5990A
15
Measurements
(cont’d)
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 Insulation Resistance
Measurement. The
GUARD
terminal is internally connected the negative test terminal (black). The
GUARD
s test clip should be connected to the test object so that most of the unwanted leakage
current is collected. See
Fig. 11
below.
Where:
U
t
........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: RINS = U
t
/ I
A
= U
t
/ (I
M
+ I
L
) …incorrect result.
Result using
GUARD
terminal:
RINS = Ut / I
A
= Ut / I
M
……correct result.
It is recommended to use the
GUARD
connection when high insulation resistance (>10G Ω)
is measured.
Note:
• The guard terminal is protected by an internal impedance (200 KΩ).
The instrument has two guard terminals to allow easy connection of shielded measuring leads.
I
L
I
L
I
L
U
t
U
t
I
A
I
A
I
M
I
M
A
A
+OUT
+OUT
-OUT
-OUT
GUARD
Fig. 11
Connection of GUARD Terminal to Measured Object
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