Your questions answered: Critical power: Designing power systems for resiliency in mission critical facilities

Questions not answered during the Nov. 12, 2019, webcast are answered here

By Tom Divine, PE, LEED AP November 21, 2019

The need for power resiliency at the mission critical facility level has been illustrated many times by devastating storm events, earthquakes or other disasters. There are many methods to design for new building power resilience and test for existing building power resilience. Ensuring that facility’s systems maintain reliability and support resiliency is the ultimate focus and goal of successful design and commissioning.

Questions not answered during the webcast Nov. 12, 2019, are answered here.


  • Tom Divine, PE, Senior Electrical Engineer, Johnston, LLC
  • Kenneth Kutsmeda, PE, LEED AP, Engineering Manager, Jacobs

Question: For emergency generators, which fuel source is becoming the norm, natural gas or diesel?

Tom Divine: For commercial and industrial systems, diesel generators have a much larger market footprint than natural gas generators for a number of reasons. Natural gas units larger than about 150 kilowatts are more expensive than comparably sized diesel units. Natural gas units in the 1-megawatt range and above still have some difficulty meeting the 10-second requirements of NFPA 101, NFPA 70andNFPA 99. Natural gas generators don’t respond to transients as well as diesel generators. And, owners seem to prefer having control over their fuel supply with on-site diesel storage.

That said, natural gas has a sizable market share in residential applications, where the cost and effort of maintaining an on-site supply strongly impact the fuel decision. And interest in natural gas is growing in nonseismic, storm-prone areas, where interruptions of the electrical supply and the natural gas supply are weakly correlated.

Question: How do you best configure and test installed automatic transfer switch equipment and existing emergency power supply system at a hospital or medical center?

Tom Divine: Today’s transfer switches implement a lot of delays and features. A detailed treatment would be much too long to show here. The most commonly implemented, and maybe the most interesting, are:

Delay from normal failure to generator start: how long the ATS waits before starting the generator(s). For Level 1 systems, this delay must be short, as the system must be up and running within 10 seconds. With no delay, though, the system responds to momentary outages, and starts the generator(s) unnecessarily. A delay of one second is reasonable.

Delay from emergency available to transfer to emergency: how long the ATS waits, after emergency power becomes acceptable, to transfer to emergency. For 10-second switches, this delay can be set to zero to facilitate meeting the 10-second deadline. For optional loads, it will typically be set longer to allow the system to stabilize from previous transfers.

Delay from normal restored to transfer to normal: This delay controls how long the switch waits after normal power returns. Its intent is to avoid unnecessary retransfers should normal power fail again, due to ongoing storm events or other unfavorable conditions. It’s often set to 30 minutes. NFPA 110-2016 appears to set the lower limit for this delay to five minutes,

Engine cooldown delay: The delay from transfer to normal until the ATS de-asserts its generator start signal. NFPA 110 6.2.10 requires a five-minute cooldown setting for generators larger than 15 kilowatts. This delay is typically set to five minutes.

NFPA 110 7.13 prescribes limited testing of transfer switches at installation, but doesn’t describe specific acceptance tests. Those requirements are generally related to making sure that each switch can start the generator(s), transfer to emergency and return to normal.

Question: Are there requirements for diesel fuel treatment (urea) to prolong the life of the generator fuel source?

Tom Divine: There’s not a specific requirement in NFPA 110, though 8.3.7 requires fuel quality testing at least annually. A., in Annex A: Explanatory Material, calls for routine testing of fuel, in order to, “make important decisions,” regarding fuel quality, but no specific actions are described. A5.5.3 suggests, but doesn’t state that fuel stored over a year should be replaced.

Author Bio: Tom Divine is a senior electrical engineer at Johnston, LLC. He is a member of the Consulting-Specifying Engineer editorial advisory board.