Electrical Requirements The requirements for the electrical safety of medical equipment are much more stringent than those for other electrical devices. The reasons
for increased precautions include:
- Patient may be connected to several medical devices simultaneously (e.g. in intensive care)
- Patient may be connected conductively to electronic circuitry (e.g. ECG monitoring)
- Contact with device may be directly to internal tissue that conducts well (e.g. through natural orifices or breaks in the skin)
The standard has both mechanical and electrical requirements intended to reduce the electrical hazards both under normal and single fault conditions. Unlike other
standards, electrical safety is not considered to be dependent on voltage, but on leakage currents. This is because even a very low voltage, when applied
to internal tissue, can cause leakage currents through the body which may be fatal. Clause 19 of IEC 60601-1 gives the requirements for leakage currents, Table IV summarizing the allowable values.Circuit separation is used as one of the means to implement electrical safety in
medical electrical equipment. Certain types of circuits must be physically separated from each other, and must be demonstrated to be electrically separated from each other by means of
dielectric strength tests, which are specified in Clause 20 of the standard. Mechanical separation is defined by the standard in terms of creepage distances and air clearances
, where
creepage distance is distance measured over the surface of insulating material, and air clearance
distance measured through air. These distances are critical in the design of transformers and for printed circuit board layout, where circuits which must be separated are on the same PCB.
Clause 17 and Appendix E of IEC 60601-1 deal with the subject of circuit separation, describing the level of insulation required between different circuit types. The
glossary
gives the definitions of basic, double
,
reinforced and supplementary insulation.
An additional means of ensuring electrical safety is protective earthing
, which is described in Clause 18 of the standard. For the designer of an electrical medical device, the most important step in acheiving compliance with the electrical requirments of the
standard, is the use of an IEC 60601-1 compliant power supply or isolation transformer. This component will satisfy many of the mechanical (spacing) and electrical (electric strength,
leakage current) requirements of the standard. An overall consideration in designing for safety is the
principle of 'single fault conditions'. A medical electrical product must be designed so that it operates safely not only in normal conditions, but also in abnormal and single fault
conditions. Single fault conditions include the shorting or opening of electrical or electronic components, the failure or locking of motors, the blockage of air vents,
etc. The shorting of basic insulation is considered a single fault, while the characteristics of double or reinforced insulation are considered such that it is not shorted under
single fault conditions. The medical device may not cause a safety hazard, e.g. danger of electrocution, mechanical hazard, fire or other hazard, as a result of a single fault and its
direct consequences. The standard does not require that the medical device remain safe under the conditions of two or more independent faults.
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