14
GMC-I Messtechnik GmbH
automatically adjusted, the measuring ranges should not
be changed manually.
The DATA function is deactivated if the
DATA|CLEAR
key is
pressed and held (approx. 1 s) or by switching the instru-
ment off and back on again.
7
Milliohm Measurement (4-Pole-Measurement)
7.1
Compensation of Cable Resistance
Electrical resistance is a dipole quantity which can
generally only be measured using two poles. This is
accomplished by directing a measuring current of
predetermined magnitude through the device under test,
and measuring the resultant voltage drop. The respective
resistance value is derived from the quotient of these two
values.
The two points between which voltage is measured are
decisive as regards the results of the measurement. All
resistances between these two points add to the
measured resistance value. These include contact
resistance, as well as cable resistance. If a very low
resistance value needs to be measured, for example
contact resistance at a contactor with a value of only a few
milliohms, the points between which voltage is measured
must be moved out of the measuring instrument and
positioned as closely as possible to the device under test.
For this reason, the measuring instrument is equipped with
separate jacks for current feed and voltage measurement.
This type of 4-pole connection is known as connection
according to Kelvin.
KC4 Kelvin clips and KC27 Kelvin probes (available as
accessories) allow for easy, correct connection.
7.1.1
Measurement with Kelvin Probe KC27
m
Ω
U+
U–
I+
red
black
black
Milliohm measurement on
conductor bars, welds,
coils, etc. with Kelvin probe
KC27 and Kelvin clip KC4
Direct the spring-loaded test
probe inward towards the DUT,
if possible, as it is connected
via the red plug with U
+
or U
–
(Sense) .
I–