Spec’ing hospital electrical distribution systems

03/18/2013


Breaker coordination

Another code requirement that adds significant complexity to the design of hospital electrical systems is that of overcurrent coordination of the emergency system. This requirement is actually found in NEC 700, which applies to all building types. However, as mentioned previously, the emergency system for most buildings is a very simple one concerned primarily with small loads such as lighting, fire alarm, and other similar loads. In a hospital, the amount of emergency power and the larger size of the distribution equipment needed further complicate this issue, as close to half (or more) of the panels and breakers in a large hospital may be connected to life safety and critical branch power. The need to provide overcurrent coordination requires much more design attention to ensure that downstream breakers will open (trip) prior to larger upstream breakers so that any disruption of emergency power is minimized. Although this may seem simple enough, the tolerances of breakers is such that often larger electrical distribution equipment is needed to provide coordination than may be required based on the loads being served. This further increases the cost of the electrical system and may also affect the physical size of the equipment. 

Figure 5: A sample one-line diagram for a hospital. Note the arrangement among the 4000 A, 800 A, 250 A, and 20 A circuit breakers. The figure shows the coordination among these breakers such that the lowest breaker should always trip first to prevent higSee Figure 5 for a sample overcurrent coordination curve on a hospital project. This figure shows four breakers that properly coordinate such that in each case the smaller breaker will trip before the larger breaker upstream of it. Graphically this is represented by the fact that none of these breaker curvesoverlap. This starts with the lowest level breaker on the left, with each level of distribution shown coordinating with the distribution level above and below. If any of these breakers were to overlap, it would indicate that at a certain current (on the horizontal axis) and time (on the vertical axis) either breaker could trip first. 

Ground fault protection 

Ground fault protection (GFP) is the sensing of current on an electrical system to ensure that there is not a dangerous ground fault occurring downstream in the electrical system. By comparing outgoing current (on the phase conductors) with neutral currents (the “return” current), GFP devices can determine if any current is being lost in the system (i.e., a fault condition). If this is the case, the GFP protection will open a breaker (typically the main breaker) and interrupt power to the system. 

The NEC requires GFP on the system’s main circuit breaker for solidly grounded systems of 1,000 A or more with a line-to-ground voltage of 150 V or more. As most electrical services in US buildings of sufficient size are 480 V line-to-line (which equals 277 V line-to-ground) and 1,000 A or more, this GFP is often required. 

For a hospital, however, NEC Article 517 expands the GFP provisions and requires two levels of GFP protection. So in a hospital, if an electrical service needs GFP due to voltage and size of the system, the main breaker and all the feeder breakers in the electrical service must be protected by GFP. This serves to enhance reliability by removing only one feeder (through which the fault is travelling) instead of removing power from the whole service (by tripping the main breaker for the system). Similar to overcurrent coordination, this is another case where the code recognizes the need to isolate an electrical condition in a hospital electrical system in order to minimize any power disruption.

Electrical systems unique to hospitals

Figure 6: A time-current curve illustrating coordination among breakers in a hospital electrical system. Current is shown on the x-axis and time on the y-axis. Any overlap between breakers would indicate a possible occurrence where a downstream breaker miAlthough most electrical equipment in hospitals is common to other building types, there are some systems unique to hospitals—notably, isolated power systems. Originally introduced in hospitals due to the use of flammable anesthetics (commonly ether) many years ago, these systems were once mandatory in all areas where anesthesia was used. Flammable anesthesia hasn’t been used in hospitals for many years, but isolated power systems remain for some applications. NEC Article 517 requires the use of isolated power systems in “wet” locations where power disruption cannot be tolerated. The NEC code leaves the determination of wet locations to the hospital’s discretion, but areas commonly considered to be wet include some general surgeries, open heart surgery, orthopedic surgeries, and cystoscopy. Please note that the 2012 edition of NFPA 99 recently included in its requirements that all operating rooms are identified as wet locations unless a risk assessment has been provided to ensure that fluids within the space will cause no danger to patient or staff (Section 6.3.2.2.8.4). As a result of this revision, the use of these complex electrical panels will likely increase significantly in future hospital projects.

Isolated power systems serve to reduce the risks of electric shock hazards from patients’ or staff members’ inadvertent contact with stray voltage and allow for the safe continuation of electrical appliance use in the event of a low-level fault condition where loss of power could affect patient safety. These systems are also designed to limit the leakage of electrical current in the system (which is very small but common in any electrical system) that may cause an electrical shock. Though such a shock is normally very small and poses little risk of harm, it becomes magnified in a wet location or with a patient more susceptible to any contact to even very minor stray voltage (such as a patient in a heart surgery where any voltage introduced to the heart could have fatal consequences). Further, these systems are capable of monitoring even these smallest amounts of leakage current (under 5 mA, or five one-thousandths of an amp) and providing alarms of dangerous levels of leakage current. As you may imagine, the use of these complex electrical systems in a hospital adds significant costs and maintenance issues.

The only constant is change

Most buildings experience some level of change throughout the years—offices are renovated, finishes are updated, etc.—but in a hospital this level of change in the building’s functions is much more frequent and can be more drastic. Anyone who lives or works at or near a hospital can tell you that construction seems to be ongoing. As new technologies and treatments come and go, the building’s infrastructure changes frequently. As a result, hospital electrical systems must be designed with extra flexibility and spare capacity to help accommodate the inevitable changes. This, coupled with the fact that the hospital electrical distribution system is a much larger and more complex system than that of other building types, means hospital electrical systems must be designed to be more robust.


Neal Boothe is a principal and electrical engineer at exp, where he specializes in the design of hospital electrical systems. He has over 18 years of experience, including over 200 projects ranging from new, greenfield hospitals to additions and renovations of existing facilities.


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BRIAN , UT, United States, 03/21/13 04:36 AM:

This article leaves much to be desired regarding hospital electrical systems. The graphic shown is only applicable for a tiny building covered under NEC 517, or smaller than 150kVA...in accordance with 517.30 B 4. I hope this isn't included in the printed version.
JITAO , NY, United States, 03/21/13 09:08 AM:

Should the three essential branches be fed from three separate ATS's?
Anonymous , 03/22/13 03:57 PM:

Some States even require bypass transfer switches.
PHILIP , PA, United States, 03/22/13 07:53 PM:

How are non-linear loads being addressed and what sizing changing were made to accomodate them?
SPENCE , VA, United States, 03/28/13 08:33 AM:

This article appears to be a broad brush to homogenize the design of hospital systems. But the author used a diagram that is applied only for small hospital facilities,typically less than 150 Kva of load. While the author aknowledged the extensive use of emergency power for various systems, he did not address the need clearer definitions for sizing the emergency systems. For instance, is a demand factor appropriate in the sizing of emergency systems?
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