Insulation Testing Of Capacitors - Saluki STB6684 Operation Manual

STB6684/A Operation Manual
GND
Precharge
Dual supply

6.4 Insulation Testing of Capacitors

There are some limiting factors to be considered when measuring the Insultion Resistance of High
Quality Capacitors.
There is a maximum value of obtainable insulation Resistance depending on the capacitance value
and the applied test voltage. See figure 6.4.1 and 6.4.2 on the following pages. This is caused by
the residual regulation noise in the 0.1Hz to 10Hz range from the H.V. power supply, which is
impossible to either reduce or filter out in the Current Input stage without prolonging the
stabilization time of the H.V. or the settling time for the Current Input Amplifier.
Most Capacitors show a memory effect by Insulation Resistance measurements. The resistance
value will grow when they are charged repeatedly to the same voltage with the same polarity, and if
the polarity is changed the resistance value will be lower than normal. i.e. if a 60sec Insulation
Resistance measurement according to the international standards has to be repeated, the capacitor
under test must be carefully discharged and the second measurement must be delayed about 24
hours in order to obtain comparable measurements.
Some Capacitors with very high Insulation Resistance f.ex. Polystyrene and Polypropylene Film
Capacitors will, when exposed to an absolute DC voltage, for a long period of time, i.e. more than
200sec. start to generate a current noise due to internal charging phenomena. This appears on the
display as if the resistance value starts decreasing and the readout becomes noisy.
Chapter 6 Measurement Procedures and Examples
HT2
HT1
GND
Charge
◇ 75
Measure current in
Measure