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insulating
members,
should
for
reliable
operation,
have
an
in-
sulation
resistance
value
above
10,000
megohms
at
20°C.
This
assumes
that
the
oil
within
the
tank
is
in
good
condition,
that
the
breaker
is
separated
from
its
external
connections
to
other
equipment,
and
that
the
porcelain
weather
shield
is
guarded.
This
means
that
each
component
such
as
the
stripped
bushing
‘itself,
cross-member,
lift
rod,
lower
arcing
shield
etc.,
should
have
an
insulation
resistance
in
excess
of
that
value.
Any
components
which
are
superficially
clean
and
dry
and
having
values
less
than
10,000
megohms
are
usually
deteriorated
internally,
by
the
presence
of
moisture
or
carbonized
paths,
to
such
an
extent
that
they
are
not
reliable
for
good
service
unless
reconditioned.
This
is
particularly
so
when
operating
under
surge
conditions
such
as
during
lightning
disturbances.
In
the
case
of
the
stripped
bushing
itself,
the
lower
stem
and
upper
|
weather
shield
must
be
either
perfectly
clean
or
guarded
before
it
is
condemned
as
unreliable
because
of
an
insulation
resistance
value
less
than
10,000
megohms.
|.
What
has
been
said
for
stripped
oil
circuit
breaker
bushings
also
applies
to
bushings
for
other
equipment
such
as
transform-
|
ers.
Since
bushings
and
other
associated
members
have
very
high
|
insulation
resistance
values
normally,
a
“Megger”
Insulation
Testing
Set
having
a
range
of
at
least
10,000
megohms
is
nec-
|
essary
in
testing
such
equipment.
“Megger”
instruments
having
ranges
up
to
50,000
megohms
will
permit
the
observation
of
deteriorating
trends
in
bushings
before
they
reach
the
question-
able
value
of
10,000
megohms.
Cable
and
Conductors
Cable
and
conductor
installations
present
a
wide
variation
|
of
conditions
from
the
point
of
view
of
the
resistance
of
the
in-
_sulation.
These
conditions
result
from
the
many
kinds
of
insu-
|
lating
materials
used,
the
voltage
rating
or
insulation
thickness,
|
and
the
length
of
the
circuit
involved
in
the
measurement.
Fur-
_
thermore,
such
circuits
usually
extend
over
great
distances,
and
-
may
be
subjected
to
wide
variations
in
temperature,
which
will
_
have
an
effect
on
the
insulation
resistance
values
obtained.
The
terminals
of
cables
and
conductors
will also
have
an
effect
on
the
test
values
unless
they
are
clean
and
dry,
or
guarded.
See
=
Section
VII.
The
Insulated
Power
Cable
Engineers
Association
gives
min-
imum
values
of
insulation
resistance
in
its
Specifications
for
va-
rious
types
of
cables
and
conductors.
These
minimum
values
[451