User Manual

GMC-I Messtechnik GmbH 49
12.1 Measurements with Constant Test Current
Start Measurement
Press and hold for
long-term measurement
Attention!
!
The test probes should always be in contact with the DUT be-
fore pressing the Start
▼ key.
If the object is energized, measurement is disabled as soon as
it is contacted with the test probes.
If the Start
▼ key is pressed first and the test object is con-
tacted 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 RLO.
Automatic Polarity Reversal
After the measuring sequence has been started, the instrument
performs 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 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 at 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 START
▼ key as
long as is necessary for the measurement.
Note
Measuring Low-Value Resistance
Measurement cable and 2-pole measuring adapter resis-
tance is compensated for automatically thanks to the four
conductor method and thus do not effect measurement
results. However, if an extension cable is used its resis-
tance must be measured and deducted from the mea-
surement 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 great conductive component
– Contact resistance
Evaluation of Measured Values
See Table 4 on page 92.
Calculation of Cable Lengths for Common Copper Conductors
If the HELP key is activated after performing resistance measure-
ment, the cable lengths corresponding to common conductor
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 If ΔRLO ≤ 10%
RLO+
RLO–
If ΔRLO > 10%