Circuit protection in hospitals
Basics understandings of circuit protection in healthcare facilities and adopting a circuit protection standard at your firm.
Consulting engineers devote their entire careers to the art of protecting the client’s electrical systems, equipment, and buildings. We achieve that goal with various forms of circuit protection. The definition of a circuit in the National Electric Code (NEC) includes feeders and branch circuits. Therefore, circuit protection is a generic term that includes any type of protective device for nearly any type of electrical distribution conductor within a facility. This article will address the most common applications of overcurrent, short-circuit and ground-fault protective devices in the healthcare environment.
Circuit protection for the healthcare environment requires more attention than conventional building design. Nearly everything from grounding requirements, quantity of devices, and lighting levels is outlined in one or more codes, often with conflicting requirements. It is up to the design engineer to assure that the most stringent requirements are always met. Healthcare design is more demanding than conventional construction because the stakes are so high. My structural engineering friend always reminds me: “In our business, failure is not an option.”
Getting the big picture
Healthcare projects are very detailed, and like most projects, the majority of the details come at the very end of the design project. This requires the engineer to define as much of the design as possible early in the project, including the circuit protective scheme.
Typically, we know the building type and square footage during pre-schematic scope definition. The design engineer needs to estimate the building loads based upon this information. With the estimated loads, it is easy to develop the initial one-line diagram.
Goals of the initial one-line diagram:
- Document the two levels of ground fault protection
- Define equipment suitable to endure the available short-circuit current
- Address early selective overcurrent coordination issues between major equipment breakers and the generator’s circuit breaker
- Estimate switchboard and panelboard ampacities based upon feeder breaker ratios, which requires a one-line diagram
- Demonstrate that the ‘N’ design for the generator system supports all life safety and mission critical systems.
It’s understandable that the engineer may need to course-correct as the design develops, but an early one-line diagram becomes the common reference point for a well-planned circuit protection system. Often riser diagrams are not suitable for this effort because it is crucial to see all of the feeder breakers at one time.
The great debate
There exists a philosophical rift in our profession regarding the selective overcurrent coordination of circuit breakers. Some engineers and code officials contend that it is not possible to selectively coordinate circuit breakers, specifically in the instantaneous region of the time-current curves less than 0.1 sec. This article will not address that debate. Most hospitals are built using circuit breaker circuit protection (which are impossible to coordinate in the instantaneous region due to their mechanical design) and are approved by the local and state authority having jurisdiction. This article will show the reader how to successfully implement a circuit protection design using circuit breakers.
Feeder circuit protection
The philosophy behind all feeder circuit protection is to localize and isolate the condition (ground-fault, short circuit or overcurrent) from the remainder of the service. The isolating breaker should be the next protective device serving the condition. It is useful to look at the initial one-line diagram and highlight the tightest coordination lines (and possible overlaps of circuit breaker time current curves) from the main service to the branch panelboard.
Most hospital main services will require ground fault protection due to their voltage and capacity. NEC 517.17 requires a second level of ground fault protection on all feeders whenever the service requires ground fault protection. The ground fault relay/trip time current curves are coordinated to assure that the feeder breaker trips and clears before the main service device starts to clear. This is easily accomplished by settings and requires little planning.
However, selective overcurrent coordination with circuit breakers requires a great deal of planning. The first general rule is to plan the initial one-line diagram using a 3:1 ratio on circuit breakers. The 3:1 ratio is a conservative ratio between upstream and downstream thermal magnetic trip circuit breakers that assures the downstream feeder breaker has enough time to “clear” before the fault condition pushes the upstream breaker into its trip curve. This ratio would apply to all breakers from the main service disconnect down to the branch circuit breaker serving the patient headwall.
As the design evolves, the designer will feel the pressure to use a tighter ratio due to various design changes (i.e., larger mechanical equipment, smaller generator, resizing the service, and larger branch panelboards). The designer can then use a microprocessor-controlled circuit breaker to tighten up the trip characteristics of the circuit breaker and permit a tighter ratio; this is common solution for circuit breakers above 250 amps. Microprocessor-controlled circuit breakers are also called static trip and LSI breakers. Some circuit breaker manufacturers have spent great effort to stack the shape of their circuit breaker curves tightly and reduce this ratio (2:1 is possible), but it is typically the designer’s role to assure that the system they design works for multiple manufacturers.
