Understanding the impact of 60601 3rd edition on power design
Earlier this year the US followed the EU and Canada in moving to the 3rd edition of the IEC 60601-1 medical device standard. This change presents several challenges and adds layers of complexity for medical device designers. Here we look at the changes and their effect on implementing power supplies in medical devices.
Designers of medical systems are faced with changes and challenges as safety-standards legislation is updated around the world. Over the past couple of years, governments in Europe and North America have decided to move to the 3rd edition of the IEC 60601-1 standard, which has important ramifications for the power supplies they incorporate.
Covering many aspects of medical-systems safety, the IEC 60601-1 standard was first published in 1977, to be replaced by the 2nd edition, which was developed in the late 1980s. In 2005, the IEC published its 3rd edition. The 2nd edition has, in a number of jurisdictions, been in force for many years, but several key regions globally have recently made the move to adopt the 3rd edition standards.
The European Union withdrew the 2nd edition standard at the start of June 2012, replacing it with the 3rd. Products already on sale as well as products new to the market now must conform to the regional version of the IEC standard, EN 60601-1:2006. If 3rd edition assessments are missing for products currently on sale government agencies in the European Union can order sales of the affected equipment to be stopped.
Canada originally planned to move to IEC 60601-1 3rd edition at the same time but delayed introduction to April of this year.
The most recent change has come with the US, which set the cessation date for UL 60601-1:2003 – based on the 2nd edition of IEC 60601-1 – at the end of June 2013. Like Canada, only new products have to meet the 3rd edition criteria.
Some of the requirements of IEC60601-1 2nd edition are in potential conflict with the 3rd. Thus, complying with the latest edition may put a product out of compliance with the 2nd, and so make a product unmarketable in regions that still adhere to the earlier version. Countries still mandating IEC 60601-1 2nd edition include Japan, Australia, New Zealand and China. As a result, many vendors are working to meet both versions in the same design, which requires additional cost and effort.
Figure 1: IEC 60601-1 3rd edition adoption
The first edition of the standard established several levels of protection to handle the different types of equipment used in medicine, as well as setting requirements for power supplies that were more stringent than those use in more generic standards such as IEC 60950-1. Equipment needing to be designed to IEC 60601-1 was classified as anything operated in an area where patients reside.
The first edition specified a number of requirements for power supplies, such as the adoption of dual-fused inputs with leakage currents of 300µA or less to earth, or 500µA in North America, and 100µA or less to the chassis, providing the effective limit for a fault condition. Other criteria included minimum creepage of 8mm and clearance of 4mm in order to prevent shorts developing between PCB tracks and air gaps, respectively.
Electrical safety parameters for different types of medical equipment are highlighted in the 2nd edition standard with the most basic form, Type B, covering non-patient systems and those with no intended electrical connection to the patient. Type BF covers systems to which the patient will be attached, such as heart-rate monitors. Type CF covers products that will be used for cardiac intervention and, as such, have more stringent requirements. For these systems, leakage is cut to a tenth for a maximum of 10µA or 50µA under a single fault condition. For DC leakage current the values are 10µA DC for all categories, or 50µA DC under a fault condition.
The 3rd edition changes the way that safety is assessed, allowing for separate means of protection (MOP) for patient and operator. In general, the patient is afforded greater protection than the operator. For power circuitry, isolation and other parameters such as creepage have higher specifications for means of patient protection (MOPP) than for means of operator protection (MOOP).
Figure 2: IEC 60601-1 3rd edition requires differing levels of MOOP and MOPP depending on the power being delivered.
Out of more than 1400 individual requirements in the 3rd edition, close to 700 are new or more stringent than those in the 2nd edition. Possibly the most significant single change in the 3rd edition, however, is the introduction of a formal risk-management procedure based on the model described in ISO 14971.
Certification without compliance to ISO 14971 is not possible. But this does not require the equipment to be certified to ISO 14971 before proceeding to IEC 60601-1 certification.
Fig 3: A simplified flow diagram showing IEC 60601-1 3rd edition’s formal risk management procedure
A further change is the way in which equipment designed to the IEC 60601-1 standard handles failures. The 2nd edition addressed basic safety issues but did not require devices to remain functional. Failsafe protection was deemed adequate, and compliance with test criteria relied upon a pass/fail result that did not take into account the performance of the device-under-test. Recognizing the critical nature of many medical systems, the 3rd edition introduces specifications for "essential performance". These require equipment to continue functioning according to operational specifications throughout the test process.
The choice of power supply for a given means of protection will have ramifications for the rest of the design. By choosing a power supply rated to provide protection only to MOOP, the equipment design team has to ensure other isolation schemes are in place between the output and the patient if the equipment is to come in contact with the patient, which is likely to complicate the design. Even though the cost of the MOOP-rated power supply might be less than an MOPP power supply, the resulting end design could easily be more expensive.
Although it may seem to make sense to provide isolation for a mainstream-market power supply to MOOP and MOPP levels, the leakage current requirements make this a less attractive scenario than it at first seems. Although the US has operated a less stringent policy for leakage current and the 3rd edition relaxes the AC leakage requirements slightly, many manufacturers want to be able to standardize on one design to allow sale around the world, which means satisfying the 300µA requirement of the IEC standard. Most off-the-shelf power supplies will need to be modified to achieve these leakage current levels, which is likely to affect electromagnetic emissions. In turn this will call for additional filtering, again adding to cost.
Instead, a good strategy is to select a power supply that not only meets the MOPP rating but exceeds it. Power supplies designed to support 2 x MOPP provide greater design flexibility with the assurance that they can minimize the risk of shock hazard and demonstrate that as part of the risk-management process.
Risk management is not necessarily a new concept to systems manufacturers, particularly in the medical domains, but it has yet to be grasped by many power-supply manufacturers. This was not an issue under certification regimes operating to the 2nd edition rules. Under those rules, the certified body would test primarily against pass/fail criteria. As risk management is now included, the systems manufacturer has to demonstrate that all of the subsystems interact in a safe way.
In summary, medical-systems manufacturers need to take account of changes to the IEC 60601-1 standard as legislation is enacted in key territories. Although many aspects of system design are affected, the changes have important ramifications for users of power supplies. To help ease the compliance process for medical device designers, CUI has certified their line of medical power supplies ranging from 15 W to 400 W to the most stringent two MOPP classification.
Figure 4: The CUI VMS medical power supply family (20-365 W) is certified for IEC 60601-1 3rd edition and its devices offer two MOPPs.
For further information on the effect of 3rd edition IEC 60601-1 medical standards on medical power supplies, including a downloadable application note, please visit www.cui.com/medical.
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