Find a downloadable version of this story in pdf format at the end of the story.
Many designers think meeting the worldwide safety standards for power supplies in mains-powered products involves following a simple checklist to ensure their designs don't run into distribution problems in some countries due to a lack of dotted i's or crossed t's. But that's a naÏve perspective. The truth is that most designers need some serious hand-holding during their first several designs. This task can't be addressed with a checklist and a positive attitude.
Also, safety is critical, since you don't want to electrocute your end user. Yet basic safety is the least of your worries. In fact, it isn't too difficult to demonstrate conformity if you use already-approved components, except in the case of medical products.
The catch is that the safety requirements are in the same standards as electromagnetic-compatibility (EMC) and electromagnetic-interference (EMI) requirements. You won't know if your product has any problems in those areas until you run the tests at the testing labs, and then you have to solve the problems ad-hoc in real time.
That's why you should ask experts questions about your specific situation and follow their recommendations. Your best bet is simply to find an OSHA-certified (Occupational Safety and Health Administration) nationally recognized testing laboratory (NRTL) or the equivalent overseas. One NRTL is Underwriters Laboratories (UL) itself. However, a number of others can be found on the UL Website, any of which can test for UL certification. There's a parallel path for overseas approval. Allowing for deviations that are familiar to the experts, the fundamental requirements remain the same.
The markings for your product will differ in each country. Core standards for power supply safety and electromagnetic compatibility and interference are the European IEC 60601 (Measurement, Control, and Lab Equipment) and 60950 (Information Technology Equipment) standards, including their various dash-numbered subparts. In the U.S., they get “UL” prefixes. In Europe, they get “EN” (for “European Norm”) prefixes.
EMC AND EMI PROTECTION
There's more to these marks than electrical safety. Industry guru Derek Krous, who has gone through a number of certifications, said obtaining UL's approval for the electrical safety of a new product using an approved power supply is usually much easier than meeting the specs for EMC and EMI.
For a detailed description of the challenges involved, check out “EMC Testing/ Immunity Testing for the CE Mark,” an article by Rodger Gensel, in the March 15, 2007, issue of Conformity at www.conformity.com/artman/publish/printer_166.shtml. Gensel's article focuses on European EMI requirements. But as in the case of electrical safety, there's more overlap than difference these days.
Essentially, Gensel said the European Union requires manufacturers of electronic equipment to meet the EMC guidelines of EC Council Directive 89/336/EEC. Thetest requirements are issued by CENELEC, the European Committee for Electro-technical Standardization.
Generic immunity standards are called out in EN 61000-6-1 and EN 61000-6-2, and generic emission requirements in EN61000-6-3 and EN 61000-6-4, and sometimes there may be specific test requirements as well. The International Electrotechnical Commission (IEC) creates “Basic Standards” that define the specific tests, test methods, setups, and test equipment.
Continue on next page
As with safety, you get European Norms for EMI. If you want to look them up, they're EN 61000-4-2, Electrostatic Discharge; EN 61000-4-3, Radiated EM Field; EN61000-4-4, Burst-Electrical Fast Transients (EFT); EN 61000-4-5, Surge; EN 61000-4-6, Conducted Radio Frequency Disturbances; and EN 61000-4-8, Power Frequency Magnetic Field.
As if that weren't complicated enough, achieving conformity takes a major leap in complexity when it comes to matching power supplies to medical electronics. XP Power explains how that works in terms of the way that product safety and EMI come together in an applications brief, “Power Supplies in Medical Electronics,” available at www.xppower.com/pdfs/MedicalPowerSupplies.pdf.
According to XP, medical equipment combines the risk of electric shock with EMC, both in terms of immunity and emissions, as critical issues. The brief said, “As a result, the design of power supplies for use in the medical industry is driven as much by legislation as it is by the technical requirements of powering the end equipment.”
The brief continues, “System designers therefore need an understanding of this legislation and of the markets into which their products will be sold if power solutions are not to be over-specified, over-engineered, and excessively expensive as a result of building in too much safety margin when it comes to meeting legislative requirements.”
The brief references EN 60950 and the European, U.S., and Canadian variants of IEC 60601-1. On safety levels, the brief said: “The degree of protection needed in any particular medical application is related to the proximity of equipment to the patient, equipment that is directly applied to patients needing the highest specification with respect to isolation.”
XP recommends three progressive safety levels to consider regarding isolation and protection when designing medical electronic equipment:
- The basic safety requirements of EN 60950 that apply to all mains-connected electronic equipment
- The more rigorous IEC60601-1 standard for equipment used in patient vicinity
- The requirement for an additional isolation barrier in equipment that is in intentional physical contact with patients.
