Selective coordination increases reliability of emergency systems
This article focuses on emergency power performance, selective coordination for fuses, and actions taken by NEC Code Panels prior to the release of NEC 2011.
Tim Crnko, manager, training and technical services, Cooper Bussmann
The 2005 and 2008 National Electrical Code (NEC) expanded the mandatory requirements for overcurrent protective device selective coordination to include power circuits supplying vital life-safety loads ( 700.27 emergency systems , 701.18 legally required standby systems , and 517.27 healthcare essential electrical systems ) and national security/public safety ( 708.54 critical operation power systems ). Since the adoption of these requirements, there has been acceptance by most, resistance by some, and new product introductions, along with application materials by electrical manufacturers to provide new solutions and approaches for easier compliance.
There are some parallels between the industry acceptance of selective coordination for elevator circuits that became mandatory in the 1993 NEC, and the industry acceptance of the expanded selective coordination requirements in the 2005 and 2008 NEC. After the adoption of 620.62-requiring selective coordination for elevator circuits -a few people in the industry fought for the ouster or dilution of that requirement for the next three or more code cycles. Over time, engineers, installers, and enforcers adapted to these requirements, and proposals to eliminate or dilute 620.62 were minimal during recent code cycles. Through the 2011 NEC Proposal Stage , selective coordination remains as a mandatory requirement for the sections added in the 2005 and 2008 NEC.
Understanding the action of the code panels illustrates the progress made to comply.
Emergency power performance
First, it is important to emphasize that selective coordination is mandatory only for a few power circuits supplying vital loads that are intended for life safety. These requirements are not for all facilities or circuits. These vital loads are powered by electrical circuits that have special NEC requirements to ensure attainment of at least minimum requirements for high reliability power. The selective coordination requirements are in addition to the general requirements of NEC Chapters 1 to 4, and are located in NEC Chapters 5, 6, and 8.
The loads on these circuits are critical during emergency egress of high-rise buildings and sports arenas. Personnel safety during emergencies is dependent on maintaining power to these vital loads. For example, emergency lighting must be maintained for building egress.
Typical high-rise building emergency egress design uses emergency lighting and pressurized stairways. Reliability of power for fire alarm systems and notification systems is critical. NFPA 72 is evolving by including mass notification systems.
The July 2009 Consulting-Specifying Engineer article " Ensuring emergency power performance " noted that many items, while seemingly unimportant, can jeopardize the operation of the emergency power supply system. Consider convenience receptacles or light fixtures at a low elevation after a flood. They may be served by overcurrent devices with poor selective coordination that can trip circuit breakers farther up in the system. As a result, portions of the system far removed from the receptacles or lights can be disabled.
Unfortunately, all too often, the design and installation meet only the minimum requirements of the NEC. It's a good idea to personalize the design by considering which loads you would want to be without if you and your family were on top of a high-rise and had to evacuate during a catastrophe. The principle of selective coordination is to minimize the loss of power to only those loads that must be removed if there is an electrical fault. If there are fires or building structure failures, you do not want to unnecessarily lose life safety loads due to a lack of selectively coordinated overcurrent protective devices. Without these NEC selective coordination requirements, during times of building system duress such as fires, floods, hurricanes, human-caused attacks, etc., cascading overcurrent protective devices would be permitted and life-safety loads could unnecessarily lose power.
Most in the industry have adjusted to the 2005 and 2008 NEC selective coordination requirements. This is in part due to the contributions by overcurrent protective device manufacturers who have published application material to assist in the design and installation of selective coordination compliant systems.
Selective coordination for fuses
Providing a selectively coordinated fuse system is relatively simple by following a fuse manufacturer's selectivity amp rating ratio guide. In most cases, selective coordination between fuses does not require a short-circuit current study or plotting the time-current curves; just follow the amp rating ratios. Also, when using the most current-limiting type fuses, normally the arc-flash hazard analysis results are acceptable and equipment protection is achieved. In some larger ampacity circuits, if there are cases where the arc flash hazard is higher than desired, other means may be deployed, such as optic sensing technology with overcurrent relays that can signal a switch to shunt trip and thereby reduce the arc flash hazard. To satisfy selective coordination requirements, the fuse industry introduced a fusible branch circuit panelboard for lighting and other loads.
Selective coordination for circuit breakers
Circuit breaker (CB) manufacturers now provide tables of circuit breakers that have been tested to achieve selective coordination. In addition, the manufacturers explain ways to comply by using circuit breakers with (1) short-time delay settings, (2) fixed high instantaneous trip settings, or (3) zone selective interlocking communications and tripping restraint. Short-time delay settings are sometimes helpful in attaining selective coordination, but they may also be associated with an increase of the arc flash hazard levels. For this reason, circuit breaker manufacturers provide options such as zone selective interlocking of CBs or maintenance switches. When maintenance is being performed on a CB with a maintenance switching option and a short-time delay, the CB can be switched to an instantaneous trip setting. CB short time delay settings also may negate equipment protection. However, the industry has responded with 30-cycle withstand ratings for automatic transfer switches and motor control centers.
