KISS Your 60601-1 Medical Power Supply

The 3rd edition of the ANSI/AAMI ES 60601-1:2005 / IEC 60601-1 standard is so complex, layered and in many instances ambiguous and contradictory that power-supply and system designers may be forgiven for losing track of its original purpose. Still, the new ‘60601, as it’s affectionately known, with all its means of protection (MOP) for both operator and patient, has more to do with classification of systems, definitions, and overall system safety than with raising the bar on design specifications or performance. That being said, picking the right off-the-shelf supply will adhere to the ‘keep it simple, stupid’ (KISS) principle by going a long way toward bypassing a lot of testing, paperwork and associated costs, while alleviating design headaches.

Before going any further, let’s first clarify the standards’ naming and derivations, which themselves can be confusing. ANSI stands for the American National Standards Institute, while AAMI is the Association for the Advancement of Medical Instrumentation. ANSI/AAMI ES 60601-1:2005 is identical to the globally accepted IEC 60601-1:2005, but with deviations to accommodate the US National Electric Grid. It has been officially added to the FDA registrar. Similarly, EN 60601-1 is IEC 60601-1:2005 with European deviations, while CSA‐C22.2 NO. 60601‐1:08UL is the Canadian variant. UL is based on the US-oriented ES 60601-1:2005, in recognition of the global acceptance of IEC 60601-1.

According to the ECRI Institute’s Medical Device Safety Reports (MDSR), medical device failures that result in injuries or deaths can be put into five main groups, two of which – device factors and external factors – have sub-categories that can be directly related to the power supply. In device factors, the MDSR calls out software deficiency, while under external factors it calls out power supplies directly.

In the context of 60601-1, and medical power supplies, software deficiency is particularly interesting as digital power supplies make their way into medical systems, which have traditionally been analog-only. With this move toward digital, designers can expect a massive step up in testing requirements to ensure both the system and the software are stable.

Until then, the focus of the 3rd edition is squarely on the means of protection for operators (MOOP) and patients (MOPP), as both face different usage and danger scenarios. An operator is typically in direct contact with a system or its interface, while a patient may be connected via sensors or probes. In the case of a very sick patient, leakage current looms large. Even for healthy humans, a leakage current of only 30mA can cause breathing difficulty and ventricular fibrillation.

Leakage Current

Leakage current is a fact of life within electrical and electronic systems and can be defined as the flow of current from a system’s conductors to ground, either directly via a properly grounded conductor, or, failing that, through direct or indirect coupling to other elements of a system — or a human body.

For supplies connected to AC mains, leakage sources include capacitive coupling from the EMI filters and from the primary to the secondary winding — or even to nearby circuits — from the power transformer. Within IEC/EN 60601-1, the leakage current limits are clearly defined under Normal Operation and in Failure Mode 1 as 0.5 and 1.0mA, respectively, for earth discharge current; and 0.1 and 0.5mA, respectively, for cabinet discharge current. For North America, UL’s ES 60601-1 stipulates 0.3mA — not 0.5mA — as the maximum leakage current.

It’s worth noting that these leakage figures apply to all classes of medical devices, from no-indirect patient contact through to direct patient contact with the heart. Therefore, while many power supplies may conform to the more generic IEC/EN 60950 regulations and so be usable within in vitro (IVD) medial diagnostic equipment, which may not even come near a patient, the 60601-1 leakage limits still apply.

However, this is where the 3rd edition gets tricky as it introduces the concept of a formal risk assessment process called out in ISO 14971. This is used to help the manufacturer determine (as it is ultimately their responsibility) whether or not a patient is likely to come in range sufficient to make contact with the equipment. If it is determined that contact may occur, then the system could require the full 60601-1 qualification usually reserved for medical devices such as heart and blood monitors, pumps, respirators and defibrillators.

IEC/EN/ES 60601-1, in essence, puts the onus upon manufacturers to develop risk assessment protocols, establish an acceptable level of risk, and show that the remaining risk is acceptable, though ISO 14971 does help by including a risk management file whereby identifiable faults can be listed and assessed.

Is Your MOP MOOP or MOPP?

While it may seem 60601-1 emphasises the design process and risk management more so than the design itself, there are very clear guidelines that have been defined as to what constitutes acceptable MOP for various situations.

An MOP can be a protective earth that is specifically designed to protect from electric shock, such as a GFCI fail-safe protector, an insulator, an air gap, a defined ‘creepage’ (the shortest distance between two conductive paths, or a conductive path and chassis/enclosure), or any high-impedance isolation barrier between the input and the output.

Edition 2 and 3 each define two means of protection, but because operators and patients have different risk scenarios, IEC/EN/ES 60601-1 3rd edition goes further by specifically defining and differentiating operator (MOOP) and patient (MOPP) protection in terms of isolation, creepage and insulation.

Both one MOOP and one MOPP can be met using standard insulation, but the two MOPP isolation test is particularly demanding at 4,000VAC and the creepage distance of 8mm is twice that of one MOPP. Also, the withstand test voltages are higher for under IEC/EN/ES 60601-1 than under the more general industrial IEC/EN 60950.

KISS More Than Ever

With all the complexity and ambiguity surrounding compliance, the designer’s KISS (keep it simple, stupid) rule applies more than ever, and may even be more advantageous in terms of cost. For example, while it may seem cheaper to opt for a supply for an IVD that can be deemed fit for medical application because it uses a MOP that fits baseline 60950 requirements, or maybe the plan is to go with a baseline 60950 device and then add 1x or 2x MOOP or MOPP at a later stage, there are two reasons this approach may be counterproductive.

The first is the paperwork and processes involved in the formal risk assessment to show the remaining risk is acceptable. This can be a time sink. The second is the matter of inventory and the associated costs. Long term, the relatively small cost of opting for a full IEC/EN/ES 60601-1 2x MOPP-compliant supply will pay dividends in terms of reduced system design, reduced compliance complexity, and lower inventory costs.

If a full 2x MOPP supply is specified, a designer not only future-proofs a system, but the cost of inventory management and paper work falls dramatically as that single supply, or series of supplies, can fit almost any medical system application. As a result of the decreasing cost of power supplies in general, it’s a good bet that new supplies will meet full 2x MOPP requirements, but as usual, it’s best to check.

Some good examples of IEC/EN/ES 60601-1 2x MOPP compliant power supplies include CUI Inc.’s VMS-365 Series open-frame, AC/DC supplies, which come in under UL (0.3mA leakage current) requirements at 0.110 and 0.275mA, for test voltages of 120 and 230 VAC/60Hz, respectively.

Other key features include a single output for 12 to 48V, an industry standard 3x5” footprint, an efficiency of 90%, a universal input (85~264 VAC) and continuous power up to 365W. The full open frame VMS line up ranges from 20 to 365W.

For external applications, CUI has 2x MOPP supplies ranging from 15 to 250W, including the ETMA 60W desktop power supply that complies with IEC/EN/ES 60601-1 requirements. The supply also emphasizes CUI’s ‘green’ focus so meet Level V efficiency standards, exceed the current US EISA 2007 efficiency regulations, and meet April 2011 ErP regulations.