Biddle MEGGER - Page 35

"4౬606 ౬౧”
CONDUCTOR UNDER
INSULATION TESTER
3 CONDUCTOR CABLE
LEAD
SHEATH =
OTHER CONDUCTORS
CONNECTED TOGETHER
AND TO SHEATH
FIGURE
38.
Showing
how
to
use
the
Guard
terminal
to
eliminate
the
effects
of
surface
leakage
across
exposed
insulation
61
one
end
of
a
cable.
See
also
Figs.
14,
15
and
40,
is
no
chance
of
an
arc-over
between
the
guarded
terminal
of
the
specimen
and
ground.
Such
an
arc-over
may
cause
unde-
sirable
arcing
at
the
commutator
of
the
instrument
generator.
The
leg
r,3,
which
shunts
the
Megger
deflecting
coil
(coil
a,
Figure
45),
should
be
at
least
100
megohms
for
a
measuring
accuracy
of
approximately
1%.
The
1%
accuracy
figure
is
based
on
the
R’
ballast
resistor
(see
Figure
45)
being
1
meg-
ohm,
which
is
typical.
For
more
precise
determinations
of
accuracy,
obtain
the
exact
value
of
R’
by
writing
to
the
James
G.
Biddle
Co.
giving
the
serial
number
of
the
instrument
in
use.
“MEGGER”
INSULATION TESTER
CONDUCTOR UNDER
3 CONDUCTOR CABLE
FIGURE
39.
Showing
how
io
use
the
Guard
terminal
to
eliminate
the
effects
of
surface
leakage
across
exposed
insulation
at
both
ends
of
a
cable
when
a
spare
conductor
in
the
cable
is
available
for
completing
the
Guard
connection.
Where
the
highest
accuracy
is
desired
in
cases
as
shown
in
Figure
42a,
or
where
the
true
resistance
of
each
leg
is
wanted
as
in
the
case
of
Figure
42b,
three
measurements
are
re-
[331