Hioki Super megohm SM7110 - Measurement with Use of a Measurement Fixture

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Measuring Components or Circuits
Measurement with use of a measurement xture
Design the circuit in order that the currents owing through the outside of all the objects to be
measured ow through the guard circuit and connect the guard circuit to the GUARD terminal of the
instrument.
Example of use: (Use a measurement xture with a guard installed)
1
Sample (Rx)
2
Terminal A
3
Insulator a (Ra)
4
Terminal B
5
Insulator b (Rb)
6
Holding plate
(guard)
1
2
3
6
4
5
Principle of the guard
Measurement voltage source Measurement voltage source
(a) Measurement without a guard (b) Measurement with a guard
Rx
Ra Rb
A B
i
1
+ i
2
i
1
i
2
A
Rx
Ra Rb
A B
i
1
A
i
2
C
i
1
The gures “Example of use” and “Principles of the guard” explains the e󰀨ectiveness of the guard.
Considering how the current ows from Terminal A to Terminal B in the “Example of use,” 2 ow
channels are found.
• Current pathway passing through the measured object (Rx)
• Current pathway passing through in the following order: Insulator a (Ra), holding plate, Insulator
b (Rb), and Terminal B.
Connect Terminal A to the OUTPUT terminal and Terminal B to the INPUT terminal of the
instrument, and then start a measurement. Thus, the sum of the insulation resistance (Rx) of the
measured object, the current that ows through the insulation resistance Ra of Insulator a, and the
current that ows through the insulation resistance Rb of Insulator b is measured. “Principle of the
guard” shows the equivalent circuit of this case.
As you can clearly see from the equivalent circuit in “(a) Measurement without a guard,” both
the current i
1
, which ows through the measured object, and the current i
2
, which ows through
Insulators a and b, ow into the input circuit through Terminal B.
Next, use a metal plate as the holding plate of the holder, and connect it to the GUARD terminal
of the instrument. This ensures that only the current owing through the measured object ows
into the INPUT terminal; the current that ows through Insulators a and b is excluded. (See “(b)
Measurement with a guard.”)
This equivalent circuit shows that the current i
2
, which ows through Insulator a, does not ow
into the input circuit, but directly ows into the common current measurement block, and that only
current i
1
, which ows through the measured object, ows into the input circuit.