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 8.7 of IEC 60601-1 gives the requirements for leakage currents, Tables 3 and 4 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 8.8.3 of the standard.
Clause 8.5 of IEC 60601-1 deals with the subject of circuit separation. The glossary gives the definitions of basic, double, reinforced and supplementary insulation.
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.
An additional means of ensuring electrical safety is protective earthing, which is
described in Clause 8.6 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.