Thermal memory, Actual long time pickup is 112% of nominal pickup, Fuse shaped steep long time overcurrent – GE Industrial Solutions EntelliGuard TU Trip Units User Manual
Page 18
EntelliGuard TU Trip Units: UL/ANSI Models
DEH-4567B
Protection
8
©2012 General Electric All Rights Reserved
Thermal Memory
The Long Time and Short Time pick up algorithm also includes a cooling cycle that keeps track of current if it oscillates in
and out of pick up range. This Thermal Memory is also active in case the circuit breaker trips on Long Time or Short Time to
account for residual heating in conductors. If a circuit breaker is closed soon after a Long Time trip or Short Time trip, a
subsequent trip may happen faster than indicated by the time current curve due to the residual cable Thermal Memory
effect. In trips without control power, the Thermal Memory is powered from the trip battery. The cooling algorithm requires
up to 14 minutes to fully reset to zero.
Table 2-1 shows the nominal clearing and commit times for X multipliers of nominal pickup.
Table 2-1: Nominal Time Delays for Thermal Shaped Long Time Bands
X =
1.5
1.5
6
6
7.2
7.2
10
10
Commit
Clear
Commit
Clear
Commit
Clear
Commit
Clear
Min CB
4.25
8.04
0.20
0.33
0.14
0.24
0.072
0.137
C-2
12.7
24.1
0.60
0.94
0.42
0.66
0.21
0.35
C-3
25.5
48.1
1.21
1.84
0.83
1.28
0.43
0.67
C-4
34.0
64.1
1.61
2.45
1.11
1.70
0.57
0.89
C-5
51.0
96.2
2.41
3.66
1.67
2.53
0.86
1.32
C-6
67.9
128
3.21
4.87
2.22
3.37
1.15
1.75
C-7
84.9
160
4.02
6.08
2.78
4.20
1.43
2.18
C-8
102
192
4.82
7.29
3.33
5.03
1.72
2.61
C-9
119
224
5.62
8.49
3.89
5.87
2.01
3.03
C-10
136
256
6.43
9.70
4.44
6.70
2.29
3.46
C-11
153
289
7.23
10.9
5.00
7.54
2.58
3.89
C-12
170
321
8.04
12.1
5.56
8.37
2.87
4.32
C-13
204
385
9.64
14.5
6.67
10.0
3.44
5.18
C-14
238
449
11.2
17.0
7.78
11.7
4.01
6.04
C-15
272
513
12.9
19.4
8.89
13.4
4.59
6.90
C-16
306
577
14.5
21.8
10.0
15.0
5.16
7.76
C-17
340
641
16.1
24.2
11.1
16.7
5.73
8.61
C-18
374
705
17.7
26.6
12.2
18.4
6.30
9.47
Max CB
408
769
19.3
29.1
13.3
20.0
6.88
10.3
• Algorithm will not commit below 1.5 cycles, clearing time will not be less than 0.088 seconds.
• Actual Long Time pickup is 112% of nominal pickup.
Fuse Shaped Steep Long Time Overcurrent
The optional steeper fuse characteristic is a straight line K=I
4
t shape for application in systems where fuses and circuit
breakers are used together. Twenty-two different time bands are available in each trip unit. Figure displays minimum and
maximum bands. Table 2-2 displays the nominal time delays for each of the 22 bands at various multiples of nominal
current pickup.
Drawn bands also include a constant time component, which accounts for the slight widening evident in the time current
curve at the lower (right) end of the faster (lower) time bands.