A Critical Look at Selective Coordination

09/01/2006


The new requirements for selective coordination of emergency and legally required standby systems in Sections 700.18 and 701.27 of the 2005 National Electrical Code (NEC) are challenging electrical engineers to intensively consider how systems respond to short circuits.

Meeting this challenge raises questions regarding the code requirements, how other aspects of system design are compromised and what constitutes compliance. While the probability of owners benefiting from selectively coordinated electrical systems is small, the likelihood of incurring significant costs to ensure selective coordination is a certainty with the 2005 NEC.

As electrical engineers consider the impact of selective coordination, there is a growing consensus that this is the wrong direction for the NEC, and that this area of design should be returned to engineering judgment. In fact, there were more than a dozen proposed changes to the 2008 NEC to delete these new requirements or better define them.

Unfortunately, the just-released NEC Report on Proposals showed that there were not enough Code Panel votes to return this design area to engineering judgment. However, there is an opportunity to address and respond to this issue, as NFPA Code Panels will vote again in December and are accepting public comments on these proposals until Oct. 20.

Is selective coordination needed?

Selective coordination might prevent a small-scale problem from becoming larger—e.g., if a fault on a single circuit were to expand and take down an entire panelboard. However, selective coordination won't prevent the smaller-scale problem from occurring, nor will it ensure that additional lives will be saved if an escalated problem was prevented. Even with selectivity, it's still possible for a large-scale problem to occur, such as a feeder fault, and create the same type of situation that would be mitigated by selective coordination.

For a selectively coordinated emergency system to benefit the occupants of a building other than a hospital, a connected chain of events must occur:

  1. A building must be occupied.

  2. There must be an event that mandates exiting the building.

  3. The event must disrupt a portion of the emergency electrical system, in which no other life safety support exists.

  4. The disruption must be of a nature that it would interrupt more of the electrical system than would occur if the system were not selectively coordinated. This type of event is usually an infrequent, high-level fault.

  5. The extended disruption of the electrical system must result in injury or death of an occupant.

The probability of all of the above occurring is miniscule. If the first four occur, it may still be possible to avoid injury or death through the use of alternative systems—e.g., using the normal lighting system to exit, using a redundant exit path or exiting in the dark.

An emergency system is most likely to fail under everyday operation. In this scenario, the building's normal power system is available to support alternatives, and an emergency system failure can be treated like the failure of any other circuit.

Extending this logic, it becomes obvious that if selective coordination were needed, it would be required as much or more for normal systems than for emergency systems! If faults on non-selectively coordinated systems frequently created unwanted large outages, we would see it in our everyday lives; building blackouts would halt workplace production and create unlimited safety hazards.

More than 99.9999% of electrical systems in existence today were designed using engineering judgment and do not meet the 2005 NEC's strict definition of being selectively coordinated; yet once energized, they operate with high reliability. While these systems are not selectively coordinated for every fault condition, the vast majority will selectively coordinate for the most common types of faults, which are low-level arcing faults.

Can selective coordinationbe verified?

Demonstrating compliance with the code places engineers and inspectors in somewhat of an untenable position. The NEC gives exact requirements for determining loads, sizing wires and overcurrent devices, and many other aspects of design, but it does not describe how to perform a coordination study or how to determine if overcurrent devices will selectively coordinate. The methodology for coordination studies is described in several publications such as ANSI/IEEE Std 242, IEEE Recommended Practice for Protection, and IEEE Coordination of Industrial and Commercial Power Systems. However, these publications are not acknowledged by the NEC.

Determination of selective coordination is dependent on information provided by manufacturers, who describe the operating characteristics of fuses and circuit breakers using graphical curves that plot overcurrent performance data vs. time curves (TCs). The de facto standard for TCs has the time axis beginning at 0.01 seconds. Most overcurrent protective devices start to operate in time periods of less than 0.01 seconds, and all current-limiting fuses operate in less than 0.004 seconds. To address time periods of less than 0.01 seconds, manufacturers have developed overcurrent device coordination tables. There are no industry standards for the test procedures that are used in developing these tables, and no guarantee that various manufacturers are developing these selective coordination tables under similar circumstances. The tables are often subject to disclaimers such as “reasonable expectation of selective coordination.”

A selective coordination standard would need to be similar to the series rating portion of UL 489. A series-rated circuit breaker can be applied at fault current levels that are above its listed rating if used with a tested upstream breaker. When applied in the field, series-rated circuit breakers are specifically labeled and referenced to an upstream protective device. Eventually, the NEC will have to include similar requirements for selectively coordinated devices and require testing per standard criteria. Until appropriate standards are in place, engineers cannot ascertain if a system is selectively coordinated.

Will there be a loss of competition?

If testing standards for selective coordination are developed, the public would benefit from the assurance of independent verification of selectively coordinated systems. Manufacturers could submit their devices to a testing laboratory, which would subsequently verify selective coordination in accordance with the standard.

Unfortunately, the quantity of devices combined with performance variations at different fault levels leads to a huge number of permutations and associated testing expense. Manufacturers would not likely submit their devices for verification of selective coordination with the devices of other manufacturers. Accordingly, it may be necessary for all overcurrent protective devices in a selectively coordinated system to be made by a single manufacturer.

Facility owners are often locked into proprietary systems that can only be modified by using the products of the original manufacturer. Fire alarm systems are a good example. The listing or communication protocol requirements for fire alarm systems force the entire system in a building to be made by a single manufacturer.

