CSE: Please explain why selective coordination in emergency/standby electrical systems is more of an issue than selective coordination in primary facility electrical systems.
Bruce Young : Selective coordination of electrical systems is one of the most critical design tools an engineer can use to achieve reliability. The public depends on emergency electrical systems to operate correctly and reliably. In the healthcare environment, lives may (and often do) depend on emergency electrical systems to provide reliable power. Power interruptions can occur from loss of utility, or operation of an overcurrent protective device (OPD). Because electrical systems are so critical, any fault in the emergency system must be cleared as close to the fault as possible, leaving the rest of the distribution system operational. A coordinated distribution system helps provide a reliable electrical distribution system.
James Degnan : The National Electric Code (NEC) mandates complete selective coordination of emergency/standby systems. For primary systems, engineers can use their judgment to consider tradeoffs between selective coordination, arc flash hazards, fault recovery time, maintenance requirements, and economic value.
Ken Lovorn : In emergency systems, a single set of uncoordinated breakers can result in the complete failure of all power to the emergency loads, which could result in endangering lives of occupants and, in the case of critical patient areas of hospitals, death of the patients. When the primary circuits are uncoordinated, power to the loads can still be lost, but the emergency/critical loads will still be operational and personnel safety can be maintained.
Brian Rener : The purpose of selective coordination is to isolate faults nearest to the point of occurrence and allow the remaining portions of the electrical system to continue to operate. In emergency systems it is critical to maintain operations that could affect the life safety of building occupants or emergency responders.
CSE: What role does electrical design software play in determining selective coordination? Can selective coordination be successfully accomplished without it?
Degnan : Selective coordination design software facilitates the use of time current curves (TCCs) to determine selective coordination. Most manufacturers' published-TCCs and all electrical design software address selective coordination for time periods beyond 0.01 sec. For time periods below 0.01 sec., the manufacturers must be consulted for specific application information. Selective coordination can be determined without electrical design software by referring to manufacturers' overcurrent device application tables.
Lovorn : Design software can automate the coordination of breakers, fuses, and overcurrent relays so that the minimum time bands may be easily maintained. By inputting the electrical single-line into the coordination software, the designer can either manually or automatically determine tripping points of all the overcurrent devices. The designer can then print out the log-log coordination curves and the set-point table for each of the breaker settings.
While this software is very useful and can save considerable design time for larger projects, using it for the smaller projects becomes somewhat tedious and it is more effective to prepare the coordination curves manually. The only problem with manually coordinating the overcurrent devices is that the coordination curves for the breakers and fuses are not as readily available for breakers and fuses as they once were.
Rener : Software readily allows the use of different scenarios of a system's operation. It also allows the easy use of manufacturers' tested and coordinated breakers. Most coordination software is available with other modules for electrical studies such as those designed for short circuit and arc flash, which greatly simplifies the whole process. However, I started out my career doing fundamental time vs. correct coordination using 11x17-in., log/log trace paper, manufacturer-printed breaker curves, and a light table, along with the IEEE Buff Book .
CSE: What, if any, recent or upcoming changes in NEC are applicable to selective coordination issues? Are authorities having jurisdiction (AHJs) using these? If so, how many and where? If not, when will the AHJs inspect/enforce these changes?
Rener : We are seeing local authorities requesting copies of the short circuit and coordination studies on emergency systems. On brand new construction this is a straightforward task, but it is a bit more challenging and time consuming to do coordination studies on modifications to existing electrical systems. In addition, the new NEC Article 708: Critical Operations Power Systems has also made an impact on the power study requirements and coordination of electrical systems.
Lovorn : The 2005 NEC requires that all electrical distribution systems be selectively coordinated as required in paragraph 240.12(1). In our experience, we have not had any AHJs require that we demonstrate that our systems are selectively coordinated. We have not heard of any timetable for the AHJs requiring that systems be coordinated.
Degnan : The NEC continues to mandate complete selective coordination despite the potential for increasing the arc flash hazard. AHJs are put in a difficult position because they are responsible for enforcing the code, but this aspect of the code cannot be readily determined by inspecting a site. AHJs have to go back to the designer, who often has to go to the manufacturers. There are no standards for manufacturers to publish selective coordination data, so it rapidly becomes necessary to simply trust the manufacturer. If multiple manufacturers are involved, there is no data. In many respects, selective coordination was not ready for prime time when it became effective.
CSE: Which systems in a typical building or facility are the most difficult electrical load paths to coordinate?
Lovorn : The systems that are the most difficult are those where the building is fed at medium voltage, 5 or 15 kV, and the service transformer belongs to the building owner. Trying to maintain complete protection throughout the transformer damage curve; locate the coordination curves so that the transformer inrush point is above the worst-case trip value; have all the transformer capacity available through the main secondary breaker; and selectively coordinate the main and tie breakers in a double-ended unit substation is easily the most difficult coordination path.
Degnan : Any building that needs more than six buses between source and load is very difficult to selectively coordinate. There's only so much time for the bandwidth of each overcurrent device. Hospitals with central generating plants are a real challenge.
Rener : I am seeing some challenges and tradeoffs between achieving a completely selectively coordinated system and limiting arc flash levels. There can be some tradeoffs between the two types of studies and goals.
