Capacitor Insulation Resistance; General - QuadTech 1865 Instruction Manual

The 1865-11 Test Cell measures resistance and with unique formulas, the resistance can
be converted to surface and volume resistivity. The formulas required to convert from
measured resistance to resistivity are given in the ASTM D257 Standard. For the circular
or concentric electrode configuration, the resisitivity is calculated by:
Surface
ρ
= =
Resistivity s
P = effective perimeter of measuring electrode
g = dimension of space between electrodes 2 and 1
R = measured surface resistance in ohms
s
Refer to ASTM D257 for formulas based on electrode configuration of the test cell.
3.3

Capacitor Insulation Resistance

3.3.1 General

The insulation resistance measurements of capacitors is different from that of resistors by
the fact that some consideration must be given to the charge and discharge currents.
Consideration should also be given to high value, low leakage capacitive devices relative
to dc resistance measurements.
measurement instrumentation is the inability to provide stable leakage measurements on
high value capacitors. This is due to the fact that a capacitive element is very ac sensitive
and the dc voltage applied to the device always contains a small percentage of low
frequency noise ripple. Measurements on pure resistive elements will tolerate a high
degree of ripple due to the voltage ratio measurements whereas a capacitor easily passes
the noise ripple. When this is amplified by the current amplifier of the measuring
instrument results can fluctuate and vary widely. To eliminate this in the 1865 two
resistance adapters are supplied which can be placed in series with the (-) unknown
(black) when measurements are made on the low current ranges (the 1MΩ, Hi Range
Adapter is recommended for ranges 1nA & 10nA and the 100kΩ, Lo Range Adapter for
ranges 100nA, 1nA & 10uA). This added resistance has negligible effect on the dc
measurement since its value is very small compared to the leakage resistance of the
unknown however it increases the ac input resistance to the current amplifier, reducing
the ac gain and thus minimizing the wide fluctuations. This added resistance will have an
effect on the charge time constant and is discussed briefly in paragraph 3.3.2 below.
Capacitors being measured may be charged and contain lethal energy. Always make sure
the CAUTION HIGH VOLTAGE
Applications
P
R
s
g
Inherent to some degree in all high resistance
WARNING
Indicator is OFF when connecting or disconnecting the
capacitor under test.
A
Volume
ρ
= = R
Resistivity v v
t
A = effective area of measuring electrode
t = average thickness of the specimen
R = measured volume resistance in ohms
v
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