Keeping up to date with medical-safety standards

The 3rd edition of the 60601 medical-safety standard was first published by the IEC in 2005 (IEC60601-1:2005), was adopted by the EU (European Union) in 2006, and published as EN60601-1:2006. The AAMI (American Association for Medical Instrumentation) published the USA version, also in 2006, which appears as ANSI/AAMI ES60601-1:2006. In Europe, as of 1 June 2012, the previous 2nd edition (EN60601-1/A2:1995) has been withdrawn and all products will need to be certified to the 3rd edition. The situation is rather different in the United States. The cessation date for 2nd edition (UL60601-1:2003 1st ed) is 30 June 2013 but, unlike the EU, the FDA only requires that new products brought to market after this date will need to be 3rd-edition certified. One of the most significant changes that the 3rd edition introduces is that equipment manufacturers must now follow a formal risk management procedure that follows the ISO 14971 model. While the 2nd edition simply addressed basic safety issues to ensure freedom from any electrical, mechanical, radiation, and thermal hazards, it did not require devices to remain functional in that fail-safe was adequate, and compliance with test criteria relied upon a pass/fail result that did not take into account the essential performance of the device-under-test. Recognizing these limitations, the 3rd edition introduces specifications for essential performance that require that equipment will continue to function as its designers intended throughout the test process. Within the electrical safety arena, the standard continues to require that equipment implement two MOPs (Means of Protection) such that if a failure occurs within one area, a second mechanism safeguards the operator and the patient against any electric shock hazard. The standard allows for three defensive approaches that manufacturers may use in various combinations: safety insulation, protective earth, and protection impedance. It is therefore essential to determine several key factors from the outset of the equipment design process, including its insulation class and its reliance on a protective-earth connection. Significantly, for power supplies, the 3rd edition distinguishes between protecting the equipment's operator and the patient within its MOOP (means of operator protection) and MOPP (means of patient protection) categories. This distinction can result in quite different safety insulation and isolation requirements for circuits that operators and patients may contact. Specifically, anything that falls within the remit of operator protection only has to meet the clearance and creepage requirements that IEC/EN 60950 specifies for general-purpose information and technology equipment. In choosing a power supply with only MOOP, one would have to ensure other isolation schemes are in place between the output and the patient if the equipment is to contact the patient. No matter whether designers choose MOOP or MOPP, the standard still demands that equipment meets power-supply leakage-current requirements: 300 μA for the USA and 500 μA for the EU. XP Power