METREL MI 3298, Instruction Manual With Exercises

Page: 195 / 232

Power Network Application Trainer
– Insulation Analysis
195
The total current I
tot
comprises of four partial currents.
I
tot
Total current
I
Riss
Surface leakage current
I
Riso
Insulation leakage current
I
RCpi
Polarization absorption current
I
Ciso
Capacitance charging current
Table 65-3: The four partial currents table
Figure 107-3: Typical current / time diagram for a real voltage source
In practice the insulation resistance measurement instrument does not include an
ideal voltage source. At the start all available instrument power is used to charge the
capacitor C
iso
for a short period. The voltage on connection points drops because of
this.
Figure 108-3: Current diagram for an ideal voltage source
When DC voltage is suddenly applied to the insulation, the test current will start at a
high value, gradually decrease with time, and finally level off to a stable value. The
leakage current does not change with time, and this current is the primary factor on
which the insulation quality may be judged.