MI 3201 TeraOhm 5 kV Plus Measurements
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5.2 Guard terminal
The purpose of the GUARD terminal is to lead away potential leakage currents (e.g.
surface currents), which are not a result of the measured insulation material itself but
are a result of surface contamination and moisture. This current interferes with the
measurement i.e. the Insulation Resistance result is influenced by this current. The
GUARD terminal is internally connected to the same potential as the negative test
terminal (black one). The GUARDs test clip should be connected to the test object so as
to collect most of the unwanted leakage current, see the Fig. 9 below.
I
L
I
L
I
L
Ut Ut
I
A
I
A
I
M
I
M
+OUT +OUT
-OUT
-OUT GUA RD
Fig. 9. Connection of GUARD terminal to measured object
Where:
Ut........ Test voltage
IL ......... Leakage current (resulted by surface dirt and moisture)
IM ........ Material current (resulted by material conditions)
IA......... A-meter current
Result without using GUARD terminal: R
INS = Ut / IA = Ut / (IM + IL) …incorrect result.
Result using GUARD terminal: R
INS = Ut / IA = Ut / IM ……correct result.
It is recommended to use the GUARD connection when high insulation resistance
(>10G ) are measured.
Note:
The guard terminal is protected by an internal impedance (200 K).
The instrument has two guard terminals to allow easy connection of shielded
measuring leads.
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