Addressing the EMC Requirements During a Digital Upgrade Installation
As stated in the NRC Regulatory Guide 1.180, "this guidance recommends design and installation practices to limit the impact of electromagnetic effects, testing practices to assess the emissions and susceptibility of equipment, and testing practices to evaluate the power surge withstand current (SWC) of the equipment." In addition, "the electromagnetic conditions at the point of installation for safety-related I&C systems should be assessed to identify any unique EMI/RFI sources that may generate local interference." Therefore, the recommended process involves more than the EMC qualification testing of the digital or advanced analog system to be installed; the system must also be compatible with the environment at the point of installation, which requires a review of the specific plant’s electromagnetic environment (EME).
To address both the qualification of the equipment and the testing of the environment in which it will be installed, the Figure 2 flow chart should be followed. This flow chart provides a framework for the digital upgrade process with respect to EMC and involves three main components: the plant’s EME characterization, the system’s EMC qualification, and installation of the system at the plant. Each component can be broken down into a subset of tasks, as explained in Figure 2.
2. Implementation process for a digital upgrade with regard to EMC qualification. Source: Analysis and Measurement Services Corp.
Electromagnetic Environment Characterization
The first step in determining the effects of EMI/RFI on a new digital upgrade is to review the performance history of the existing system that will be replaced. Have there been any previous issues caused by EMI/RFI that could affect the new digital system? If there have been any issues, what modifications were made to prevent the disturbances, and should those considerations be integrated into the new system design or left in place during the new system installation?
Performing a walkdown of the affected areas of a digital upgrade can identify the location of typical plant sources of EMI/RFI in the vicinity. If significant EMI/RFI sources are identified, it may be possible to avoid potential interference by selecting a different location for installation of the digital upgrade. Once a location is selected, a site survey can be performed to capture an emissions profile of the area. There are several advantages to having this information from a site survey, including these:
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The emissions data will provide a baseline measurement of the specific location’s EME.
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The site survey can locate the source of significant emissions and either recommend addressing the emissions in the design of the new system to increase its immunity, or modifications can be made to the existing equipment to minimize its emissions.
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If the measured plant emissions from the site survey exceed the EMC qualification susceptibility test limit, the higher level can be used during immunity testing to ensure that the equipment will perform its intended function given the plant’s EME.
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During qualification testing, if the measured equipment emissions exceed the allowable equipment limit but would not increase the site’s emissions profile at the point of installation, it could be justified that since the existing plant equipment has operated reliably in its current EME, the emissions due to the installed system should not impact operation of the existing equipment. However, an assessment of the characteristics of the added emissions is necessary.
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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.