118 ELECTRICALCONTRACTOR | SEP. 14 | WWW.ECMAG.COM
10” or “× 100,” to adjust the axis to represent larger magnitudes
of current. Figure 2 uses a scale of × 10, which means each
value of current listed is multiplied by 10. Therefore, 0.5A on
the graph becomes 0.5 × 10 or 5A, 1 multiplied by 10 is 10A and
so on. The vertical axis on the left side of the graph represents
time plotted in seconds.
The TCC of the 70A breaker is shown as a wide band. The
sloped part of the curve toward the upper left of the graph is
often referred to as the thermal or overload region. The upper
left of the TCC lines up with 70A on the graph, and a dashed
vertical line is drawn at this point. If a current is less than 70A
(i.e., to the left of the 70A vertical line), the circuit breaker
should not trip. This is labeled “No Trip.” However, if the current exceeds 70A, the breaker will operate for a specific amount
of time defined by where the current value intersects the curve.
The thickness of the curve is a function of manufacturing
tolerance and circuit-breaker-opening time. With mass produced protective devices, the possibility that every device will
trip exactly the same is not realistic. A plus and minus tolerance that defines the left and right sides of the curve is used.
A device should not operate for current and time points below
and to the left of the left side of the band. It should have tripped
and cleared the fault by the time the right side of the band is
reached. The actual operation should occur somewhere in
between the left and right sides.
The vertical band in the middle of the TCC defines the
instantaneous pickup. This corresponds to the minimum
amount of current required for the breaker to trip as fast as
possible in the instantaneous region. In the power-system
world, the term “instantaneous” means no intentional time
delay. However, there will be some minor delay as a result of
the breaker physically opening. This is illustrated by the thickness of the horizontal band on the lower right side of the TCC.
Many circuit breakers have a setting function that permits
the user to select the magnitude of current where the breaker
begins to operate instantaneously. On a TCC, this appears as a
movable vertical instantaneous band. Figure 3 illustrates the
TCC of a 225A adjustable molded-case circuit breaker with
both “low” and “high” instantaneous pickup settings.
Example—coordinating two circuit breakers
To evaluate selective coordination between the 70A and adjustable 225A circuit breakers as shown on the single-line diagram
in Figure 1, each device’s TCC must be plotted on the same
graph. The objective is to eliminate or minimize any overlap
of the device’s TCCs by selecting an appropriate instantaneous
pickup setting for the adjustable breaker. If the current falls
within the overlapping vertical bands, it is uncertain whether
each device would time-delay or trip instantaneously. However,
if the current is beyond the vertical band of both devices, they
should trip simultaneously.
To improve coordination, the available adjustable circuit-breaker settings become very important. Figure 4, on page
120, illustrates coordination between the two devices when
the 225A breaker is set to “low.” With this setting, the curves
begin to overlap at currents just above 900A, shown by the
vertical dashed line. If a fault current is less than 900A, and
occurs downstream of the 70A breaker, good coordination
exists between the two devices. If the current is above that
level, both devices are likely to trip together.
> FOCUS SELECTIVE COORDINATION
Continued on page 120 →
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Figure 2: The device should have tripped and cleared the fault
by the time the right side of the band is reached.
Figure 3: The TCC of a 225A “adjustable” molded-case
circuit breaker with both the “low” and “high” instantaneous
pickup settings is shown below.