UFC 3 -520-01
June 10, 2002
APPENDIX B
CALCULATION METHODS AND EXAMPLES
B-1
SHORT CIRCUIT CURRENT EFFECTS .
B-1.1
Electrical distribution systems must be designed to withstand the maximum
expected fault (short circuit) current until the short circuit current is cleared by a
protective device. This is a fundamental electrical requirement. NEC Article 110.9
(2002 Edition) requires that all protective devices intended to interrupt current at fault
levels must have an interrupting rating sufficient for the nominal circuit voltage and the
current that is available at the line terminals of the equipment. For this reason, the
maximum available short circuit current must be determined for all locations throughout
the electrical system.
B-1.2
Figure B-1 shows a simplified short circuit study for a small section of an
electrical distribution system. The available fault current is shown at the service bus
and at an MCC bus. As can be seen, the bulk of the short circuit current is provided by
the distribution system through the transformer, with a lesser amount of current
provided by each of the motors.
Figure B -1. Sample Short Circuit Results--1 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-1.3
The transformer size has a significant effect on the available short circuit
current. Whenever a transformer is replaced with a larger transformer, perform a short
B-1
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