79
Measuring Components or Circuits
Measurement without use of a measurement xture
Connect the optional Pin Type Lead or Clip Type Lead to the measured object and perform
measurement.
Both the measuring leads do not have a guard on the end connected to the OUTPUT terminal but
have a guard on the other end connected to the INPUT terminal.
Pin Type Lead The structure of the lead is designed to guard also the test rods, so that you can
perform measurement while holding the test rods in your hands.
Clip Type Lead The clip parts have no guard, connect them to the terminals of the object to be
measured and perform measurement with your hands kept away from them.
See “2.2 Connecting Measurement Leads to Instrument” (p. 30), and “3.5 Connecting
Measurement Leads or Electrode to Object to Be Measured” (p. 45)
The insulation resistance value varies depending on the insulation material. However, the measurement
time, measurement voltage, and temperature also have a signicant impact on the measured value.
Impact of the measurement time
A circuit of an insulator is schematically considered as a complex circuit in which resistance and capacitance
are included serially or parallel; thus, applying a voltage to the insulator results in the dielectric absorption
current and leakage current equivalent to the charging current owing.
The insulation resistance can be obtained by measuring the leakage current and converting it into a
resistance value. Generally, the dielectric absorption current is considerably larger than the leakage current,
and the time constant may also be very large depending on the type of the insulator. Therefore, in some
cases, it may take a long time for the dielectric absorption current to become smaller than the leakage
current. If a resistant value of such an insulator is measured, measuring for a short time results in a small
insulation resistance value; the longer the measurement time, the higher will be the insulation resistance
value.
Impact of measurement voltage
The time constant and insulation resistance value of the dielectric absorption current may considerably vary
depending on the voltage applied to the insulator. Generally, the higher the measurement voltage, the lower
will be the insulation resistance value.
Impact of temperature
Temperature also has an impact on insulation resistance values. Although they will vary depending on
the type of the insulator, generally the higher the temperature, the lower will be the insulation resistance.
Accurate insulation resistance not only it takes time, but also it is dicult to perform measurement.
In some cases, you cannot freely determine the measurement voltage due to the dielectric strength of the
object toe be measured. However, a measurement time value of 1 minute is used, and 1-minute insulation
resistance value is expressed with the measurement voltage as, for example, “1000 M
Ω
at a voltage of 500
V DC.”
To compare the insulation resistance, you must determine the measurement voltage and time to perform
measurement. However, it may be important to reduce the measurement time (or inspection time) in a
production process. In such cases, it is important to determine the time in the range that allows correlation
with the 1-minute value.
5
Measurement Methods Suitable for Various Objects to Be Measured
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