This section is separated into three first level subdivisions:

(A) Two Independent Power Sources

(B) Power Sources for the EESS

(C) Location of EES Components

Also, this section is in Part III of Article 517 which is dedicated to the essential electrical system.  The essential electrical system includes the equipment branch, life safety branch, and the critical branch.

(A) Two Independent power Sources

By definition in Article 100, the essential electrical system is a system that is comprised of alternate power sources and all connected distribution systems and ancillary equipment, designed to ensure continuity of electrical power to designated areas and functions of a health care facility during disruption of normal power sources, and also to minimize disruption within the internal wiring system.

Clarity was added during the 2023 code cycle that the EES must have two or more independent sources or sets of sources.  One has to be on-site.  The Other has to be an independent source sized to supply the entire ees and is “permitted” to be on-site or off-site.  You can have any additional sources beyond this bare minimum.

(B) identifies the permitted power sources for the EES

In this first level subdivision permitted power sources are identified.  (4) and (5) were added during the 2023 cycle.

(1) Utility Supply Power

(2) Generating Units

(3) Fuel Cell Systems

(4) Energy Storage Systems

(5) Health Care Microgrid

The first 4 of these are clearly a power source.  The microgrid is not solely a power source by definition.  Article 100 defines a health care microgram as “A group of interconnected loads and distributed energy resources within clearly defined boundaries that acts as a single controllable entity with respect to the utility.”  This microgram can include various types of power sources and must be designed with “sufficient reliability” to provide effective facility operation constant with the facility emergency operations plan.  NFPA 99 has additional information on the health care microgram.  The term sufficient reliability is subjective and it is important that the designer understand what “sufficient reliability” is.

(C) Location of EES Components

The goal is to not let the location of the EES components compromise the reliability of the system.  They have to be located to minimize the interruptions. This is separated into two second level subdivisions:

(1) Services 

(2) Feeders   

This entire first level subdivision is pretty vague with such a general requirement of “minimize interruptions”.