Electrical safety and grounding
6.1 General discussion
This chapter discusses the basic engineering involved in obtaining electrical safety in health care facilities through proper design of the electrical distribution system. It also provides a perspective on the electrical hazards in hospitals so the engineer has the background to design an electrical systemto mitigate these hazards. Factors involved in making design decisions regarding electrical safety and some of the various options will be discussed.
During the period from 1940 to 1970 there were serious problems and accidents in hospitals that required major additions and changes to the codes and standards for health care facilities. Many of these changes involved thefacilities’ electrical distribution systems and were a result of the increase in the complexity of procedures, equipment, and instrumentation used.
The design engineer should know and understand the latest codes and standards so that designs and options regarding electrical safety may be presented to the hospital accurately.
The general level of safety requirements for electrical devices andinstallations has increased over the past years. Examples of this are the continuing changes to UL standards, the National Electrical Code¨ (NEC¨) (NFPA 70-1996) 1 , and NFPA 99-1996. The NEC or any other NFPA code or standard can only be enforced if it is adopted as being applicable by an authority having jurisdiction. Any governmental body or administrative agency may or may not incorporateNFPA codes or standards into its own health building or construction codes.
Even if the NFPA codes or standards are not incorporated into local codes, it is good practice to at least review them. Good electrical safety often means going beyond the code as this is only the minimum requirement. Understanding local standards is mandatory for any designer.
This chapter will concentrate on specialrequirements of a health care facility over and above those normally required for a commercial facility. Principal differences are in layout, care of installation, quality of components, and special devices required.
Since all of the details contained in referenced documents cannot be duplicated in this chapter, it is highly recommended that these be obtained, studied, and their applicabilityto a particular facility determined.
The patients in a hospital are either ill or incapacitated and rely on others for care, and as such, they are often physically incapable of taking self-protective action. These patients may be brought in contact with electrical equipment routinely, and this may take place in a wet environment or in the presence of flammable vapors, such as alcohol or ether,plus supplemental oxygen. The patient may also be subjected to invasive procedures of various types.
The design shall take into consideration the electrical safety of not only the patient, but also of the nurse, doctor, and any other health care provider working in the environment. Hazards can exist not only in the familiar ampere ranges, but also at the milliampere/microampere levels andshort duration short-circuit kiloampere ranges.
Among basic safety features required are insulation, overcurrent protection, adequate dependable power (especially for life support equipment), reliable grounding, and coordinated protection to guard against shock or burns from leakage and fault currents.
6.2 Physiological parameters
An understanding of some of the physiological parametersinvolved should give the designer of hospital care facilities a better understanding of the factors involved in the development of a safe environment for the patient.
6.2.1 Cell excitability
The individual nerve and muscle cells have a small inside-to-outside potential of the order of 90 mV because of chemical differences. Excitation of the cell for transmission of a nerve impulse or muscle...