Feeder protection and hospital branches
NEC 517 requires that the emergency feeders for a hospital be broken into a life safety branch, a mission critical branch, and an equipment branch for emergency loads over 150KVA. There are plenty of exceptions in the code about combining or adding multiple other branches to the system, but the majority of large facilities will have these three branches (normally with multiple automatic transfer switches). Unlike the normal side distribution, the emergency side distribution does not require ground fault protection on the feeders (NEC 517.17), however, realize that ground fault monitoring is always required at the generator set.
When designing with the coordination ratios of circuit breakers, start on the emergency side of the electrical distribution system prior to establishing the normal side of the distribution; because the emergency side of the distribution is typically smaller, a coordinated emergency side will all fit “beneath” the larger normal side of the system.
As mentioned earlier, establishing the size of the largest breaker in the equipment branch is critical to determining the minimum size of the generator circuit breaker. Multiple generator set installations have many advantages; however, the designer must realize they always negatively impact circuit breaker coordination because the largest breaker in the emergency side coordination study will be smaller amperage. This will drive down the designer’s maximum equipment feeder size and make a potentially problematic coordination line. Preliminary selective coordination studies are always helpful to help guide the development of the one-line diagram.
The equipment branch is required to serve certain loads: boilers, exhaust fans, all air-handlers necessary for maintaining pressure relationships, heating for all critical care, and so forth. All of this is well documented in the NEC; however, it is becoming common to put some or all of the building’s cooling capacity on the generator system. Most modern hospitals quickly overheat without cooling capacity (even in winter months), and the potential for loss of life due to this condition has been recognized. Normally, the circuit breaker serving a chiller will be the largest circuit breaker in the electrical distribution system. It can wreak havoc on the coordination study.
The equipment branch also is seeing more impact from the medical diagnostic equipment than in the past. There are advantages to providing a dedicated automatic transfer switch (ATS) for the medical diagnostic equipment in the hospital. Medical diagnostic equipment requires the highest power quality with the most demanding operators/technicians. The equipment in this department is also the most likely to change in the next couple of years. Dedicating a distribution switchboard for this department is always a good idea. Keep the following in mind when developing this design:
- Medical equipment manufacturers typically require a ground conductor sized to match the phase conductors
- Medical equipment has a high-inrush load for a very short duration (0.1 msec)
- Medical equipment is sensitive to generator maintenance testing.
The life safety branch is easiest to estimate the loads on. Most of us are familiar with a lighting power budget due to the increased attention to energy efficiency, but it is typically the second most difficult branch to coordinate. Because the loads and panelboard sizes are small (rarely does the branch require more than 200 amps at 480 V), there is little room for variation with thermal magnetic circuit breakers. Anything more complicated will require an adjustable trip microprocessor controlled circuit breaker in a 200 amp panel, which looks like poor planning and can be unnecessarily expensive.
The critical branch is the real workhorse of the system. It serves all the emergency “red” receptacles within the hospital including the operating room, post-anesthesia care unit, pre-operation areas, and other high-acuity areas. Since the majority of the load on this branch is 120 V, the overcurrent protection planning needs to account for the large inrush current (6 to 8 times the input current) on each step-down transformer. We typically subdivide this branch into smaller step-down transformers (less than 112.5 kVA) to ease upstream coordination issues. The critical branch is also used for some medical equipment applications (i.e., catheterization labs and other critical care applications).
The normal branch is not exempt from selective coordination. Design of normal circuitry in the hospital requires as much care as the emergency systems. Typically the coordination is easier, because tight coordination issues can normally be resolved by making the feeder and corresponding circuit protection bigger.
Branch circuit protection
The NEC defines a branch circuit as: “The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).” The NEC, NFPA 99, the Guidelines for Design and Construction of Health Care Facilities (commonly referred to as the AIA guidelines), and other related codes typically give minimum quantities of wiring devices and indicate which branch of the hospital distribution should be used for the application. Often the codes disagree, and the design engineer determines the “worst-case” number of receptacles and uses that in his or her design.
Unfortunately, the codes adopted in most jurisdictions don’t give the engineer direction on how to design a branch circuit for a patient room or any other area within a hospital. There is the short description and exhibit in NEC 517.19, but it’s not nearly enough information for the engineer to use when designing an entire hospital. It basically requires one normal and one emergency circuit per patient bed location.
