Achieving effective selective coordination design



Determining coordination

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.

Figure 2: (a) This schematic shows a conventional bus design based on loads, physical layout, reliability, cost, safety, owner’s standards, and code minimums. Design results are system layout, component sizes, and device selections. Courtesy: ASCO Power TFigure 2: (b) This time-current curve represents traditional device selections. Note that the time scale does not go to zero. The conventional design is considered to be selectively coordinated with the devices. The red dotted line indicates good coordinaFigure 2: (c) Greater control can be gained by using different trip units for A and B. Using larger frames for A and B produces greater withstand (higher instantaneous override) and also avoids an instantaneous trip. This design means more energy is releaFigure 2: (d) Increasing the distance between bus 1 and bus 2 reduces the available fault current at bus 2. Selective coordination is achieved if the available fault current at C is less than the instantaneous trip on B. Courtesy: ASCO Power Technologies

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.

Figure 3: Modern circuit breakers with electronic trip elements provide adjustability to selectively coordinate power systems. Courtesy: ASCO Power TechnologiesIn 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. 

Design optimization

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.

No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
2017 Lubrication Guide; Software tools; Microgrids and energy strategies; Use robots effectively
Prescriptive maintenance; Hannover Messe 2017 recap; Reduce welding errors
Safety standards and electrical test instruments; Product of the Year winners; Easy and safe electrical design
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Future of oil and gas projects; Reservoir models; The importance of SCADA to oil and gas
Diagnostic functions for system safety; Specifying industrial enclosures; Effective decision support for a crisis
Transformers; Electrical system design; Selecting and sizing transformers; Grounded and ungrounded system design, Paralleling generator systems
Natural gas for tomorrow's fleets; Colleges and universities moving to CHP; Power and steam and frozen foods

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
Featured articles highlight technologies that enable the Industrial Internet of Things, IIoT-related products and strategies to get data more easily to the user.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me