GE GCX51A - Page 21

GEI
-
98328
to
make
up
the
line
impedance
in
the
test
circuit
,
its
value
will
be
twice
that
of
the
phase
-
to
-
neutral
relay
reach
.
The
percent
tap
of
the
test
box
autotransformer
,
which
should
cause
the
OHM
unit
to
just
close
its
contacts
with
the
fault
switch
closed
,
is
given
by
:
2
(
xRelavl
%
(
100
)
XL
For
this
test
the
value
of
RL
may
be
made
zero
(
0
)
since
the
OHM
unit
responds
only
to
a
reactance
quantity
.
The
load
box
,
however
,
may
be
adjusted
to
give
a
fault
current
of
approximately
15
amperes
,
or
whatever
fault
current
is
expected
during
three
-
phase
and
/
or
phase
-
to
-
phase
fault
conditions
.
To
illustrate
the
above
,
assume
that
a
standard
-
reach
relay
is
being
tested
and
that
the
OHM
unit
range
selection
taps
have
been
set
for
a
basic
minimum
reach
of
0.5
ohms
.
zone
1
reach
of
approximately
0.72
ohms
phase
-
to
-
neutral
.
used
in
the
example
in
the
section
on
CALCULATION
OF
SETTINGS
.
Further
assume
that
the
No
.
1
taps
are
set
at
70
%
,
providing
a
These
values
were
2
XRelay
=
2
(
.
72
)
=
1.44
ohms
Therefore
it
will
be
necessary
to
use
the
2
ohm
reactor
tap
,
since
this
is
the
nearest
tap
above
twice
the
unit
phase
-
to
-
neutral
ohmic
reach
.
Assume
now
that
the
calibration
curve
for
the
particular
test
reactor
in
use
has
been
checked
and
that
the
reactance
of
the
2
ohm
tap
at
the
current
level
to
be
used
in
the
test
is
actually
2.1
ohms
.
The
percent
tap
of
the
test
box
autotransformer
at
which
the
OHM
unit
contact
will
just
close
can
be
calculated
as
follows
:
(
100
)
=
68.6
%
%
Tap
=
The
OHM
unit
should
therefore
theoretically
close
its
contact
with
the
test
box
autotransformer
taps
set
at
68
%
and
remain
open
with
the
taps
at
69
%
.
A
range
of
67
%
to
70
%
is
acceptable
.
The
phase
angle
of
the
OHM
unit
reactance
characteristic
can
be
readily
checked
by
adding
resistance
RL
in
series
with
the
line
reactance
XL
in
the
test
circuit
Figure
12
.
This
resistance
,
RL
,
should
be
non
-
inductive
and
about
3
to
4
times
as
large
as
the
line
reactance
,
XL
-
The
source
impedance
should
also
be
readjusted
so
that
when
the
pickup
point
is
checked
the
fault
current
will
be
approximately
the
same
as
in
the
previous
test
with
reactance
alone
.
When
the
fault
switch
is
closed
,
the
OHM
unit
should
close
its
contacts
at
the
same
test
box
percent
tap
setting
as
in
the
previous
test
.
A
check
of
the
OHM
unit
second
-
zone
setting
may
be
made
in
the
same
manner
,
except
that
the
transfer
relay
OX
must
be
picked
up
by
applying
rated
DC
voltage
between
studs
12
and
13
(
be
sure
the
DC
voltage
selection
link
position
agrees
with
the
voltage
to
be
used
)
,
different
test
reactor
tap
setting
.
It
may
also
be
necessary
to
use
a
b
.
Testing
the
MHO
Unit
The
MHO
unit
is
tested
in
a
manner
very
similar
to
the
OHM
unit
above
,
the
major
difference
being
in
the
manner
in
which
the
test
-
box
autotransformer
21
Courtesy of NationalSwitchgear.com