Your questions answered: Know how to design power systems in mission critical facilities

Learn about the choices and criteria for the planning and design of mission critical facility switchgear, automatic and static transfer switches and UPS in this Q&A.

By Consulting-Specifying Engineer September 26, 2023
Courtesy: Consulting-Specifying Engineer

Power system insights 

  • NFPA 70: National Electrical Code is the primary resource for electrical engineers when designing power systems in mission critical facilities.  
  • Engineers should determine options for mission critical switchgear, automatic and static transfer switches (ATS), and uninterruptible power supplies (UPS). 

Mission critical facilities typically require electrical engineers to design and specify significant amounts of power to increase reliability, limit outages and provide for redundancy.

With the need for extensive power, these facilities often require the engineer to design medium-voltage primary services, primary service transformers, low-voltage distribution gear and uninterruptable power supplies. In some smaller mission critical facilities, the local utility may furnish primary medium-voltage equipment, leaving the engineer to focus on low-voltage systems design.

Watch the webcast on Critical power: Know how to design power systems in mission critical facilities from July 27, 2023, and refer back to these additional responses.

Responses provided by: Radames Cocco, PE, NCEES, LEED AP, Principal, DLR Group

Question: Can solar power be considered for mission critical power?

Answer: It can be considered to augment normal sources, but should not be considered a reliable source. That means your normal and/or alternate sources should be sized for the full load. There would also be controls considerations when on alternate source.

Question: Can you use a static transfer switch in a hospital?

Answer: Most hospital loads are usually not on an uninterruptible power source. Also, the higher tier ratings for data center distribution topologies are usually not found in hospitals, other than their data components. However, for such loads which would be downstream of UPSs, other than cost, I am not aware of a reason why one could not do so.

Question: Can you use a second utility connection as a source of redundancy?

Answer: For hospitals, the normal source is usually the utility and the alternate source is usually a generator. If the normal source consists of a generator, the alternate source can be a generator, or an external utility source. Reference NFPA 70: National Electrical Code (NEC) Article 517.30.

Question: For a static transfer switch, how is uninterrupted power provided without batteries?

Answer: One option can be a rotary uninterruptible power supply (UPS) where the sored energy is a rotating mass. This usually results in shorter backup times.

Question: Have you noticed any increase of natural gas generators over diesel gensets being specified? Are there any benefits of one over the other?

Answer: Not yet. Diesel fuel has a higher energy content, which aids in ramp-up speeds and transient response. Also tends to have a smaller footprint. Natural gas can be connected to a utility (with a minimum on-site fuel source) So, for gas it can be more economical to run.

Question: In a hospital, does the NEC and/or NFPA 99: Health Care Facilities Code require that the life Safety and critical branch be two-hour rated? Will the equipment branch also be required to be two-hour rated?

Answer: NEC Article 517.31(C) has physical separation and physical protection requirements for life safety and critical branch feeders and branch circuits. However, I am not aware of specific fire rating requirements similar to 700.10(D).

Question: Is NEC Article 708 critical operations power systems (COPS) applied on mission critical facilities?

Answer: Yes.

Question: Is there a building or electrical code mandate that specifically which installations must be designed as a COPS facility?

Answer: Not specifically. The relevant governmental authority will designate which functions constitute a designated critical operations area (DCOA).

Question: What do you think of a fuse downstream and breaker upstream system?

Answer: Because a fuse has a narrow, defined curve in the instantaneous region, it is a good candidate for downstream overcurrent protection. I use the noted combination often.

Common examples of mission critical facilities include hospitals, laboratories and data centers. Courtesy: Consulting-Specifying Engineer

Common examples of mission critical facilities include hospitals, laboratories and data centers. Courtesy: Consulting-Specifying Engineer

Question: Can you comment on the 0.1 sec coordination in hospitals?

Answer: It seems a bit counterintuitive. However, the NFPA report on proposals (ROP), or draft reports at the time noted that it was done to align the NEC with the requirements in NFPA 99. A couple of considerations, not specifically noted in the ROP. Higher selectivity can have the consequence of higher destructive energy passing through upstream overcurrent devices. Also, 0.1 sec is achievable with breakers, which are quicker to reset than replacing fuses.

Question: What are the requirements for distribution in the DCOA for COPS building?

Answer: The minimum requirement for the distribution is noted on the slide in the presentation, which was drawn from Part III of Article 708. There are also selectivity requirements, on-site fuel sources and bypass isolation transfer switch requirements.

Question: What distinguishes a COPS (NEC Article 708) system versus a system needing high reliability?

Answer: National security, public health or security, the economy or where continuity of operation has been deemed necessary by the government authority. This can be federal, state or local.

Question: What do you assume is the usual amount of time between a power failure and a generator starting?

Answer: For Level 1 installations, which are systems where failure of the equipment to perform could result in loss of human life or serious injuries, the load side of the transfer switch must obtain alternate source power within 10 seconds of loss of normal power. Other systems can take longer and often are staggered to limit the step loads on generators.

Question: What is the difference between life safety and nonlife safety back up loads?

Answer: In a health care setting, life safety loads include illumination of the means of egress, exit signs, alarms and alerting systems, communications systems where used for issuing instructions during emergency conditions, generator set locations and accessories, elevator cab lighting and communications and automatic doors. In a nonhealth care setting, these similar loads are called emergency loads.

Question: Where does the ideal short-circuit current fall in a properly coordinated system?

Answer: The short circuit current value is a function of available fault current from the source and impedance of the distribution system. The available fault current at various locations of the distribution help define the value up to which the devices need to coordinate.

Question: Which governing body/bodies in the U.S. determines if a facility is mission critical?

Answer: For designated critical operations areas, it would be the controlling government authority. This can be federal, state or local.

Question: Why do we consider the utility as a single point of failure?

Answer: In most utilities, it is very difficult to get multiple and different incoming sources.

Question: Can we consider the generator as a primary power source to not rely on a single utility power source?

Answer: NEC Article 517.30(B)(1) recognizes a generator as option for a prime source of power. If a generator is the prime source, the alternate source would then be another generator, or an external utility source.

Question: Would the location of a mission critical facility near a large electrical substation allow a lower level of on-site fuel for generators?

Answer: I would not expect so.

Question: Would the panel discuss the recent implementation of NEC 700.5 (D) requirements for redundancy of the life safety branch?

Answer: I am not opposed to it, just going to be expensive for most noncritical projects. But the exceptions seem to be very loose. There are already similar requirements (e.g., Article 708.24 for COPS systems) and it is common practice for transfer switches in health care facilities, if not at least for those serving life safety and critical loads. I suspect less critical occupancies may opt for Exception 4, which permits a written emergency plan to address mitigation actions for recognized site hazards.