“Levels of Protection” (LOPs) are used to define the safety specifications for all electronic equipment, according to XP. Insulation or a protective earth and fuse can provide an LOP. Insulation is defined as one of five types with varying LOP ratings (see the table). XP also said that an earth can be similarly classified as functional or protective, with no protection provided by a functional earth and one level of protection by a protective or fused earth.
The brief states the basic principle is to provide two LOPs against electric shock. Each insulation type is defined in terms of air clearance and creepage distances. (For definitions of terms like creepage, see the sidebar “A Brief Safety Lexicon: Selected Definitions From EN 60950,” p. 42.)
The brief also details the fine-grain differences between EN 60959 and EN 60601-1 and current leakage specs depending on the specific kind of medical equipment in terms of whether it never touches the patient, whether it just touches the outside of the patient, or whether it gets stuck inside the patient. The power supply in the case history was based on XP's ECM series of small-footprint supplies for rapid qualification in medical applications that can be configured for outputs from 40 to 60 W (see the figure).
Continue on next page
Beyond current EMC/EMI specs, there's a further catch. An upcoming third edition of IEC 60601-1 will introduce several new concepts to the medical approval process (see “Evolving Standards Reshape Medical Power Supplies”).
According the document's authors, Peter Resca and Dave Murray of Astrodyne, these new concepts first and foremost will include risk management, followed by “essential performance.” New testing and design processes will be required to support these approaches. “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 authors said. “The purpose is to allow the manufacturer to identify the appropriate 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.”
The new standard introduces the concept of means of protection, which describes the isolation protection between the electrically charged circuitry and any equipment that may come in contact with the device. Resca and Murray also note that isolation protection includes the creepage/clearance distances, insulation, and protective earth connections.
The authors further separate the means of protection into means of operator protection (MOOP) and means of patient protection (MOPP). These classifications add protection for patients who may be more vulnerable to the medical device in use.
This article was previously published in Electronic Design
KEY ELECTRONIC PRODUCT CONFORMANCE MARKS
ALL PRODUCTS MUST MEET VARIOUS REGULATORY COMPLIANCE requirements for safety, emissions, and other criteria before they can be sold globally. All industrial nations require specific marks on products before they can be sold there. Testing and certifying compliance are the keys to getting those marks. For electronic products, safety and electromagnetic compatibility (EMC) and electromagnetic interference (EMI) are key certification issues.
In the U.S., the Federal Communications Commission (FCC) dictates the testing requirements for EMC and EMI. Products can be Class A, marketed for commercial or industrial use and not intended for home use, or Class B, targeting home use. Class B requirements tend to be more strict than Class A requirements.
Stateside, the registered certification marks of Underwriters Laboratories (UL) mean that UL or a nationally recognized testing laboratory (NRTL) has tested and evaluated representative samples of the product and determined that it meets ULÍs specified product safety requirements(a). The negative impact of lacking a UL mark may be more de facto than de jure, but it's just about as crippling.
Continue on next page
The Occupational Safety and Health Administration wonÍt let products without the mark be used in businesses. Also, under the National Fire Code, electrical inspectors wonÍt allow the product to be installed in buildings. You may be obligated contractually to have a UL mark on your product, or some customers might not sell it. And, savvy consumers look for the UL mark on products they buy.
In Europe, all products must have a CE or “Conformite Europeenne” mark (b). The CE mark is all-inclusive. It shows that the product complies with the “essential requirements” of European laws or directives. It also indicates the product's conformance to legal requirements with respect to safety, health, the environment, and consumer protection in the European Union.
The CE marking is mandatory for certain product groups, but it can be achieved either by using an external test house like an NRTL in the U.S. or by a companyÍs internal self-certification process. More information is available from the U.S. Department of Commerce at www.export.gov/cemark/index.asp.
In Canada, standards are determined and required by law by the Standards Council of Canada (SCC). In general, products require the cUL, a mark from UL, expressly for Canada(c). Products also require the Canadian Standards Association (CSA) mark. U.S. FCC approval is generally accepted for emissions.
UL issues the Recognized Component mark(d) or its Canadian variant to transformers, relays, and other subcomponents of power supplies. Using recognized components in your power supply doesn't guarantee conformance. That's where clearance and creepage and all of the other good practices, along with EMC/EMI performance, come in. But it does provide a foundation.
Japan uses the Voluntary Control Council for Interference by Information Technology Equipment (VCCI) mark, which certifies EMI compliance(e). It also employs the Denan/PSE mark, which targets electrical safety(f). “Denan” comes from “denki youhin anzen hou,” as denki means “electrical,”and anzen means “safety.”
Korea's Ministry of Information and Communication offers the MIC mark (g). Most MIC standards are based on IEC standards. Taiwan's Bureau of Standards, Metrology and Inspection offers the BSMI mark. The China Compulsory Certificate (CCC) integrates the “CCIB” Mark and the “CCEE” mark for electrical commodities.
Download the story in pdf format here.