Manual

78 Gossen Metrawatt GmbH
17.1 Measurement with Constant Test Current
Start Measurement
Press and hold for long-
term measurement
Attention!
!
The test probes should always be in contact with the device
under test before the Start key
▼ is activated.
If the DUT is energized, measurement is disable as soon as it’s
contacted with the test probes.
If the start key
▼ is pressed first and the DUT is contacted with
the test probes afterwards, the fuse blows.
Which of the two fuses has blown is indicated in the pop-up
window with the error message by means of an arrow.
In the case of single-pole measurement, the respective value is
saved to the database as R
LO
.
Automatic Polarity Reversal
After the measuring sequence has been started, the instrument
performs the measurement with automatic polarity reversal, first
with current flow in one direction, and then in the other. In the
case of long-term measurement (press and hold ON/START ▼ key),
polarity is switched once per second.
If the difference between RLO+ and RLO– is greater than 10%
with automatic polarity reversal, RLO+ and RLO– values are dis-
played instead of RLO. The respectively larger value, RLO+ or
RLO–, appears at the top and is saved to the database as the
RLO value.
Evaluating Measurement Results
Differing results for measurements in both directions indicate volt-
age at the DUT (e.g. thermovoltages or unit voltages).
Measurement results can be distorted by parallel connected
impedances in load current circuits and by equalizing current,
especially in systems which make use of overcurrent protection
devices (previous neutralization) without an isolated protective
conductor. Resistances which change during measurement (e.g.
inductance), or a defective contact, can also cause distorted
measurements (double display).
In order to assure unambiguous measurement results, causes of
error must be located and eliminated.
In order to find the cause of the measuring error, measure resis-
tance in both current flow directions.
The instrument’s batteries are exposed to excessive stress during
insulation resistance measurement. For measurement with cur-
rent flow in one direction, only press and hold the ON/START ▼ key
as long as necessary for the measurement.
Note
Measuring Low-Value Resistance
Measurement cable and 2-pole measuring adapter resis-
tance is compensated automatically thanks to the 4-wire
method and thus doesn’t effect measurement results.
However, if an extension cord is used its resistance must
be measured and deducted from the measurement
results.
Resistances which do not demonstrate a stable value
until after a “settling in period” should not be measured
with automatic polarity reversal, but rather one after the
other with positive and negative polarity.
Examples of resistances whose values may change
during measurement include:
– Incandescent lamp resistance, whose values change
due to warming caused by test current
– Resistances with a large conductive component
– Contact resistance
Evaluating Measured Values
See Table 4 on page 100.
Calculation of Cable Lengths for Common Copper Conductors
If the HELP key is activated after performance of resistance mea-
surement, the cable lengths corresponding to common conduc-
tor cross sections are displayed.
If results vary for the two different current flow directions, cable
length is not displayed. In this case, capacitive or inductive com-
ponents are apparently present which would distort the calcula-
tion.
This table only applies to cables made with commercially available
copper conductors and cannot be used for other materials (e.g.
aluminum).
Polarity Selection Display Condition
+ pole to PE RLO+ None
– pole to PE RLO– None
 Pole to PE
RLO Where RLO  10%
RLO+
RLO–
Where RLO > 10%