Manual

46 Gossen Metrawatt GmbH
8.14 Milliohm Measurement – m/4 (4-wire measurement)
8.14.1 Compensation of Cable Resistance
Electrical resistance is a dipole which can generally only be mea-
sured using two poles. This is accomplished by directing a mea-
suring current of predetermined magnitude through the device
under test and measuring the resultant voltage drop. The respec-
tive resistance value is derived from the quotient of these two val-
ues.
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 mil-
liohms, 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 measur-
ing 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 accesso-
ries) allow for simple, correct connection.
Measurement with the KC27 Kelvin Probe
Measurement with KCV100 Cable Reel for 4-Wire Measurement
with 200 mA
The KCV100 is a 100-meter, 2-pole extension cord which permits
4-wire 200 mA measurement at large objects, for example bond-
ing tests, testing of lightning protection and wick tests at large
objects such as wind power turbine blades and lattice towers.
A Kelvin measuring device is also required, for example the KC27
Kelvin probe.
S– A
V
Tem p
m/4
COIL
S+
red
black
Milliohm measurements at busbars, weld-
ing seems, coils with up to roughly 1 H
etc. with the KC27 Kelvin Probe
(or KC4 Kelvin clip)
Insofar as possible, tips which are not spring-
loaded should be aligned inwardly to the
device under test because they’re connected
via the red plugs labeled S
+
and S
–
(sense).
red
black
S– A
V
Tem p
m/4
COIL
S+
Measuring Instrument in Direct Proximity to the Inspector
The measuring instrument, including the part of the Kelvin measuring device con-
nected to it, is in direct proximity to the inspector. The cable reel, and the second part
of the Kelvin measuring device which is connected to it, are located at a distance from
the measuring site.
Example: Aircraft wings and fuselage.
black
red
black
red
red
black
2
1