GE EntelliGuard G Application Manual page 50

Section 6. Application Data
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 mechani-
cally interlocked such that equivalent frames may
be installed within a specific cubicle or space. When
exchanging circuit breakers for maintenance pur-
poses, 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 accom-
modate, 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. UL 1066
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 635 volts. Any UL 1066
listed device must be tested at these higher voltages
if listed at the normal nominal voltage of 240, 480
and 600 Volts. UL 489 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.
Table 6.1
First Half-Cycle Peak at Specific Fault X/R Ratios
(excerpted from UL 489)
Power Factor X/R ratio Maximum Peak @ 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
17% 5.8 2.26
18% 5.5 2.23
19% 5.2 2.21
20% 4.9 2.18
100% 0.0 1.41
48
Courtesy of NationalSwitchgear.com
Figure 6.1
CB's Test Peak
= Derate Ratio
Fault Peak
Short circuit interrupting ratings are a measure-
ment 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, UL 489 circuit
breakers are tested at 20% power factor and UL
1066 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 2, 1.41.
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 UL 489 rating at