DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

98 DESIGN OF SAFE MEDICAL DEVICE PROTOTYPESthe construction of a number of useful test instruments suitable for assessing the electricalsafety of medical electronic instruments.STANDARDS FOR PROTECTION AGAINST ELECTRICAL SHOCKIt has been a long time now that medical electronic devices left the realm of experimentationand were transformed into irreplaceable tools of modern medicine. This widespread use of avery diverse variety of electronic devices compelled countries to impose regulations thatensure their efficacy and safety. In the United States, the Food and Drug Administration(FDA) is responsible for the regulation of medical devices. In the European Union (EU), aseries of directives establishes the requirements that manufacturers of medical devices mustmeet before they can obtain CE marking for their products, to authorize their sale and use.In addition, however, individual nations of the EU may impose local regulations throughinternal regulatory bodies. Other countries, including Canada, Japan, Australia, and NewZealand, have their own regulations, which although similar to the harmonized European andU.S. standards, have certain particulars of their own.Safety standards are sponsored by organizations such as the American NationalStandards Institute (ANSI), the Association for Advancement of Medical Instrumentation(AAMI), the International Electrotechnical Commission (IEC), and Underwriters’Laboratories, Inc. (UL), among many others. These standards are written by committeescomprised of representatives of the medical devices industry, insurance industry, academia,physicians, and other users in the medical community, test laboratories, and the public. Thepurpose of creating these broad-spectrum committees is to ensure that standards address theneeds of all parties involved in the development, manufacture, and use of medical devices.Thus, through a consensus process, emerging standards are deemed to capture the state ofthe art and are recognized at national and international levels.In general, safety regulations for medical equipment address the risks of electric shock,fire, burns, or tissue damage due to contact with high-energy sources, exposure to ionizingradiation, physical injury due to mechanical hazards, and malfunction due to electromagneticinterference or electrostatic discharge. The most significant technical standard is IEC-601,Medical Electrical Equipment, adopted by Europe as EN-60601, which has been harmonizedwith UL Standard 2601-1 for the United States, CAN/CSA-C22.2 601.1 for Canada, andAS3200.1 and NZS6150 for Australia and New Zealand, respectively.According to IEC-601, a possible risk for electrical shock is present whenever an operatorcan be exposed to a part at a voltage exceeding 25 V RMS or 60 V dc, while an energyrisk is present for circuits with residual voltages above 60 V or residual energy in excessof 2 mJ. Obviously, the enclosure of the device is the first barrier of protection that can protectthe operator or patient from intentional or unintentional contact with these hazards. Assuch, the enclosure must be selected to be strong enough mechanically to withstand anticipateduse and misuse of the instrument and must serve as a protection against fires thatmay start within the instrument due to failures in the circuitry.Beyond the electrical protection supplied by the enclosure, however, the circuitry of themedical instrument must be designed with other safety barriers to maintain leakage currentswithin the limits allowed by the safety standards. Since patient and operator safetymust be ensured under both normal and single-fault conditions, regulatory agencies haveclassified the risks posed by various parts of a medical instrument and have imposedspecifications on the isolation barriers to be used between different parts. The first type ofpart is the accessible part, a part that can be touched without the use of a tool. Touchingin this context not only assumes that contact is made with the exterior of the enclosure orany exposed control knob, connector, or display, but that it could be made accidentally: forexample, by poking a finger or pencil through an opening in the enclosure. In fact, most