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 circuit breakers should be selected to be no higher than necessary to override transient
overcurrents associated with the energizing of load equipment. They should also coordinate with
downstream protective devices. The adjustable instantaneous trip elements of trip devices on
molded-case circuit breakers and low-voltage power circuit breakers should be set no higher than
necessary to avoid nuisance tripping.
4.6.8 Coordination. Circuit breaker performance must be coordinated with upstream and
downstream circuit breakers and protective devices. The objective in coordinating protective
devices is to make them selective in operation with respect to each other. Coordinating device
operation reduces the effects of short circuits by disconnecting only the affected part of the
system (i.e., the zone of protection containing the faulted equipment). Normally, coordination is
demonstrated by plotting the time-current curves of the circuit breakers showing that no
overlapping occurs between the curves of series circuit breakers and fuses. Often selectivity is
possible only when circuit breakers with delayed trip devices are used in all circuit positions
except the one closest to the load. System coordination is discussed in the next section.
4.7 SYSTEM COORDINATION STUDY. 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 source. This study compares the time it takes the individual devices to operate
when certain levels of normal or abnormal current are sensed.
4.7.1 Objective. The objective of a coordination study is to generate a comprehensive
one-line-diagram of the distribution system. Additionally, the study will determine the
characteristics, ratings, and settings of overcurrent protective devices. This will 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.
The coordination study provides data useful for the selection of instrument transformer ratios,
protective relay characteristics and settings, fuse ratings, 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.
4.7.2 Short-Circuit Currents. To obtain complete coordination of the protective equipment
applied, it may be necessary to obtain some or all of the following information regarding
short-circuit currents for each local bus.
4.7.2.1 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
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