Like all things left to the engineer’s discretion, there is constant debate about the standard of care for the design of these circuits. If the phrase “standard of care” isn’t in your vocabulary, check with your company’s risk assessment group.
In the absence of any other standard, I typically adhere to the specific guidance of Chapter 419 in the Florida Building Code (FBC) as a defined and defendable standard of care for healthcare design. It’s conservative; that is, appropriate for healthcare design, but more importantly it’s a published document that an engineer can reference in court.
Chapter 419 requires the following branch circuits:
- Throughout the hospital, a maximum of six duplex receptacles on a 120 V/20 A branch circuit
- Half of all headwall branch circuits shall be on emergency (66% in critical care headwalls)
- In critical patient care areas, a maximum of two duplex receptacles per 120 V/20 A branch circuit
- In critical care patient rooms, a minimum of one dedicated emergency and one normal 120 V/20 A branch circuit per headwall.
I would also add the following general statement that summarizes the branch circuit requirements written into the NEC:
- Never connect patient room branch circuits to a wiring device in another area.
Life safety branch circuits are easily identified in the NEC and are supported by the requirements of NFPA 99 and 110. They are:
- Means of egress lighting
- Exit signs
- Alarm, alerting control systems
- Communication systems
- Generator set and transfer switch locations
- Generator accessories
- Door hardware essential to egress.
It is worth stating that the loads listed in the NEC and NFPA 99 and 110 are the only loads permitted on the life safety branch. Realize that the only emergency convenience receptacle permitted on this branch is for the fire alarm panel and at the generator set; any other emergency outlet branch circuit belongs to either the critical or equipment branches.
Critical branch circuits serve the majority of emergency “red” receptacles installed in the facility. When the code requires emergency power at areas of patient care, a critical branch circuit is required. Unfortunately, critical branch circuits are often used to serve all the emergency receptacle needs of the facility. Please reference the description in the NEC where it limits all critical branch emergency outlets to “patient care.” Any other emergency receptacle cannot be on the critical branch.
All emergency lighting branch circuits that are not used to illuminate the path of egress should be connected to the critical branch. Examples of areas that would require this branch circuit include nurse stations, soiled and clean holding rooms, patient exam lights, and operating rooms.
An operating room shall contain both normal and critical branch circuits. Often the branch circuits in an operating room are on an isolated, ungrounded power system. Isolation power systems serve the dual need for ground fault monitoring and maintaining power that might be necessary to sustain life. The NEC does not require the branch circuits in an operating room to be served by an isolated power system (i.e., ungrounded system) unless the operating room is identified as a “wet area” by the hospital administration. Isolated power systems are expensive to purchase and maintain; always verify the use of the operating room prior to determining the branch circuit design within the room. Don’t forget that you will need to provide two units (one for normal, or a separate critical branch) to satisfy NEC requirements.
In general, the engineer isn’t always aware of what the end user will be plugging in to the receptacles provided by the design. Always ask! Most young engineers are shocked at what the staff can roll out of closet; a portable neonatal warming light is typically rated at 1800 W. The current receptacle guidelines require only a single duplex outlet to be shared among four bassinets. Do the math—it doesn’t work.
Putting it all together
Using the quantity of wiring devices required by the Guidelines and your local jurisdiction and then connecting them to branch circuits as described by FBC chapter 419, you can develop a minimum standard chart for all healthcare rooms. See Table 1 for an example.
Healthcare design is regulated by multiple codes and guidelines that have murky and even conflicting requirements. The requirements are full of recommendations on the quantity of wiring devices and information on the branches of the emergency power system. Unfortunately, the codes and guidelines are conspicuously silent on feeder and branch circuit design specific to healthcare design, except for general requirements like selective coordination or ground fault protection. It is often left to each engineer’s individual experience and best judgment to interpret the standard of care required for the circuit protection on the project.
In today’s economy, the engineer is under increasing pressure to make each project as cost-effective as possible. This usually involves a “code minimum” design. I strongly recommend adopting a circuit protection standard for all in-house healthcare design, publishing it, and adhering to it. Design to a higher standard and protect the lives of those who trust our professional abilities.
Versluys is a senior electrical engineer and principal at TLC Engineering for Architecture. His areas of expertise include sustainable healthcare design and campus distribution systems. He is a member of Consulting-Specifying Engineer's editorial advisory board.