Understanding the New EMC Standard for Medical Devices: What Manufacturers Need to Know Now

Originally Published MDDI August 2002

ELECTRONICS

Understanding the New EMC Standard for Medical Devices: What Manufacturers Need to Know Now

Meeting the new requirements set forth in the latest edition of 60601-1-2 might prove challenging for OEMs.

Don Sherratt

In 1993, the International Electrotechnical Commission (IEC) published the first edition of 60601-1-2, the international electromagnetic compatibility (EMC) standard for medical devices. Soon after, the European Committee for Standardization (CENELEC) adopted it as a European Norm (EN) standard—with only minor changes. It should therefore come as no surprise that IEC's latest revision of 60601-1-2, published in September 2001, was adopted by CENELEC shortly thereafter and published as EN 60601-1-2:2001 It will appear in the Official Journal of the European Community in August 2002.

IEC (EN) 60601-1-2:2001 is considered a major rewrite of the original and will have profound effects on compliance with regard to two important EU directives: the Active Implantable Medical Devices Directive (90/385/EEC) and the Medical Devices Directive (93/42/EEC), as well as CB Scheme requirements and related standards. While the phaseout date of the 1993 EN standard will be two years from now, manufacturers that export their products globally—or who plan to—and wish to remain competitive should immediately begin developing strategies to address the new standard's impact on product design and development.

To help manufacturers prepare for this transition, this article addresses the most significant differences between the first and second editions of IEC 60601-1-2, and outlines key steps to compliance.

Tougher criteria, higher limits

The most significant revisions to IEC 60601-1-2 include increased testing limits for electrostatic discharge (ESD), radiated radio-frequency (RF) immunity, surge immunity, and electrical fast transients (EFTs). There are also required tests for conducted RF immunity, magnetic field immunity, voltage dips, harmonic distortion, and voltage flicker. In addition, the standard contains more-comprehensive pass/fail criteria, which require that clinical utility be maintained during immunity tests and that the manufacturer define acceptable criteria of clinical utility.

Electrostatic Discharge. The standard outlined in IEC 60601-1-2:2001 for measuring ESD is IEC 61000-4-2. It defines the range of test levels in relation to different environmental and installation conditions and establishes the appropriate test procedures. The objective of the standard is to establish a common reproducible basis for evaluating the performance of equipment when subjected to ESD. Table I compares the old ESD measurement limits to the new ones.

These new requirements will likely demand that insulation thicknesses and creepage and clearance distances be greater than before to prevent arcing through and tracking over insulation. They might also require enclosure materials to possess increased antistatic properties, or that equipment specifications detailed in user manuals mandate higher relative humidity levels for operating environments.

Radiated RF Immunity. In the first edition of the standard, RF immunity requirements were defined by IEC 801-3. In the updated version, they are defined by IEC 61000-4-3 and are outlined separately for life-supporting and non-life-supporting medical electrical equipment. The frequency range of the requirements has been widened and allows for devices specified for use only in shielded locations. Other noteworthy differences include an intentional RF receivers' operation-mode exemption and a mandate that patient-coupled equipment must meet the same requirements as other equipment. Table II presents a comparison of the two standards.

There are different test methods defined in the new standard for measuring RRFI of different medical electrical equipment; manufacturers should discuss the most acceptable methods with technical experts at their testing lab.

Electrical Fast Transients. IEC 801-4 determined the EFT immunity requirements and test methods in the previous standard, but it will be replaced by IEC 61000-4-4 in the new edition. The new standard defines the test voltage waveform, range of test levels, test equipment, test setup, and test procedure, as outlined in Table III. One notable change is the lack of differentiation between permanently and temporarily installed equipment. There also are significant differences in the requirements for testing ranges and repetition rate, and pass-fail criteria are much tighter under the new standard.

Manufacturers should also be aware that the new standard requires that handheld equipment or parts of handheld equipment be tested with an artificial hand to simulate the capacitive coupling effect of the operator, as specified in CISPR 16-1. The artificial hand is connected to the ground reference plane during the tests. This requirement is new for medical electrical equipment and might result in dielectric stresses on signal or patient cables carrying high-frequency or high-voltage signals. To reduce the potential for this occurrence, manufacturers should keep in mind the type of cables used in interconnecting leads and note that patient leads might need to be changed.

Surge Immunity. IEC 61000-4-5 is the new standard that specifies test levels relating to different environmental and installation conditions. It establishes a common reference for evaluating the performance of equipment when subjected to high-energy disturbances on the power and interconnection lines, and specifies sweeping phase angles over particular voltage ranges. The determination of compliance is based on the response of the equipment, considering each surge individually, and taking into account the effects of any coupling between cables that are tested directly and those that are not. Table IV presents a comparison of the old and the new requirements.

These new requirements could necessitate circuit redesign and component changes in medical devices. While the upper voltage requirement remains the same, the new lower voltage requirement means more than one level of surge protection will be needed and will have to begin at ±0.5 kV.

New tests, methods, and prerequisites

In addition to a significant difference in test limits, the new standard also specifies changes in the classification of products and outlines the methods and new equipment necessary to determine emissions and immunity compliance.

Product Classification. The new standard stipulates that nonmedical equipment used in a medical system need only meet the applicable CISPR requirements—in most cases CISPR 14, 15, and 22. Most medical electrical equipment or systems will remain subject to the requirements of CISPR 11 and must be defined as Group 1 or 2 and Class A or B. Manufacturers should keep in mind that hospitals are often considered residential or domestic; therefore, Class B limits are often imposed.

