Most aspects of daily life depend on electricity—a fact most of us take for granted until the power suddenly goes out.
Brief outages may bring nothing more than temporary inconvenience when we’re at home, but there are circumstances in which a power failure can be life threatening. Who wants to be on a hospital operating table when the room suddenly goes dark? Or in an elevator stuck between floors in a high-rise building? Preventing system failures is a key element in the design of every electrical system, and planners and engineers have many options to reduce the likelihood of systemwide blackouts.
Since publication of the 2005 National Electrical Code (NEC), selective coordination has become a much-discussed and controversial method of preventing failures of vital electrical systems that could cause serious risks to safety.
Simply stated, a selectively coordinated system is designed so that in the event an overcurrent condition occurs, the failed circuit is isolated from other circuits in the system. In this case, only the protective device closest to the overcurrent opens, leaving other circuits operational and preventing total system shutdown. The protective devices in a selectively coordinated system can be either fuses or breakers; the kind of devices selected depends on the design of the system.
Although selective coordination is not new, the 2005 NEC was the first to include it with articles requiring overcurrent protective devices in specific vital circuits, including emergency systems, legally required standby systems, and certain essential circuits in healthcare facilities.
As work progressed on the 2008 NEC, selective coordination became hotly debated and highly controversial.
Proponents believe selective coordination is essential for preventing blackouts in critical systems. Others believe it should not be included in the NEC and have taken the position that it removes an important aspect of system design from engineers and can lead to compromises in other parts of the design.
The selective coordination requirement affects electrical contractors on many projects. In order to provide selective coordination of overcurrent protection, it is necessary to use the correct fuse types and ratings and the types and settings of breakers that are specified. Substituting fuses or breakers from a manufacturer other than the one on which the selective coordination study is based cannot be done. It also is important that breakers, ground fault relays and other protective devices be set in accordance with the selective coordination study. Many manufacturers ship such products with adjustments set low, and a system will not perform as intended unless these settings are readjusted.
In addition, contractors accustomed to recommending value engineering to lower project costs find that option restricted on selective coordination projects, and replacing specified components will require additional engineering evaluation and approval.
With changes in articles, including selective coordination possible in the 2008 NEC, contractors will need to understand new articles and revisions and how they will impact future projects.
Representatives of two companies that are primary suppliers of fuses and breakers used to selectively coordinate systems agreed to summarize the benefits and concerns about this issue.
Supporting selective coordination
Vince Saporita, Cooper Bussmann vice president, technical marketing and services, explained the position of Cooper Bussmann and others who support requirements for selective coordination in all circuits where continuity of service is a life safety issue.
“Selective coordination,” Saporita said, “is important so that critical loads are not unnecessarily disconnected when something happens in another portion of the electrical distribution system. For that reason, selective coordination is required by the NEC in several very specific locations covering areas where life safety is paramount, and loads must continue to be powered or lives could be lost.”
In order to have a selectively coordinated system, none of the upstream or line-side overcurrent protective devices may open while the downstream or load-side overcurrent protective device opens the circuit under an overcurrent condition, he continued. This covers all kinds of overcurrents, both overloads (current stays in its normal path) and short circuits (current goes outside the normal path).
Selective coordination is required where two or more elevators are fed from a single feeder, for example.
“The logic,” Saporita said, “is that a short circuit in one elevator branch circuit should take out only that one elevator. It could become catastrophic if, for example, a short in one elevator branch circuit took out an entire bank of elevators. There has been a requirement for selective coordination in elevator circuits for the past 12 years [long before the NEC addressed selective coordination in emergency and legally required standby systems].”
“These systems supply very critical loads,” Saporita said. “Emergency generators and transfer switches are utilized to help ensure that power is available for as long as possible. When these systems go down, people can and do die. It is only natural that a requirement be in place in the NEC that limits the loss of power only to problem circuits.”
Saporita believes most objections to selective coordination boil down to increased material costs, extra space requirements and additional time needed to perform the selective coordination study.
“None of these reasons,” he said, “justifies an emergency system, legally required standby system, elevator system or essential healthcare system being installed with a built-in blackout, especially when selective coordination is so easily achievable. The biggest challenge, mainly for new engineers, is learning how to specify selectively coordinated systems.”
Ed Larsen, Square D/Schneider Electric industry standards manager, discussed concerns about selective coordination and why his company and other organizations oppose its inclusion in the NEC. Larsen said his company and other organizations recognize the importance of selective coordination but believe it should be left to the engineering community whether selective coordination in a system should be at all levels of fault current or optimized.
“I think the core of the debate has been whether the Code should tell engineers how to design their systems or if engineers should have the flexibility to make the tradeoffs that are so often necessary in a complex system, depending on objectives and design conditions,” Larsen said.
The implementation of selective coordination is simply not practicable in many instances, he added.
“Selective coordination,” Larsen said, “can in some instances drive system-design parameters that may make the system less reliable. It will also place restrictions on the design unnecessarily with no added benefit.”
Larsen said in order to achieve selective coordination, upstream breakers or fuses must be increased in size, making upstream equipment larger and more costly, and could mean increasing the size of low-voltage transformers, cable and raceway, busway, automatic transfer switches and other equipment.
“In some instances,” he said, “this could increase the arc flash hazard or necessitate the use of electronic trip breakers with zone selective interlocking. In addition, the need for total selective coordination can impact the selection and configuration of generators. It could force the use of a single large generator rather than several smaller generators, decreasing overall system reliability. It might not be possible to achieve adequate selective coordination with the overcurrent device on the generator or indeed with the generator controller itself.”
Changes in 2008?
No matter what changes the 2008 NEC includes, it should be apparent the requirement for selective coordination must be addressed upfront in the design process.
With Code Making Panel balloting and review by the Technical Correlating Committee complete, all that remains is voting at the NFPA annual meeting this month and approval by the NFPA Standards Council in July. Then, the 2008 NEC will go to press.
“The major change I expect is the addition of a requirement for selective coordination for critical operations power systems [Article 585, Code Making Panel 20],” said Cooper Bussmann’s Saporita. “During the comment stage, before recirculation of negative votes, Code Making Panel 13 has reaffirmed its desire to require selective coordination in emergency and legally required standby systems. At a recent report on comments meeting for the 2008 NEC, both proponents and opponents for the requirements for selectively coordinated systems agreed that consulting engineers are paying much more attention to selectively coordinating these systems than ever before.”
Larsen believes the 2008 Code may add exceptions to 700.27 and 701.18 to negate the need to selectively coordinate two devices in series where there are no other loads connected in-between, such as devices on the primary and secondary of a transformer (see the diagram). Also, Article 517 may be changed to limit the application of the Article 700 requirements, including selective coordination, to the life safety branch only. Currently, the status of these changes is uncertain. EC
GRIFFIN, a construction and tools writer from Oklahoma City, can be reached at 405.748.5256 or firstname.lastname@example.org.