Hioki DSM-8542, Instruction Manual, Page: 68 / 140

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When a jig is used, the insulation resistance of the jig is much higher than the sample
insulation resistance, such as higher by more than two digits. Guard must be provided
for the part holding section of the jig, except when insulation resistances of both a
sample and jig are measured together without any problem.
Structures of actual jigs and holders will be varied depending on shapes of parts.
However, a guard circuit to let escape all currents that do not pass through parts
collectively to the “GUARD” terminal is needed.
Using an example of a holder with a guard function illustrated in Fig. 6.1 and Fig. 6.2.
The diagrams show an example of connection between CH1 equipment and the
DSM-8542.
Fig. 6.1 Example of Holder with Guard
Fig. 6.2 Principle of guard
In measurement using a holder with a guard illustrated in Fig. 6.1 and considering the
flow of a current from Terminal A to Terminal B, one channel passes through Sample
(Rx), while the other channel flows to Terminal B passing through Insulating Material a
(Ra), the holder plate and Insulating Material b (Rb).
Starting measurement after connecting Terminal A to the “OUTPUT” terminal of this
meter and Terminal B to the “INPUT” terminal of the meter, currents that flow insulation
resistance (Rx) of the sample, as well as insulation resistances Ra and Rb of Insulating
Materials a and b, are collectively measured. This flow can be illustrated by the
equivalent circuit shown in Fig. 6.2 (a) without a guard.
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