Basic Elements of EMI/RFI
This article provides designers with an understanding of EMI/RFI root causes. EMI/RFI is said to exist when unwanted voltages or currents are present so that they adversely affect the performance of an electrical device or electronic system. These voltages/currents can reach the victim circuit or device by conduction or by non-ionizing radiation. In all cases, EMI/RFI occurs because of a combination of three factors:
- A source
- A transmission path
- A response (at least one response is unplanned)
EMI/RFI control refers to the process of making design changes or adjustments of the signa or noise levels in order to achieve Electromagnetic Compatibility (EMC).
The main reasons that EMI/RFI are becoming an important aspect of electronic equipment design are: a) Electronic equipment and devices are becoming more prolific these devices are the mainstay of the modern environment and will not go away, b) Current and future electronic equipment will operate at higher and higher frequencies and will utilize higher data rates, and c) Current and future electronic equipment will use lower voltages that reduce power consumption and render the equipment more vulnerable to EMI/RFI. Analog signals are a thing of the past most data signals and data links today are digital, and are designed to encompass as many separate channels as possible using up bandwidth and limited spectrum.
Most electronic devices are susceptible to emissions generated either internally or by other devices and many act as both emitters and receptors of EMI/RFI. Dependent upon the emitter and receptor, the resulting effects of EMI/RFI can take many forms (e.g., interference on the telephone line due to an electrical storm or radio interference due to an arc welder).
The methods of coupling between emitters and receptors of electrical signals are divided between radiation and conduction. The emissions can be transmitted as electromagnetic radiation (signals radiated through space) or conducted through network cables or wires (signals traveling along interconnecting cables).
Emitters are natural or man-made sources of noise and interference. Natural sources can be terrestrial such as atmospheric or precipitation static; or extra-terrestrial such as the sun, cosmic noise, or radio stars. Man-made sources can be devices such as communications-electronics (C-E) equipment or ignition systems.
Receptor refers to a class of devices, equipment, and/or systems which, when exposed to conducted and/or radiated electromagnetic energy from emitting sources, will either be degraded or malfunction in performance. Examples of receptors susceptible to EMI/RFI include communications receivers, computing devices, electronically guided/triggered ordnance, radars and new systems.
The EMI/RFI which develops between two or more discrete systems is called antenna-coupled EMI/RFI. It occurs as a result of different types of equipment and systems interacting through the electromagnetic radiation that enters or leaves the antenna from another source.
Any electromagnetic disturbance that interrupts, obstructs, or otherwise degrades or limits the effective performance of electronics and electrical equipment can be induced intentionally, as in some forms of electronic warfare, or unintentionally, as a result of spurious emissions and responses, inter-modulation products, and the like.
EMI/RFI Problems/Solutions
Causes of EMI problems are: a) Failure to adequately define environments, b) Failure to assess EMI/RFI impact on key performance parameters, c) Failure to accomplish adequate Electromagnetic Compatibility (EMC) testing evaluation, d) Failure to grow sufficient numbers of qualified EMC engineers and, d) Communications Reduce Intelligibility Bit Error Data and distortion.
EMI/RFI always starts with current flow through a conductor and also shows up in the victim equipment in the form of a current or voltage. The coupling path (including a conducting gas or air) might be a conduction or radiation path. The actual paths can include common wiring, capacitance between devices, and mutual inductance between adjacent wiring, non-ionizing radiation, or wires in an Electro-magnetic (EM) field. This type of coupling is aided by the fact that all conductors exhibit resistance and inductance.
Guidelines to consider for the EMI/RFI elimination includes: a) Review the revise new consensus EMC standards (i.e., domestic and international), b) Assess the applicability and equivalence of each technical element embodied in the standards, c) Re-evaluate the electromagnetic environment characteristic of NPP, d) Evaluate the current operating envelopes, e) Determine required test methods that can address the open EMC issues, and f) Identify equivalent suites of test methods from the alternative standards and the conditions under which they may be applied.
EMI/RFI Interference Sources:
Designers do not plan and design their equipment with the intention of being sources of interference. However, unintentionally, what is a desired signal in one path is an undesired signal (considered noise) in an inadvertent coupling. Interference may pose as an arc discharge, radiation from a lightning strike, a corona discharge from power lines, or a noise caused by a sudden change in current flow in a conductor.
Functional interference often includes: sine waves, computer clock pulses, speech or video waves, or pulses forming data trains. An example of this interference type is signal leakage from cable TV systems. Fluorescent lamps, car ignition systems, and industrial, scientific and medical equipment all constitute sources of interference
Designer’s purchase specification should indicate whether the equipment should meet the latest NRC Regulatory Guide 1.18 or the EMI US Military Standards (MIL-STD- 461E) and EMI International Electro-technical Commission Stds (IEC 61000) test methods. It is essential to analyze the system and the locations of the devices and cables to minimize the noise disturbances on the circuits. To properly address a probable complex situation, one must start with an analysis that begins with the individual electromagnetic-interference links and their components and then considers their interactions. The acronym, FATTMESS, should be applied to each of the often unsuspected sources, transfers, and receptors of interference. FATTMESS stands for frequency, amplitude, time temperature, mode, energy, size or structure, and statistics. Not all criteria are significant for each entity, but should be checked so that nothing is overlooked.
