Standards For Protection Against Electrical Shock

It has been a long time now that medical electronic devices left the realm of experimentation and were transformed into irreplaceable tools of modern medicine. This widespread use of a very diverse variety of electronic devices compelled countries to impose regulations that ensure 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), a series of directives establishes the requirements that manufacturers of medical devices must meet 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 through internal regulatory bodies. Other countries, including Canada, Japan, Australia, and New Zealand, have their own regulations, which although similar to the harmonized European and U.S. standards, have certain particulars of their own.

Safety standards are sponsored by organizations such as the American National Standards 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 committees comprised of representatives of the medical devices industry, insurance industry, academia, physicians, and other users in the medical community, test laboratories, and the public. The purpose of creating these broad-spectrum committees is to ensure that standards address the needs 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 of the 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 ionizing radiation, physical injury due to mechanical hazards, and malfunction due to electromagnetic interference or electrostatic discharge. The most significant technical standard is IEC-601, Medical Electrical Equipment, adopted by Europe as EN-60601, which has been harmonized with UL Standard 2601-1 for the United States, CAN/CSA-C22.2 601.1 for Canada, and AS3200.1 and NZS6150 for Australia and New Zealand, respectively.

According to IEC-601, a possible risk for electrical shock is present whenever an operator can be exposed to a part at a voltage exceeding 25 VRMS or 60 V dc, while an energy risk is present for circuits with residual voltages above 60 V or residual energy in excess of 2mJ. Obviously, the enclosure of the device is the first barrier of protection that can protect the operator or patient from intentional or unintentional contact with these hazards. As such, the enclosure must be selected to be strong enough mechanically to withstand anticipated use and misuse of the instrument and must serve as a protection against fires that may start within the instrument due to failures in the circuitry.

Beyond the electrical protection supplied by the enclosure, however, the circuitry of the medical instrument must be designed with other safety barriers to maintain leakage currents within the limits allowed by the safety standards. Since patient and operator safety must be ensured under both normal and single-fault conditions, regulatory agencies have classified the risks posed by various parts of a medical instrument and have imposed specifications on the isolation barriers to be used between different parts. The first type of part is the accessible part, a part that can be touched without the use of a tool. Touching in this context not only assumes that contact is made with the exterior of the enclosure or any exposed control knob, connector, or display, but that it could be made accidentally: for example, by poking a finger or pencil through an opening in the enclosure. In fact, most standards define rigid and articulated probes that must be used to verify the acceptability of enclosure openings.

The second type of part is the live part, a part that when contacted can cause the leakage current to ground or to an accessible part of the equipment to exceed the limits established by the standard. One form of live part is the mains part, defined as a circuit connected directly to the power line.

The third type of part comprises signal-input and signal-output parts, referring to circuits used to interface a medical instrument to other instruments: for example, for the purposes of displaying, recording, or processing data. The fourth and most critical part of a medical instrument is that which deliberately comes into physical contact with the patient. Such a part, called an applied part, may include a number of patient connections which provide an electrical pathway between it and the patient. The patient circuit comprises all patient connections as well as all other parts and circuits of the medical instrument that are not electrically isolated from these connections.

The level of electrical shock protection provided to patients by the isolation of applied parts classifies them as follows:

• Type B: applied parts that provide a direct ground connection to a patient

• Type BF (the F stands for "floating"): indicates that the applied part is isolated from all other parts of the equipment to such a degree that the leakage current flowing through a patient to ground does not exceed the allowable level even when a voltage equal to 110% of the rated power line voltage is applied directly between the applied part and ground

• Type CF: similar to type BF, but refers to applied parts providing a higher degree of protection, to allow direct connection to the heart

The use of F-type applied parts is preferable in all cases to type B applied parts. This is because patient environments often involve simultaneous use of multiple electronic instruments connected to the patient. In any case, type B applied parts are prohibited whenever patient connections provide either low-impedance or semipermanent connections to the patient (e.g., through recording bioelectrodes as in ECG or EEG, or for stimulation of tissues, such as TENS). Furthermore, all medical electrical equipment intended for direct cardiac application (e.g., intracardiac electrophysiology catheters, invasive cardiac pacing) must contain only CF-type applied parts. Additionally, the applied parts of instruments for cardiac diagnosis and therapy are often designed to withstand the application of high-voltage high-energy shocks, such as those used for cardiac cardioversion and defibrillation.

These classifications have more than academic purpose. The standards provide the designer with clear indications regarding the minimal level of circuit separation and the application of insulation between these parts to accomplish acceptable levels of isolation. As such, insulation is not only defined as a solid insulating material applied to a circuit, but also to spacings that establish creepage distances and air clearance between parts. The separation of two conductive parts by air alone constitutes a clearing distance, while the separation of conductive parts on a nonconductive plane (e.g., tracks on a printed circuit board) is a creepage distance. The minimum separation distance between elements of two parts is determined by the working voltage between parts as well as by the insulation rating required to afford protection against electrical shock.

A basic insulation barrier is applied to live parts to provide basic protection against electrical shock. For example, its use applies to the separation between a live part and an accessible conductive part that is protected by connection to ground. Supplementary insulation is an independent insulation barrier applied in addition to basic insulation in order to provide protection against electrical shock in the event of failure of the basic insulation. Double insulation and reinforced insulation provide protection equivalent to the use of both basic and supplementary insulation.

TABLE 3.1 Spacings (Millimeters) Required to Provide Various Levels of Insulation between Parts of a Medical Device

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