I.L. 40-386.4
3-14
For the best sensitivity, “GB Pickup” should be set at 0.5 amperes, this is normally ade-
quate for most applications.
c.
“GBT Curve” is the time delay setting of the ground backup function. As shown in Figures
2-32 through 2 -38, the “GBT Curve” is settable in steps of one from 1 to 63. As with any
time delay overcurrent function, the time delay setting must be coordinated with other over-
current devices.
d.
“GB Dir.” is the setting for directional control selection. The ground backup function be-
comes a forward-directional torque control overcurrent ground function if “GB Dir.” is set
to “Yes”. If “GB Dir.”is set to “No”, the overcurrent ground backup function is non-direc-
tional and will produce a trip output for faults in the forward and reverse directions.
The following equation can be used to calculate the trip time for all CO curves from CO-2
through CO-1 :
T (sec) =
(for 3I0 >1.5 “GB Pickup”)
T
(sec) =
(
for 1< 3I
0
< 1.5 “GB Pickup”
)
Where 3I
0
=
IF = Applied fault current
“GB Pickup”
= Pickup setting
“GBT Curve”
= Time curve dial setting (1 to 63).
T
0
, K, C, P and R are constants, and are shown in Table 3-1.
Taking the CO-8 curve set as an example (see Figure 2-36), assuming that the maximum
3Io of unbalanced load is 0.2A, the minimum ground fault current for a fault two buses away
is 10A, and 0.7 seconds is required for coordination with current of 20 times the “GB Pick-
up” setting, then the settings of the ground overcurrent backup function should be as
shown below:
10/2 > “GB Pickup” > 2 x 0.2 set “GB Pickup” = 0.5
Choosing from the curves, in Figure 2-36
, for 0.7 seconds at 20 times the “GB Pickup”
setting, “GBT Curve” should be set to “24”. Set “GB Type” to “C0-8” and set “GB Dir”
to “YES” if directional control is required.
REL 301/302 offers two reset characteristics for the ground backup overcurrent function,
instantaneous or time delayed. Instantaneous reset, as the name implies, means reset with
no intentional time delay. Time delay reset function is a linear characteristic shown in Fig-
ure 1 and is intended to replicate the reset characteristics of electromechanical overcurrent
relays.
T
0
K
3
I
0
C
–
(
)
P
--------------------------
+
GBT Curve
24 000
,
--------------------------------
×
R
3
I
0
1
–
(
)
---------------------
GBT Curve
24 000
,
--------------------------------
×
I
F
GB Pickup
-------------------------------