Harmonic Distortion. There are two new requirements for determining emissions compliance. The first, IEC 61000-3-2, Limits for harmonic current emissions, addresses the effect on the sinusoidal waveform of the main supply of products with a rated input current up to and including 16 A per phase that are intended to be connected to the public mains network.

In addition to changing the power filter to accommodate the frequency range increase, it may be necessary for manufacturers to redesign the power supply circuit so that it will not affect the sine wave quality of the public mains network or the product harmonics on the public mains network.

Voltage Fluctuations and Flicker. IEC 61000-3-3, Limitations of voltage fluctuations and flicker, is the other new emissions requirement. It specifies the testing and limits of short interruptions or dips in the supply voltage caused by other apparatus connected to the same electrical supply (known as flicker). The new requirement specifies that compliance must be determined for all medical electrical equipment or systems with a rated input current up to and including 16 A per phase and those that are intended to be connected to the public mains network.

If the equipment has long-time and momentary current ratings, the higher of the two ratings shall be used in determining the
applicability of the requirement.

Manufacturers should note that the holdup times for power supplies will need to be improved in machines for which voltage dips and interruptions can cause the product to lose function.

Conducted RF Immunity. There are several new immunity requirements in the new IEC document. The first is a new standard for measuring conducted disturbances induced by RF fields in start frequency: IEC 61000-4-6. Under this standard, equipment is evaluated according to its respective testing category; these categories include life-supporting equipment, non-life-supporting equipment, shielded-location-use equipment, intentional RF receivers, and battery-powered equipment. Manufacturers are responsible for choosing the appropriate test and severity level applicable to the product.

Magnetic Field Immunity. Using IEC 61000-4-8 to determine magnetic-field immunity at various power frequencies is another new immunity requirement under IEC 60601-1-2. Equipment rated for both 50 and 60 Hz is subjected to a magnetic field of 3 A/m at 50 Hz and again at 60 Hz. If a piece of equipment is rated for operation at only one frequency, the tests are conducted at the rated frequency. There is no allowance for loss of performance or interruption of operation in this requirement.

The manufacturers most effected by this new test requirement will be those with equipment containing magnetic-sensing devices, such as hall-effect sensors or bimetal switches. Specific shielding of these components may be required to reduce unwanted operation. Magnetic field immunity levels may also need to be raised for equipment intended for use in the proximity of powerful magnetic fields.

Voltage Dips, Interruptions, and Variations. A third new immunity requirement outlined in IEC 61000-4-11 defines immunity test methods and the range of preferred test levels for voltage dips, short interruptions, and voltage variations of equipment connected to low-voltage power supply networks. The requirement applies to equipment and systems that have an input power rating of up to 1 kVA, or an input current of 16 A or less per phase. There is no allowance for component failures or equipment malfunctions requiring repairs.

There is some leeway within the compliance levels for non-life-supporting equipment, if the manufacturer can justify lower levels based on the hazard the product will pose when exposed to voltage dips or short interruptions in the supply voltage. If a lower compliance level is sought and justified by the manufacturer, it may be necessary to use (or make recommendations for the use of) uninterruptible power supplies, batteries, or power-conditioning equipment.

Crucial next steps

Manufacturers should act now to ensure their readiness for the new requirements in 60601-1-2. The following suggestions are measures that companies can take to help prepare themselves.

Test products currently under development. Manufacturers should begin to identify potential noncompliance in products that are under development by testing them against the current IEC standard in a 10-m semianechoic chamber. Potentially top-money-earning products should be identified and tested immediately to determine what changes—if any—will be necessary before the new edition of the standard is officially established.

Partner with a third-party testing lab as soon as possible. Testing of medical devices or systems requires time to complete the conformity assessment phase. only a few conformity assessment organizations accredited to perform the testing exist to serve hundreds of manufacturers. What's more, few labs are equipped with the full array of test equipment needed to determine compliance with the new standard. To avoid production delays, manufacturers should work with a third-party tester to ensure that their most important products are either compliant—or very close—by November 1, 2004.

Develop tables of acceptable criteria for each operating mode. The tables should identfy possible indicators of performance degradation and describe how such degradation is likely to manifest itself. This information will assist the testing lab in detecting problems objectively so they can be accurately reported to the manufacturer. It would also be wise for the manufacturer to witness the immunity tests so that noncritical errors are correctly interpreted as such. Manufacturers seeking guidance in completing these tables will benefit from an early partnership with their third-party test lab.

Decide whether or not to continue Offering legacy Products. Manufacturers must make decisions soon about legacy products, for which redesigning for compliance could be cost-prohibitive. Older products might need to be replaced with redeveloped models to meet the new standard.

Check CE Marking. Following a successful product conformity reassessment, the declaration of conformity for those devices previously bearing the CE marking will need to be renewed and the technical files will need to be revised, along with the device master records and design history files. The relevant notified body (for devices in Class I, IIa, IIb, or III) will need to be informed of any significant design changes, and it may be necessary to resubmit a summary of changes made, along with a risk analysis and essential requirements checklist.

By keeping abreast of the changes in the new EN EMC standard and preparing products for compliance, manufacturers can avoid critical time-to-market delays and gain a global competitive edge.

Don Sherratt is the director of business and technology for medical devices at Intertek ETL SEMKO in Boxborough, MA.

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