Critical operations power systems application

How NFPA Standard 70 and Article 708: Critical Operations Power Systems (COPS) is being interpreted and applied, and how to respond with standby systems design when COPS is levied on a project.

08/15/2011


Recent natural and man-made events have prompted people to re-evaluate the safety and reliability of facility infrastructure. In 2008, Article 708: Critical Operations Power Systems (COPS) was added to NFPA 70 (NEC) to provide mission critical facilities with a higher level of protection. Article 708 provides requirements for the installation, operation, control, and maintenance of electrical equipment and wiring serving designated critical operation areas that must remain operational during a natural or man-made disaster.

COPS are generally installed in vital facilities that if destroyed or incapacitated would disrupt national security, the economy, public health, or safety. The local authority having jurisdiction (AHJ) can also require a facility to meet Article 708 where it deems an enhanced electrical infrastructure is necessary for continuity of operation. Examples of facilities that would use COPS include police stations, fire stations, emergency management centers, emergency call centers, and government facilities involved in national security.

There are two common approaches when applying Article 708: COPS to a facility. One approach is to designate a specific area within the facility as the designated critical operations area (DCOA). Under this approach, only the power systems serving the DCOA must meet the requirements of Article 708. The other approach is to designate the complete facility as a critical operations area. Under this approach, the entire electrical power infrastructure has to meet the requirements of requirements of Article 708.  

Requirements of Article 708 include the following:

Risk assessment: Perform a risk assessment to identify hazards (natural and human), the likelihood of their occurrence, and the vulnerability of the system. Based on the risk assessment, develop and implement a plan to mitigate the potential hazards.

Commissioning and maintenance: Develop and implement a commissioning plan and a preventive maintenance program for COPS equipment. Means shall be provided to test all critical power systems under maximum anticipated load conditions.

Identification: Boxes, enclosures, and receptacles for COPS systems shall be permanently marked and easily identified.

Feeder physical protection: Feeder wiring shall be protected from physical damage by one of the following methods: rigid metal conduit, intermediate metal conduit, or MI cable. PVC schedule 40 or 80, EMT, reinforced thermosetting resin conduit (RTRC), flexible nonmetallic raceway, flexible jacketed metallic raceway, and jacketed metallic cable assemblies (listed for installation in concrete) also can be used when encased in a minimum of 2 in. of concrete.

Feeder fire protection: Feeder cable shall meet one of the following conditions: be listed electrical circuit protective systems with a minimum 1-hour fire rating, be protected by a fire-rated assembly listed to achieve a minimum fire rating of 1 hour, be embedded in not less than 2 in. of concrete, or be a cable listed to maintain integrity for not less than 1 hour.

Branch circuit wiring: COPS wiring shall be kept entirely independent from other wiring. Branch circuits installed outside the DCOA shall comply with the physical and fire protection requirements for COPS feeders. Branch circuits installed inside the DCOA shall comply with any code-approved wiring method.

Distribution equipment: Branch circuit distribution equipment shall be located within the same DCOA as the branch circuits it supplies. Feeder distribution equipment shall be located in spaces with a 2-hour fire-resistance rating and above the 100-year flood plain.

HVAC, signaling, and communications wiring: Signal and communications wires shall be shielded twisted pair. Riser communications cables shall be 2-hour fire resistive cable or a listed 2-hour protective system. Control, monitoring, and power wiring to HVAC systems shall be 2-hour fire-resistive cable or a listed 2-hour protective system.

Power source: All sources of power shall be grounded as a separately derived source in accordance with Article 250.30. Surge protection devices shall be provided at all facility distribution voltage levels.

Ground fault: An additional step of ground fault protection shall be provided downstream of any service or feeder disconnect ground fault protection. Service and feeder ground fault protection shall be fully selective with a 6-cycle minimum separation.

Coordination: All COPS overcurrent devices shall be selectively coordinated.

Alternate power source: Provide an alternate source of power with the capacity to operate at full load continuously for a minimum of 72 hours. A generator, UPS, or a fuel cell system can be used as the alternate source of power.

Emergency and/or standby generators can be used as the alternate source of power in COPS. This will have a great effect on the fuel tank size for the generator (sized for 72-hour continuous operation). Additional requirements that must be incorporated into the design when using a generator as an alternate power source under Article 708 include:

  • The alternate source of power shall be permitted to serve other loads where it has adequate capacity or where automatic selective load pickup and load shedding is provided.
  • The load shedding shall be set up to ensure adequate power to (1) COPS and emergency loads, (2) legally required loads, and (3) optional standby loads, in that order of priority.
  • Transfer equipment for COPS loads shall be separate from other emergency, legally required, and optional standby loads.
  • Means for connecting a portable or vehicle-mounted generator shall be provided when COPS is supplied by a single generator.
  • On-site fuel supply shall be provided and protected in accordance with the risk assessment.
  • Fuel transfer pumps and battery chargers shall be connected to the COPS branch distribution system.

No matter which approach you use, compliance with Article 708: COPS has significant financial implications. Implementing physical protection, fire resistance, and selective coordination into the power system can be very expensive. It is often with these types of requirements that interpretations vary between those designing and those inspecting the project.

For example, consider the requirement for all control, monitoring, and power wiring serving HVAC systems to be 2-hour fire resistive cable or a listed 2-hour protective system. Article 708 states that branch circuits within the DCOA do not need to be fire rated. By definition, a circuit feeding equipment is a branch circuit. Therefore, some have interpreted this to mean the branch circuits serving the HVAC equipment do not need to be fire rated if you designate the entire facility as DCOA.

Another interpretation that has been used to alleviate the fire-resistive cable requirement is the installation of redundant equipment. People have made the argument that installing redundant power and HVAC equipment in separate fire-rated compartments meets the intent of fire-resistive cabling by providing protection against dropping the critical load in an event of a fire.

Due to the possibility of differences in interpretation and the limited number of projects that have been COPS compliant, it is recommended that you review the requirements of Article 708 and your design intent to meet those requirements with the local AHJ prior to completing the design. The information and requirements contained in this article were extracted from NFPA 70 - 2008 National Electrical Code. Therefore, prior to designing a COPS, it is also recommended that you refer to the actual NEC Article 708 for complete details and guidelines.

Kutsmeda is an engineering design principal at KlingStubbins in Philadelphia. For more than 18 years, he has been responsible for engineering, designing, and commissioning power distribution systems. His project experience includes 7x24 mission critical facilities, emergency operations, and call centers; highly specialized research and development buildings; and large-scale technology projects.



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