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 (a) Apparent power and voltage ratings, as well as the impedance and connections of
all transformers.
(b) Normal and emergency switching conditions.
(c) Nameplate ratings and subtransient reactance of all major motors and generators, as
well as transient reactances of synchronous motors and generators, plus synchronous reactances
of generators.
(d) Conductor sizes, types, and configurations, and type of insulating material.
(e) Current transformer ratios.
(f) Relay, direct-acting trip, and fuse ratings, characteristics, and ranges of adjustment.
(g) Cable lengths, particularly if an impedance diagram is not included.
4.7.7.2 Short-Circuit Currents. The second requirement for a coordination study is a
complete short-circuit current detail, as described earlier. The short-circuit current study should
include maximum and minimum expected three-phase and ground fault duties, as well as
available short-circuit current data from all sources.
4.7.7.3 Time-Current Characteristics. The third requirement for a coordination study is
the determination of time-current characteristics of all the devices under consideration.
4.7.7.4 Maximum Loading. The fourth requirement for a coordination study is the
determination of expected maximum loading on any circuit considered. Any limiting devices
such as utility settings on relays must be noted.
4.7.8 Plotting Procedures. The practice of using overlays for making coordination curves
removes much of the tedious work necessary in making coordination studies. Once a specific
current scale has been selected, the proper multipliers for the various voltage levels considered in
the study are calculated. Protective device curves for the various devices are then plotted on
graph paper. Preferably, the curves are plotted progressively as each circuit is studied, starting
with the device furthest from the source. This procedure will show whether or not the proposed
time-current characteristic of each successive device coordinates with the one on its load side.
Short circuit calculations can now be accomplished using computers. Use of computers provides
increased accuracy, shorter computation time, and accurate plotting. Several sophisticated
software programs are now available for personal computers (PCs), as well as for mini and
mainframe computers. With the inception of computers and associated software programs, short
circuit studies can be performed in conjunction with voltage drop calculations. The main
advantages realized in using this approach are system protection and system accommodation for
future load growth, and the sizing of down line devices, accordingly.
4-36
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