For designers of power supplies for medical equipment in the United States, as well as for devices to be used in the rest of the world, the 2nd Edition of IEC 60601-1 has long been the standard to which products and equipment had to adhere. Just about all of these types of devices are switched-mode power supplies (SMPS), more commonly and simply referred to as “switchers.” For a discussion on the technical aspects of switchers, see “Switching Power Supplies.”
The new standard, IEC60601-1, Edition 3.0, along with Amendment 1, is now being phased in on a global scale. Its adaptation is proceeding more quickly in some nations than in others. Here in the U.S., established Nationally Recognized Test Laboratories (NRTL), such as Intertek and Underwriters Laboratories (UL), have been approved by OSHA to provide what is called an NRTL Mark for the Standard AAMI (American Association for Medical Instrumentation) ES 60601-1 3rd edition, Amendment 1, as it is formally designated in the U.S. It is important to note that in the United States, devices certified under the old standard need not necessarily be recertified to the new standards as those standards become established, but some manufacturers are doing so anyway.
New Standard Brings a New Philosophy
Compliance is complex, in part because different parts of the standard are being phased in, piece-by-piece, in the world’s advanced nations. Among the changes wrought by the new standard is a requirement for more active communications between the manufacturer and the testing organization. Under the old regime, the process under has been outlined as follows:
Figure 1: 2nd Edition of IEC 60601. (Source: Intertek)
However, the flow of progress under the new, higher standard is much more involved. A new concept that has been introduced in this edition of the standard is that of Essential Performance, or EP, which can be thought of as any parameter that, when compromised, would subject the medical practitioner or the patient to an unacceptable risk.
In addition, another change is that not only are adherences to specific safety requirements required, but also that a process of risk evaluation, such as described in ISO 149971, is required as a part of the path to the new IEC 60601-1. A full ISO 14971 certification, however, is not a requirement.
Evaluation of a device through the new standard, involves the production of Risk Management Files (RMF). These files are a written record of all known risks, as well as evaluations and plans to control those risks.
Figure 2: 3rd Edition of IEC 60601-1. (Source: Intertek)
Much Remains the Same in the New Standard
And while there are many substantial changes, some elements of the standards are staying the same. An important factor in medical power supply safety is the MOPP, or means of patient protection. A MOPP is defined in terms of physical separation distance between two points in the physical power supply that, if they touched, would create a shock hazard. There are also MOPP standards for electrical insulation and dielectric strength.
The requirement for two MOPPs to protect the patient at each point remains the same in the new standard. What this means is that if one of the MOPPs fails, the other will be able to protect the patient. As illustrated below, if one isolation barrier fails, the other capable of safeguarding the patient. These and other technical specifications regarding medical power supplies and their standards are discussed in-depth in “Power Supplies in Medical Applications.”
Figure 3: Two Means of Patient Protection. (Source: XP Power Blog)
Exploring the Changes
One big change in IEC60601-1, Edition 3.0 is that different standards of protection for operators (MOOP) of the equipment as well as for patients (MOPP) are herein specified. The rationale is two-fold. First, it is assumed that the patient may be of diminished health, and may be more vulnerable to hazards of all sorts than is the operator. The other reason is that sensors and probes from the instrument under power may be in contact with the patient’s mouth or internal organs, leaving the patient glaringly vulnerable to electrical shock and perhaps even electrocution. The operator will only be touching the controls, or other easily protected surfaces with his or her hand(s).
The new standard is actually less strict than the old is for MOOP. However, the distinctions between MOPP and MOOP are subtle, and an unwary designer can easily fall afoul of these differences if the greatest care isn’t exercised.
Another liberalization of the standard’s third edition is in regards to ground-leakage current. With SFC defined as single-fault condition, and NC defined as normal condition, the old standard was 0.5 mA for NC, and 1.0 mA for SFC; the new standard is for NC and SFC of 5.0 mA and 10 mA, respectively. This may mean that, in some cases, designers need not use the more expensive power supplies designed specifically for use in medical equipment.
Standards for electrical isolation have also become a bit less stringent, but complying with them can be difficult and complex. To avoid potential difficulties, the designer may want to stay with older standards, which are still acceptable.
Standards of the Future
It should also be noted that regulators have also published a fourth edition of 60601, although at the time of this writing, it has not truly been implemented. Its impetus is the wide proliferation of mobile phones, and their use in medical settings where medical equipment is being utilized. As such, the focus of this embryonic standard is the reaction of medical equipment to the RF signals inherent to mobile phone operation, as well as the effect that other wireless systems increasingly in use in medical environments may have.
Compliance with the third edition is a complex and involved process, with many pitfalls, even for organizations well-versed in the specific issues involved in medical electronics. The standard has come under criticism for its complexity and for the added costs that it imposes on the pathway of bringing a product to market. It is highly recommended that manufacturers involve a testing laboratory as early as possible with the process to avoid significant delay and disappointment later on.