Your questions answered: Critical power: Backup power systems

Design engineers have many factors to consider when designing a backup system for a facility. Additional insights from the June 30, 2016, webcast are provided.


Douglas Lacy, PE, LEED AP, vice president, electrical engineer, WSP + ccrd, Dallas. Courtesy: WSP + ccrdTom Divine, PE, senior electrical engineer and project manager, Smith Seckman Reid Inc., Houston. Courtesy: Smith Seckman Reid Inc.Safety, maintainability, code compliance, and economics play crucial roles in determining the topology of a backup system for a critical facility. Standby and emergency power systems provide power to ensure that life safety systems and critical equipment can operate during a power outage. NFPA 70: National Electrical Code (NEC) defines the categories that apply to generator power sources as emergency, legally required standby, optional standby, and critical operations power systems (COPS).

Design engineers as well as authorities having jurisdiction must know the effects these classifications have on how generators are applied within an electrical distribution system. Due to the requirements of the NEC and NFPA 110: Standard for Emergency and Standby Power Systems, design engineers must carefully consider the implications of combining emergency, legally required, and optional standby systems to ensure code compliance with maintainability and economics in mind.

Tom Divine, PE, senior electrical engineer and project manager, Smith Seckman Reid Inc., Houston; and Douglas Lacy, PE, LEED AP, vice president, electrical engineer, WSP + ccrd, Dallas, present additional information about designing backup power systems.

The June 30 "Critical Power: Backup power systems" Webcast presenters addressed questions not covered during the live event.

Question: Which is better, a centralized system or a decentralized system?

Douglas Lacy: There are distinct advantages and disadvantages to both centralized and decentralized standby power strategies that should be evaluated on a project-by-project basis. Centralized standby systems allow for monitoring systems, transfer and test equipment, fuel storage, maintenance, and supervisory personnel to be located in one location and thus reducing the overall cost and complexity of the components of the standby power system. Centralized systems also allow exhaust and noise generation of the prime equipment to be placed in a position away from other occupied spaces, which is typically desirable. Overall, centralized systems usually have a smaller aggregate footprint because they take advantage of the consolidation of common support systems. Decentralized systems have the advantage of a diversity of geographic location of equipment, thus making failures due to certain natural and manmade disasters less likely to affect the overall system integrity during a catastrophic event.

Q: NFPA 110 and The Joint Commission on Accreditation of Health Care Organizations require load bank testing of hospital generator systems. When a load bank is required, how do we avoid exposing the generator system to overload if normal power fails during the test?

Tom Divine: If a load bank is used as the only load on the generating system during load testing, a failure of normal power could lead to the transfer of building load to the system. If the total building load and supplemental load are greater than the generator capacity, it's possible that the generating system would shut down due to overload. With the generating system already at speed and voltage, the highest priority transfer switches can operate very quickly, making it fairly difficult for personnel to disconnect the load bank before additional load is applied. Some strategies to deal with this situation are to provide automatic controls to trip the load bank if normal power fails, or to disable transfer for lower-priority transfer switches.

NFPA 110 A.8.4.2, in Annex A: Explanatory Material suggests that all essential loads be transferred to the generating system before connecting a load bank, and that the size of the load bank be selected or controlled to supplement the building load as required to meet the load levels specified by the testing requirements. If normal power fails during the test, no additional building load will be transferred to the generating system, and there won't be any immediate danger of overload.

Q: Should generators be located in the basement?

Lacy: Placing a standby diesel or natural gas generator inside a building comes with certain design considerations that must be evaluated. Mechanical, exhaust, flue, and air intake conditions must be addressed. Also, consideration for access, maintenance, and removal of equipment if there is system failure should be evaluated. In addition, many standards, including those used in health care licensing, also restrict the placement of standby generators below the 100-year flood plain level. A risk assessment regarding the likelihood of flooding should be conducted before placing any standby system in an area that may be prone to flooding, regardless of standards that are enforced.

Q: What are the run-time requirements for various applications?

Divine: For emergency loads and legally required standby loads served by internal combustion engines—typically diesel, gasoline, or gas—the system must continuously operate for 2 hours after the loss of normal power, and it must have adequate fuel onsite to operate for that period at full demand. Fuel cell systems have the same requirement. Storage battery systems, and systems using unit equipment have a 1.5-hour requirement. There is no specified run-time requirement for optional loads. However, where a single generator serves both optional loads and higher-priority loads, the presence of those optional loads will affect the fuel usage rate of the entire system, so they must be taken into account when sizing fuel storage systems.

