UFC 3 -520-01
June 10, 2002
Determination of system components' ability to withstand mechanical and thermal
stresses.
Evaluation of time -current coordination of prote ctive devices.
2-4.4.3
Calculate short-circuit currents for the following short circuit duration times:
2-4.4.3.1 First-cycle maximum symmetrical values are always required. These are
often the only values needed for low voltage breakers with instantane ous trip devices
and for fuses.
2-4.4.3.2 Obtain maximum values (1.5 to 4 cycles) for medium-voltage and high-
voltage circuit breaker applications.
2-4.4.3.3 Obtain reduced fault current values (about 30 cycles) for estimating the
performance of time-delay relays and fuses and for low voltage power circuit breakers
without instantaneous trip devices. These values must be calculated so that the proper
current is known for setting time delay protective relays. Also, minimum values must be
calculated to de termine whether sufficient current is available to open the protective
device within a satisfactory time.
2-4.4.4 Appendix B provides examples of how the maximum available short circuit
current can be affected by system modifications.
2-5
POWER FLOW ANAL YSIS.
2-5.1
A power flow analysis (also referred to as a load flow analysis) determines if
electrical system equipment is potentially overloaded and confirms that adequate
voltage is available throughout the system. Perform power flow analyses by a computer
program designed for this purpose. The computer program used to perform a short
circuit study usually also has power flow analysis capability.
2-5.2
Perform a power flow analysis to determine if the following equipment types
are potentially overloaded at expected design conditions:
Cables and conductors.
Busways.
Circuit breakers.
2-5.3
For small systems or for small modifications to large systems, this analysis
can be performed manually by summing loads and comparing load currents to
2-8
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