Kikusui TOS9213S Operation Manual page 65

Select GUARD only when DUTs, such as small electronic components and jigs, are
completely "floating" electrically.
As shown in Fig. 5-5, the GUARD mode enables high-sensitivity, high accuracy
measurement, as it excludes the current by insulation resistance of the test leadwire
and jigs.
TOS9213S/ TOS9213AS
HIGH VOLTAGE terminal
Ammeter
LOW
terminal
TOS9213S
TOS9213AS
Setting to GUARD
Fig.5-5 Selecting GUARD
Insulation
resistance of
the test lead
D
wire and jigs
U
T
5-17