Conformity Magazine - February 2009 - (Page 16)

temperature, supply voltage and load current that are within its specified ratings, when compared with the results of traditional immunity tests. Highly-accelerated life tests are often used by vehicle manufacturers to verify that functionality will be maintained over the lifecycle, despite the physical environment. But the resulting aged units are rarely, if ever, tested to see if their EM characteristics have degraded, although this is understood to be common practice for Russian military equipment. 7. Quality of EM Design Is Ignored Manufacturers apply the traditional immunity tests to their products, iterating their designs until they pass. But this approach cannot distinguish between a pass that was achieved by good EM design, or by something that would not be adequately controlled in serial manufacture over the production life of a vehicle. EMC standards ignore design issues. So, if a product’s EM design does not cope with component tolerances, semiconductor die-shrinks, variations in assembly (e.g., cable harnesses, grounding, etc.), replacement of obsolete components, firmware bug fixes, etc., the fact that some samples passed EMC tests means nothing at all for the EM characteristics of the ESAs or vehicles supplied to customers. 8. Assembly Errors are Ignored Safety engineering generally requires verifying each manufactured product to make sure that assembly errors have not made it unsafe. But traditional EMC standards do not include any requirements for manufacturers to perform routine checks in serial manufacture on the EM characteristics that are necessary for achieving tolerable safety risks. Automotive EMC test laboratories say that it is not uncommon for ESAs and vehicles that function correctly to fail EMC tests because of a misbuild. When this happens, the manufacturing errors are corrected and they are retested. Although most manufacturers employ rigorous end-of-line testing, including in-circuit test that will discover misbuilds that affect functionality, they do not generally design them to discover misbuilds that could affect EM characteristics. So, based on type testing, a customer could receive a vehicle that includes one or more assembly errors that could prevent it from having the EM characteristics claimed by its manufacturer. 9. The Maximum Test Level is Not Necessarily the Worst Electronic devices are non-linear, and circuits, firmware and software can be very complex. So ESAs can fail when tested with EM disturbances at a low level, but fail in a different way, or even pass, when tested at the specified levels. But most EM tests only expose equipment at the highest specified level to save testing time and cost. The likelihood of lower disturbance levels occurring is usually much higher than that 1 Conformity february 2009 of higher levels, so the immunity to low level disturbances could be much more significant for achieving tolerable safety risks. 10. Reasonably Foreseeable Faults are Ignored Immunity to EMI can be significantly affected by faults, for example: • Intermittent electrical connections; • Dry joints, open or short circuits; • Out-of-tolerance or incorrect components; • Missing or damaged conductive gaskets; • Loose/missing fixings in enclosures or cable shielding; • Failure of a surge protection device. But traditional automotive EMC testing ignores all faults; only perfect specimens of ESAs and vehicles are tested. 11. Reasonably Foreseeable Use and Misuse are Ignored Tolerable safety risk levels must be maintained despite reasonably foreseeable use or misuse over the life-cycle. Of course, it is impossible to make anything perfectly safe, but people are known to behave in certain ways, so safety engineering should take this into account. But traditional EM testing assumes vehicles are driven perfectly at all times, and are not damaged or modified. 12. Systematic Effects are Ignored Many system designers incorrectly assume that, if all the ESAs incorporated into a system pass their immunity tests, those systems will also be immune enough. But performance degradations that are perfectly acceptable when an ESA is EMC tested, or are not even measured during the testing, could have significant implications for the functional safety of systems that use those ESAs. Agreement between the EMC test results on ESAs, and on the systems that incorporate them, is frequently found to be poor. This is often attributed to the principle known as emergence, which states that the characteristics of complex systems cannot necessarily be predicted from the characteristics of its component parts. What Needs to Be Done The IET’s new guide [9] provides a comprehensive and detailed practical approach to dealing with the issues described above by applying modern risk management principles to EMC. It adopts the principles of [7], but uses an application-neutral language that makes it useful whichever functional safety standard is being applied (e.g., IEC 61508, or ISO 26262), or not. Unlike [7], it includes suggestions for how to take EMC into account when using modern risk assessment methods (e.g., FMEA, fault tree analysis, brainstorming, etc.), and adds checklists that will be useful for management,

Table of Contents Feed for the Digital Edition of Conformity Magazine - February 2009

Conformity Magazine - February 2009 Contents Editor's Note FCC Clarifies Closed Captioning Requirements for DTV Receivers Commission Cracks Down (For the Fifth Time!) On Junk Fax Marketer FDA Requests Comments on Global Harmonization Documents Australia Now Requires Solar Panel Testing and Certification EU Drafting New Directive on Toy Safety EU Commission Revises Standards List for EMC, R&TTE Directives Additional Standards Published For the EU's Medical Devices Directive EU Commission Publishes Additional Standards for Directives on Active Implantable, In Vitro Medical Devices EMC For the Functional Safety of Automobiles ESD Open Forum The ILAC Arrangement: Facilitating Trade Efficient Methodology for the EMI Analysis of Optical Transceivers Including PCBs and/or Flex Circuits The EuP Directive - Implementation, Design Requirements and Issues Chambers, Shielding, and Accessories Buyer's Guide CPSIA Lead Requirements Apply to Existing Inventory, Says CPSC CPSC Clarifies Certifcation Requirements Other CPSC Actions in the News IEC Standards Update Product Reviews UL standards Update Telcordia Standards Update From Our "You Can't Make This Stuff Up" Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - February 2009

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