A COPS is required to be able to operate continuously for an unlimited number of hours under variable load. A reasonable interpretation of this requirement might be that the generators must be rated for operation at any load that can be expected with a standby rating, as opposed to a prime rating. There is no specific requirement for the capacity of the fuel storage system. The onsite fuel supply is required to be sized in accordance with the risk assessment. Typically, the risk assessment would evaluate an upper limit for the period of time that fuel deliveries would be unavailable, and the storage system would be sized to operate for at least that period.

Q: Do fire pumps have to get connected to a standby generator if it is only a Level 2 optional standby? Can natural gas supply a Level 1 generator in lieu of onsite diesel?

Lacy: The connection of a code-required fire pump may change the standby system to a Level 1 system. You still are able to serve optional standby loads from the same generator source, but the generator would need to meet Level 1 requirements and the requirements of NEC Article 695. In addition, the optional and fire pump loads would need to be on separate transfer equipment, but usually, this is taken care of by the fire pump having its own UL listed combination automatic transfer switch (ATS) controller.

Q: Please discuss the use of standby generators for demand response.

Divine: From early 2010 until recently, the U.S. Environmental Protection Agency (EPA) ruled that owners of emergency generators have been able to obtain compensation from electric utilities in return for shifting a portion of their demand from the grid to their internal generation under system emergencies. Under these rules, an "emergency generator" is defined as "a unit that operates only when normal power is unavailable, and for a defined number of hours for specific purposes." Until May of 2016, one of those allowable purposes was demand response, defined as "generator operation at the request of a utility or other qualified entity under emergency conditions."

In response to a lawsuit from the State of Delaware, the Washington, D.C., Circuit Court of Appeals abrogated the rule that allowed operation for demand response. Based on that ruling, a generator that operates under those conditions would lose its qualification as an emergency generator, and fall under the EPA emissions requirements for nonemergency units. Because nonemergency generators may operate for extended periods, their emissions requirements are much more stringent than those for emergency units. The costs of bringing an emergency unit into compliance are so high that the rule change effectively prohibits standby generators from operating under demand response.

The actual effect of the court's action isn't entirely clear. It may have eliminated demand response entirely, or it may have reduced the number of hours available for demand response to 15 hours per unit per year. The EPA issued a guidance letter saying that it expected a formal ruling from the court on May 1 or 2, 2016—the effective date of the change—and that the court's order would eliminate demand response operation entirely for emergency generators. It's not clear that the court actually issued such an order, and it's not completely clear whether the previous 15-hour rule has been reinstated, or whether the rule allowing for demand response has been eliminated entirely. Any entity that contemplates operating an emergency generator under demand response should consult an attorney to determine the actual effect of the court's ruling.

Q: Are emergency elevators a legally required item?

Lacy: There are many types and use cases where elevators would be required to be connected to the emergency or legally required system. These include elevators necessary for accessible means of egress in buildings over a certain height as well as fire-service elevators in certain occupancies, including, but not limited to high-rise buildings. Refer to the International Building Code for requirements concerning standby power requirements for elevators and other means of egress components.

Q: Do hospital systems have to be selectively coordinated in the instantaneous region also? NFPA 99: Health Care Facilities Code doesn't require coordination below 0.1 seconds.

Divine: For most circuit breakers, the tripping characteristics for high-level faults are poorly characterized, in that well-defined curves that can be coordinated aren't known for faults with duration of less than approximately 3 cycles at 60 Hz. On time-current curves, this region often is informally called the "instantaneous region." In general, where coordination is required in this region, coordinated breakers and settings are determined empirically by the manufacturer. The definition of selective coordination in the 2014 edition of NEC states that selective coordination covers all tripping times, and the full range of available fault currents, and Articles 700 and 701 require selective coordination for emergency and legally required standby systems. Systems covered by those articles require coordination in the instantaneous region.

Under that definition, hospital systems don't have to be coordinated in the instantaneous region. The requirements for selective coordination in hospitals have been defined in NFPA 99-2012, and in the 2014 edition of the NEC, in Article 517. NEC Article 517.30(G) requires selective coordination "for the period of time that a fault's duration extends beyond 0.1 second," with an informational note specifically stating that the requirement does not cover the full range of overcurrent conditions. The intent of relaxing the selectivity requirement for hospitals is to allow other considerations, such as protection or arc flash, to influence the selection of overcurrent devices and settings. If the strict definition applied to hospitals, the requirement for selective coordination would drive all, or nearly all, of the decisions affecting the design of the protection system at the expense of those other functions.

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