Selective Coordination - Cooper Bussmann

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Selective CoordinationEssential Electrical Systems In Healthcare Facilities, Emergency Systems and LegallyRequired Standby SystemsIn recent history, there has been an increasing demand for reliability ofelectrical power for buildings spanning the entire spectrum from single-familyresidences to places of assembly to industrial facilities. Many buildingsystems have gone to the extent of installing emergency back-up or standbysystems to ensure maximum possible reliability and continuity of theirelectrical system. There are some types of buildings that require an emergencysystem or legally required standby system such as hotels, theatres,sports arenas, healthcare facilities and many more similar institutions. Theselife safety electrical systems are used to ensure electrical power in the case ofpower loss due to an outside source (utility loss) or an inside source(overcurrent condition). Very important loads must remain energized as longas possible during times of normal power outage and emergencies that maybe caused by equipment failures, nature or man. However, simply installing aback-up system is not the entire solution, the electrical system must bedesigned to maximize power to these important loads and minimize outageseven under physical catastrophes. It is very important to analyze thecharacteristics of the overcurrent protective devices (OCPDs) of these lifesafety systems to ensure they will perform as desired.The NEC ® has special requirements for these life safety electrical systems.These include several requirements that are based upon providing a systemwith reliable operation, reducing the probability of faults, and minimizing theeffects of an outage to the smallest portion of the system as possible. Beloware a few of these sections:• 700.4 maintenance and testing requirements• 700.9(B) emergency circuits separated from normal supply circuits• 700.9(C) wiring specifically located to minimize system hazards• 700.16 failure of one component must not result in a condition where ameans of egress will be in total darkness.The objective of these requirements is to ensure system uptime with the goalof safety of human life during emergencies or for essential health carefunctions.The 2005 NEC® has adapted to this demand and has modified a few articlesto address the issue of selective coordination and to ensure by design andinstallation that the intended life safety electrical systems will stay in servicefor the longest possible period of time.The installation of a back-up system could easily be negated by the use ofovercurrent protective devices that are not selectively coordinated. Selectivecoordination helps ensure by design and installation that some life safetyelectrical systems will stay in service for the longest possible period of time.There has been a requirement for selective coordination in the NEC ® for manyyears. NEC ® 620.62 has had a selective coordination requirement for multipleelevators in a building. This requirement is crucial for a few specific reasonssuch as not stranding passengers for normal operation and for emergencyegress as well as keeping elevators in use for emergency firefightingoperations. The 2005 NEC ® selective coordination requirements have beenexpanded and a definition Coordination (Selective) has been added to Article100:New Article 100 DefinitionCoordination (Selective). Localization of an overcurrent condition to restrictoutages to the circuit or equipment affected, accomplished by the choice ofovercurrent protective devices and their ratings or settings.NEC ® 700.27 (Emergency Systems) and 701.18 (Legally Required StandbySystems) have added selective coordination of overcurrent protective devicesto help ensure life safety. Examples of circuits that require selectivecoordination would be emergency and egress lighting for the safe evacuationfrom a building and to assist in crowd and panic control. Also, in many cases,some elevators and ventilation equipment may be classified as an emergencyor legally required standby system by the “Authority Having Jurisdiction” (AHJ)or the locally adopted building code. Also, 527.26 requires the essentialelectrical systems of healthcare facilities have overcurrent protective devicesthat are selectively coordinated.The New NEC ® Requirements517.26 Application of Other Articles. The essential electricalsystem shall meet the requirements of Article 700, except asamended by Article 517.700.27 Coordination. Emergency system(s) overcurrent devicesshall be selectively coordinated with all supply side overcurrentprotective devices.701.18 Coordination. Legally required standby system(s)overcurrent devices shall be selectively coordinated with all supplyside overcurrent protective devices.System RequirementAs stated in the NEC ® requirements above, selective coordination is not justbetween the branch circuit and feeder. It is the entire circuit path from themain overcurrent protective device to the branch circuit device. See figurebelow.100 ©2005 Cooper Bussmann
Selective CoordinationEssential Electrical Systems In Healthcare Facilities, Emergency Systems and LegallyRequired Standby SystemsA given emergency branch circuit load has a circuit path up through to themain on the normal source and another circuit path up through to the main onthe emergency source. The requirement for selective coordination means thatall the overcurrent protective devices must be selectively coordinated witheach other, from each emergency branch circuit up through to the main of thenormal power supply and up through to the main on the emergency powersupply. See following example:3. A coordination study should fully investigate, interpret, and present thelevel of coordination achieved for the full system and for the determinedamount of available short-circuit currents. This requires a person qualified forthis task. Too often, coordination studies are performed where the deliverable isjust time current curve plots with no interpretation or discussion on the level ofcoordination achieved (or sacrificed). Also, the studies should be performed in thedesign phase so that if the study results prompt a change, the change can bewithout major ramifications.Fusible SystemsCooper Bussmann makes it easy to design fusible systems that are selectivelycoordinated. For the modern current-limiting fuses, selectivity ratios arepublished (see Fuse Selectivity Ratio Guide section). It is not necessary to plottime current curves or do a short-circuit current analysis; all that is necessaryis to make sure the fuse types and ampere rating ratios for the mains, feedersand branch circuit meet or exceed the selectivity ratio. These selectivity ratiosare for all levels of overcurrent up to the interrupting ratings of the respectivefuses. The ratios are valid even for fuse opening times less than 0.01seconds. This means with current-limiting fuses, it is not necessary to do anyanalysis for less than 0.01 seconds when the fuse types and ampere ratingratios adhere to the selectivity ratios. The industry standard for publishing fusetime current curves is to plot the times from 0.01 seconds and longer. Thefollowing example illustrates all that is necessary to achieve selectivecoordination with a fusible system.How Can Selective Coordination Be Achieved?Selective coordination of the overcurrent protective devices for these lifesafety systems for both the normal power source path and the alternate powersource path can be achieved with either modern current-limiting fusetechnology or modern circuit breaker technology. However, not all fuse typesor circuit breaker types can be used in any and all situations to ensureselective coordination. The ability to achieve selective coordination dependson the specific circuit parameters (short-circuit currents available at variouspoints in the system) and the fuse or circuit breaker opening characteristicsand possible setting options. Proper overcurrent protective device choice andselective coordination analysis is absolutely necessary. However, if the properchoices and analysis are not made so that selective coordination is ensured,the system may be built with deficiencies that may negatively impact lifesafety.Other Important Considerations1. Either a system has overcurrent protective devices that are selectivelycoordinated or the system does not. There is no middle ground. When termssuch as “enhancing” coordination, “optimizing” coordination, coordination "to thebest degree possible," or similar terms are used, it generally means theovercurrent protective devices are not selectively coordinated over the full rangeof short-circuit currents that are available in the application.2. Selective coordination is for the full range of overcurrents available at allmains, feeders and branch circuits. It is not acceptable to arbitrarily considerselective coordination is applicable only above certain times such as 0.01, 0.1, or1.0 seconds. The coverage of selective coordination in the sections on SelectiveCoordination for Fuses and Circuit Breakers of this SPD publication explains howto assess fuses and circuit breakers for the full range of overcurrents irrespectiveof the times. Also, these preceding sections provide some easy, quick methods toassess selective coordination for both fuse and circuit breaker systems.©2005 Cooper Bussmann101
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