Achieving effective selective coordination design
Effective selective coordination can be determined by two types of studies: a short-circuit current study and an overcurrent coordination study. A short-circuit current study identifies the maximum available short-circuit currents throughout the distribution system at the line-side terminals of each overcurrent protective device. This type of study is typically considered to be part of a facility’s required electrical documentation.
An overcurrent coordination study compares the timing characteristics of various protective devices under consideration in relation to each other. These studies determine the degree of coordination, but only guarantee that selective coordination is achieved if all levels of overcurrent are considered—including bolted line-to-line faults.
Mandated selective coordination demands selective coordination for the full range of overcurrents. It is the responsibility of the design engineer to provide substantiated documentation showing that the design achieves this goal.
The two methods of achieving selective coordination among overcurrent protective devices are the graphical and the table or chart methods. Time-current curves indicate the response of overcurrent protective devices to a range of fault current magnitudes. Typically, time-current curves can be divided into overload and instantaneous or short-circuit regions (see Figure 2). The graphical method examines curves for fuses and circuit breakers. The horizontal axis represents current, while the vertical axis shows the time it takes for the device to interrupt the circuit.
Using the graph method, two circuit breakers crossing at any point in their respective instantaneous trip regions indicates that those two circuit breakers do not coordinate for fault currents above the crossover point.
For current levels in the overload region, time-current curves for overcurrent protective devices can be overlaid for a visual indication of whether selective coordination is achievable. In the overload region, fault currents are relatively low, and device response time is usually not much faster than 1 sec. In this region, selective coordination can be relatively easy to accomplish and the time-current curve is typically an adequate tool for determining selective coordination between devices. The curve must include the level of available short-circuit current. However, the fact of no overlap on the graph does not definitively prove selective coordination.
At higher short-circuit current levels, the time-current curves alone do not show the total picture. The results do not include the effect of the added impedance of the downstream circuit breaker if it begins to open faster than the upstream circuit breaker, as well as the resulting higher coordination levels.
If curves overlap, the consulting engineer should reference the manufacturers’ circuit breaker tables to determine if selective coordination is achieved. The tables show results of tests of overcurrent protective devices connected together.
Overlapping curves can indicate a potential lack of selectivity. Conversely, a lack of overlap indicates selectivity. However, time-current curve analysis alone ignores how current limitation affects the load-side overcurrent protective device. The load-side circuit breaker will react to the peak let-through current allowed to flow by the smaller—or faster—overcurrent protective device for a given prospective fault current.
The true time-current curve for overcurrent protective devices, such as circuit breakers and fuses, is really a band or region extending to either side of a single line. This variation from the ideal is due to the time difference between minimum response time and total clearing time as well as manufacturing and temperature variations. Consideration of all the time-current curve variations is required to eliminate possible errors when examining selective coordination.
A table/chart-based method can also be used to determine coordination. It uses a matrix that shows response time in sec versus current in Amps. This method shows the level of short-circuit current to which the two breakers (upstream and downstream) coordinate.
Both time-current curves and tables are necessary to achieve proper selective coordination.
Imaginary systems, worst-case scenarios
The problem is that design engineers need to conduct a preliminary study based on an imaginary system including worst-case scenarios to ensure the design will be acceptable—before manufacturers and their products are chosen. Engineers typically base the design on standard, generic equivalents. After the contractor chooses the material, the engineer requires manufacturers to conduct their own studies with the selected breakers to ensure they still coordinate and that the correct breaker is provided.
In many cases, achieving coordination with breakers requires specifying breakers with electronic trip, which are more expensive than standard molded case breakers (see Figure 3). Selective coordination can also be achieved with zone-selective-interlocking (ZSI) protection. This method allows two or more ground fault breakers to communicate over a network so a short circuit or fault clears by the breaker closest to the fault in the shortest time possible, regardless of the location of the fault.
If there is a fault on the main bus, there will be more current coming in on the main and less current going out on the feeders. ZSI protection would open the main, which would protect against a bus fault in the substation.
While the design engineer may select an overcurrent protective device that may seem well suited for satisfying the short-circuit study requirements, it may not be the best choice for selective coordination. If the system has been expanded or upgraded over time, it may include both circuit breakers and fuses—possibly from different manufacturers. In these cases, ensuring selective coordination becomes more problematic because a manufacturer’s tables provide data only for its products. Effective selective coordination during a system’s lifecycle could require using the same type of overcurrent protective devices from the same manufacturer over time.
Optimizing selective coordination is an iterative process. Depending on the system’s complexity, the analysis may suggest that device selection indicates an imbalance or tilted trade-offs among selective coordination, equipment protection, and personnel safety.