Thermal Memory; Short Time Protection; Short Time Pickup - GE EntelliGuard G Application Manual

DET-653C
24-May-12

Thermal Memory

The Long Time and Short Time pickup algorithm also
includes a cooling cycle that keeps track of current if it
oscillates in and out of pickup 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 Unit battery. The
cooling algorithm requires up to 14 minutes to fully reset to
zero.

Short Time Protection

Short Time Pickup

The EntelliGuard TU provides a wide range of Short Time
pickup settings, I 2 t characteristics and time bands to
optimize selectivity while not sacrificing clearing time
unnecessarily. Short Time pickup settings range from 1.5 to
12 times the Long Time pickup setting for the EntelliGuard G
circuit breakers in Envelopes 1 and 2. The maximum Short
Time pickup for Envelope 3 is 10 times. In Power Break,
WavePro, AKR and other circuit breakers, the maximum
Short Time pickup may be lower. The transition between
Long Time band and Short Time function may happen at the
horizontal Short Time Delay Band rather than the vertical
Short Time pickup, depending on the following:
•
How fast or what type of Long Time band is chosen
•
What Short Time pickup is selected
•
What Short Time Delay is selected
An example of this may be seen in Figure 3.5 for the
combination of Long Time Delay Band and Short Time Delay
Band shown in Figure 3.6, any Short Time pickup setting
larger than 6X will result in the Long Time band intersecting
with the horizontal Short Time delay band rather the vertical
Short Time pickup.
Short Time Bands
The EntelliGuard TU comes with a wide range of adjustable
Time Delay Bands, ranging from a minimum of 25ms (clears
in 80ms, 55ms for Sensing, Operating and Clearing Time) to
417ms (clears in 472ms). The bands are specially designed
to pickup above various circuit breakers and trip systems to
provide required selectivity while not sacrificing any more
clearing time than required to provide the best possible arc
flash protection.
The time bands are shown in Table 3.6 and Figure 3.7.
©2012 General Electric All Rights Reserved
EntelliGuard
Section 3. EntelliGuard TU Trip Unit System
Figure 3.5. Short Time Pickup
Figure 3.6. Short Time Transition
® G Circuit Breaker Application Guide
19