Biddle MEGGER - Page 62

(
\
with
it.
All
current
models
of
Megger
Insulation
Testers
are
equipped
with
Guard
terminals.
However,
on
motor-
driven
models
and
on
2500
volt
models,
and
higher,
there
is
a
resistor
in
series
with
the
Guard
terminal
to
protect
the
generator
in
case
of
short
circuit.
On
some
models
this
resis-
tance
is
as
high
as
100,000
ohms.
Therefore,
even
with
the
Guard
and
Line
terminals
shorted
the
maximum
shunting
ratio
would
be
approximately
40,
limited
by
the
guard
re-
sistance
in
series
with
the
short.
As
was
found
in
the
example,
“this
ratio
must
be
1,000
or
more
to
check
the
upper
end
of
the
scale.
Therefore,
shunting
is
impractical
on
Instruments
hav-
ing
a
protective
resistance
in
the
guard
circuit.
Where
the
Line
terminal
guard
ring
is
not
accessible,
the
shunting
method
can
only
be
used
by
opening
the
Instrument
to
reach
the
guard
system
ahead
of
the
guard
resistance.
In
this
case
it
is
recommended
that
the
Instrument
be
returned
to
fac-
tory
for
the
calibration
check.
Practical limits must be observed in selecting the values of
the standard resistance and the shunting resistance. If the
standard resistance selected requires a correspondingly low
value shunt resistance, the load on the generator may be ex-
cessive, which will cause overheating and poor commutation
and at the same time reduce the voltage across the control
coil. This in turn will reduce the current through this coil, -
causing loss of control. Likewise, if the shunting is carried to
extremes, errors may result because the adjustment of the
shunt becomes very critical. The resistance values of the stand-
ards used in the previous examples are found to be most
practical.
[601
12/27, το ΝΜ Printed in U.S.A