MI 3205 TeraOhm XA 5kV
Measurements
35
5.3 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 the 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 Figure below.
I
L
I
L
I
L
Ut
Ut
I
A
I
A
I
M
I
M
A
A
+OUT
+OUT
-OUT
-OUT
GUARD
Figure 5.2: Connection of GUARD terminal to measured object
where:
Ut ................................... Test voltage
I
L
.................................... Leakage current (resulted by surface dirt and moisture)
I
M
.................................... Material current (resulted by material conditions)
I
A
.................................... A-meter current
Result without using GUARD terminal:
)
(
I
I
U
I
U
R
L
M
t
A
t
ins
Incorrect result.
Result using GUARD terminal:
I
U
I
U
R
M
t
A
t
ins
Correct result.
It is recommended to use the GUARD connection when high insulation resistance
(>10 G
) are measured.
Notes:
The guard terminal is protected by internal impedance (400 k
).
The instrument has two guard terminals to allow easy connection of shielded
measuring leads.
