Equipment EMC Qualification
Once the EME of the installation location has been characterized, the digital system can be selected based on these design considerations. If the system has already been selected, then its design can be reviewed to determine if it complies with the EME specifications.
Next, the regulatory requirements regarding EMC qualification testing must be identified. They determine whether emissions and susceptibility tests are required (such as for safety-related systems or systems that could impact safety functions) or if only emissions testing is required (such as for systems in the vicinity of safety-related equipment) to ensure that the new system will not increase the plant’s EME in that area. In certain cases, plants choose to perform immunity testing on nonsafety-related equipment even though it is not required because the system may be deemed important to plant operation.
EMC qualification testing is performed to determine the new equipment’s ability to withstand the plant’s EME (susceptibility testing) and the impact the equipment will have on the EMI/RFI at the point of installation (emissions testing). This requires that the equipment setup for testing must be as similar to the in-plant installation as possible. If an emissions or susceptibility test fails to meet the acceptance criteria, then an EMC expert should review the data to determine the ability of the system to perform its intended function in the presence of the plant’s EME without affecting the existing plant equipment. If it is determined that the equipment must meet the equipment test criteria, then the system must be modified to limit its emissions or increase its immunity through techniques such as filtering and shielding. The testing process must be performed until it is demonstrated that the equipment meets the acceptance criteria.
Besides qualification of the equipment, the goal of EMC testing is to provide recommendations and requirements of the system and its installation in the plant to ensure a successful implementation. This information would include requiring that any modifications made to the equipment during the EMC qualification testing in order to meet the acceptance criteria be included in the final design for site installation. A recommendation may also be made for ways to improve the immunity or reduce the emissions of a piece of equipment if results of qualification testing approach the acceptance limit.
Equipment Installation and Follow-up
The system must be installed as close as possible to the final test setup during EMC qualification and must include modifications to the system made to gain a passing level. The installation should also adhere to the recommendations provided by the EMC testing and best practices for minimizing EMI/RFI.
After the upgrade, a site survey can be performed to determine if installation of the digital system caused an increase in its emissions compared with the qualification testing or if it raised the plant’s EME from the previous site survey. If the emissions are determined to potentially affect the operation of the surrounding equipment, or if the system has caused or experienced performance issues, then troubleshooting techniques can be used to identify the cause of the disturbance. Modifications can then be implemented to prevent its reoccurrence. After resolving all of the issues, the system should continue to be monitored for malfunctions that could indicate degradation in its immunity to EMI/RFI or an increase in surrounding EME levels.
—H.M. Hashemian (hash@ams-corp .com), C.J. Kiger, J.C. Seibel, and R.J. Wunderlich work for Analysis and Measurement Services Corp., AMS Technology Center, in Knoxville, Tenn. R.D. Meininger (richardm@emifix.com) works for CHAR Services, a division of AMS Corp. in Annville, Pa.
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Several of the worst plant upsets in process plant have been ship or tugboat radars effects on transmitters. In one case the transmitter EMC filtering had been damaged by lighting without noticeably changing the 4-20 mA signal.