Conformity Magazine - November 2007 - (Page 36)

Non-standardized checks and tests—Because EMC testing has become standardised, many people tend to think of EMC testing only in terms of the standard test methods, such as MIL-STD-461, IEC 61000-4-x, etc. But there are very many non-standard EMC checks and tests that can (and often should) be done to improve confidence in safety integrity. For example, a low-cost portable spectrum analyzer and close-field probe can be used to check the correct assembly of shielded enclosures, shielded connectors, and filters. This is a qualitative technique, rather than a quantitative one, but nevertheless can be very useful in improving confidence. It can also be usefully applied during the operational lifetime to check that shielding and filtering performance is being maintained. Similar close-field probing techniques can check purchased devices (e.g., integrated circuits) or equipment (e.g., power supplies, computers, etc.) to detect bad batches or errors in assembly, before they are incorporated into the safety system. Many other EMC “checking” methods can be designed and used to improve confidence without adding significant cost. Individual and/or integrated hardware tests—Different parts of the safety system are assembled step-by-step, with checks and tests applied to ensure that they function correctly at each step. Validated computer modeling—Computer-aided EMC design has made large strides in recent years, and is now routinely used in certain critical industries (for example, see [12]) to successfully reduce design and test timescales without sacrificing reliability. All computer modeling is based on simplifications, so it is important to validate any predictions by appropriate testing. But once the model is shown to replicate the test results with sufficient fidelity, it can be used to quickly simulate the results of numerous similar tests that would be too costly or time-consuming to perform in real-life. Testing (e.g. factory acceptance test or on-site testing)—Most engineers automatically think of EMC testing as the only way to prove adequate EM performance. But as mentioned above, an EMC test plan that could – on its own – give sufficient confidence in EM performance for safety reasons, will always be much too lengthy and much too costly. EM immunity testing is supplementary to the other validation and verification measures. Clause 9.1 of IEC CD 61000-1-2 says: “In most cases there will be no simple or practicable way to verify by means of testing that EM immunity is achieved.” Despite this, appropriately-designed testing is a powerful validation/verification technique, and some suitable techniques are discussed below. EM Immunity Test Methods EM measures required for the achievement of adequate system safety should be evaluated using EM testing and highly36 Conformity noVEmBEr 2007 accelerated life testing (HALT), to demonstrate sufficient confidence that individual EM design aspects (e.g., circuit, shielding, filtering, surge transient or ESD suppression, etc.) will reliably achieve at least their minimum EM performance requirements over the anticipated lifetime of the final system. Such tests should be carried out as early in a project as possible, to reduce technical risks and save time and cost. Some of them will not need to have a functioning unit available – for example the effectiveness of filters, and shielded enclosures, cables and connectors, can be tested in isolation. It is also good practice to apply the immunity tests to the final safety system, after installation and commissioning. For smaller systems this may be possible in a test laboratory, but larger systems may need to be tested on site. On-site EMC test methods exist (such as those found in [13]), but some might prove too difficult, in which case they should be applied at the highest practicable level of system integration. Care should be taken to apply them so that they realistically simulate the way in which EM phenomena will affect the whole system. For example, when testing systems that use redundant channels, all of the channels should be exposed to the EM environment simultaneously – testing one channel at a time proves nothing at all about the system’s safety. All immunity tests should be based upon accepted test methodologies, such as the IEC 61000-4-x series, or the test methods in MIL-STD-461, all of them modified as necessary to better simulate the real-life EM environment where the system is to be operated, and/or to improve confidence that the test results are meaningful for real-life safety. For instance, the IEC’s basic test method for radiated RF immunity, IEC 61000-4-3, is limited in terms of angle of incidence, frequency range, modulation type, modulation frequency, and numbers of simultaneous modulated frequencies, any or all of which could have a significant effect on the performance of electronic devices and software. Real-life radiated RF environments are always more complex than those simulated by the unmodified IEC 61000-4-3 test method, and can cause very different and complex effects. Similar considerations apply to the other IEC 61000-4 series standards, and this problem is recognized in IEC CD 61000-1-2. Equipment is especially susceptible at the operating frequencies of its internal hardware and software processes. But high-enough levels of interfering signals can overdrive devices, causing errors, malfunctions, maybe even damage, at any frequency. A continuous RF test method currently used in some safetycritical industries uses unmodulated signals stepped in small increments over the range 0 to 30kHz, with a one-second pulse OFF then ON again at each step. Some test methods

Table of Contents Feed for the Digital Edition of Conformity Magazine - November 2007

Conformity - November 2007 Contents Editor’s Note Commission Sets Date, Terms for 700 MHz Auction FCC Reaffirms Roaming Obligations Commission Moves to Improve USF Management FCC Proposes Fines for Violations of E911 Handset Requirements Commission Levies $1 Million Plus Fines for Unsolicited Fax Advertisements GR-3150 - Lithium Battery Certification Levels Based on Criteria for General Product, Safety, and Performance, Part 2 ESD Open Forum Machinery Field Evaluation for the United States Validation, Verification, and Immunity Testing Techniques for EMC for Functional Safety Thermal/EMC Simulation Addresses New Mechanical Design Challenges Product Safety Equipment Buyer’s Guide FCC Releases Report on Telephone Numbering Resource Utilization Canada Still Has PCS Spectrum to Assign Updated Standards List for the EU’s Medical Devices Directive Updated Standards List for EU Directive on Active Implantable Medical Devices Call for Papers for the 2008 EOS/ESD Symposium Announced Mobile Phone Use in Hospitals May Be Dangerous CPSC Actions in the News IEC Standards Update UL Standards Update Telcordia Standards Update From Our “You Can’t Make This Stuff Up” Department Product Reviews Looking Back: Items from Past Issues of Conformity Upcoming Events Advertisers

Conformity Magazine - November 2007

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