Application
Generally, electrical power system engineers calculate the
></R ratios of circuit rather than power factor. The procedures
for calculating short-circuit current and X/R ratios are de
scribed in Publication G ET-3550.
Envelopes of peaks are not
symmetrical about zero axis
Zero axis
Oscillogram of typical short circuit
Asymmetrical ac waveform
The figures below show the initial fault current displacement
of a pure circuit (symmetrical) and a reactive circuit with a
high X/R ratio. The actual current displacement of a reac
tive circuit depends on the exact moment the short circuit is
a pplies. There is a simple relationship between power factor
and X/R of a faulted circuit:
R
PF = - x 100
z
And: Z
=
+
(XJZ
R
Therefore PF =
+
x 100
PF = Power factor, %
R = Resistance i n ohms
Z
=
Impedance in ohms
>< = Reactance i n ohms
I nterrupting Rati ngs - Sta ndard And Hi-Break® Brea kers
Power Break® I I breakers are available i n two classifications
of interrupting ratings: Standard and Hi-Break breakers. All
ratings are without the need for external fuses or current
limiters. Hi-Break breakers' interrupting ratings are for those
circuits with unusually high levels of available fault current.
46
Table 46.1
Interrupting Rating Breaker Performance In RMS Symmetrical kA
Frame Size
U L 489
1600 -
50/60 Hz
800A
2000A
Standard
240V
65
85
480V
65
65
600V
50
50
Hi-Break'"'
240V
100
125
480V
100
100
600V
65
65
Short Time
10 5 sec.)
25
40
IEC 947 -2 Ratings
415V, 50/60 Hz
leu
-
7 5
lcs
-
56
lew (1 sec)
-
40
Q) Requires 4000A construction
Table 46.2
Estimated 400 Hz Interrupting Ratings
RMS Symmetrical Amperes - Not UL Listed
Standard Break
Frame Size
Max. Rating Plug
120/240
(Amps)
Ampacity@
800
600
6500
1600
1000
8500
2000
1200
8500
2500
1600
10000
Hi-Break
Frame Size
Max. Rating Plug
120/140
(Amps)
Ampocity@
800
600
10000
1600
1000
12500
2000
1200
12500
2500
1600
20000
2500 -
3000
100
100
85
200
150
100
42
7 5Q)
45Q)
50Q)
277/480
5000
6500
6500
10000
277/480
10000
10000
10000
15000
4000A
100
100
85
200
150
100
42
85
25
50
346/600
4200
5000
5000
8500
346/600
6500
6500
6500
10000
@
No additional thermal derating is required for above-maximum rating
plug ratings far frame sizes noted. Note: 400Hz interrupting ratings ore
based on engineering judgment, taking into consideration the operating
characteristics of insulated case circuit breakers and the worldwide lack
of test facilities to verify performance. Power+™ trip units and the energy
management functions in MicroVersaTrip PM™ trip u nits are suitable for
50/60 Hz applications only.
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