DET-653C EntelliGuard
®
G Circuit Breaker Application Guide
24-May-12 Section 3. EntelliGuard
®
TU Trip Unit System
©2012 General Electric All Rights Reserved
The EntelliGuard TU trip unit has Short Time bands under
100msec designed to optimally fit above the Instantaneous
clearing times of various GE circuit breakers. Table 3.4 lists
the suggested Short Time band that may be used above
specific GE circuit breakers for the fastest coordination and
consequently fastest EntelliGuard G circuit breaker
interruption.
Also listed is the Hazard Risk Category for various available
fault currents calculated at the listed Short Time band
clearing times and Instantaneous clearing times provided by
the EntelliGuard TU trip unit’s Waveform Recognition
Selective Instantaneous and its Reduced Energy Let-Through
(RELT) Instantaneous trip. The Hazard Risk Category (HRC)
levels shown are calculated for a solidly grounded, 480V
system, 32mm arc gap, 18 in. working distance and arc in
box. Using the optimized Short Time band allows the system
to stay at HRC2 most of the time and always less than HRC4.
However, use of the RELT Instantaneous trip allows the
system to protect at HRC1 or 2 for available fault currents as
high as 90kA.
The Waveform Recognition Selective Instantaneous
The Instantaneous algorithm in the EntelliGuard TU trip unit
will allow a large switchboard or switchgear circuit breaker
to be set low yet maintain selectivity. For example, when set
to protect the conductors to a motor control center, an 800A
feeder circuit breaker’s Instantaneous may be set as low as
8kA nominal. This should provide complete selectivity above
the typical 150A and smaller circuit breakers or fuses found
in a motor control center.
Allowing for 10% pickup tolerance in the Instantaneous
pickup of the feeder circuit breaker would indicate that the
feeder would pickup Instantaneously for arcing fault
currents approximately 9kA or higher. From Figure 3.1 it can
be seen that fault currents of 20-25kA and higher would
seem to reliably produce arcing current above 9kA in a
variety of configurations. From Table 3.4 it can be seen that
an EntelliGuard G circuit breaker clearing instantaneously
can maintain HRC1 in a system with up to 42kA available
and HRC2 in systems with up to 100kA available – excellent
arc flash protection and selectivity simultaneously!
Note: These incident energy calculations do not take into
account motor contribution or other factors that may affect
expected levels of incident energy or hazard. For a thorough
understanding of the incident energy available or HRC in a
power distribution system, an Arc Flash Study should be
conducted by engineering personnel qualified to provide
such analysis.
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