Developing Medical Power Systems That Conform to IEC60101-1

The updated IEC 60601 standard will bring some real changes including a focus on EMI and how medical equipment will be used in the home.

The latest standards for the development of medical power supplies are set to have a significant impact on how system designs are carried out, particularly in relation to their use within the domestic environment.

The 4th edition of the IEC 60601 standard comes into force in early 2019 and moves away from specifying types of equipment and instead defines a set of ‘intended use environments’. These cover professional healthcare facilities of course, but will also take into account the places where patients live, where equipment will be used by non-specialists and also locations with potentially poor electrical systems.

The updated IEC 60601 standard will be more concerned with electromagnetic interference (EMI) and how the equipment will be used in the home, both from safety perspective and in terms of the interaction with other electrical/electronic systems situated there. This is potentially a huge issue in smart homes(with lots of wireless connections present), and in homes with poor wiring (which thereby cause lots of emissions).

As a result, engineers will have to be more adept at risk analysis as part of the design process. The new standard will lead to a higher degree of compliance testing and equipment makers will need to submit risk analysis documentation ahead of testing. All of this will have to factor in any reasonably foreseeable electromagnetic interference compatibility (EMC) disturbances that might arise.

Another change is that compliance with the 4th edition needs clear pass/fail criteria prior to testing. These have to be outlined in the Risk Management File with a test plan that includes what is to be monitored in the equipment during testing procedures. There are higher levels for immunity tests, as well as requirements for the impact of nearby RF wireless communications equipment - which in the home environment will be very hard to accurately define. There are higher electrostatic discharge (ESD) levels to be aware of too, up from 6kV/8kV for contact isolation and air discharge to reaching 8kV/15kV levels.

Within IEC 60601-1, there are also collateral standards have to be considered within the context of these use cases. For example, IEC 60601-1-2 is the EMC collateral standard, IEC 60601-1-9 relates to the environmental design and IEC 60601-1-11 covers home healthcare equipment.

All of this will have a significant effect on the design of the power supplies for such systems, particularly in defining the safety criteria involved. The means of patient protection (MOPP) and means of operator protection (MOOP) were introduced with the 3rd edition and left to the manufacturer to estimate the likelihood of a patient coming into contact with the equipment. This determined that the insulation between the primary and secondary coils must meet at least 2 MOPP under the 3rd edition (3000Vrms) and at least 1 MOPP (1500Vrms) between primary to protective earth at normal conditions.

In the home, it will be much more difficult to establish who the user might be and who is the operator - so it makes thing more difficult in the advent of the 4th edition. It blurs the previous separation that existed between MOOP and MOPP, with major implications also for the architecture of the power supply and whether an internal or external supply is preferable.

The immunity updates being brought in with the 4th edition will also have a substantial influence on how medical power systems are designed. In order to ensure compliance with IEC 60601-1, design engineers should specify a power supply that is explicitly intended to comply with edition 3.1 safety standards and 4th edition EMC standards. The supply will then have acceptable isolation, leakage and EMC characteristics.

Developing a supply that is compliant with both the safety and emission requirements of the 4th edition can be difficult as the safety and EMI performance are tightly linked. Higher levels of isolation and lower leakage will both reduce the emissions, limit the susceptibility and provide higher levels of safety.

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Defining suitable use cases

Using a supply with a 1 MOPP rating might be seen as a more cost-effective approach than one with 2 MOPP classification, but in a number of use cases it may be more sensible to go for the latter. What does help designers is that many supply makers have already taken this on and launched versions with 2 MOPP classification. However, these are more expensive than the 1 MOPP versions and will result in a premium price bracket product.

To some extent it also depends who the customer is. Systems for the home that are provided by a healthcare supplier to be utilised by carers will have very different use case and economic considerations from a system that has been bought in a supermarket or an electronics shop (such as a simple blood pressure monitor or heart monitor).

Such systems can also be implemented in different ways. Some will have wireless sensors powered by small batteries that link back to a central hub powered from the mains. Others will have wired connections from the hub to the user. These will both have distinctive safety and EMI use case scenarios associated with them. The former will have a 1 MOPP requirement with low voltages, smaller creepage requirements and less isolation, but there will be more focus on the EMI requirements. Conversely, the latter case will probably require a 2 MOOP specification of 3000Vrms and higher ESD requirements, but EMI will be less of a concern.

While the main challenges for the 4th edition of IEC 60601-1 is on home-based healthcare, it also applies to environments where EMC is a serious issue, such as providing medical assistance in an industrial setting (for example defibrillation equipment). Here it is also necessary to meet the MOPP and MOOP safety criteria, while at the same time being fully compliant with tough EMC and EMI requirements.

There are also regional differences between Europe, North America and Asia that must be allowed for. Many healthcare systems are defined for specific countries’ legislative guidelines, and so developers are used to handling the regional differences that occur, but industrial systems are generally designed for global operation.

Conclusion

The IEC 60601-1 standard is inherently complex and the collateral standards often present additional requirements. Knowing which ones to follow and to be formally certified as complying with can be challenging, as there are many potential areas of conflict, confusion and ambiguity. Simply shipping a medical device with basic documentation to a certification lab is no longer enough. The development process has to be carefully structured to provide comprehensive documentation across the architectural analysis, design process and the rationale for why certain elements were or were not included or undertaken.

This emphasis on risk assessment and considerations about use environments adds significantly to the challenges of developing systems. To help with this process, power supply manufacturers offer lines of external and embedded medical power supplies in varying power ratings that have been certified to IEC 60601 edition 3.1’s 2 MOPP safety standards and the 4th edition’s EMC standards.

Mouser Electronics

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