Operation Manual

8 GMC-I Messtechnik GmbH
5.3 Measurements with the Guard Cable (with Accessory Guard 5000A)
The measurement of very high impedance resistances presupposes
extremely small measurement currents, and can be rendered problematic
by influences such as electromagnetic fields, moisture or surface currents.
For this reason, the measurement set-up must be implemented in an
accurate fashion.
For measurements within a range of 100 G (10 G) ... 1 T a guard
cable must be used in order to prevent the distortion of measurement
results caused by surface currents. The guard rings prevent current at the
surface of the insulation material from flowing from the +measurement
cable to the –measurement cable, instead of through the insulation mate-
rial itself.
Ð Insert the guard cable plug into the appropriate jack at the tester.
Ð Plug the crocodile clip onto the guard cable test prod.
Ð Clip the crocodile clip onto the guard ring located between the two
measuring points of the insulation material to be measured.
Ð See Chapter 5.1, page 6 for measurement procedures.
Note

The following materials can be used as guard rings:
aluminum foil, copper foil or metal tubing clamps.
6 Characteristic Values
Insulation Resistance
ShortCircuit Current I
K
1.3 mA
Direct and Alternating Voltage
Protective Devices
1)
with reference to scale length 97.5 mm (100 M range) or 109.8 mm (1 T range)
2)
PTC thermistor cool-down period until start of new measurement:
at least 2 minutes must be observed!
+Measurement Cable
– Meas. Cable
Guard Cable
Conductor
Insulation
Material
Guard Rings
Contact Ring
Measuring
Range
Nominal
Range of Use
Nominal/Test
Voltage
U
N
/ U
T
Nom./Test
Current
I
N
/ I
T
Intrinsic Un-
certainty
1)
Measuring Un-
certainty
100 k...
100 M
100 k...
10 M
100 V
250 V
500 V
1000 V
1mA
 2.5%
30%
of measured
value
10 k... 1 T
100 k...
100 G
100/1500 V
250/ 2000 V
500/
2500 V
1000/5000 V
1mA/0.7mA
1mA/0.5mA
1mA/0.4mA
1mA/0.1mA
 5%
Measuring Range Frequency Internal
Resistance
Max. Allowable
Voltage
Intrinsic
Uncertainty
1)
0 ... 2000 V DC/AC 15 ... 500 Hz 5 M
2200 V DC/AC
max. 10 s
5%
Terminal Internal
Resistance
Max. Allowable
Voltage
Protective Device
–Measurement
Cable
—
to +meas. cable/
to guard cable:
2000 V DC/AC
max. 10 s
via grounded
damping diodes
+Measurement
Cable
Insulation
Measurement
—
to –meas. cable/
to guard cable:
2000 V DC/AC
max. 10 s
diodes in high-voltage
cascade, PTC thermistor
2)
and series resistors
Guard Cable
between guard and
meas. cables 90 k
to meas. cable
2000 V DC/AC
max. 10 s
PTC thermistor
2)
and
series resistors
Battery — 10 V
pole protection
with diodes voltage limiting
in battery charger (optional)