UFC 3 -520-01
June 10, 2002
circuit study for the larger transformer to verify that all equipment is properly sized for
the increased short circuit current. Figure B -2 shows an example of the increase that
might be observed as a transformer size is increased from 1 MVA to 2 MVA.
Comparing Figure B -1 to Figure B -2, the MCC bus fault current has increased from
18,000 amperes to over 30,000 amperes. Although the system breakers might have
been adequately rated for use with the 1 MVA transformer, the larger 2 MVA
transformer could allow a short circuit current in excess of the breakers' ratings. This
example illustrates the importance of evaluating the entire electrical system whenever a
change is made.
Figure B -2. Sample Short Circuit Results--2 MVA Transformer
Utility
Service Bus
Note:
All currents are
in kilo-amperes.
Transformer
MCC Bus
M
M
M
Motor 1
Motor 2
Motor 3
B-1.4
The computer program used for short circuit analysis should be capable of
identifying overduty breakers (breakers in which the short-circuit current, including
asymmetric current effects, exceeds the breaker interrupting rating). Figure B -3 shows
an example of overduty breakers. The feeder breaker to the MCC bus is 7 percent
below its interrupting rating and the downstream load breakers are 33 percent over their
interrupting rating.
B-2
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