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Vil
USE
OF
THE
GUARD
TERMINAL
All
Megger
insulation
testers
having
ranges
of
1,000
megohms
and
higher
are
equipped
with
Guard
terminals.
The
purpose
of
this
terminal
is
to
provide
facilities
for
making
a
so-called
three-terminal
network
measurement,
so
that
the
resistance
of
one
of
two
possible
paths
can
be
determined
directly.
It
has
the
further
or
secondary
purpose
of
providing
a
source
of
d-c
voltage
of
good
regulation
and
of
limited
current
capacity.
The
insulation
of
all
electrical
apparatus
has
two
conducting
or
leakage
paths—one
through
the
insulating
material
and
the
other
over
its
surfaces.
By
providing
a
third
test
terminal
in
the
path
of
the
surface
leakage
it
is
separated
into
two
parts,
forming
a
three-terminal
network
as
shown
in
Figure
42a.
In
practice,
this
third
terminal
may
be
provided
as
shown
in
Figures
18
and
38
to
41.
There
are
also
cases,
such
as
found
in
two
winding
trans-
formers
or
multi-conductor
cables,
where
a
three-terminal
net-
work
is
formed
as
shown
in
Figure
42b.
Figures
14,
20
and
others
show
practical
applications
of
this
form
of
three-terminal
network.
In
making
a
three-terminal
test
involving
only
one
measure-
ment,
the
Line
terminal
of
the
Megger
instrument
is
con-
nected
to
Terminal
1,
Figure
42a,
the
Guard
terminal
to
Terminal
3,
and
the
Earth
terminal
to
Terminal
2.
This
will
give
the
true
value
of
Τι»,
provided
reg
and
Τι
are
not
too
low
in
value.
The
leg
rə,
which
is
connected
across
the
Megger
instrument
generator
(see
Figure
48),
should
be
about
1
meg-
ohm
or
higher
to
prevent
excessive
load
on
the
generator,
and
maintain
satisfactory
generator
voltage.
In
using
the
Guard
terminal,
particularly
in
the
case
of
motor-driven
or
rectifier-
operated
Megger
instruments,
also
make
certain
that
there
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