116 ELECTRICALCONTRACTOR | SEP. 14 | WWW.ECMAG.COM
> FOCUS BY JIM PHILLIPS
Production came to a screeching
halt. Except for a few faint alarms
in the distance, an eerie quiet fell over a large part of the
facility. It was quite an ominous sight to see the emergency lights attempting to penetrate the darkness from
the power outage.
What happened? The investigation that followed
determined that an insulation failure caused a fault on a
branch circuit. Instead of only that circuit’s overcurrent
device tripping, several other devices closer to the utility
(upstream) also tripped, which resulted in a much larger
outage. The report listed the cause of such a large outage as a lack of “selective coordination.”
What is selective coordination?
Simply put, selective coordination is an attempt to have protective devices trip in sequence so that only the device closest
to a faulted circuit interrupts. This minimizes the extent of an
outage by keeping the other circuits energized. I use the word
“attempt” because, although perfect selective coordination is
the desired objective, it will rarely happen perfectly. Sometimes, several devices trip together.
For example, if a fault occurs on a branch circuit fed from
panel PP- 1, as shown in Figure 1, only the 70-ampere (A) device
should interrupt. The 225A feeder device upstream should not
trip and neither should the main.
However, the other upstream devices, including the 225A
feeder and the main, also see the short-circuit current flowing
to the fault. Depending on each device’s operating characteristics and the magnitude of current, they may also trip.
The concept of selective coordination is pretty simple. It is
much like a race between protective devices to see which will
interrupt first. The most common approach for evaluating the
“race” is to use special graphs known in the industry as time-current curves (TCCs).
Every overcurrent protective device, such as circuit breakers,
fuses and overcurrent relays, will have its own unique TCC
that defines its tripping characteristic as a function of current
and time. Since a protective device’s TCC depends on many
factors—such as the device rating, design and type—the TCCs
are usually obtained from the device manufacturer or from
databases included with many computer programs used for
performing coordination studies.
Figure 2, on page 118, illustrates the TCC of a 70A molded-case circuit breaker. The industry convention is that every TCC,
including this one, uses a logarithmic scale, which means current and time increase by orders of magnitude such as 1, 10, 100,
1,000 instead of 1, 2, 3, 4.
The horizontal axis of the graph represents current in
amperes. Although this axis begins on the left side at 0.5A, a
“scaling factor” shown at the bottom is often used, such as “×
Continued on page 118 →
Figure 1: If a fault occurs
on a branch circuit fed
from panel PP- 1, only
the 70A device should
interrupt. The 225A
feeder device upstream
should not trip and
neither should the main.
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