
4.2 Memory Menu
Measurement result with all relevant parameters can be stored into the instrument’s
memory.
The instrument’s memory space is divided into 2 levels: Object and Number of Results.
The upper Object level can contain up to 199 locations. The number of measurements
stored under single object location is not limited.
Figure 4.2: Memory Menu
4.2.1 Saving results
After the completion of a test, the results and parameters are ready for storing. By
pressing the MEM key the user can enter storing menu.
Figure 4.3: Storing Menu
Keys:

Selects Object number.
MEM
Saves measurement result to selected object number and returns to the
Measurement result screen.
ESC
Returns to the Measurement result screen without save.
The instrument will beep in order to indicate that result is successfully saved into the
memory.
Note:
Every stored test result also includes date and time stamp (dd:mm:yyyy, hh:mm).