Ground fault relays and phase overcurrent devices
Selectively coordinating a system with ground fault relays and phase overcurrent devices sometimes provides a challenge for the inexperienced design engineer. Inverse time ground fault relays often provide the right amount of delay in order to achieve selective coordination with the phase overcurrent protective devices. Another design option is to eliminate the need for ground fault relays by using high-resistance grounded systems to supply all the non-neutral loads. High-resistance grounded systems do not require ground fault protection (GFP) relays and can increase system reliability. The loads requiring neutrals can then be supplied from a feeder of the high-resistance grounded system through a transformer with a grounded secondary. If the secondary rated current is kept to 800 A or less, no GFP relay is required.
All of the objections to mandatory selective coordination were vetted by the 2005 and 2008 NEC Proposal and Comment process, and Code Panels 13 and 20 retained selective coordination as a mandatory requirement.
Still, some are advocating that selective coordination should be mandatory only for times greater than 0.1 sec. (6 cycles). Such a change would permit systems to be designed and installed where overcurrent protective devices would be coordinated only for overload conditions. Selective coordination only for times greater than 0.1 sec. would ignore the instantaneous trip of circuit breakers and the current-limiting range of fuses. Proposing selective coordination only for times greater than 0.1 sec. is an attempt to gut the requirement. For more information, read the sidebar, "Why selective coordination only for times greater than 0.1 sec. is flawed."
Code Panel action
The NEC Code Panels were clear in their definitions, requirements, and panel statements that selective coordination performance is for the full range of overcurrents (overloads and faults) at the point of application, regardless of how fast the overcurrent protective devices open. In considering the proposals and comments in the 2005 and 2008 NEC process, the Code Panels considered proposals for only times greater than 0.1 sec. To understand the NEC requirements, it is necessary to also study the panel statements. In the 2008 NEC process, Proposal 13-146 proposed a Fine Print Note to 700.27 essentially stating that selective coordination was applicable only for times greater than 0.1 sec. Code Panel 13 rejected this proposal with the following statement: "The instantaneous portion of the time-current curve is no less important than the long time portion."
Selective coordination is necessary for the continued operation of critical loads under emergency situations. Code Making Panel (CMP) 13 appropriately addressed the issue with its statement, "Selective coordination increases the reliability of the emergency system." Challenges to requirements for selective coordination in emergency systems, legally required standby systems, and critical operations power systems have been addressed by the CMPs in the 2005 and 2008 NEC cycles and in the 2011 Report on Proposals stage and are expected to continue in the Report on Comments stage for the 2011 cycle. These challenges are expected to decrease over time as design engineers develop a deeper understanding of the design techniques and as manufacturers introduce new products and solutions to meet designers' needs.
Why selective coordination only for times greater than 0.1 sec. is flawed
With a selective coordination requirement only for times greater than 0.1 sec. and a system designed to the minimum, low-level to high-level faults would be permitted to cascade (trip or open) multiple levels of overcurrent protective devices (branch, feeder, and main). The result would permit emergency loads to unnecessarily be interrupted due to a lack of selective coordination, even though compliant with a requirement only for times greater than 0.1 sec.
To illustrate this, Figure 1 and Figure 2 depict the time-current curves of the same 20 A, 200 A, and 800 A system.
Figure 1 shows no crossover or intersection of the circuit breaker curves above 0.1 sec. If the requirement for selective coordination were only for times greater than 0.1 sec., Figure 1 would be used as evidence to show these circuit breakers would comply.
Figure 2 depicts the circuit breaker curves with the analysis for the full range of overcurrents as required by the present NEC 700.27 text. Figure 2 shows the crossover of the circuit breakers in their instantaneous trip region.
It shows a lack of coordination between the 20 A and 200 A circuit breakers for ground, arcing, and any combination of phase faults as low as 800 A or greater. Any type of fault as low as 2,200 A or greater on the 20 A circuit can trip the 800 A circuit breaker as well. If the fault is on the 200 A feeder circuit, any type fault current of 2,200 A or greater can trip the 800 A circuit breaker. These are low available fault currents easily achieved in almost every essential electrical system via a line-ground fault, line-line fault, or three-phase fault, arcing or bolted.
The same situation can occur if fuses are not chosen and installed properly. Figure 3 shows fuse time-current characteristics where the curves are evaluated only for times greater than 0.1 sec. Figure 4 shows the same fuse curves, but below 0.1 sec. There is a noticeable lack of coordination for fault currents greater than where the fuses cross.
Unless the selective coordination analysis is for the full range of available overcurrents and the installer uses the proper settings, it is expected that low- to high-level faults will be permitted to cascade multiple levels of overcurrent protective devices. The 2008 NEC requires selective coordination for the full range of overcurrents available, regardless of the opening time of the protective devices.
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