DET-653C EntelliGuard
®
G Circuit Breaker Application Guide
24-May-12 Section 6. Application Data
©2012 General Electric All Rights Reserved
under the applicable UL, IEC and ANSI standards as shown in
the various ratings tables. Switchgear equipment is typically
sized to match the frame ratings and physical size.
Switchboards may be sized to match the installed circuit
breaker’s sensor rating.
Equipment cubicles or device mounting provisions are
normally mechanically interlocked such that equivalent
frames may be installed within a specific cubicle or space.
When exchanging circuit breakers for maintenance
purposes, care should be exercised so that there is no
attempt to install a circuit breaker with a rating higher than
what the equipment was designed to accommodate, even if
the physical size seems similar.
Short Circuit Interrupting Ratings
The EntelliGuard G family of UL devices is available with short
circuit ratings as high as 150kA at 480V, and withstand
ratings as high as 100kA. UL1066 and ANSI C32 ratings are
based on various tests performed at rated voltage plus 6%.
Hence it is common to see low voltage circuit breakers listed
as suitable for 254, 508 and 635V. Any UL 1066 listed device
must be tested at these higher voltages if listed at the
normal nominal voltage of 240, 480 and 600V. UL489 circuit
breakers are tested at the nominal voltage. Both circuit
breakers are subject to dielectric testing after interrupting
faults to ensure the ability to sustain voltage.
Short circuit interrupting ratings are a measurement of the
circuit breaker’s ability to interrupt a particular value of fault
current at a maximum power factor (also expressed as X/R
ratio). For fault currents above 20,000A RMS, UL489 circuit
breakers are tested at 20% power factor and UL1066 circuit
breakers are tested at 15% power factor. When fault
currents have power factors lower (higher X/R) than the test
power factor, the circuit breaker’s short circuit rating must
be adjusted to compensate for the fault’s lower PF. The
adjustment is performed as shown in Figure 6.1. Table 6.2
shows various asymmetrical peak ratios for various power
factors and X/R ratios. A power factor of 100% yields a
symmetrical fault and hence the peak over RMS ratio is 1.41.
Figure 6.1. Short Circuit Rating Adjustment Equation
CB’s Test Peak = Derate Ratio
Fault Peak
Table 6.1. First Half-Cycle Peak at Specific Fault X/R
Ratios (excerpted from UL489)
Maximum Peak at 1/2
Cycle
5% 20.0 2.63
6% 16.6 2.59
7% 14.3 2.55
8% 12.5 2.52
9% 11.7 2.50
9% 11.1 2.49
10% 10.0 2.46
11% 9.0 2.42
12% 8.3 2.39
13% 7.6 2.36
14% 7.1 2.34
15% 6.6 2.31
16% 6.2 2.28
Consider a system where the fault current has an X/R ratio
of 10 and the calculated RMS value is 92,000. A circuit
breaker with a stated interrupting capability of 100,000 is
being considered. Since the fault’s X/R ratio is higher than
the UL489 rating at 4.9, the circuit breaker’s rating, 100kA,
must be multiplied by 2.18/2.46 (from Table 6.1). The ratio
is .89. Hence, the circuit breaker must be considered as if it
is only rated for 89,000A. The UL489 100kA rated circuit
breaker is not suitable for the application because its de-
rated 89kA rating is below the available fault current of
92kA.
Consider an equivalent UL1066 circuit breaker tested at an
X/R ratio of 6.6. The UL1066 circuit breaker must be de-
rated by 2.31/2.46, which yields 0.94. The UL1066 circuit
breaker is then de-rated to 0.94 times 100kA. The UL1066
circuit breaker is rated higher than the available fault
current of 92kA and hence could be considered for the
application.
Withstand Ratings, Selective Waveform Recognition-,
RELT- and Override-Instantaneous Protection
Instantaneous adjustment capability for any circuit
breaker is intrinsically connected to the circuit breaker’s
withstand rating and the trip system’s sensing methods. In
the case of the EntelliGuard G family of circuit breakers,
high withstand ratings allow for a broad range of
instantaneous trip options. The trip system relies on high
accuracy air core sensors not encumbered by the
saturation concerns associated with iron core sensors
used in many other trip systems. This allows the
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