120 ELECTRICALCONTRACTOR | SEP. 14 | WWW.ECMAG.COM
> FOCUS SELECTIVE COORDINATION
To improve coordination in this example, the instantaneous
pickup setting of the adjustable 225A circuit breaker can be
set to “high,” as shown in Figure 5. With this setting, the overlap
is reduced and now begins at a higher value of current shown
by the dashed vertical line.
Although coordination is not perfect, the higher setting
has increased the range of currents where good coordination
exists. It should be recognized that, in most cases, coordination will not be perfect for all levels of current. However, with
appropriate device settings, the range of currents where good
coordination exists can be increased.
Coordination vs. protection
Improving coordination often requires that devices further
upstream must operate more slowly or be less sensitive than
devices closer to the load. However, it is also important not to
have devices operate too slowly, or electrical equipment may
not be adequately protected.
Equipment—such as transformers, motors, generators and
conductors—often has its own form of a time-current graph or
other protection criteria. This defines an upper limit of how much
current, and for what duration, the equipment can sustain a fault
before damage begins. A coordination study should strike a balance between setting devices with more time delay to improve
coordination with downstream devices and setting devices faster
to provide optimal protection of electrical equipment.
Care must also be exercised because setting a device too
low can introduce problems, such as when energizing a transformer or starting a motor across the line. A transformer can
experience a magnetizing inrush current, which can be 10 to
12 times its rated primary full-load current. However, it typically only lasts about 0.1 second. When a motor starts across the
line, it can draw a starting current (also known as locked-rotor
current), which typically can be four to six times its full-load
current or even greater, depending on the design.
In each case, if the instantaneous trip of a breaker is set too
low or if a selected fuse is too small, it may operate during these
momentary conditions and cause an unnecessary outage often
referred to as “nuisance tripping.”
Same concept, different devices
There are many other types of protective devices, including
electronic-trip circuit breakers, protective relays, fuses and
ground-fault devices. Although each type of device may have
a different design, settings and operating characteristics, the
fundamentals are the same. You should attempt to minimize
the overlap of the TCCs to reduce the likelihood of multiple
devices tripping, which can result in a larger outage.
Can short circuits and overloads be eliminated? No. Failures
happen, but with properly selected and adjusted overcurrent
protective devices, the extent of the outage that can be greatly
reduced. It is also likely that the stress level of your customers,
as well as your own, will be reduced, too.
PHILLIPS, P.E., founder of www.brainfiller.com and www.
ArcFlashForum.com, conducts training programs around the world
and is author of “Complete Guide to Arc Flash Hazard Calculation
Studies.” He is secretary of the IEEE 1584 Arc Flash Working Group
and a member of many other national and international standards
organizations. Reach him at firstname.lastname@example.org.
← Continued from page 118
Figure 4: With the “low” setting, the curves begin to overlap at
currents just above 900A, shown by the vertical dashed line.
Figure 5: With a “high” instantaneous pickup setting of the
adjustable 225A circuit breaker, the overlap is reduced and now
begins at a higher value of current shown by the dashed vertical line.