Eaton Cutler-Hammer Digitrip RMS 510 - Page 77

AD
33-855-4
Page
76
The SHORT
DELAY
TRIP
TIME
curve
in
Figure
14-1
shows
the
time
to trip
response
of the
Trip
Unit
to FAULT
currents.
There
are
actually
eight
different
SHORT
DELAY
TRIP
TIME
curves
avail-
able
on
the
Trip
Unit
(selected
by
the
Short
Time
-
switch
setting),
three
of
which
follow
a constant
lzt
slope
for best
coordination
with downstream
devices,
five of
which
are
FLAT
(fixed
trip
time)'
The
lowest
level
of FAULT
current
to bring
on
this
response
is selected
by the
Short
Delay
Pickup
setting
on
the
TriP
Unit.
The
SHORT
DELAY
TRIP
TIME
curves
available
on
the
Trip Unit
(selected by
the Short
Time
switch
setting),
can also
be
modified
by a
feature
of
the
Digitrip
RMS
system
called
ZONE
SELECTIVE
INTERLOCKING.
The
INSTANTANEOUS
TRIP
TIME curve
in Figure
14-1 shows
the
time
to
trip response
of the
Trip
Unit
to
FAULT
currents.
This
curve
is
FLAT,
provid-
inq
the
most
rapid
trip time
possible
(with
no
inientional
delay)
to be able
to clear
the
fault
as
fast
as
possible
and
minimize
the
damage
to
the
svstem.
The
lowest
level
of FAULT
current
to
bring
on thi"
respon""
is selected
by
the
lnstantaneous
Pickup
setting
on
the
TriP Unit.
ln
most
applications,
main
Breakers
will have
Long
Delay
and
Short
Delay
functions,
but
will omit
the
lnstantaneous
function
to allow
a
downstream
feeder
Breaker
to clear
the
fault
without
tripping
the
main
Breaker,
thereby
enhancing
system
stability.
ln many
applications,
feeder
Breakers
will
have
Long
Delay,
Short
Delay,
and
lnstantaneous
func-
tioni
to allow
a
downstream
device
to clear
the
fault
without
tripping
the
feeder
or
the main
Breaker,
thereby
maximizing
system
stability.
ln some
applications,
feeder
Breakers
will
have
Long
Delay
and
lnstantaneous
functions,
but
omit
the Shod
Delay
function.
This is
fine
for coordina-
tion
with
upstream
main
Breakers,
but
the feeder
would
not
normally
allow
a
downstream
device
to
clear
the
fault
without
tripping
the
feede[
thereby
compromising
the system
stability.
ln some
applications,
feeder
Breakers
will
have
Long
Delay
and
Short
Delay
functions,
but
will
omit
the lnstantaneous
function
to allow
proper
opera-
tion
of an
unusual
load or
coordination
with down-
stream
devices.
LONG
DELAY
TRIP
TII\4E
\
\t2t
FLAT
SHOBT
DELAY
TBIP TIME
INSTANTANEOUS
TRIP
TIIV]E
J
>l
=l
-.-->
CURRENT
OPERATING
CONDITIONS
Fig.
14-1
Time-Cunent
Curves
EtT.il
Ftfective 9/01