Eaton Cutler-Hammer Digitrip 3000 Instructions For Installation, Operation And Maintenance page 28

Page 22
Fig. 3-2 Digitrip 3000 Time-Current Characteristic Curves
Note that the scope of protection offered by 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 3-7 shows a typical time-
current curve which has both inverse time overcurrent
and short delay protection, and an I
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 func-
tion 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 inter-
sect 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
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 Over-current 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
2
t curve shape
). It could, however,
n
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 a inverse time and
short time cooperative curve, the minimum trip time can-
not be less than the short delay time setting.
Instantaneous Protection
Instantaneous (short circuit) protection reacts to high
level fault currents. The instantaneous pickup setting
establishes the current level at which the relay's instanta-
neous trip relay will be energized with no time delay and
is the instantaneous setting times (I
Table 2.2 specifies the possible settings, which are from 1
through 25 and "NONE." If "NONE" is selected, the
instantaneous trip function is disabled and the discrimi-
nator option is offered. See Paragraph 2-1.4 (Phase
Instantaneous) to review the discriminator option details.
If an Instantaneous Setting other than "NONE" is
selected, the instantaneous portion of the overall curve
can be moved independently in a horizontal direction.
Figure 3-9 graphically illustrates this horizontal move-
ment.
The instantaneous protection (INST) is designed to typi-
cally provIde no less than 2 cycle total response time. To
provide this fast response time the rms current detection
level and display readout may differ somewhat from a
true rms ampere value, if a significant percentage har-
monic current is present.
I.B. 17555C
).
n
Effective November 1999