Hioki DSM-8104, Instruction Manual

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Fig. 6.3 Example of Holder with Guard
Fig. 6.4 Principle of guard
In measurement using a holder with a guard illustrated in Fig. 6.3 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. 6.4 (a) without a guard.
The equivalent circuit shows that Current i
1
flowing through the sample and current i
2
that
flows through Insulating Materials a and b both flow to the input circuit through the
Terminal B.
Next, using a meal plate for example as the holder plate of the holder and connecting it
to the “GUARD” terminal of the meter, the current flowing to the “INPUT” terminal will be
only the current that flows through the sample after the current that flows through
Insulating Materials a and b is eliminated. The equivalent circuit in this case is the
circuit in Fig. 6.4 (b) with a guard.
This equivalent circuit shows that Current i
2
that has passed through Insulating Materials
a flows directly to the common side of the current measuring unit without entering the
input circuit, to which only Current i
1
that has passed through the sample is input.