Selectivity With Molded Case Circuit Breakers And Other Devices; Figure 3.1. Arcing Current - GE EntelliGuard G Application Manual

DET-653C
24-May-12

Figure 3.1. Arcing Current

Figure 3.1 demonstrates the wide range of possible arcing
currents predicted by the IEEE standard's calculations. The
actual arcing currents may be lower or higher when
consideration is given to the potential error in short circuit
calculations, fault current data provided by the utility
company, and variance in the actual arcing gap or
enclosure's geometry where the arc occurs. Of particular
concern should be the lower end of the possible range of
current that may fall below the Instantaneous pickup of a
circuit breaker or the current limiting threshold of a fuse.
Dangerous incident energy may quickly increase when
interruption time increases from a few cycles to a few
seconds, even for low arcing current.
Selectivity with Molded Case Circuit Breakers and Other
Devices
The EntelliGuard TU trip unit's Waveform Recognition
Selective Instantaneous provides a unique selectivity
capability. When used in conjunction with current limiting
devices downstream, the trip unit may be set low, and still
maintain significant levels of selectivity. Table 3.1 lists the
minimum instantaneous setting above specific GE circuit
breakers and the levels of selectivity achieved. Testing and
documentation of the selectivity capability of the
EntelliGuard Trip unit will be expanded as more test data
becomes available. Always consult GE Selectivity reference
publication DET-537 for the latest information or contact
your GE sales representative for up to date information.
Selectivity above current limiting fuses may also be
expected. The required minimum setting may be estimated
by examining the peak let-through curve for the fuse. Find
the peak let-through current value that is 1.41 times the
prospective RMS current. The trip unit should be set such
that the instantaneous pickup is above that peak value. The
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EntelliGuard
Section 3. EntelliGuard
circuit breaker and fuse may then be selective up to the
values identified in Table 3.1. The fuse and circuit breaker
combinations have not been tested so performance cannot
be guaranteed at this time; however, selectivity is expected
for arcing and bolted faults up to the values indicated in the
above selectivity table.
Protection between circuit breakers using the EntelliGuard
TU trip unit may be further improved using zone selective
interlocking. The trip unit's unique instantaneous zone
selective interlocking which can increase selectivity for UL
489 EntelliGuard G mains while allowing to trip
instantaneously for faults within their zone of protection
allows significant selectivity and arc flash protection at the
same time.
Selectivity for an EntelliGuard TU ground fault function and a
circuit breaker below may be enhanced by the trip unit's
selective ground fault function and zone selective
interlocking. Figure 3.2 demonstrates an 800A circuit
breaker under a 1200A ground fault set to be fully selective.
See also Tables 3.2 and 3.3.
Figure 3.2. Selectivity Curve
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® G Circuit Breaker Application Guide
® TU Trip Unit System