Instruction Leaflet
DT3000
Page 22
Effective: Date 07/02
I.B. 17555D
For more information visit:
www.cutler-hammer.eaton.com
Supersedes I.B. 17555C dated November 1999
other portions of the curve. Figure 3-4 graphically
illustrates the vertical time line movement with an I
2
t curve
shape selection. Similar movement occurs for the
remaining curve shapes.
Short Time Protection
Short time (fault) protection responds to short circuit
conditions. Similar to the inverse time overcurrent function,
the short time function is comprised of a short time current
pickup setting and a short delay time setting. The Short
Delay pickup setting establishes the current level at which
the relay’s short time tripping function begins timing. The
Short Delay Time setting establishes the amount of time a
short-circuit will be carried before the protective relay’s trip
relay is energized. As is the case with inverse time
overcurrent protection, short delay protection also offers a
range of settings for both pickup and time. Refer to Tables
2.2 and 2.3 for available selections.
Two points should be made concerning the available
selections:
•
If “NONE” is selected, the Short Delay function is
disabled and there will be no Short Delay protection.
Also, if “NONE” is selected, a Short Delay Time
selection is not offered.
•
There is no curve shape selection for the Short Delay
Time portion of the curve. A flat response curve is
automatic.
When a short delay pickup setting other than “NONE” is
selected, the Short Delay pickup and the Short Delay Time
portions of the overall curve are moved horizontally and
vertically in a similar manner to the inverse time protection
functions. Refer to Figures 3-5 and 3-6 for graphic
illustrations of this movement.
Note that the scope of protection offered by the Digitrip
3000 is a coordinated effort. This is especially true when a
number of protective functions, such as inverse time
overcurrent and short delay protection are combined into
one cooperative curve. Figure 3-8 shows a typical time-
current curve that has both inverse time overcurrent and
short delay protection, and an I
2
t curve shape selected.
Because of the pickup, time and curve shape selections
made for this illustration, a triangle (shaded area on the
illustration) is formed by the intersection of the different
time and pickup lines. Internally, the Digitrip 3000 design
looks at this particular curve as if the shaded triangular
area does not exist. Therefore, in an actual performance
situation, the short delay time function would take
precedence over that portion of the inverse time
overcurrent line forming the one leg of the triangle.
This does not create a problem from a protection or
coordination standpoint. In fact, it is recommended on
certain applications to set the minimum time the Digitrip
3000 Protective Relay can respond and where it will
intersect the inverse time overcurrent curve. If only the
Short Delay Time is required, it is recommended that the
Short Delay setting be set at 11 times (I
n
). It could,
however, cause confusion if the combination of protection
functions is not viewed as a coordinated activity. For
example, an individual might expect a tripping action based
on a selected low value for Inverse Time Overcurrent Time
Multiplier. The expected tripping action will not take place
at the expected time, if the Short Delay Time selected is in
the higher end of time selection possibilities. It should also
be noted that this situation is similar for other curve
selections. The only thing that changes with different curve
selections is the general shape of the triangle. When the
Short Delay Time setting is low enough, this situation will
not exist. In summary, for an inverse time and short time
cooperative curve, the minimum trip time cannot be less
than the short delay time setting.
Fig. 3-1 Digitrip 3000 Time-Current Characteristic Curves
