MI-10KV
ELECTRONIC
MEGOHMMETER
• TECHNICAL SPECIFICATION
• OPERATING INSTRUCTIONS
WARNING
Hazardous voltages exist on the terminals during operation.
Although output current of the megohmmeter is limited,
external capacities are charged to very high potentials and there
is no protection against them. So, this equipment should only be
used by a suitably trained and competent person, strictly
applying corresponding security standards.
M I- 10KV
DESCRIPTION
The CIRCUTOR MI-10KV electronic megohmmeter is a versatile,
portable, user friendly, rugged instrument. It uses an efficient, well
experienced technology providing reliable, safe and accurate insulation
resistance measurements up to 2.000.000MΩ, with four test voltages:
1.000, 2.000, 5.000 and 10.000 V.
Due to its compact size and reduced weight, mechanical strength, selfcontained battery supply, this apparatus is particularly suitable for field
tests under severe environments. It is easily to be carried, very simple
to be operated and stands severe handling conditions including
frequent shocks, extreme temperatures, vibrations during transportation
through hard roads, long direct exposure to solar radiation, dust, sand
and other air-borne impurities, etc. Accuracy is not affected by all these
adverse conditions and it is still comparable with that of the best
laboratory instruments.
IMMUNITY AGAINST ELECTRIC AND MAGNETIC FIELDS
CIRCUTOR megohmmeters feature an effective filtering system that
improves immunity against electromagnetic disturbances and prevents
variations induced by industrial frequency fields.
OPERATOR’S SAFETY
Due to the high voltages involved, operator’s safety is a must.
CIRCUTOR megohmmeters were designed considering this
outstanding aspect:
• ENCLOSURE: Molded in high dielectric strength plastic. There are no
conductive parts accessible to the operator with the exception of the
output terminals. This terminals are located in a hidden and protected
position on the enclosure.
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M I- 10KV
• HIGH-VOLTAGE INDICATOR LIGHT: An indicator light (LED) is
warning the presence of high voltage at the output terminal during a
measurement and remains lit until the discharge process is completed.
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TECHNICAL SPECIFICATIONS
MEASURING INTERVALS, TEST VOLTAGES
M I- 10KV
Test
Voltage
(volts)
NKMMM MJ=OM =OM=J=QMM OMM=J=OMKMMM OKMMM=J=OMMKMMM =ñ=O
OKMMM MJ=QM =QM=J=UMM QMM=J=QMKMMM QKMMM=J=QMMKMMM ñ=R
RKMMM MJNMM NMM=J=OKMMM NKMMM=JNMMKMMM NMKMMM=JNKMMMKMMM ñ=NM
NMKMMM MJOMM OMM=J=QKMMM OKMMM=JOMMKMMM OMKMMM=J=OKMMMKMMM =u=OM
^ _ ` `=ñ=NM
MEASURING INTERVALS (MΩ)
Scale
Multiplier
Output.
Resistance
=jΩ
==O
K
jΩ
Q=
=jΩ
NM=
jΩ
OM==
TEST VOLTAGE STABILITY
1 % during full battery service life.
SHORT-CIRCUIT CURRENT
500 µ A.
CLASS
Class 2 (related scale size) @ 30 º C.
BATTERY TEST
Allows checking battery status under real consumption conditions
without interrupting the generation of test voltages.
GUARD TERMINAL
Allows the measurement of very high resistance values, avoiding the
effect of stray resistances.
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M I- 10KV
CARRYING CASE
A strong synthetic leather carrying case provides easy transportation for
the instrument and attachments. The megohmmeter may operate
without being removed from the carrying case.
DIMENSIONS AND WEIGHT
290 x 155 x130 mm (outside dimensions without carrying case)
8 kg. (including apparatus, carrying case, battery and test probes)
ENVIRONMENTAL OPERATING CONDITIONS
This apparatus is designed to be used in outdoors field work under very
unfavorable weather conditions. Room temperature may vary between
0°C and +50°C, with relative humidity close to saturation point without
affecting the instrument operation.
ATTACHMENTS SUPPLIED
Every megohmmeter is supplied with a full set of test leads,
rechargeable battery, charger, carrying and operating manual.
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KEYBOARD
When pressed, the battery test is performed under
actual consumption conditions
Low resistance range. Read values at scaleA
Read values at scaleB
Read values at scaleC
M I- 10KV
Guard
Terminal
Higher resistance range. Read values at scale,
then multiply it by 10.
ON/OFF Switch
C
Return
Terminal
High voltage Terminals
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M I- 10KV
OPERATING INSTRUCTIONS
1) Be sure that there are no voltage differences between the points at
which the megohmmeter will be connected, nor between them and
ground.
2) Determine the value of test voltage to be used in the insulation
resistance measurement.
3) Connect the red banana pin of the red cable to the 1.000, 2.000,
5.000 or 10.000 V terminal in accordance with the selected test voltage.
