Silence! 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).


Time-current curves


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 “× 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.


Improve 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.


It happens


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.