Critical power: Transfer switches and switchgear: Q&A Session

Webcast Q&A session with presenters Ken Lovorn, PE, Lovorn Engineering Associates, Pittsburgh; and Danna Jensen, PE, LEED AP BD+C, ccrd partners, Dallas.


Ken Lovorn, PE, Lovorn Engineering Associates, Pittsburgh; and Danna Jensen, PE, LEED AP BD+C, ccrd partners, Dallas.CSE: Is there a difference between emergency loads and life safety loads?

Ken Lovorn: The term “life safety loads” has a specific definition in the NEC and is generally associated with NEC Article 517, while emergency loads are generic and are associated with NEC Article 700.

CSE: Regardless of whether there is a combination of life safety, critical, or normal loads in the building, isn't there a situation where one transfer switch is allowable, such as when the building has a power transformer that’s 150 kVA or smaller?

Danna Jensen: The answer varies depending on the facility type. In a hospital (NEC Article 517), if the total emergency power load is less than 150 kVA, the critical and life safety branches are allowed to share one transfer switch, but the delayed equipment branch must be kept separate. In a high-rise office building, only one transfer switch is required for legally required loads, as defined in NEC Article 701. But if you want to include optional loads (as defined by NEC Article 702) such as security and IT components, a separate transfer switch is required. It is different for facilities such as nursing homes, which require code due diligence to determine the total required quantity of switches.

CSE: What are typical timings for restaurant, office, and daycare buildings according to NFPA 110? What type of ATS (closed/open) should be used?

Lovorn: For a restaurant, office, or daycare building, the life safety loads (exit lights, egress lights, and fire alarm system) must be connected within 10 sec. Because short outages are not particularly critical, an open transition transfer switch without isolation/bypass would serve quite nicely.

CSE: As I understand it, you have 3 sec to prove loss of power, and then at least 4 or 5 sec to start the generator and get it ready to accept loads. Aren't we too close to the 10-sec limit to start life safety loads anything other than first?

Lovorn: The timings that you offered in your example are adjustable, but you are correct that there is scant little time to bring on anything other than emergency loads (exit lights, egress lights, fire alarms) and still meet the 10 sec time limit. Our example was intended to show that starting generator loads in multiple steps had the possibility of reducing the size of the generator. It was not intended to require that emergency loads be delayed after the 10 sec time limit.

CSE: Whose scope is it under to program the generator load steps?

Jensen: It is ultimately the construction manager’s or general contractor’s responsibility to “scope” a project. Typically, this is held under Division 26 and the electrical contractor has the manufacturer program the steps. Again, it is up to the construction manager/general contractor, but my recommendation is to have the manufacturer perform the on-site programming and also have a manufacturer’s representative present during the commissioning start-up process in case anything goes awry.

CSE: I believe generators must have capacity to start fire pumps after all other loads are energized, correct?

Lovorn: Yes, the generator must have the capacity to start the fire pump with a maximum of 15% voltage dip. No matter where it is in the sequence, the pump is required to start.

CSE: My experience with generator software allows for steps, but does not allow us to state or limit the time duration of the steps. How do you know from the software that the time to safely start the fire pump within 10 sec is achieved?

Lovorn: The generator sizing software does not give starting times for steps. Engineering judgment based on fire pump starting data, generator data, and other information is required to determine if your particular combination of generators and fire pumps will permit the fire pump to be operational within 10 sec.

CSE: Are there any side clearances required for ATSs?

Jensen: Typically, standard (non-bypass isolation) ATSs require only front clearance. Bypass isolation switches typically require either front and rear access or front and side access. It depends on the manufacturer, the exact configuration, and their required clearances.

CSE: Regarding genset sizing and fire pumps, I think that, based on the NEC, the voltage dip must be limited to 15%. By putting the fire pump in a separate step, is the limit for the step or for the genset?

Lovorn: In our calculations, the fire pump step was limited to a 15% voltage dip on the generator. Because the generator is the only source during a power outage, the voltage dip of 15% must be applied to the generator. When starting the fire pump on the utility source, the 15% dip must still apply, but with typical utilities and distribution systems, there would be no issue on meeting this voltage dip with the utility as the source.

CSE: With regard to generator sizing, fire pumps, elevators, and random loads, how do you account for this random need? Clearly, both of these can be enabled to run any sequence we want, but when they actually start is random and will most likely be elevator next to last and fire pump last. Does the fire pump have a separate breaker at the generator in this example, and how does that affect the calculations?

Lovorn: The calculations were intended to show examples of worst-case starting of various loads that are typically found in commercial buildings. You are correct that there is a significant probability that the elevator and fire pump would not start at the same time as the loss of utility power. It is possible that the fire pump could have a separate breaker, but not necessarily. It is, however, required that the fire pump have a separate transfer switch. Having a separate breaker for the fire pump would not impact the calculations.

CSE: Please elaborate on 3-cycle and 30-cycle transfer switches.

Jensen: In 2007, UL 1008 (the standard for short circuit testing criteria) was amended to allow transfer switches to be protected with circuit breakers incorporating short time ratings as long as the transfer switch was tested for the longer time duration (longer than the original standard of 3-cycles). Having a switch that is rated for 30 cycles allows UL 1066 breakers (which incorporate short time delays up to 30 cycles as opposed to 3 cycles of the 489 breakers) helps in selective coordination, which was a new requirement first issued in the NEC in 2008. Also, 30-cycle switches tend to be more expensive and take up more space, and while they allow for better coordination, also could potentially increase the arc flash hazard and should be carefully planned for when using in an emergency power system.

CSE: Do all manufacturers specify a 30-cycle ATS rating?

Jensen: I cannot speak for all manufacturers. However all of the major manufacturers do offer 30-cycle rated transfer switches.

CSE: Please provide links to white papers and articles for 3- vs. 30-cycle switches.


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