Selective coordination ensures proper isolation and localization for all possible overcurrents (from overloads to maximum short-circuit currents at the point of application) to the nearest upstream overcurrent protective device and prevents unnecessary loss of power to other loads, especially loads essential for life safety. The Code requires selective coordination for all supply-side overcurrent protective devices in the circuits to a limited number of life-safety related loads. Selective coordination is just one of many requirements in the Code that ensures that these vital loads that are essential for safety of human life remain energized without interruption. This is especially important in times of emergency such as might occur during fires, natural disasters, man-caused disasters, building failures, loss of utility power, and other situations. The investment of an alternate power source can be negated by the application of overcurrent protective devices that are not selectively coordinated. The life safety benefits of the selective coordination requirements include, but are not limited to:
- Increased system reliability by maintaining power availability to vital loads
- Reduced unnecessary power outages
- Increased safety for building inhabitants
- Quicker power restoration when a fault occurs
Figure 2
As with many Code requirements, a proactive approach by the authority having jurisdiction (AHJ) is the key factor as to whether circuits supplying these vital loads employ overcurrent protective devices that are selectively coordinated for all eventualities. There are four important considerations to keep in mind:
- It concerns the integrity of the life safety system
- Three code panels have now included selective coordination requirements in four Code articles
- Selective coordination can be achieved using circuit breaker systems, fusible systems, or various combinations.
- To enforce the requirements, the AHJ only needs to know the requirements and understand the concept of selective coordination. The AHJ does not have to be an expert at device coordination—that is the responsibility of engineering and design.
Requirements
Selective coordination requirements for life safety are not a new concept for the Code. There has been a Code requirement to coordinate selectively the overcurrent protective devices for elevator circuits since 1993. The overcurrent protection selectivity in 620.62 is crucial for a few specific reasons such as not stranding passengers for normal operation and for emergency egress as well as keeping elevators in use for emergency firefighting operations. The original elevator requirement evolved from the Canadian Code where the substantiation recognized that selective coordination ensures faster restoration of power (the electrician does not have to search for upstream panels where the overcurrent devices unnecessarily cascaded open).
Figure 3
NEC-2005 expanded the selective coordination requirements for emergency system loads and legally required standby system loads via 700.27 and 701.18. Also, the definition coordination (selective) was moved to Article 100 from Article 240. NEC-2008 includes a selective coordination component in the exception to 700.9(B)(5). Section 517.26 includes selective coordination for the essential electrical systems in health care facilities since it is required to comply with Article 700 requirements, unless otherwise amended in Article 517. Examples of loads that require selectively coordinated overcurrent protective devices in the supplying circuit paths would be emergency and egress lighting for the safe evacuation from a building, and to assist in crowd and panic control.
Also, in many cases, ventilation, pressurization systems, automatic fire detection systems, voice/alarm communication systems, smoke control systems, fire command center systems, fire alarm systems, elevators, elevator car lighting, fire pumps, public safety communications, and continuous processes may be classified to be supplied by an emergency or legally required standby system by the authority having jurisdiction or the locally adopted building code. Where hazardous materials are manufactured, processed, dispensed, or stored, ventilation, treatment systems, temperature control, alarm, detection, or other electrically operated systems may be classified to be supplied by emergency or legally required standby systems. The essential electrical systems of health care facilities include the loads on the critical branch, life safety branch, and equipment branch. The emergency system of a hospital is made up of two branches of the electrical system that are essential for life safety and for the health and welfare of patients receiving critical care and life support. These two branches are the life safety branch and the critical system branch.
Figure 4
In the 2008 Code process, every selective coordination requirement was challenged. With plenty of pro and con proposals and comments submitted, after presentations at the code panel meetings by interested parties on both sides of the argument and after deliberate and thorough discussion by the respective code panels, the existing requirements were overwhelmingly retained; a clear message by the NEC technical committees. Panel 12 voted unanimously (11–0) to retain the requirement for selective coordination in elevator circuits. Panel 13 voted 11–2 to add exceptions to 700.27 and 701.18 for two devices of the same ampere rating in series and single devices on the primary and secondary of a transformer. (It was felt that neither exception reduced life-safety because no additional parts of the electrical system would be shut down unnecessarily.) These exceptions were added by comments 13-185 and 13-238. In addition, the new Article 708 Critical Operations Power Systems included selective coordination requirements via 708.54. Article 708 concerns itself with systems powering loads vital to public health and safety, national security, and the economy. The threat of events such as 9/11 and similar natural disasters as Hurricane Katrina were the catalyst for the creation of Article 708. There were four comments to reduce or eliminate the selective coordination requirement for critical operations power system loads. Panel 20 voted 16–0 (three times) and 15–1 (one time) to reject all attempts to reduce or eliminate this key life safety requirement.
