IET Labs, Inc. 1864-1644, User And Service Manual

Page: 24 / 46

APPLICATIONS
1864-1644 Positive Polarity Megohmmeter
14
WARNING
Capacitors being measured may be charged
and may contain lethal energy. Always set
the function switch to DISCHARGE before
connecting or disconnecting the capacitor
under test.
4.3.2 Charging Time Constant
The time constant for charging a capacitor in the
CHARGE position is determined by the value of
the capacitor times the effective source impedance
of the supply. The supply resistance is approximately,
R
0
=
E
I
MAX
Ω =
E
0.005A
Ω =
E
5
k Ω
where E is the indicated test voltage in volts and I
MAX
is the short-circuit current, which is approximately
5 mA. Therefore, the time constant is:
T = R
0
C
x
=
EC
X
5000
seconds
where C
x
is in μ F. As an example, on the 500 V range,
R
0
is approximately 100 k
Ω
so that the time constant
for charging of a 1 μ F capacitor is 0.1 s.
The time necessary for full charging depends on the
type of capacitor and the leakage current that is to be
measured. A capacitor with no dielectric absorption
will have a charging current that decreases by a factor
of 2.72 (the natural logarithm to the base e) for every
time constant it is left in the
CHARGE position.
Thus, the effective resistance at any moment is
R
0
E
(
t
R
0
C
X
)
. The capacitor could be considered fully
charged when this resistance is substantially higher
than the true leakage resistance, even though the
charging current theoretically never reaches zero. As
an example a 1
μ F capacitor, with a leakage resis-
tance of 10
10
Ω
measured at 500 V, would have less
than 1% error due to charging current, if measured
after seventeen time constants, or 1.7 s.
4.2 Test Sample Resistivity
Measurements
The megohmmeter can be used for measuring the
resistivity of test samples as described by ASTM
Standard D257, which describes in detail the tech-
niques for both surface-and volume-resistivity mea-
surements. The most common electrode arrangement
is that shown in Figure 4.1. In this con
fi guration
surface resistivity is measured with terminal 1 tied
to the
-UNKNOWN terminal, terminal 2 tied to
the +UNKNOWN terminal and terminal 3 tied to
GUARD . For volume resistivity measurements,
terminal 1 is tied to the -UNKNOWN terminal,
terminal 2 to the GUARD and terminal 3 to the
+UNKNOWN terminal. The formulas required to
convert from measured resistance to resistivity are
given in the ASTM standard. Contact IET regarding
the availability of resistivity test
fi xtures.
4.3 Capacitor Insulation Resistance
4.3.1 General
The insulation resistance, IR, of capacitors is mea-
sured by either the search or sort method (paragraph
3.2.2 and 3.2.3) used for resistors, except that some
consideration must be given to the charge and dis-
charge currents.
Figure 4-1. Electrode arrangement for resisitivity
measurements