Guard Terminal - Greenlee 5990A Manuel D'instructions

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
GUARDs test clip should be connected to the test object so that most of the unwanted leakage
current is collected. See Fig. 11 below.
+OUT
U
Where:
U ........Test voltage
t
I .........Leakage current (resulted by surface dirt and moisture)
L
I ........Material current (resulted by material conditions)
M
I .........A-meter current
A
Result without using GUARD terminal: RINS = U
Result using GUARD terminal: RINS = Ut / I
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
M
I
L
I
A
-OUT
A
t
Fig. 11 Connection of GUARD Terminal to Measured Object
= Ut / I
A
I
M
+OUT
U
t
/ I = U / (I + I ) ...incorrect result.
t A t M L
......correct result.
M
5990A
I
I
A
L
-OUT GUARD
A
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