Figure 5
In the 2008 Code cycle, Proposal 13-135 proposed the elimination of the selective coordination requirement for 700.27 and moving the language to a fine print note. Code Panel 13 rejected this proposal 9–4. The panel statement to this proposal sums up the matter well:
Panel Statement:This proposal removes the selective coordination requirement from the mandatory text and places it in a non-mandatory FPN. The requirement for selective coordination for emergency system overcurrent devices should remain in the mandatory text. Selective coordination increases the reliability of the emergency system. The current wording of the NEC is adequate. The instantaneous portion of the time-current curve is no less important than the long time portion. Selective coordination is achievable with the equipment available now.
Code Panel 20, which was responsible for the new Article 708, summed up the need for selective coordination in their statement to Comment 20-13, which proposed deletion of the selective coordination requirement. This comment was rejected 16–0. The actual panel statement to Comment 20-13:
Panel Statement:The overriding theme of Articles 585 (renumbered to 708) is to keep the power on for vital loads. Selective coordination is obviously essential for the continuity of service required in critical operations power systems. Selective coordination increases the reliability of the COPS system.
Following are the 2008 Code requirements for selective coordination:
Article 100 Definitions
Coordination (Selective)
Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings.
Article 517 Health Care Facilities
517.26 Application of Other Articles
The essential electrical system shall meet the requirements of Article 700, except as amended by Article 517.
Article 620 Elevators, etc.
620.62 Selective Coordination (2005)
Where more than one driving machine disconnecting means is supplied by a single feeder, the overcurrent devices in each disconnecting means shall be selectively coordinated with any other supply-side overcurrent protective devices
Article 700 Emergency Systems
700.9(B)(5)(b), Exception
Overcurrent protection shall be permitted at the source or for the equipment, provided the overcurrent protection is selectively coordinated with the down stream overcurrent protection.
700.27 Coordination
Emergency system(s) overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices.
Exception: Selective coordination shall not be required in (1) or (2):
(1) Between transformer primary and secondary overcurrent protective devices, where only one overcurrent protective device or set of overcurrent protective devices exist(s) on the transformer secondary,
(2) Between overcurrent protective devices of the same size (ampere rating) in series.
Article 701 Legally Required Standby Systems
701.18. Coordination
Legally required standby system(s) overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices.
Exception: Selective coordination shall not be required in (1) or (2):
(1) Between transformer primary and secondary overcurrent protective devices, where only one overcurrent protective device or set of overcurrent protective devices exist(s) on the transformer secondary,
(2) Between overcurrent protective devices of the same size (ampere rating) in series.
Article 708 Critical Operations Power Systems
708.54 Selective Coordination
Critical operations power system(s) overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices.
So, What is Selective Coordination?
Figure 6
Article 100 provides the Code definition. Here is another approach to describe selective coordination: “For the full range of possible overcurrents, the act of isolating an overloaded or faulted circuit from the remainder of the electrical system, thereby eliminating unnecessary power outages.” The circuit causing the overcurrent is isolated by the selective operation of only that overcurrent protective device which is the closest upstream to the overcurrent condition.
Three important conditions for selective coordination:
- Achievable for the full range of possible overcurrents for the system involved
- Only the closest upstream overcurrent protective device opens
- No other upstream (larger ampacity) overcurrent protective devices open
Probably the best way to understand the concept of selective coordination is with graphics. There are multitudes of possible scenarios and these represent a few. Figures 1 and 2 illustrate selective coordination: figure 1 for a branch-circuit fault and figure 2 for a feeder-circuit fault. Figures 3, 4, 5, and 6 illustrate non-selective coordination for both branch-circuit and feeder-circuit faults. The most important aspect to visualize is the affect on the availability of power to individual loads for each scenario. Some disruption is necessary due to a fault. However, non-selective coordination causes unnecessary power disruption to some loads.
Key Aspects to Selective Coordination
Figure 7
1. With today’s many options, it is possible to design and install either fusible or circuit breaker systems that comply with the Code requirements for selective coordination. This article will not get into how to use fuses or circuit breakers to comply. If the reader is interested, contact the fuse or the circuit breaker vendors. The fuse industry has published selectivity ratio tables for decades. Since the 2005 Code requirements became effective, the circuit breaker vendors have published materials to facilitate designing selectively coordinated circuit breaker systems. This includes tables for circuit breakers that provide the maximum available short-circuit current for which specific circuit breakers are selectively coordinated.
2. These requirements are about reliability of the power system supplying the designated vital loads thus ensuring as much load availability as possible. This is the case especially in emergency situations or when the building or electrical system may be imperiled due to unusual or catastrophic conditions. The selective coordination requirement (which also includes 517) in Articles 700, 701, and 708 encompasses the overcurrent protective devices in the circuit paths of the normal source supplying these vital loads as well as the circuit paths of the alternate source supply. The requirements state, “overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices.” These loads can be powered by the normal source or the alternate source. So, the fuses or circuit breakers must all be selectively coordinated from the vital load branch-circuit devices up through the circuit paths to the main overcurrent protective device of the normal source supply as well as from the vital load branch-circuit devices up through the circuit paths to the alternate source supply.
