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Testing
o
Exitation Test
The purpose
of
an
exitation test
is
to
reveal shorted turns
in
the
current
sensorwinding.
Sixty
(or50)
Hertz ac
voltage
is
applied
to
the
secondarywinding
and the
exciting current
is
monitored.
Short circuit turns
will be
revealed
by
excessive
exciting
current
and possibly by overheating
of
the winding.
A
l20V
variable transformer
(Variac)
and a low range ammeter
is
all
that
is
required, although
a
0-150
Volt ac
voltmeter
is
desirable
to
monitor
the applied voltage.
The ammeter
in
the
PTS4
test
set
can be
used
by
passing current through
the
external
current
input.
ln
the
absence
of
a
voltmeter the
dial reading
on
the variable
transformer can be used
as an
approximate voltage indicator.
See
table
5.
Table
5
Current Sensor
Exciting
Current Test
Again each phase
must be
tested
in
turn and
the
trip device
must
be disconnected.
Connect the
ac supply
to
terminals
l
and 5
for
Phase
1,
terminals
2
and
5 for
Phase
2
and
from
terminals
3
and
5
for
Phase 3.
The output
selector
switch can
be
used
to
select each phase when
the
PTS4
test set
is
used
as the
AC ammeter,
To
test the
ground
sensor or neutral
sensor
it
is necessary
to
have
the
breaker in the
cubicle,
in
the
TEST position.
Again check the breaker wiring
diagram,
for any
special connections that
may have
been
made.
Table 5 gives the applied voltage and test limits of
exciting
current for all
standard current
sensor ratings.
The value
of
.02
ampere
is
about
the
lowest
readable current
using the ammeters
in
the
PTS4
portable test sets, and
the
sensor that meets this
limit
will give satisfactory performance.
Normal
exciting currents
for sensors
rated 400 amperes
and
above may
be well
below this value.
Therefore
when
checking high
rating sensors
it is
essential
to
perform
the
continuity
check.
For
the lower
ratios
the continuity
check
can
be
omitted since
the
indicated
magnetizing current
would
verify continuity.
Sensor Polarity
Forthe ground fault detection elements
to
operate
properly
the
sensors must be
connected
with the
proper
polarity. Siemens
current sensors and ground/neutral sensors
are
checked
for
polarity
at
the
factory
and terminals are marked with
a
polarity
dot.
Therefore
a
simple check
for
proper connection
according
to
the
breaker wiring diagram is all that should normally
be
required
to
verify that the
trip
unit
is
receiving
the
correct
ground
fault signal.
lf
there is doubt that
a
given sensor
is
properly marked,
its
polarity
can
be
checked
by "flashing" the primary with
a
short
duration
pulse of known
dc
polarity and observing the polarity
of the induced secondary voltage on
make
and break of
the
primary.
A
6
volt dry
cellcan
be used
for
the
primary source and
the
polarity indicated
by
a dc
voltmeter
connected
to
the
secondary
winding.
Refer to the
equipment one
line or
breaker
wiring diagram for the correct neutral
or
ground return
sen-
sor polarity.
Primary Current
Testing
Testing
with
primary
current applied
to the
circuit
breaker
requires
a
low voltage
high
current supply
that can
deliver
currents
up
to
4
or
6 times
the sensor rated current.
There
is
commercially available
equipment
made
for
this purpose, This
equipment
generally includes the
ammeters
and
timers
needed
to fully test
the
breaker.
Primary current
testing can provide the
ultimate
in
assurance
that the entire protective system
is
functioning
properly, since
the entire
system
is
tested
at
one
time. However primary
current
testing
may present
additional problems.
One major
difficulty
arises from the non-linear
impedance
of
the
trip device
power
supply circuit.
This
circuit is designed
to
provide
power
to
operate
at
low levels of current then
change
impedance
for
higher
levels
of current to protect
the
trip
device.
When sup-
plied from
a
low
voltage source this changing
impedance
alters
the
waveshape
of
the current
supplied
to
the
trip device.
This can result
in
insufficient power
to
properly
operate
the tripped
device,
or it
may affect
the
indication
of
the
source ammeter,
depending on the
ammeter
design,
lead-
ing to the erroneous assumption that the
trip
device calibra-
tion
is incorrect.
U
Sensor
Rating
Applied
Volts
(V)
120
Volt
Variac
Setting
Maximum
Exciting
Current (A)
150
204
300
400
600
800
1200 & up
JJ,C
67
67
67
67
134
134
50
50
50
50
100
100
0.25
015
0.10
0.10
0.05
0.03
o.o2
ca
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