Stress Considerations - AVO MEGGER BM25 Guide De L'utilisateur

Stress Considerations

Measurement above 100 GΩ
Figure 1. shows the stresses and subsequent leakages which
will occur between the test leads if neither is connected to earth
(ground). These leakages have significant effect and occur
through the air itself.
( +2,5 kV )
Figure 2. shows the effect of connecting the guard lead to the
ground. This reduces the stray leakage into the negative
(measurement input) terminal considerably, but this technique is
only permissible if the item under test is isolated from the ground.
('Isolated' means insulated by a resistance of at least 5 MΩ for
the positive terminal or 10 kΩ for the negative terminal).
( +5 kV )
( -2,5 kV )
GROUND
Figure 1.
( 0 V )
GUARD
GROUND
Figure 2.
Figure 3. shows a problem which can occur. If one end of the
sample is grounded and this is required to be connected to the
positive terminal, then the negative (measurement) lead is
surrounded by a 5 kV field. This is likely to cause at least 1 nA of
unwanted leakage current, representing a 5 TΩ resistance in
parallel with the sample under test.
( 0 V )
When taking measurements above 100 GΩ therefore, the user
should where possible ground the Guard Lead as shown in
figure 2, otherwise parallel leakage paths may occur.
( -5kV )
GROUND
Figure 3.
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