Eaton Cutler-Hammer Digitrip RMS 510 Instructions Manual page 76

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Page 75
SECTION
14
EXPLANATION OF TIME
CURRENT CURVES
14.1 Explanation of the Time Gurrent
Curves (LSl)
Digitrip RMS Trip Units and Digitrip Retrofit Kits
have been designed
to upgrade existing Breakers
to
meet the same protection requirements of the
most modern Power Circuit Breakers. ln fact, the
Trip
Unit and Direct Trip
Actuator
(DTA) are
electri-
cally identical to those found on current DS Break-
ers. They are
built
in
the same manufacturing
facilities, with the same high level of Quality Con-
trol,
as
those on the DS Breakers. ln this way, we
assure
that the Retrofit Designs are as up
to
date
as our current production.
All Digitrip RMS Trip Units are designed
to
meet
the protection requirements of a modern power
distribution system. As
is
shown in Figure
14-1
,
Application Current Levels can be broken into
three typical regions:
.
NORMAL currents, the magnitude of which
is
within the system continuous current rating;
.
OVERLOAD currents, which exceed the system
continuous current rating and where conductor
heating
is
the primary concern (such as with
an
overloaded motor, for example);
.
FAULT
currents, which are usually caused by
an unintended current path (either phase
to
phase or phase to ground) and can lead
to
currents as high
as
the maximum short circuit
available from the system.
Digitrip RMS Trip Units,
when set up according to
the results of a Power System study of the specific
distribution system on which they
are applied, will
respond to various current levels
to provide accu-
rate system protection as well as maximum system
stability by properly coordinating with upstream
and downstream devices (Figure 14-1).
.
For NORMAL currents, the Trip Unit will
accurately monitor the current, regardless of
current waveform (due
to
its true RMS sensing
system), assuring
that
up to full rated current
may
flow through the system without falsely
tripping the
breaker. This is
shown
as
the
NORMAL OPERATING CONDITION
in
Figure
14-1
.
.
For OVERLOAD currents, the Trip Unit will
accurately assess the true heating value of the
current (again a distinct advantage of its true
RMS sensing system), and
trip
out following
the LONG
DELAY TRIP TIME curve in Figure
14-1
.
There are actually eight different LONG
DELAY TRIP TIME curves available on the Trip.
Unit (selected by the Long Delay Time switch
setting), and each curve follows a constant lzt
slope for excellent heating protection and
coordination with other devices. The lowest
level
of OVERLOAD current
to
bring on this
response is selected by
the
Long Delay Pickup
setting on the Trip Unit.
.
For
FAULT
currents, the Trip Unit will respond
according
to either the SHORT DELAY TRIP
TIME or the INSTANTANEOUS TRIP TIME
shown in Figure
14-1 ,
depending on the fault
current magnitude and whether the Short Delay
function or lnstantaneous function or both
are
present on the Trip Unit.
Efiective 9/01
FlT.il