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GEK-34055
APPLICATION
The extremely inverse time/current characte ristics make
the
1AC77
and
1AC78
relays
particula rly
well
suited
for
the
protection
of
primary
distribut ion
circuits.
In
such
applications,
because
the
relay
characte ristics
closely
parallel
those
of
the
power
fuses
normally
employed
in
these
circuits,
it
is
possible
to
obtain
selective fault protection with a minimum time delay.
The unique time—current characte ristics of
these relays also make
them well
suited for picking up cold
loads.
For any given cold
load
pick
up
capability,
the
resulting
setting
will
provide
faster
protection
at
high
fault
currents
with
the
extremely
inverse
relay
than with less inverse devices.
The
general
practice
is
to
use
a
set
of
three
relays
for
interphase
faults.
A
separate
relay,
residually
connected,
can
be
utilized for single—phase—to—ground faults.
Use of a separate ground
relay
is
advantageous
because
it
can
be
adjusted
to
provide
more
sensitive
protection
on
ground
faults.
Typical
connections
for
applying these relays are shown in Figures 14 and 15.
In
the
application
of
1AC77
and
IAC78
relays
with
automatic
reclosing
devices,
the
reset
time
should
be
considered.
The
reset
time of
all
relays covered by
these
instructions
is approximately 60
seconds
from the fully—closed to the fully—open position,
when set at
number 10 on the time dial.
When setting these relays to coordinate with "downstream's
relays,
a
coordination time of
from 0.25
to
.40
second
is generally allowed.
These
coordination
times
include,
in
addition
to
breaker—clearing
time,
0.10
second
for
relay
overtravel
and
0.17
second
for
safety
factor.
For
example,
if
the breaker-clearing
time
is
0.13
second
(8
cycles)
the
coordination
time
would
be
0.40
second.
If
the
relay
time
is
set
by
test
at
the
current
level
in
question,
the
safety
factor
may
be
reduced
from
0.17
to
0.07
second.
Then
with
relay
overtravel
of
.10
second,
if
the
'downstream"
breaker
time
is
5
cycles
(0.08
second),
a minimum of
0.25
second could
be
allowed
for
coordination.
If
the
downstream
devices
are
fuses
rather
than
circuit
breakers,
then
the maximum clearing
time of
the
fuse
should
be
used
and
the
time
allowed
for
breaker—clearing
time
should
be
omitted.
CONSTRUCTION
The
induction
unit
is
the
basic
unit
in
all
Type—IAC
relays.
Figures 2 and 3 show the induction unit mounted in the cradle.
These
units are of the induction—disk construction type.
The
disk
is
actuated
by
a
wattmetric—type
current—operating
element.
This is similar to the standard element as used in watthour
meters,
except the actuating coils above and below the operating disk
are connected in series.
A capacitor and variable resistor connected
in series with
the
inner
coil on
the upper
laminated structure makes
up
the
phase—shifting
circuit.
The
disk
shaft
carries
the
moving
contact,
which completes
the
trip or
alarm circuit.
The
disk
shaft
is
restrained
by
a
spiral
spring
to
give
proper
contact—closing
4
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