Your questions answered: How to specify backup, standby and emergency power in mission critical facilities
When designing backup, standby and emergency power systems for mission critical facilities, there are several considerations beyond NFPA 70: National Electrical Code and other building code requirements that must be addressed. Electrical engineers must understand the specific owner project requirements for the building’s power systems and ensure that the generator specification and system topologies meet all those requirements for cost, construction schedule, component performance and system maintainability.
Review additional responses to questions from the Feb. 27, 2020, webcast “How to specify backup, standby and emergency power in mission critical facilities.”
- Bart Hogge, PE, ATD, LEED AP, principal and mission critical market leader, Affiliated Engineers Inc., Chapel Hill, N.C.
- Danna Jensen, PE, LEED AP BD+C, principal, Certus Consulting Engineers, Carrollton, Texas
Question: What about fuel polishing and the associated pumps and how they are powered for redundancy/resiliency
Danna Jensen: Depending on the quantity of on-site diesel fuel provided (see question regarding engine run-time for additional details), it could add up to a massive amount of fuel stored on-site. Depending on the conditions it is stored, diesel fuel has an average shelf life of 6 to 12 months. In a facility such as a hospital or other “emergency standby” generator applications where the system is used solely for normal power outages and testing, it could take much more time than this to use up the fuel.
For facilities with large quantities of on-site diesel that anticipate they will not use all of the fuel within this timeframe or are regularly refueling new fuel mixed with old fuel, a fuel maintenance program should be in place. This could a fuel maintenance contract with the supplier where they will come and cycle or change out the fuel or and on-site polishing system.
Either way, this is a consideration for emergency power system designs that must not be overlooked. When it comes to powering the associated pumps, NFPA 70: National Electrical Code Article 517.32(F) and NFPA 99: Health Care Facilities Code 22.214.171.124.2.6 both state that “loads dedicated to a specific generator, including the fuel transfer pump(s) … shall be connected to the life safety branch or to the output terminals of the generator with overcurrent protective devices.”
Bart Hogge: We see the benefits and it is often implemented on our projects where bulk storage is involved. The quality of the fuel is vital to system operation and long-term health of the machines. The fuel storage and distribution should be aligned regarding resiliency and redundancy goals to match the owner’s project requirements (to include maintenance expectations). The fuel polish may or may not be included in that, based on the owner’s sensibilities. It isn’t required for the fuel system to operate. If the project should choose to invest in redundancy for the fuel polish capacity and distribution components, be careful to be consistent with the power sources (avoid single points of failure) and isolation valves (concurrent maintainability).
Question: In a medical facility, how do we determine generator run time?
Danna Jensen: NFPA 99: Health Care Facilities Code defines a hospital emergency power supply system as a Class X. You must refer to NFPA 110: Standard for Emergency and Standby Power Systems to determine what Class X means, which indicates that the run time required is determined by the application, code or user. Therefore, you must turn to another guideline to determine the amount of run time. The Facility Guidelines Institute for the design and construction of Hospitals is the adopted standard for most jurisdictions. This standard state that where stored fuel is required, the capacity shall permit continuous operation of the emergency power system for at least 24 hours. However, the appendix in this same standard states “storage of fuel for at least 96-hours should be considered for facilities in locations likely to experience an extended power outage.”
Similarly, The Joint Commission, which is the nation’s oldest accrediting body in health care, requires that all licensed hospitals have a plan in place to achieve 96 hours of run time. There are other jurisdictions that have varying rules as well. The bottom line? It really varies from facility to facility and having those discussions up front with both the facility operators and the local authority having jurisdiction will be the ultimate deciding factor for generator run time.
Question: What are the options to bypass transfer switches for maintenance?
Bart Hogge: To completely bypass the automatic transfer switch enclosure, look into a solution similar to a uninterruptible power supply maintenance wrap. Investigate using a remote-control cabinet to control electrically operated circuit breakers or switches for a closed transition, electrically operated main-main input to the downstream distribution panel. The second main bypasses the ATS. Use a sync check relay to inhibit closed transition transfer if the sources are out of sync. Ensure that the two sources inherently will or can be made to be in sync.
Question: What are the fuel system run times and sizing requirements for the different applications?
Danna Jensen: NFPA 110: Standard for Emergency and Standby Power Systems defines different classes of emergency power supply systems that provides the minimum time, in hours, that the system is required to operate without being refueled or recharged. The requirements run anywhere between five minutes to 48 hours, so a class 5 requires five hours, 48 requires 48 hours, and so on. And there is one rating beyond 48-hours called “Class X,” which means the ultimate decision is up to the facility. Other codes mandate the class requirements: for instance, NFPA 99: Health Care Facilities Code says that hospital systems must be Class X, and then NFPA 110 defines what Class X means.
