Selective coordination studies for mission critical environments


Understanding the code 

To understand the full potential impacts of recent changes to the NEC, it is important to quote the new definition of “selective coordination” and the new codes in sections 100, 517, 700, and 701 as they apply to emergency and standby systems, which are a part of mission critical systems: 

NEC 100 definitions – coordination (selective): Localization of an overcurrent condition to restrict outages to the circuit or equipment effected, accomplished by the choice of overcurrent protective devices and their ratings and settings. 

NEC 517.26 - application of other articles: The essential electrical system shall meet the requirements of Article 700, except as amended by Article 517.

NEC 700.27 - coordination: Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices. 

NEC 701.18 - Coordination: Legally required standby system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices. 

For clarity, it is important to include the NEC definition (Section 100) of overcurrent and the fine-print notes defining emergency systems and legally required standby loads in Sections 700 and 701:

Overcurrent: Any current in excess of the rated current of the equipment or ampacity of the conductor. It may result from overload, short circuit, or ground fault. 

A “short circuit” is noted as one of the items that can cause an overcurrent. The typical molded case circuit breaker combination with the upstream breaker somewhat larger than the downstream breaker does not have a problem coordinating in the overload area of the time current curve, but a high level of current in the short circuit area of the time current curve can represent significant problems to selective coordination. 

NEC Section 700.1 “fine-print note” (FPN) No. 3: Emergency systems are generally installed in places of assembly where artificial illumination is required for safe exit and panic control… Emergency systems may also provide power for such functions as ventilation, fire detection and alarm systems, elevators, fire pumps, public safety communication, and industrial processes. 

NEC Section 701.2 FPN: Legally required standby systems are typically installed to serve loads, such as heating and refrigeration systems, communication systems, sewage disposal, lighting systems, and industrial processes, that, when stopped during any interruption of normal electrical supply, could create hazard or hamper rescue or fire fighter operations.

Arc flash studies 

We are seeing more requirements for arc flash studies for critical infrastructure. The amount of arc flash energy levels that can be produced at any point in an electrical distribution system depends on the amount of fault current that is available and the speed at which the overcurrent protective device operates. 

The arc flash calculation will determine the amount of thermal incident energy to which an electrician’s chest and face can be exposed to at working distances. This energy level is measured in Joules/cm2 or calories/ cm2

These calculations are provided to determine the amount of personal protective equipment (PPE) that is required to operate or maintain the equipment with exposed live parts. The flash boundary is the approach distance limit to the exposed live parts during maintenance, operation, or testing. If the electrician is not appropriately protected in this flash boundary, he could receive second-degree burns. 

Figure 3: This graph illustrates the ground fault setting for a 2,500 amp main breaker and the ground fault setting for a 400 amp sub breaker. There is clear separation between the ground fault curves. A ground fault on the 400 amp feeder would trip the 4Typically, a fault current study is first performed on a project. The second step is to perform a coordination study to determine the optimum settings of the overcurrent protective devices and protective relays to provide for as much selective coordination as possible. Then as a third step, after the settings of the breakers and relays are determined, the arc flash study is performed. Therefore, settings that provide the optimum separation between the time current curves and isolate a fault to the smallest area possible may actually cause higher levels of arc flash energy. 

NEC 517-17 requires that if ground fault protection is provided for the service disconnecting means, then an additional step of ground fault protection shall be provided in the next level of feeder disconnecting means downstream toward the load. Figure 3 is an example of a properly coordinated ground fault study.

Ground fault settings

NEC 230-95 indicates that all 480 V, 3-phase services 1,000 amps and over must be installed with a ground fault relay. The setting of the ground fault relay cannot exceed 1,200 amps, regardless of the size of the overcurrent protection device. In addition, the time delay cannot exceed 1 second (60 cycles) for ground fault currents of 3,000 amps or more. There shall be a minimum of 6 cycles (0.1 second) ground fault delay between ground fault devices in healthcare facilities. 

Ground fault settings for main breakers serving downstream motors that are set too low or too fast may trip a main overcurrent protection device before tripping the local thermal magnetic overcurrent protection device during motor starting ground faults. On the other hand, ground fault settings that are too high can cause undue damage before a ground fault is interrupted. It is important to provide the ground fault setting that will not permit nuisance tripping, but will protect the electrical equipment from excessive damage during an event. 

It has been my experience that sometimes perfect coordination between a set of devices cannot be obtained. Certain settings may be required on a breaker that could affect the settings of many breakers. In some cases, there may be many levels of breakers that cause overlap of the breaker curves within the tolerance of the curves. In these cases experience will help the engineer make judgment calls as to compromises in coordination between devices. The engineering behind providing protective coordination studies is not a perfect science. 

Often, completed projects have no protective device study. In such cases the breaker manufacturer will ship the breakers with all settings set to the most sensitive. This will ensure the most protection but will increase false trips and is typically not good for the reliability and uptime of the systems. As soon as the owner complains of a false trip, the facility personnel will probably set all of the dials to least sensitive. This will reduce false trips, but may not adequately protect the electrical system and reduce selective coordination of the system. 

A coordination study is typically required to ensure that the most reliable electrical system has been installed. In addition, there are instances where NEC requires that a study be performed. In either case, the cost of a coordination study is pretty cheap insurance for most installations that would be adversely affected by an extensive power outage.

Keith Lane is president and CEO of Lane Coburn & Assocs. He is a member of the Consulting-Specifying Engineer editorial advisory board, and was a 2008 40 Under 40 award winner. Lane has more than 20 years of experience designing, commissioning, and optimizing mission critical facilities

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