For the most part, once you get outside of a single panelboard or switchboard, electrical power systems are immune from proprietary restrictions, with series ratings being an exception. However, this may change due to further developments in selective coordination. For expansions, renovations and similar changes to an existing system where selective coordination is mandated, it's quite possible that a manufacturer can now—or in the near future—set pricing with limited or no competition. Competition would only exist if a complete change-out was an alternative. If a proprietary bid was set for the emergency system, it would also be possible for a manufacturer to extend additional costs onto the normal power system as part of a packaged system quote.

What are the design consequences?

When one aspect of a design becomes an absolute requirement, compromises are often made to other aspects of the design. Ideally, engineers could simply improve a system. However, it's likely that the following compromises will arise with selectively coordinated systems:

  • Electrical equipment will increase in size, either to meet a manufacturer's minimum ratios between upstream and downstream equipment or to allow upstream equipment to pass through more energy while waiting for downstream equipment to operate.

  • Larger equipment will increase the maintenance requirements.

  • Multiple panels will increasingly be strung together to achieve selectivity ratios.

  • If circuit breakers are used, arc-flash hazards will increase, and automatic transfer switches may be forced to withstand fault currents longer than the three-cycle duration required by UL Standard 1008.

  • Introducing additional impedance into a system helps circuit breakers to selectively coordinate. Some undesirable methods of achieving this include higher impedance transformers or current-limiting reactors that increase energy losses; extending circuit runs longer than is necessary; and forgoing spare capacity to minimize wire size.

  • If fuses are used where circuit breakers may have been the original choice, the advantages of circuit breakers are lost, including rapid resetting after a known overload trip; compact size; prevention of single phasing; and no risk of incorrect fuse or unavailable fuse replacements.

  • Selective coordination increases the cost of electrical systems.

Selective coordination of systems with ground fault protection is very problematic. Ground fault protection is set below the rating of its associated overcurrent device, which often makes it impossible to maintain trip ratios that permit selective coordination with downstream devices.

Reconsidering selective coordination

Selectively coordinated systems will benefit building occupants only under a narrow set of circumstances for which there is no record of any significant occurrences. While the NEC requirements for selective coordination are clear, the methods of compliance are vague, and the groundwork has been set to further limit competition in the industry. Prior to 2005, the benefits of selective coordination could be compared against other safety and performance aspects of an electrical system. With the 2005 NEC, selective coordination takes precedence and overrides engineering judgment.

Selectivity and the 2008 NEC

To respond to the NFPA proposals on selective coordination for the 2008 NEC, visit www.nfpa.org . The deadline for comments is Oct. 20, 2006.

A summary of key proposals is as follows:

Proposal 13-77 . This proposal adds selective coordination to fire pump feeders as part of a proposed alignment with a draft of NFPA 20. At this time it is being rejected by NEC Code Panel 13.

Proposal 13-135 (also see Proposal 13-137). This proposal was to delete Section 700.27 and the requirements for selective coordination from the 2008 NEC. At this time the proposal is being rejected by NEC Code Panel 13. Unless the panel receives comments that substantiate a reversal, selectivity will be retained for the 2008 NEC. Code Panel 13 refers to their statement in Proposal 13-135 for all other proposals regarding article 700.27.

Proposals 13-139 (also see Proposal 13-146). This proposal would keep selective coordination in the NEC but only require it for specific types of faults, most notably high-resistance faults, which are the most common type. If accepted, the concerns listed in this article regarding verification, competition and design consequences would be significantly relieved. At this time the proposal is being rejected by Code Panel 13.

Proposal 13-159 . Similar to proposal 13-135, this proposal calls for removal of Section 701.27, which calls for selective coordination of legally-required standby systems. At this time the proposal is being rejected by Code Panel 13, and selective coordination of legally required standby systems will be retained in the 2008 NEC.

Be heard

Whether you're for or against selective coordination, you can provide useful feedback to the appropriate Code Panels by commenting on the above proposals. Even if the Code Panel is supporting your viewpoint at this time, they would still like to hear your input. Of particular value to the Code Panels is verifiable information that identifies specific instances where selective coordination has or has not been a problem. Inadequate ground fault settings are the most common problem and should be distinguished from other problems.



ASHRAE Updates Energy Conservation Code, Partners with ICC

The International Code Council (ICC) and ASHRAE recently signed a Memorandum of Understanding that will seek ways to optimize development of codes and standards to improve public safety. The organizations will work together on industry advocacy and public policy and will explore joint business opportunities.

“This MOU will enhance both ASHRAE's and ICC's capabilities to have a positive impact on new construction and renovation activities in not only North American, but also the international markets,” said ASHRAE President Terry Townsend.

In other ASHRAE news, the organization has updated its standard on energy conservation in existing buildings, designed to help owners maximize efficiency by improving operation, maintenance and energy monitoring.

ANSI/ASHRAE/IESNA Standard 100-2006, Energy Conservation in Existing Buildings, was updated to bring it in line with other ASHRAE guides, specifically ANSI/ASHRAE/IESNA Standard 90.1-2004, Energy Standard for Buildings Except Low-Rise Residential Buildings, and the ASHRAE Handbook, HVAC Applications.

Other changes incorporated in Standard 100-2006 include updated requirements for compliance that allow for newer technology, such as more efficient lighting, that has been made available since the last update, and a revised bibliography to reflect current documents and new publications.



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