GE STD15C - THROUGH-CURRENT RESTRAINT CHECK

GEK-45307
Example
WDG
1
Tap
=
3.5A
12
(DC)
=
0.80
x
3.5
= 2.8
amperes
II
=
0.90 x
3.5
to
1.10
x
3.5
Ii
=
3.15
to
3.85
amperes
If
DC
meter is
not
available,
12
(AC)
would in
this
example
be
2.25
x
2.8
=
6.30 amperes.
THROUGH-CURRENT
RESTRAINT
In
order
to
check
the
service
tap
slope setting,
the test
current
values
indicated
in
Table
IV
must be
modified
to
take
into
account
any
difference
in
tap settings.
Furthermore,
the
test
circuit
shown
in
Figure
15
must
be
set
up
such
that
the
lead
from
ammeter
13
to
the
test
plug is
connected
to
the
stud
that connects
to
the
winding
with
the
lowest
tap
setting.
The
common
lead,
of
course,
is
connected
to
the
stud
corresponding
to
the
winding
with
the
higher
tap
setting.
For any
combination
of
taps,
the
percent
slope
is
given
in
the
following
equation.
%
Slope
=
[i (L+
i)
-1
]
x
100
where
T 1
=
smaller
tap
setting
T2
=
higher
tap setting
Ii
=
differential
current
13
=
smaller
of
the
two
through
currents
Table
VI,
derived
from
the
above
expression,
is
based
on
a
multiple
of
tap
current
of
6
times
the
lower
tap
setting
for
all
combinations
of
taps except
those
which
involve
the
8.7
amp
tap.
For
the
latter
case,
a
4—times
tap setting
is
used, since
the
total
test
current
for
a
6—times
tap
setting
may
be
as
high
as
75.2
amperes,
which
is
not
only
prohibitively
high
for
many
installations,
but also
may
subject
the
relay
to
excessive
heating.
For
a
given
tabular
value of
13
corresponding
to
a
given
combination
of
winding
and
percent
slope
taps,
the
values
of
I
(mm.)
and
I
(max.)
correspond
to
the
minimum
and
maximum
percent
slope
tolerance
limits
given in
Table
V.
However,
for
a
4-
times
tap
setting,
both
the
upper
and
the
lower
percent
tolerance
limits
have
been
raised
by
a
value
equivalent
to
the
difference
between
the
true slope
and
the
nominal
slope
at
4-times
tap
value,
indicated
by
the percent
slope
characteristic
curves
shown
in
Figure
7.
Example
WDG
1
Tap
=
3.5
A
Wdg
2
Tap
5.0
A
Slope
Setting
=
40%
Since
WDG1
has
the
lower
tap setting,
the
lead
from
ammeter
13
to
the
test
plug
should
be
connected
to
stud
6,
and
the
common
lead
connected
to
stud
4.
From
Table
VI
13
=
21.0 amps
1 i
(mm.)
=
21.0 amps
1
(max.)
22.2
amps
25