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Biddle MEGGER - Page 45

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serve
as
a
guide
for
equipment
in
service.
However,
periodic
tests
on
equipment
in
service
will
usually
reveal
readings
con-
siderably
higher
than
the
suggested
minimum
safe
values.
It
is
therefore
strongly
recommended
that
records
of
periodic
tests
be
kept,
because
persistent
downward
trends
in
insulation
resistance
usually
give
fair
warning
of
impending
trouble,
even
though
the
actual
values
may
be
higher
than
the
sug-
gested
minimum
safe
values.
Conversely,
allowances
must
be
made
for
equipment
in
ser-
vice
showing
periodic
test
values
lower
than
the
suggested
min-
imum
safe
values,
so
long
as
the
values
remain
stable
or
con-
|
sistent.
In
such
cases,
after
due
consideration
has
been
given
to
|
temperature
and
humidity
conditions
at
the
time
of
the
test,
there
may
be
no
need
for
concern.
This
condition
may
be
caused
by
uniformly
distributed
leakages
of
a
harmless
nature,
and
may
not
be
the
result
of
a
dangerous
localized
weakness.
Here
again,
records
of
insulation
resistance
tests
over
a
period
of
time
reveal
changes
which
may
justify
investigation.
The
|
“trend
of
the
curve”
may
be
more
significant
than
the
numerical
values
themselves.
|
The
“One-Megohm”
Rule
For
many
years
One
Megohm
has
been
widely
used
as
a
fair
_
allowable
lower
limit
for
insulation
resistance
of
ordinary
in-
|
dustrial
electrical
equipment
rated
up
to
1000
volts,
and
is
still
|
recommended
for
those
who
may
not
be
too
familiar
with
in-
|
sulation
resistance
testing
practices,
or
who
may
not
wish
to
ap-
proach
the
problem
from
a
more
technical
point
of
view.
For
equipment
rated
above
1000
volts
the
“one
megohm”
rule
is
usually
stated:
“A
minimum
of
one
megohm
per
thou-
sand
volts.”
Although
this
rule
is
somewhat
arbitrary,
and
may
|
be
criticized
as
lacking
an
engineering
foundation,
it
has
stood
the
test
of
a
good
many
years
of
practical
experience.
It
gives
some
assurance
that
equipment
is
not
too
wet
or
not
too
dirty
and
has
saved
many
an
unnecessary
breakdown.
More
recent
studies
of
the
problem,
however,
have
resulted
in
formulas
for
minimum
values
of
insulation
resistance
that
are
based
on
the
kind
of
insulating
material
used,
and
the
elec-
trical
and
physical
dimensions
of
the
types
of
equipment
under
|
consideration.
[431