It appears that common EMI/RFI sources include large electrical rotating machinery producing strong time varying magnetic fields, large relays, contacts, SCR circuits, and power lines. Most EMI/RFI is best stopped at its source. Effective grounding and Shielding separation of signal wiring from noise producing equipment for the DCS will provide the best protection against EMI/RFI. EMI/RFI sources include radio frequency transmitters and fluorescent lights. The system should be designed to tolerate EMI/RFI noise with adequate shielding and filters. Moreover, use of fluorescent lights with EMI/RFI filters should be considered.
EMC considerations such as radiated emissions and susceptibility in the frequency band from 1 KHZ to 10 GHz have been identified as open issues that have been addressed by the IEC which has been issued recently. The need to develop and maintain specific practices for the NPP industry to addressed the effects of EMI/RFI and power surges on electrical safety-related and non-safety related system to support System Design Criteria and Specifications.
Vulnerability to the NPP EMI/RFI environment commonly exists on Instrument and Control (I&C) systems. The US NRC Reg. Guide 1.18, “Guidelines for Evaluating EMI/RFI in Safety Related Instr. & Control System” latest revision (2004) provides an acceptable method for qualifying digital and advanced analog systems for the projected electromagnetic environment in nuclear power plants. The typical environment in a nuclear power plant includes many sources of electrical noise. For example, hand-held two-way radios, arc welders, switching of large inductive loads, and high fault currents.
Fiber Optic Cable/System
Used of Fiber Optic cables are free from electromagnetic interference and immune to lightning and surge voltages. Shielding confines the dielectric field within the cable and eliminates high voltage stress points on the insulation; it can also reduce or control the electromagnetic and electrostatic effects on the conductor control.
Fibers act as waveguides in which the light is confined to the core, by an outer cladding. Both core and cladding are transparent to light but are of different refractive indices. The boundary between core and cladding appears as a partial mirror that totally reflects light rays back into the core, thus causing it to propagate down the fiber.
Thus Fiber Optic system is not subject to atmospheric effects such as fading. Both environment stability and low susceptibility to interference that leads to a very high quality signal. Thus there is no stray radiation from the fibers that eliminate cross-talk and preventing unauthorized monitoring of signal traffic. Additionally, they exhibit very low losses that allows for long spans, and minimizing maintenance cost and the use of repeaters. Fiber optics offers significant advantages over other communication system and is ideally suited for NPP requirements.
EMI/RFI US Military Standards Test Methods
The EMI/RFI US Military Standards (MIL-STD-461E) stipulate test methods associated with operating envelopes that serve to establish test levels. General operating envelopes that are acceptable are discussed and programmed on MIL-STD-461E test methods. These operating envelopes are acceptable for locations where NPP safety-related I&C systems either are or are likely to be installed in control rooms, remote shutdown panels, cable spreading rooms, common equipment rooms, auxiliary instrument rooms, relay rooms, and other areas (e.g., the turbine deck) where safety-related I&C system installations are planned. The operating envelopes are acceptable for analog, digital, and hybrid system installations source (emitter) and a susceptible receptor are required to make EMI/RFI /RFI possible.
EMI/RFI International Electro-technical Commission Standards Test Metods
EMI/RFI International Electro-technical Commission Stds (IEC 61000) test method provides the most recent international guidance for emissions test practices and incorporates by reference the test methods “Limits and Methods of Measurement of Electromagnetic Disturbance Characteristics of Industrial, Scientific and Medical (ISM) Radio-Frequency Equipment.” It is intended that either set of test methods be applied in its entirety, without selective application of individual methods (i.e., no mixing and matching of test methods) for emissions testing. Because of the absence of IEC 61000 test methods to address low frequency conducted emissions testing, low-frequency (magnetic field) radiated emissions testing, and high-frequency conducted emissions testing in the frequency range from 10 kHz to 150 kHz, the IEC emissions testing option is only acceptable under conditions that correspond to the special exemption conditions for the MIL-STD emissions testing option related to power quality control and proximity to equipment sensitive to magnetic fields.
Conclusion
It is important that the designer should coordinate closely and effectively with the vendor as possible. Thus enhancing EMI/RFI problem solving and process improvement effort. This will ensure that EMI/RFI are dealt with as they arise, and that all changes are addressed in a very timely manner.
The critical nature of communication is difficult for the end user to verify EMI/RFI, as it is generally proprietary and specific to the Vendor’s equipment, since the electrical requirements and the level of noise reduction technology employed by the various DCS suppliers differ, the vendor's site planning manual and installation instructions must be reviewed to determine the specific details and consideration with regards to EMI/RFI reduction and the electrical installation. Electrical requirements for a Distributed Control System (DCS) must incorporate EMI/RFI analysis, safety of personnel and equipment reliability and quality of power.
BIOGRAPHY
Armand L. Rogado, P.E., is a senior electrical engineer in the Electrical Engineering Department working for KOPEC. He has worked in several nuclear and combined cycle power plants in the US, Iraq, Philippines, Saudi Arabia and So. Korea as Nuclear Safety Engineer, Project Manager, Project Construction Engineer, Startup System Engineer, PLC&DCS Instrument & Control Engineer, and Maintenance Electrical Engineer for the past 35 years. He is a registered professional engineer and INPO Certified System Engineer for PWR and BWR Nuclear Power Plant.