4) Connect the black cable to the -R megohmmeter terminal and the
yellow terminal to terminal G.
5) The green GUARD terminal is not always used. Technical Note #32
explains the use of GUARD terminal in order to minimizing the effect of
stray resistances. When measurement is carried out between parts
which none of them is grounded, (like between high-side and low-side
windings of a transformer), GUARD terminal must be connected to
ground in order to fix the apparatus potential. At any time a
measurement is performed, either the “-R " or GUARD terminals
must be connected to ground but never both simultaneously. If
none of these terminals are connected to ground, the megohmmeter
can reach a high potential that may result in an unstable non-reliable
reading. If both terminals are simultaneously connected to ground,
there is a short-circuit between them and consequently the
megohmmeter will measure with error.
6) Connect the free ends of cables (alligator clips) to the element to be
measured.
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M I- 10KV
7) Before turning on the apparatus, be sure that the key corresponding
to A scale is pressed. Turn on the apparatus by pressing the ON/OFF
key. Then the high-voltage generator starts operating and the
corresponding indication light turns on at the front panel. The meter
pointer will indicate the value of the unknown resistance. If the element
to be measured is strongly capacitive it will initially indicate a low
resistance value, which will be gradually increased while the charging of
that capacitance takes place.
8) When the measured resistance exceeds the maximum value in range
A, press range B key, and if still the value is not achieved, press keys of
ranges C or Cx10, as required.
9) Always remember to multiply the reading by the factor stated in the
following table, depending on selected test voltage:
1.000 V x 2
2.000 V x 4
5.000 V x 10
10.000 V x 20
10) When key C x 10 is used, reading shall be carried out in range C
and shall be multiplied by 10, in addition to the factor corresponding to
the test voltage.
11) To finish measurement press again and release ON/OFF key. The
megohmmeter will start discharging the potentials accumulated in the
apparatus internal capacitances and in those of the element under test
as well. When this discharging process is over (up to 60 seconds after
turn off) the high-voltage indicator will turn off automatically. Then, test
leads may be disconnected.
12) In certain instances, when the apparatus is disconnected the pointer
exceeds the infinite position to the right side. This is a normal behavior.
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M I- 10KV
13) Checking battery status. Battery measurement can be performed
without interrupting high-voltage generation, which will provide a better
evaluation of the battery status, by pressing the BATTERY key during
the measurement. So, the battery test is performed under actual
consumption conditions and, for long lasting measurements, (i.e.
Polarization Index), the evolution of battery status can be checked
without affecting the measurement.
The meter pointer should stop over the blue zone. If the pointer stop
over the red zone this means that the battery is discharged and shall be
charged
14) Infinite setting. The mechanical zero of galvanometer must be
periodically checked. In order perform this checking, be sure that the
megohmmeter is off. The pointer should stay on the right end of the
scale just over the infinite mark on scale C. In other case, the plastic
screw at the bottom of the galvanometer acrylic cover shall be adjusted.
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M I- 10KV
TECHNICAL NOTE # 32
USE OF GUARD TERMINAL IN MEGOHMMETERS
When insulation resistance measurements are performed
with megohmmeters, specially with high-sensitivity
instruments measuring high resistance values, the use of
the GUARD terminal avoids the harmful influence of stray
resistances.
In order to better explain the function of this terminal, let us
start reviewing the megohmmeter basic circuit diagram of
Fig. 1.
Vt
Where:
Ri
-R
+V
I
A
V : DC high-voltage generator
Ri: Generator internal resistance
A : Indicator meter (microammeter)
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Rx
Guard
Fig. 1
M I- 10KV
The unknown resistance (Rx) is connected between Vt and
R terminals. Its value determines the current passing
through the circuit, which in turn is indicated by the
microammeter. The value of Rx can be determined as
follows:
V
Rx = - Ri
I
In many cases the resistance to be measured is in parallel with other
stray resistances which influence on Rx should be minimized.
A typical example of this situation is when the insulation resistance
between primary and secondary windings of a transformer mounted
inside a metal housing is to be measured.
Rx: Insulation resistance between primary and secondary
winding.
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M I- 10KV
R1: Insulation resistance between primary winding and
housing.
R2: Insulation resistance between secondary winding and
housing.
If megohmmeter (terminals Vt and R) is connected to
transformer terminals A and B, and considering that the
resistance of the coils on each side of the transformer may
be disregarded, Rx appears to be in parallel with (R1 + R2).
The situation is changed if we connect the transformer
housing to GUARD terminal.
Then the circuit will be:
R
R
i
V
R
I
A
x
A
R
1
R
2
G
FIG. 3
In the circuit of Fig. 3 it may be noted that R1 is in parallel
with a low-value resistance (the one of the microammeter)
therefore its influence is minimized during reading.
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