Medical Power Supplies are Re-Shaped by Evolving Standards
New medical equipment standards will improve safety and facilitate the process of international compliance.
The design of medical-grade power supplies or the integration of such supplies into medical equipment has long posed unique challenges to engineers and system designers. The performance of power supplies in such applications can be potentially life saving. With the burgeoning market for medical apparatuses, designers have become more familiar with the particular compliance standards required. However, advances in technology have led to changes in standards that will continue to significantly affect the power-supply designer and system integrator.
New standards are being proposed that would change the medical-device approval process, literally turning the current method upside down. These new standards would change the method of approving medical equipment from simple parameter testing to an involved collaboration of risk declaration and the methods applied to identify and minimize those risks.
Current Medical-Grade Power Supply Standards
Today, global compliance for medical power supplies is based on IEC 60601-1 'Medical Electrical Equipment, Part 1: General Requirements for Safety.' Most of the medical power-supply compliance standards are based on this, including UL 60601-1, EN 60601-1, JIS T 0601-1, and CSA C22.2 No. 606.1 (Table 1).
The IEC standard has helped define and ensure that the components and systems designed for medical equipment are safe. The IEC 60601 document sets the parameters for the design of medical power supplies as the unit shown in Figure 1.
This document, as well we, the IEC 60950 for information technology equipment (ITE), use a common approach to power-supply evaluation. The medical supply standard includes the definition of requirements for creep-age, clearance and isolation. The requirements in medical design are more stringent, designated for cardiac content.
IEC 60601 National Standards by Country
UL 60601-1(U.S. national deviations)(Note: UL 60601-1 was previously numbered UL 2601-1)
CAN/CSA C22.2 No. 606.1(Canadian national deviations)
EN 60601-1 (Identical to IEC 60601-1)(Known as BS EN 60601-1 in the United Kingdom)
JIS T 0601-1 (Japanese national deviations)
AS/NZ 3200.1.0 (Australian and New Zealand national deviations)
IEC 60601 also included other related standards of importance to any medical supply design effort. The standards are classified as collateral and particular, and attempt to address unique aspect of given applications. Collateral standards are designated 60601-1-1-X, also referred to as horizontal standards because they provide additional considerations outside of the base standard. Particular standards are identified by 60601-2-X, also sometimes referred to as vertical standards because they add detailed requirements for specific medical devices, such as X-ray machines and hospital beds.
Perhaps the most importance collateral standard for power-supply design is IEC 60601-1-2 for electromagnetic communicability. This standard impacts all steps of the development process, from power-supply design to interrogation and product testing.
Proposed Medical-Grade Power Supply Standards
The third edition of the IEC 60601-1 standard will force a philosophical shift in designing for compliance. The proposed medical standard is a response to the rapidly changing medical device market. The market has taken advantage of technology and material science advances to develop more sophisticated and complex products. Current medical standards are challenged to keep pace with these changes while maintaining the purpose of their existence; providing safe products for use in the medical industry.
To address current and future technology advances, the proposed standard formally introduces several new concepts to the medical approval process. First and foremost is the include of risk management. A second new important term is that of 'essential performance.' To support these approaches, new testing and design processes are required. The net result is that the certification process will be less about type tests with defined limits - although these will still be important - and more about the manufacturer identifying potential hazards and documenting how they are addressed.
What is the IEC 60601 Third Edition Standard?
The new standard, as mentioned previously, still includes type tests for leakage, isolation and creep age/clearance. The difference is that the determination of the correct category and values is guided by the concept of 'essential performance.' Essential performance identifies operating characteristics that can impact the safety of operators or patients. This will tie into the risk analysis performed under the risk-management system employed with the new standard. The purpose is to allow the manufacturer to identify the approach levels to ensure safe medical devices. In some cases, this may be a reduction in limit from the current standard, but in many cases it will require additional protection or analysis.
A second concept introduced in the new standard means of protection (MOP) which describes the isolation protection between the electrically charged circuitry and any equipment that may come in contact with the device (Figure 2). The isolation protection includes the creep age/clearance distances, insulation, and protective earth connections. This means of protection is further separated into two categories: means of operator protection (MOOP) and means of patient protection (MOPP).
As the term suggests, the classifications provide greater protection for the patient who may be more vulnerable to the medical device in use. MOOP is more closely aligned to the traditional IEC 60950, which is the standard protection for ITE, while MOPP maintains the more stringent requirements similar to the current IEC 60601 standard (table 2).
