5.1.2 INTERNAL SWITCH BETWEEN GROUND AND GUARD (UST)
The sole function of this mode of operation is to separate the paths of leakage
currents. These leakage currents are leakage to ground and leakage across the test
specimen. The leakage currents to GROUND are bypassed around the current
meter, measuring only the specimen leakage current. The low side (RETURN)
connection of the specimen must be "floated" (isolated from GROUND). This
mode of operation should be used only when extremely sensitive measurements
are desired.
This is typically used for discrete components or when leakage to GROUND is to be
disregarded. For example, when testing a transformer for leakage (or resistance)
between the secondary and the primary, the guarded return mode allows leakage
to the core or frame to bypass the meter, thus reading only leakage between
the two coils.
It is not possible to test installed cables in this mode of operation, as the cable shields are always
grounded, making it impossible to see the leakage current.
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