Conformity Magazine - November 2007 - (Page 42)

to be as severe as the final safety system’s overall EM specification. For example, if the worst-case lifetime radiated RF threat, plus the test margin to achieve sufficient test confidence, required a test level of 1000V/m, employing an enclosure that can be relied upon to achieve at least 40dB attenuation (over the anticipated lifetime despite the worst-case physical environment) would require the equipment housed within it to be tested at only 10V/m. The effectiveness of the mitigation measures also needs to be verified, but for techniques such as filtering or shielding over their linear regions there is usually no need to test at the worstcase levels plus the test margin. Their effectiveness in dB can be measured using low test levels. Where appropriate data is provided with a purchased enclosure, it may not be necessary to test the effectiveness of its EM mitigation at all. In such cases it is always necessary to ensure that the manufacturers” data can be relied upon. Where it seems impossible to avoid RF testing at very high levels, reverberation chamber methods (such as those described in [18], or recommended for civil aircraft by [19]) can be much more cost-effective than “traditional” anechoic chamber tests. Methods of using low-level RF tests to predict the outcomes of high-level tests, to avoid the cost of highlevel tests, are being developed in IEC 61000-4-23 and by the military, and might be able to be used. Testing with high levels of surges and transients often requires finding or making suitable test equipment. Because of the complexity and non-linearity of modern electronics, software and systems, passing an immunity test at the highest level does not always ensure that the test would be passed at a lower, more commonplace level. Confidence can be improved by repeating all types of immunity tests using a range of levels up to the highest. Simultaneous Phenomena Simultaneous phenomena are a feature of real-life EM environments (e.g., transmissions on multiple radio channels; continuous RF fields plus mains transients or ESD, etc.). They are also a feature of real physical environments (e.g., temperature plus vibration, temperature plus humidity, etc.). These issues should have been captured in the specification used to control the design and formulate the validation/ verification plans. Testing with multiple simultaneous RF threats is already used for testing military aircraft and digital TV receivers, and multiple-signal RF generators are commercially available. So testing using simulated real-life RF environments is an option that should be considered. Testing that applies different types of phenomena simultaneously is not uncommon in physical/climatic testing, and is normal in HALT, but is (almost) unknown in EMC testing. Appropriate analysis techniques can generally be used to achieve sufficient confidence in safety performance despite simultaneous EM phenomena, without the need to test more than one phenomenon at a time. However, it is possible to test with different types of EM phenomena at the same time, and some such tests might need to be employed in some cases, to improve confidence when higher SILs are required. Any such tests would need to be very carefully planned, to achieve the desired confidence without disproportionate increases in timescales and costs. Emissions Testing It is usually assumed that all that is needed for EMC for Functional Safety is to ensure that the final safety system is immune enough. But it is possible for the emissions caused by an item of equipment to exceed the levels and/or frequencies assumed when the intra-system effects were analyzed to help create the original EM specification. So it is also important to employ validation and verification techniques like those discussed above, to ensure that the emissions from the parts of the overall safety system are within their design limits, after assembly, installation and commissioning. It is also important to have sufficient confidence (given the SIL required) that they will remain so over the system”s anticipated lifetime in its physical environment. Figure 3: Uncertainty, statistics and test margins 42 Conformity noVEmBEr 2007

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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.