The Third Edition IEC 60601-1 Classification
Basic, or one layer of insulation (at 240 Vac)
Test voltage: 1500 Vac, Creepage: 2.5 mm
Double, or two layers of insulation (at 240 Vac)
Test voltage: 3000 Vac, Creepage: 5 mm
Basic, or one layer of insulation (at 240 Vac)
Test voltage: 1500 Vac, Creepage: 4 mm
Double, or two layers of insulation (at 240 Vac)
Test voltage: 4000 Vac, Creepage: 8 mm
The compliance measures for leakage current have been altered to ensure patient and operator protection in contact with medical devices. First, the present standard definition of 'enclosure leakage circuit' is now described as 'touch current.' Touch currents are the leakage paths from an enclosure that may be in contact with a patient or operator. The levels within the third edition of IEC 60601 are 100 A for normal operation and 500 A for single fault condition, which is the same as the enclosure leakage limited for the current IEC 60601 standards.
In addition to the terminology change, a new leakage test has been added called total patient leakage current. The basis for the total patient leakage current test is to measure the leakage current when all applied parts required for the operation of the medical device are in contact with the patient (Figure 3). This new test further protects the patient in light of devices that may have multiple connections and leakage paths.
What Does ISO 14971 Stand For?
ISO 14971:2000 'Medical Devices - Application of Risk Management to Medical Devices' was developed in a joint ISO/IEC committee along with the third edition 60601 standards. As such, they are tied closely together in defining the procedure and requirements for the certification of medical devices. Specifically, Clause 4.2 in the third edition standard states: 'A risk management process complying with ISO 14971 shall be performed.' The goal and major shift in this approach, summarized in table 3, is to guide the development engineer through a process that ensures hazards are identified and mitigated. In this way, that standard can last over time and include current and future advancements in medical technology.
The process begins by analyzing potential hazards the medical device may cause in its application. It also considers possible misuse and fault conditions to account for all potential dangers. Once presumed risks are identified, an evaluation is made to determine the level of each risk. During the risk-control phase, the manufacturer must consider methods to mitigate or identify the potential hazards. This includes design considerations (design in safety and/or protection and alarms), system considerations (design in safety in application) and notification (markings, notes and instructions). Following the risk-control phase, the potential dangers are reevaluated with the identified measures taken. If the residual risk is deemed acceptable, the process continues to the report, which documents the steps taken and considerations made. The concluding steps completes the loop by evaluating the medical device data in production.
The importance of ISO 14971 cannot be understated in the application of the third edition IEC 60601-1 standard. The role of risk management is mentioned more than 100 times in the standard. To support device compliance, the third edition describes a risk-management file, which is the supporting body of work that the risk management systems was applied to under evaluation. Documented evidence, including the analysis of hazards, steps taken to mitigate hazards and identification of unavoidable hazards, would be found in the file.
Risk Management Process Steps to Comply with ISO 14971
Step 1: Risk analysis
Step 2: Risk evaluation
Step 3: Risk control
Step 4: Overall residual risk
Step 5: Risk-management report
Step 6: Post-production information
US Adoption of Medical-Grade Power Supply Standards
For the United States, there is a more subtle change that would occur with the adoption of the new medical standard. The decision was made that the American Advancement for Medical Instrumentation (AAMI) would publish the standard as opposed to Underwriters Laboratory (UL) who had published the previous editions base on the IED 60601 medical standards. This is a minor shift, as the prior UL standards were based on the IEC standard and included input from the AAMI ES-1 standard and both AAMI and UL have long histories with the common goal of promoting the development of safe devices for the market.
However, the AAMI has attempted to publish a document more in line with the harmonized standard published by the IEC. The AAMI 60601 standard has reduce the U.S country deviations to 2 pages. This is a significant change as the current UL 60601-1 has 32 deviations of pages for the United States. The deviations primarily include requirements outlined by the National Fire Protection Association (NFPA) and the National Electrical Code (ELC). Although it is likely the deviation list may grow slightly to resolve a few outstanding issues, it will maintain its goal of minimizing the differences for global compliance. The standard has been published as American National Standards Institute (ANSI)/AAMI ES 60601-1:2005, and is available through a variety of outlets including the AAMI website, www.aaami.org
Meeting New Medical-Grade Power Supply Design Challenges
The environment for electrical designers in the medical arena is challenging. Understanding, interpreting and applying the wide variety of global standards for the product requires extensive knowledge of the standards. This knowledge must include general requirements, collateral standards, type specifications and an understanding of the regulatory differences between individual countries, which can disrupt the development and marketing process. This is a difficult challenge and now designers must begin to understand the new requirements being proposed.
These new challenges require design engineers, system engineers and manufacturers to be away of the changes. It also requires significant changes to the certification of medical power supplies. The product compliance test, which starts at the product, is now inverted to start with compliant processes. To comply with the proposed standards, managers would have to institute risk-management processes from which compliant products could be spawned.
Peter Resca, Director of Engineering, and Dave Murray, Senior Product Engineer, Astrodyne
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