Stress Considerations - Megger BM25 Guia Del Usuario

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.
Figure 1.
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).
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.
Figure 3.
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.
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