UFC 3 -520-01
June 10, 2002
selectivity is possible only when circuit breakers with delayed trip devices are used in all
circuit positions except the one closest to the load.
9-2.1.3 Evaluate electrical coordination and include the evaluation as part of the
system design package by the system designer , if applicable . The construction
contractor is responsible for installing a system that meets the electrical coordination
design requirements. Medium voltage relay coordination is often a separate task after
completion of project design and installation; determine who will perform this analysis
and specify required settings.
9-2.2
Coordination Study Description.
9-2.2.1 The system designer will perform a coordination study as part of the system
design. The coordination study of an electric power system consists of an organized
time-current study of all protective devices in series from the utilization device to the
facility source (the service entrance). The objective of a coordination study is to
generate a comprehensive one -line-diagram representation of the electrical system
performance to abnormal currents. Additionally, the study must determine the
characteristics, rati ngs, and settings of overcurrent protective devices. This is intended
to ensure that protective devices will isolate a fault or overload anywhere in the system
with the least possible effect on unfaulted sections of the system. At the same time, the
devices and settings selected must provide satisfactory protection against overloads on
the equipment and must interrupt short circuits as rapidly as possible.
9-2.2.2 The coordination study provides information necessary for the selection of
instrument transformer ratios; protective relay characteristics and settings; fuse ratings;
and low voltage circuit breaker ratings, characteristics, and settings. The coordination
study also provides information regarding relative protection and selectivity,
coordination of devices, and the most desirable arrangement of these devices. To
obtain complete coordination of the protective equipment applied, determine the
following short -circuit currents for each bus (refer to paragraph 2 -4 for other criteria
associated with short circuit studies).
9-2.2.2.1 Momentary Duty. The maximum and minimum 0 to 1 cycle momentary duty
currents are used to determine the maximum and minimum currents to which
instantaneous and direct -acting trip devices must respond. They also verify the
capability of the applied apparatus to withstand the maximum electromechanical
stresses to which they could be subjected.
9-2.2.2.2 Interrupting Duty. The maximum 3 to 8 cycle interrupting duty current, at
maximum generation, is used to verify the rati ngs of circuit breakers, fuses, and cables.
This is also the value of current at which the circuit protection coordination interval is
established. The maximum 3 to 8 cycle interrupting duty current, at minimum
generation, is needed to determine whether the circuit protection is sensitive enough to
protect against damage that could result from low level faults.
9-2
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