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 naive 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 Web site, 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 powersupply 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, says obtaining UL’s blessing for the electrical safety of a new product using an approved power supply is usually a much easier process 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,” a long article by Rodger Gensel, in the March 15, 2007, issue of Conformity at www.conformity.com/artman/publish/printer_166.shtml. This magazine is unique in focusing exclusively on the engineering and politics involved in meeting government standards around the world. In his article, Gensel focuses on European EMI requirements. But as in the case of electrical safety, there’s more overlap than difference these days.
Essentially, Gensel says the European Union requires manufacturers of electronic equipment to meet the EMC guidelines of EC Council Directive 89/336/EEC. The test requirements are issued by CENELEC, the European Committee for Electrotechnical Standardization.
Generic immunity standards are called out in EN 61000-6-1 and EN 61000-6-2, and generic emission requirements in EN 61000-6-3 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.
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; EN 61000-4-4, 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/MedicalPower-Supplies.pdf.
According to XP, medical equipment combines the risk of electric shock with EMC, both in terms of immunity and emissions, as critical issues. “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 explains.
“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,” it continues.
The brief goes on to reference EN 60950 and the European, U.S., and Canadian variants of IEC 60601-1. It then has something interesting to say about safety levels: “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 EN60950 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.
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