Agilent Technologies 4339B - Page 214

Insulation
Resistance
Measurement
4339B
Insulation Resistance
Measurement
Residual
Charge
Eect
Once
a
high resistance
insulation material
is charged
with a
voltage,
a residual
voltage will
still
remain
on
the
material
for
a
while after
the charging
voltage is
removed. When
you
want
to
measure
a
high
resistance
material
after
charging and
measuring it
once,
you must
wait
long
enough
to
discharge
the
voltage
across
DUT from
the previous
measurement before
making
another
measurement,
or the
measurement value
may be
incorrect.
Absorption
Phenomena
When
a
high
resistance
insulation
material
remains
charged
with
a
voltage
for
a
long
time
,
its
resistance
value
will
increase
.
So
record
the
time
from
which
the
charge
was
applied
to
the
DUT
to
the
start
of
the
measurement.
V
oltage
Coecient
and
T
emperature
Coecient
The
resistance
value
of
an
insulation
material
depends
on the
measurement
voltage:
the
resistance
value
of
most
insulation
materials
will
increase
as
the
measurement
voltage
increases
.
The
resistance
of
an
insulation
material
depends
on
its
temperature
.
T
o
measure
accurately
,
place
the
DUT
into
an
environmental
test
chamber
to
keep
the
DUT's
temperature
at
a
constant
known
level
when
making
a
measurement.
Record
the
measurement
voltage
and
temperature
when
making
a
measurement.
Shielding
V
ariation
in
total
current
seen
by
the
measurement
circuit
can
be
caused
by
power-line
noise
pickup
,
adjacent
electro-magnetic
elds
of
excessive
strength,
exing
the
test
leads
beyond
reasonable
limits
,
or
by
changing
the
position
of
the
leads
relative
to
their
surroundings
. Most
noise
problems
of
this
type
are transient,
and contribute
more inconvenience
than error
.
Any
suitable
enclosure
will
suce
to
reduce
unwanted noise
pick-up.
Connect the
shield
to
the
guard
(ground).
When
using
sensitive
measurement
ranges,
lead movement
or operator
proximity
will
aect
the
stability
of
the
measurement. Stationary
leads and
shielding are
recommended.
7-2 Measurement Basics