Question: Do you see the implementation of the ring bus design for data center picking up or is it more for high-speed data centers (e.g., Tier 4)?
Bart Hogge: I have not seen a trend in implementation one way or the other. It is certainly a popular choice that we see implemented for projects we are involved in. My experience is that the size of the system (megawatt and quantity of capacity components) dictates the viability for our clients based on their budget. If the final infrastructure is installed on Day One and substantial future expansion and testing won’t be necessary, this solution may become more attractive.
Question: Have you seen any move toward natural gas generators or BLOOM fuel cells to backup data center loads so as to avoid the refueling issues you mentioned?
Bart Hogge: I am aware of those alternative fuel sources being implemented, certainly, but I have not observed a trend one way or the other and presently they are less often implemented. For natural gas, the owner and AHJ need to align on expectations for bulk fuel storage (if required). The capital expenditure versus operational expenditure and physical footprint comparison based on the size of the generator needs to be understood. We have more often seen fuel cell and other alternative energy technology used in microgrid and extended utility paralleling applications.
Question: Do you have experience with putting elevators and escalators on a generator? Considerations?
Danna Jensen: Including elevators on emergency power, specifically in a hospital, is not only highly recommended, but in most cases, it is required. If the elevator is considered part of an accessible means of egress per International Building Code (which an architect or life-safety consultant will typically define), it must be on emergency power.
Also note that for high-rise buildings greater than 120 feet high, a fire service access elevator will be provided and this must be on emergency power as well. The concern could be the effect this has on the sizing of the emergency power system. However, there are options with most elevator manufacturers that allow for an “emergency power mode,” which will basically move only one elevator at a time, alleviating that full load on the emergency power source. This must be closely coordinated with the Masterspec Division 14 specifier and requires additional signal wiring between the elevator controller and the transfer switch feeding it.
As far as including an escalator on emergency power, if the facility deems the motion of the escalator important enough to put on emergency power, that is their option and the code will permit optional loads on the equipment branch. However, it is not required because the escalator still allows for safe evacuation when not in motion.
Question: For N+1 design, is a different rated kVa capacity allowed?
Bart Hogge: In my opinion, yes. The design must plan for removal of the largest capacity component and maintain service to the critical load. For example, if a 1 megawatt and a 1.5 megawatt generator are used in a redundant scheme the critical load is limited to 1 megawatt or less.
Question: How do you comply with ground fault detection when there are more than three power sources?
Bart Hogge: There are several methods to consider. The three I’ve observed most commonly are detailed below. Investigate using a differential ground fault system. This is common, however can be complicated and care must be taken to see that the system is set corrected and tested. Alternatively, certain applications may not need a neutral beyond the service entrance disconnecting means (and deriving a neutral close coupled to the load if necessary) and a 3-watt system can be implemented where multiple sources are connected. In this situation a traditional ground fault solution can be implemented. Lastly, 4-pole circuit breakers to disconnect the neutral if a 4-watt system is required can be implemented in an option. However, the additional cost for the circuit breakers and space footprint impact must be considered.
Question: Distribution planning needs to account for flooding of the electrical equipment, correct?
Danna Jensen: Depending on the location of the facility, flood-proofing considerations may be warranted. The codes are not necessarily specific in this instance, rather they state the equipment must be placed in a location to minimize damage. Installing critical equipment in a basement subject to flooding is never a good idea. In areas such as this, an option may be to install equipment on a second level, however additional considerations to refueling are then warranted. All of this should be discussed at the onset of the project.
Question: Should you consider the priority one load block in the generator sizing?
Danna Jensen: To clarify, when dealing with a hospital, the life safety and critical branches are typically considered “priority one” and not permitted to be shed. Does the minimum size of the generator need to be able to support all priority one loads? The answer is: maybe. Per NFPA 99: Health Care Facilities Code, a hospital emergency power system is a Type 10 system, which means power must be restored to all of the nondelayed (or all of the life safety and critical loads) within 10 seconds. Depending on the system size, it may only be possible to get one engine fired up and closed to the bus to deliver power in that allotted 10 second time frame.
However, different systems have different operating characteristics and if it can be proven through repeated testing without fail that two or more engines are able to deliver the power within the 10 seconds, then it is acceptable to spread the priority one loads over more than one engine from a capacity standpoint.