Allen-Bradley Rockwell Automation 857 User Manual page 129

Table 3.49 – Maximum Secondary Current Sealing
Current input
I , I , I and I
L1 L2 L3 0Calc
I = 5 A
0N
I = 1 A
0N
I = 0.2 A
0N
The availableI values depend on the order code. The Allen-Bradley 857-3C7____ has 1A
0N
and 5 A I inputs while the Allen-Bradley 857-3D7____ has 0.2 A and 1 A I
0
Example 1 of limitation:
CT = 750/5
Application mode is Feeder
Example 1 of limitations
CT = 750/5
Application mode is Feeder
CT = 100/1 (cable CT is used for residual current)
0
The cable CT is connected to a 1 A terminals of the available I
inputs.
For overcurrent stage I> the table above gives 12.5 A. Thus the
maximum setting for I> stage giving full inverse delay range is 12.5
A / 5 A = 2.5 xI = 1875 A
N
For earth fault stage I > the table above gives 0.5 A. Thus the
0
maximum setting for I > stage giving full inverse delay range is 0.5
0
A / 1 A = 0.5 xI = 50 A
0N
Example 2 of limitations
CT = 750/5
Application mode is Motor
Rated current of the motor = 600 A
I (= I +I +I ) is used for residual current
0Calc L1 L2 L3
At secondary level the rated motor current is 600/750*5 = 4 A
For overcurrent stage I> the table above gives 12.5 A. Thus the
maximum setting giving full inverse delay range is 12.5 A / 4 A =
3.13 xI = 1875 A
MOT Primary
For earth fault stage I > the table above gives 12.5 A. Thus the
0
maximum setting for I > stage giving full inverse delay range is 12.5
0
A / 5 A = 2.5 xI = 1875 A
0N
Protection Functions
Maximum measured
secondary current
250 A
50 A
10 A
2 A
.
Primary
.
Primary
.
.
Primary
3-79
Maximum secondary
scaled setting enabling
inverse delay times up to
full 20x setting
12.5 A
2.5 A
0.5 A
0.1 A
inputs.
0
0
857-UM001A-EN-P – July 2009