The role of medical safety standards in power-supply design
03 November 2016
Legislation to ensure the development of safe medical systems has seen a number of changes over the past decade and impacts every part of the design down to the power supplies that lie at the heart of any electrical or electronic healthcare device.
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The differences in legislation around the world, because countries adopt standards at their own rate, add complications to the approvals process. We can expect those complications to increase with the imminent arrival of the latest revision to the key medical-power standard over the next couple of years. Experienced designers of power supplies for medical applications, such as Stadium, have built up a base of knowledge around this area that can guide device makers through the changing standards landscape.
For power supplies, the key standard for designing safe power-delivery subsystems for medical applications is IEC 60601-1. Enforced in most jurisdictions, equipment that needs to be designed to IEC 60601 is anything that will be operated in an area where patients reside.
IEC 60601 was first published in 1977 and written to handle what were seen as the main forms of electrically powered equipment used in medicine. It set requirements for power supplies that reflected medical use and so would be more appropriate than more generic standards such as IEC 60950, which was introduced for information-technology (IT) equipment. This first edition was replaced by a second developed in the 1980s. From a legislative standpoint, this edition remained in force until relatively recently. Although the third edition was published by the IEC in 2005, governments did not bring it into force until the start of the current decade.
For example, the European Union chose to withdraw the second edition in June 2012 in favour of the third edition. The US did not move to adopt the third edition of IEC 60601 until June 2013, enforcing its use for new products from the beginning of 2014. China represents a major market territory that continues to work according to the second edition. The first edition specified a number of requirements for power supplies, such as the adoption of dual-fused inputs and 100µA or less of enclosure leakage current. Key criteria were devised to limit the risk of arcing or short-circuit.
One criterion is creepage. This defines the minimum distance between two uninsulated conductors, such as PCB tracks, for a given voltage. The creepage distance for a given maximum voltage needs to be sufficient to prevent a short developing across the separating insulator. To prevent arcing between conductors through air, the IEC 60601 standard also defined a clearance distance. This is the shortest distance through air between two conductors.
Some medical equipment may need to provide some level of electrical contact with the patient – sometimes direct to the heart. An electrocardiograph (ECG) is one example. In other cases, electrical power is only needed to maintain operation, such as a bed that contains motors to allow the patient to sit up easily or a blood pressure instrument. These systems have different protection requirements, which were taken into account from the first edition of IEC 60601.
The Type B, or Body, category was defined in the standard for devices that had no electrical contact with the patient. Type BF, standing for Body Floating, was intended for equipment with a floating electrical connection to the patient but not one that provided a connection direct to the heart. Type CF, standing for Cardiac Floating, was reserved for the subset of equipment that does provide a floating electrical connection to the heart.
Whereas the initial focus of IEC 60601 was on the concept of “basic safety” – ensuring that the product could not cause injury through electric shocks or short-circuits if it malfunctioned – the second edition expanded the scope of the standard. This expanded scope was intended to handle situations where power problems may affect the “essential performance” of the systems. If such problems could cause injuries, they needed to be handled by the designer.
The subcommittee responsible for the IEC 60601 changed the approach of the standard when preparing the third edition. The first two standards took a largely prescriptive approach, specifying the types of construction that would be needed to ensure safe operation. It is difficult for prescriptive standards to keep pace with changes in technology. The medical device market has seen major developments over the past few decades. The third edition introduced a greater degree in flexibility by moving to an approach based on risk management performed by the design team rather than guidelines and specifications for construction.
The third edition of IEC 60601 follows the approach set out in the ISO 14971 standard for risk management in medical devices. Under the risk-management regime, the design team must analyse the hazards and risks that might result from use of a device and address them. The advantage of the risk-management approach is that it allows for the use of novel solutions to safety hazards, as long as the designer can demonstrate their efficacy. However, if a medical system – including its power supply – is to be sold globally, designers will still need to take into account the prescriptions of the second edition because of the differences in legislation around the world. Having a specialist design team on board, such as Stadium, will ensure the many different factors are taken into account.
In addition to the three categories for electrical-isolation requirements in the first and second editions, the third edition of IEC 60601 introduced the concept of means of protection (MOP). There are two forms of protection that need to be identified. One is the means of operator protection (MOOP), intended to define the isolation and other mechanisms that prevent harm coming to doctors, nurses and other people who will control and monitor the medical device. To a large extent, the requirements for MOOP-level protection are similar to those defined in the IEC 60950 standard for IT equipment.
The other category introduced in the third edition of IEC 60601 is the means of patient protection (MOPP), which defines, in general, a more stringent set of requirements intended to prevent harm coming to the patient who may be attached to the medical device. It is not sufficient to design a subsystem to the MOOP requirements if analysis shows that this subsystem may have contact with the patient. In many cases, the power supply will need to be designed to satisfy MOPP requirements to ensure that the necessary protection is achieved.
Under the third edition of IEC 60601, electrical medical equipment should have two MOPs in order to prevent applied parts or other operator- or patient-accessible parts from exceeding voltage, current or energy limits. The reason for this is to ensure that if one MOP fails there is another MOP that will protect the patient or operator from harm.
The distances defined for creepage and clearance are affected by the choice between MOOP and MOPP for each part of the system. For example, the two-MOPP creepage distance is 8mm compared to 5mm for a two-MOOP situation.
When considering the requirements of the third edition of IEC 60601, it is important to check that power supplies conform to the appropriate MOP level. A supply advertised as “approved for medical applications” may be designed to meet MOOP rather than MOPP requirements – and so may not be acceptable in the end system. Due to these considerations, the support of specialists with experience in custom design, such as Stadium, is important to ensure that the right type of power supply is designed into the end equipment. A pure off-the-shelf solution with limited design-in support may not meet the needs of the final system, especially if it is to be approved under different standards regimes.
The greater flexibility of the third edition provides opportunities to improve safety without an increase in cost. For example, an experienced designer may choose to add an isolated DC/DC converter to the system to operate with a one-MOPP AC/DC front-end. Since this may result in higher overall cost, a better solution is likely to be a custom design for the front-end that takes the full requirements of the device into account.
Although the third edition of IEC 60601 recently came into force in a number of major markets, there is likely to be a further tightening of the regulations within several years following the February 2014 publication of the fourth edition of IEC 60601. Within less than a year, the US Food and Drink Administration (FDA) signalled its willingness to move quickly to an approvals regime based on the fourth edition. The FDA issued a guidance note that informed manufacturers they should start developing and testing medical products to the fourth edition in expectation of a change in regulations within a few years.
There are a number of changes made for the fourth edition. Although the major alterations are to ensure more stringent protections against hazards caused by electromagnetic interference, the revised version calls for more robust risk analysis. Designers must take account of situations that may result from voltage dips and power interruptions as well as the effects of electrostatic discharge (ESD).
A further distinction made in the fourth edition of IEC 60601 is the environment in which the device is used. There are different electromagnetic immunity requirements depending on whether the system is used in a professional healthcare environment, such as a hospital, in the home or in more specialised locations, such as military installations.
As with previous editions of the IEC 60601 standard, countries and regions around the world will adapt to the publication of the fourth edition at different rates. The US FDA is likely to lead but others are expected to follow relatively quickly. As a result, it is important to have expert advice and design support for the power subsystems, from a supplier such as Stadium, to ensure that your next medical device or system has the best chance of obtaining approvals in as many markets as possible.
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