Application
Table 47.1
Interrupting Rating Multipliers-
For High X/R Rations (Low Power Factors)
When the X/R ratio of a faulted circuit is higher than 4.899 (power factor of
0.20) the breaker"s interrupting rating must be derated. The multiplying fac
tor is shown in the table below.
Power Factor
X/R Ratio
I nterrupting Capacity
Derating Factor
0.20
4.899
1.000
0.19
5.167
0.98
0.18
5.465
0.97
0.17
5.797
0.96
0.16
6.169
0.95
0.15
6.169
0.94
0.14
7.072
0.93
0.13
7.627
0.91
0.12
8.273
0.90
0.11
9 036
0.89
0.10
9.950
0.88
0.09
1 1066
0.87
0.08
12.460
0.85
0.07
14.251
0.84
0.06
16.637
0.83
0.05
19.974
0.82
0.04
24.980
0.81
Example Data
A Power Break® II stationary feeder breaker consisting of
770A of continuous loads @ 480Vac required. Available
short-circuit current at the breaker is 63kA rms symmetrical
with a power factor of 0.13 IX/R ratio of 7.6271. Average tem
perature of the room air could be as high as 45a C (air sur
rounding the enclosed breaker will be 15° C higher.) Altitude
above sea level at the installation site is 6500 feet. The loads
are considered critical.
Selection
Since the Power Break
II
breakers are labeled as suitable to
carry 100% of their ampere rating continuously,* there is no
need to multiply the continuous load by 1.25 as would be
required when using a non 100% rated breaker.
Per U L 489, Power Break
II
breakers are short-circuit tested
with a power foetor of 20% (X/R ratio of 4.889); the actual
faulted circuit power factor in this example, however, is 13%
IX/R ratio of 7.627) and this requires derating of the breaker's
interrupting rating per Table 47.1. Multiplying the standard
break IC rating of 65kA by .91 gives 59.1kA rating when the
power factor is 13% and not 20% minimum.
The available short-circuit current is 63 kA and thus the stan
dard break breaker is not adequate and could not be used.
The Hi-Break® units are rated 100kA and are acceptable.
Further derating must be done for high ambient conditions.
Per Table 44.2 Factor "B", 60°C breaker ambient tempera
ture requires a derating factor of 1.10. Similarly, the 6500-ft.
altitude requires a 1.04 derating factor- see Table 45.1 Factor
"D".
This feeder is a critical circuit and therefore a Safety Factor F
of 1.10 will be applied.
Next multiply the load, lA, by the several factors AxBxCxDx
ExF to determine the minimum frame ampere rating:
lc8=1A X
A
X
B
X
C
X
D X E X F
=
770A
X
1.00
X
1.1
X
1.00
X
1.04
X
1.00
X
1.1
=
770 A
X
1.258 or 969A.
Select a 1600A H i-Break frame with either a 1000A or a
1600A sensor rating with a n 800A rating plug because the
rating plug is not affected by a ny of the load characteris
tics. An 800A rating plug, however, may not permit the 10%
safety factor for additional current-carrying capacity above
the actual 770A. An a lternate choice would be to select a
1000A rating plug and set the current setting (long time pick
up) to 0.9 and cable or bus for 900A a m pacity. If the alter
nate choice is used, some local inspecting authorities may
require sealing of the sealable trip unit and accessory door to
prevent upward adjustment of the 0.90 current setting.
Finally, if insulated conductors are to be used, the high -am
bient temperature (60°C) will require using 125°C minimum
insulation rating with the conductors sized per the 75°C
a mpacity tables.
Conductors may be sized to the 800A rating plug value,
if used, or to 900A current setting value if a 1000A rating
plug is used. Additional derating of the conductors may be
required for (1) more than three conductors in a raceway
and (2) ambient temperatures other than 30°C. Refer to the
National Electrical Code, Article 3 10.
B reaker Selection
User may select a front connected stationary, Hi-Break
breaker, 1600-amp frame, standard instantaneous with
1000-amp current sensor. Rating plug is 1000 amps. An
enhanced M icroVersaTrip Plus
TM
trip unit with LT. ST, and
1.
•
Breaker:
Cat. No. SHF16B210
•
Rating Plug:
Cat. No. TR10B 1000
•
Trip Unit:
Cat. No. B210LSI
*4000A Stationary 80% rated.
47
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