Young : Smaller life safety distribution systems are the most difficult. Larger systems have proportionately larger budgets and can justify using solid-state trip units with multiple adjustments. However, use of these trip units for smaller (and less expensive) systems can be difficult to justify. Achieving coordination with no (or limited) adjustable trip units presents many challenges, and even though it is possible, it may require more expensive distribution equipment.
CSE: How do choices of whether to use fuses, breakers, or a combination of both for selective coordination vary among consultants and engineers and/or AHJs?
Young : We specify breakers for larger (2,000 amps or larger) critical power system distribution systems (like those in data centers) because the devices can be reset after operation. For smaller systems, fuses are a very viable alternative and can help coordination because they are less expensive than solid-state breakers. However, it is very important to ensure that any replacement fuses be matched (operating time, melting characteristics) with the original fuses.
Lovorn : For us, the use of fuses or breakers comes down to several issues:
If the owner or building occupant does not have an on-site maintenance staff, we typically use breakers for all locations.
If the available fault current on the incoming utility service exceeds 50,000 amps, we lean toward using fuses to limit the fault current at the distribution panels so that the downstream branch circuits may use standard interrupting duty breakers.
If the available utility fault current exceeds 150,000 amps, we always use current limiting fuses.
Some distribution systems are nearly impossible to coordinate without the use of both circuit breakers and fuses, so we do so in these cases.
We have had no requirements for the use of fuses or breakers by the AHJs on any projects we have designed.
Rener : It is an “apple and oranges” situation. The breaker vs. fuse debate has been going on for some time, and both are suitable. I would give a nod to adjustable trip breakers in being adaptable to future changes and additions in an electrical system. At one time, fuses also offered a higher level of fault protection and response time, but breakers are now closing that gap. Some AHJs have in the past required fuses on emergency systems.
CSE: What are the relationships between one-line diagram accuracy and effective and reliable selective coordination?
Young : Accurate one-line diagrams are essential to ensure an accurate selective coordination study. Errors in the type of OPD, feeder characteristics, and available fault currents can result in improper settings and possibly cascading failures in the event of a fault in the system. Inaccurate one-line diagrams, incorrect shop drawings, incorrect relay information, mistakes in types of settings, and incorrect current transformer ratios can also lead to failures. The accuracy and dependability of any selective coordination study depends on this information being 100% accurate.
Degnan : Engineers must design one-line diagrams with selective coordination in mind. If selective coordination is an afterthought, there is an excellent chance that the one-line diagram cannot be selectively coordinated.
Lovorn : Without the one-line accurately defining the distribution system, it is impossible to design an effective and reliable coordination.
Rener : An accurate one-line is an important part of a coordination study. However, a complete and accurate coordination study requires detailed floor and site plans, equipment cut sheets, and information from the utility company.
CSE: Do you think selective coordination is code overkill, or is it really needed?
Young : I believe selective coordination is absolutely needed in any legally required system as well as most critical power distribution systems. Losing power to an operating room or stairway during an evacuation because of an incorrectly coordinated system can be potentially life threatening. Losing your desktop computer because a water heater faulted and tripped the main is just an inconvenience.
Lovorn : Based on a review of many existing electrical distribution installations, very few of them were not selectively coordinated or use series ratings to provide protection up to the level of available fault current. So in my opinion, selective coordination is needed and the AHJs should aggressively enforce this requirement. The lack of full overcurrent protection and selective coordination is the most common hazard in electrical systems.
Rener : Selective coordination for emergency systems is a good mandate. However, coordination is as much art as science, and achieving selective coordination in every type of electrical system can be unnecessary and expensive. It really comes down to understanding the need a building has for continuous operations and the costs of downtime. A small commercial office building might not need a full selectively coordinated system, whereas a data center should.
CSE: What changes to the NEC would you recommend regarding selective coordination? Why?
Young : I would recommend that selective coordination still be required on all of the current legally required systems. However, on occasion, 100% selective coordination is just not practical for a project, and the professional engineer should be allowed to use his or her judgment in those cases.
Rener : Expansion on the integration of protective device coordination with arc flash studies and improved labeling of hazards. Additionally, a requirement to keep updating and revising studies as the systems are updated, maintained, repaired, or modified over their operational lifetime would be a major advantage.
Degnan : Selective coordination was added to the 2005 NEC without any real-life statistical verification that it was needed or that the lack of selective was costing lives. The justification for it is pure conjecture, and it could make maintaining electrical systems more hazardous. It's unfortunate that selective coordination is in the NEC as an absolute mandate.
Lovorn : The NEC provides adequate rules and regulations on selective coordination, but the enforcement of these requirements is inadequate. Due to the serious hazards of the lack of full overcurrent protection and selective coordination, more stringent enforcement will result in a safer environment for both facilities and their occupants. This would fulfill the basic mandate of the NEC as stated in article 90.1, “The purpose of this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity.”
James E. Degnan , PE, LEED AP, Principal, Sparling, Seattle
Ken Lovorn , PE, President Lovorn Engineering, Pittsburgh
Brian Rener , PE, LEED AP, Senior Manager M+W Zander U.S. Operations Inc., Chicago
Bruce W. Young , PE, Electrical Dept. Manager Bala Consulting Engineers Inc., King of Prussia, Pa.