Eaton Cutler-Hammer Digitrip RMS 510 Instructions Manual page 77

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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