Conformity Magazine - November 2007 - (Page 44)

Testing for Faults and Misuse The design of the system should have taken into account reasonably foreseeable faults, use and misuse, and the effects that these could have on EM performance and hence on the system”s safety. To achieve sufficient confidence in system safety, it may be necessary to devise verification methods to determine whether the design adequately deals with such events. For example, EM and/or physical checks and/or tests could be repeated whilst simulating the various faults, use or misuse. Careful planning will be required to ensure that such tests add usefully to the confidence in the system”s safety without disproportionate increases in timescales and costs. Testing Safe Shutdowns, Alarms, and Similar Safety engineers often seem not to care whether a system fails, as long as it remains safe enough. But in real life, a system that shuts down or alarms too frequently will cause annoyance and/or financial costs to its operators or owners, and is likely to be modified in an unapproved manner, for example by disabling the safety shut-down or alarm functions. Such modifications by the user are a reasonably foreseeable outcome of unduly sensitive shut-down or alarm functions so, if an accident resulted, the system”s manufacturer could possibly be found liable. So where safety shut-down, alarm and similar protective functions are to be tested, they should be tested twice. One test is required to ensure that they do not operate when they should not; the second test is conducted with the safety faults simulated, to ensure that they will operate reliably enough when they should. Verification After Installation The design and validation/verification of the safety system are based on assessments of the worst-case EM and physical environments, which often include assumptions that should be verified after the system”s installation. EM and physical mitigation measures can degrade over time, and certain assumptions will have been made in their design. EM and physical environments can also change unpredictably over the anticipated lifetime. Safety is required over the whole lifetime of a system, so it can be necessary to verify the EM and physical environments, and/or performance of any mitigation measures, regularly throughout the life of the system. Automatic or manual methods may be used, taking appropriate actions to maintain the required safety levels when unanticipated events are detected. Planning the Validation and Verification In conclusion, it should be readily apparent that proving that a system”s EM performance is adequate for lifetime safety requires a great deal more than simply asking a test laboratory to perform some standard EM tests on shiny new equipment. The planning of the validation and verification techniques needs to be performed by competent and knowledgeable personnel, in parallel with the design phase. It can be possible to avoid lengthy test sequences by doing the design in a different way. Since no organization can afford the time and cost of an EM test plan that, on its own, gives sufficient confidence for the SIL required, it is necessary to use a wide range of design, validation and verification techniques to reduce the amount of standardised EM testing whilst achieving the required level of confidence in functional safety performance over the anticipated lifetime. Keith Armstrong is the principal of Cherry Clough Consultants, and can be reached at keith.armstrong@cherryclough.com. References 1. The IET, “Guidance on EMC and Functional Safety”, 2000, www.conformity.com/1151. Keith Armstrong, “New Guidance on EMC-Related Functional Safety”, 2001 IEEE International Symposium on EMC, Montreal, August 13-17, ISBN 0-7803-6569-0, pp. 774-779. Surge protection for: • AC Power UL 1449 2nd Edition • Data lines like T1/ E1, RS232, RS422, RS485, POE and HDSL • COAXIAL lines N, 7/16 DIN, TNC, SMA Ideal for Radio, Wireless and Cellular base stations. Surge Arrester Gas Tubes for PCB. Available in surface mount and through hole. Any voltage from 75V to 2500V. (305) 621-0022 • Fax (305) 621-0766 • e-mail: citel@citelprotection.com 1-800-CITEL-4U 2. www.citelprotection.com 44 Conformity noVEmBEr 2007 http://www.citelprotection.com http://www.conformity.com/1151 http://www.citelprotection.com

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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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