Figure 8
Why care about this? (see figure 7). If there is a fault on the load side of the transfer switch at X1 or X2, only the closest upstream circuit breaker or fuse (EB for X1 and EF1 for X2) is to open. Circuit breaker or fuse EF2 or the main fuse or circuit breaker is not to open (blue path on left). The overcurrent protective devices in the normal path must be selectively coordinated when fed from the normal source, because if they are not selectively coordinated, vital loads can be unnecessarily “left in the dark” (power loss). Let’s look at a couple of scenarios:
Scenario (A):If a fault occurs on a branch circuit at X1, and feeder overcurrent device EF1 unnecessarily opens, the ATS will not transfer to the alternate source because power is still available on the ATS’ line side. Yet many vital loads will be unnecessarily without power.
Scenario (B):If a fault occurs on a branch at X1, and feeder overcurrent devices EB, EF1, and EF2 all open, the ATS will transfer to the alternate source (there is no power to the line side of the ATS due to EF2 unnecessarily opening). This transfer is unnecessary and would not have to occur if the overcurrent protective devices are selectively coordinated, but vital loads will still be blacked out because EF1 has opened unnecessarily. The unnecessary transferring of the power from the normal source to the alternate source reduces the reliability of the system and can lead to unavailability of power to vital loads, if, for instance, the generator does not start or the transfer switch does not operate properly.
3. The definition and requirements for selective coordination are very clear, understandable, and enforceable. During the 2008 code-making process, proposals and comments were submitted to alter the selective coordination requirement to phrases such as “optimize selective coordination tripping,” “selective coordination for overcurrent protective device opening times of 0.1 seconds or greater,” “only apply for faults times greater than 3 cycles,” “coordination shall not be required in the current-limiting or instantaneous portions of the time-current curves,” or other similar language. In reality, these proposals and comments would have changed the requirement to coordination for only overloads or very low-level faults and ignored selective coordination for the full range of overcurrents. Moderate and higher level fault currents can and do occur. Code Panel 13’s statement in rejecting Proposal 13-135 was clear in that the requirement includes all possible overcurrent levels: “…the instantaneous portion of the time-current curve is no less important than the long time portion.” And well it should. In time of fire, catastrophes, failure of components, improper installation of components, and worker incidents, overcurrents can run the full range of values up to and including the maximum three-phase bolted fault current. High faults are less probable, but they do occur. Another phrase that is unenforceable is “selective coordination where practicable.” (Where is it delineated as to what or when selective coordination is practicable? Is it impracticable because of cost? What is the cost of a life? Is it impracticable because a selective coordination analysis may require more effort to engineer? Is it impracticable if selectively coordinated gear takes up more space?) The present selective coordination definition and requirements are very clear and achievable. These proposals and comments to change the requirement were thoroughly discussed by the code panels and overwhelmingly rejected.
Practical Approach for the AHJ
The AHJ needs to know the requirements and enforce compliance. It is nice if the AHJ understands what selective coordination means. However, it is not necessary for the AHJ to know how to achieve selective coordination at the engineering level. That requires specific know-how best left to the engineering community. The AHJ in carrying out his or her responsibilities, should require engineers to submit a selective coordination analysis for the circuit paths required to be selectively coordinated per Articles 517, 620, 700, 701, and 708. The analysis should have a summary statement clearly stating that selective coordination has been achieved and the analysis should include the engineer’s seal. (Although not all-inclusive, an example checklist is provided at the end of this article. It simplifies the record-keeping of the AHJ and puts the onus on the consulting engineer.) During installation, the AHJ should verify (spot check) that the specified overcurrent protective devices with correct ratings or settings have been installed and match the engineering design.
Conclusion
In recent Code cycles, safety for persons is being taken in a broader context, illustrated by requirements expanding the use of AFCIs and now requiring tamper-proof receptacles. Selective coordination of overcurrent protective devices is another requirement increasing safety for persons. The requirements pertain to the overcurrent protective devices in the circuit paths supplying a limited number of loads which are vital during emergencies or which have critical availability requirements. Selective coordination requirements entered the Code in 1993 and the requirements have continued to expand where there are life safety concerns. Circuit breaker or fusible systems can be designed and installed to comply with the selective coordination requirements. The AHJ does not have to be an expert in overcurrent protection coordination to enforce these requirements. Just be sure the engineer provides suitable documentation that includes an analysis. The analysis must encompass all the circuit paths supplying the designated loads, including the normal source path and the alternate source path to those vital loads, and for the full range of overcurrents possible.
