Locating the source of an electromagnetic interference (EMI) problem is often like trying to find a needle in a haystack. But despite the difficulty, it's not impossible. Although a number of different variables make these situations more complicated, you can simplify the process by following a time-tested, methodical investigative plan. By sticking to the steps outlined in this article, you'll save time and money and make it easier to diagnose and resolve these elusive disturbances.
The investigative process for solving EMI problems is nearly identical to the approach used for other types of power quality problems. We can break down this approach into seven basic steps. It's important to follow the steps as outlined, without skipping one or changing the sequence.
Gather background information.
Develop a plan.
Select the proper tools.
Conduct the investigation.
Analyze the data and locate the source.
Select a solution.
Verify solution performance.
This is the most important step in an EMI investigation. In the beginning, you know little about the operation and characteristics of the affected equipment. Equipping yourself with meaningful information will make the investigation easier.
A coordinated meeting between all affected parties is usually an appropriate and necessary first step. The Institute of Electrical and Electronics Engineers' (IEEE) Emerald Book provides a comprehensive description of how to conduct a power quality audit and the importance of obtaining input from everyone involved. Although the book's current version focuses primarily on wiring and grounding audits and general equipment problems caused by voltage sags, interruptions, and other variations, the basic concepts apply equally well to EMI investigations. The table (left) describes some of the information you might obtain during this process.
Of course, the best background information comes from asking the right questions and taking detailed notes during the meeting. It's common for different representatives to disagree with each other, especially if they've already formed opinions. Establish common ground by maintaining an open mind and urging the other parties to do the same. What may seem like unrelated information could become a major clue in identifying the EMI source. Worksheet A contains a list of important questions that will put you on the right track.
You may direct some questions to a specific individual and others to several people. The operator spends more time with the malfunctioning equipment than anyone else, so this person will probably be more familiar with the problem. He will likely provide new information that will help trigger even more information from the others.
Use the information gathered in the meeting to determine whether the investigation will focus on the radiated environment (the space inside and outside the facility) or the conducted environment (the metallic structures inside the facility). If you didn't obtain enough information to determine this, don't panic. Most EMI problems are associated with radiated environments, so odds are you'll need to start there.
Worksheet B lists the items you should include in your plan. If you need special access to secured areas, make these arrangements in advance. If possible, obtain a layout of the facility. A good one will allow you to connect or locate testing or measurement instruments without interrupting facility operations. Of course, some interruptions may be unavoidable while taking measurements.
To detect radiated emissions, investigators typically start by using a pocket AM radio, followed by radio-frequency field-strength meters, and even spectrum analyzers and specialized antennas. For conducted emissions, you may need to use more complicated and expensive measuring instruments such as spectrum analyzers, high-frequency clamp-on CTs, and in some cases, power-line monitors and oscilloscopes. These may require basic off-site training for proper setup and operation.
Spectrum analyzers, field-strength meters, oscilloscopes, and some antennas contain components that are sensitive to mechanical shocks and vibrations. Transport and handle these with care. You don't want the equipment to sustain any damage that could require costly repair and recalibration.
Some devices may be too bulky or heavy to transport, and others require 60 Hz power, which may be unavailable. When possible, use portable instruments to simplify movement.
If your instrument requires the use of an antenna, select one carefully. Many antennas are large and may pose problems to facility operations. It's equally important to make sure the antenna's frequency range encompasses the range of the suspected EMI problem. Broadband antennas, which offer frequency ranges from 100 Hz to 3 GHz, are well-suited for EMI investigations.
Most antennas are designed specifically to measure either radiated electric or magnetic fields. You will most likely measure radiated electric fields because many EMI problems result from radiated emissions occurring in the far field.
In the far field, with respect to distance, the electric field is 377 times stronger than the magnetic field. When the emissions occur in the near field (where the field strength is, with respect to distance, comparable), you may need to measure magnetic fields using loop antennas.
Current probes, also called high-frequency current transformers (CTs), measure emissions flowing in power, data, and signal cables. These probes range in frequency from 20 Hz to 2 GHz and may reveal important clues about the EMI problem. You may need a CT to measure conducted emissions flowing in a metallic object such as a single wire, wire bundle, steel member, pipe, or plenum. Insulated and uninsulated CTs are available with various diameter openings. CTs are designed so that they may be coupled around the object without cutting or disconnecting it. Large objects will require CTs with larger openings. Care should be taken when clamping around power conductors, especially if you use uninsulated CTs.
Tracking down the source of an EMI problem may seem difficult, even to experienced investigators. But a thorough job during the information gathering process (and succeeding steps) will typically reveal the necessary clues to effectively narrow down the possibilities. As the investigation progresses, additional clues will become evident and, in the best cases, lead to several possible causes.
In most cases, you should begin at the affected equipment. There are a few simple measurements you can perform to determine if radiated or conducted emissions (or a combination) are causing the problem. For example, insert an uninterruptible power supply (UPS) with a charged battery between the building's power source (receptacle) and the equipment. A UPS on battery backup can power AC-powered end-use equipment affected by EMI. Don't move the affected equipment from the location where it malfunctions. If it continues to malfunction, then it's reacting to radiated emissions. If it doesn't malfunction, then conducted emissions are the cause.
When you determine the primary type of emissions involved, you may take more measurements inside and outside the facility to close in on the EMI source, whether it be a piece of equipment or an arcing component in a nearby power distribution network.
Another key factor is how the affected equipment operates. Many types of electronic equipment contain indicators, displays, meters, and readouts that may indicate when and how the EMI source affects the equipment.
For example, if you observe these devices and can correlate your observations with noise detected by an AM radio, you may become familiar with the sounds of radiated emissions received by the radio. You can do the same for emissions captured by a spectrum analyzer. However, it may be necessary to spend several hours becoming familiar with the equipment before discovering a correlation. If the occurrences are sporadic, a nearby piece of infrequently used equipment could be the source. If upsets are periodic, a commonly used piece of equipment may be the culprit.
In some cases, powered but unused equipment with degraded input filter components have been found to generate upsetting emissions. This type of source may be more difficult to locate and may require powering down a number of facility circuits individually. If the equipment is portable and used regularly, then locating the source may be even more difficult as the level of emissions will vary as the equipment is moved from one area to another.
Fig. 1 shows various types of radiated and conducted emission sources and their approximate corresponding frequency ranges. Intentional radiators, such as broadcast transmitters, generate emissions for a specific purpose. Unintentional radiators generate emissions as a byproduct of their normal or abnormal operations. You also may classify sources as natural or man-made. Most are man-made, such as end-use equipment that generates radiated and conducted emissions.
After completing the investigation, assemble the data and look for any patterns. The frequencies of the components measured by an analyzer will provide helpful clues. In addition, knowledge of operating frequencies for broadcast stations in the area and for various end-use devices will help you determine which components result from operating which devices. Even if you determine the source during an investigation, take some time to review the data to learn more about the EMI problem. This will lead you to possible solutions for resolving it.
When choosing a solution, carefully consider the facility's operations, the electromagnetic environment, and the operation of the affected equipment. You also must evaluate the safety, cost, installation, maintenance, aesthetics, and performance record of the possible solution. Review your choice with all involved individuals before procurement and installation to ensure that any effects on facility operations and equipment performance are known and kept to a minimum.
The most common solutions to EMI problems include shields, filters, and enhanced grounding techniques. Filtering is commonly used to solve conducted emissions problems, while shielding may be used to solve radiated problems. Correcting grounding problems may reduce both radiated and conducted emissions depending on the circumstances of the EMI problem.
Before applying these solutions, you should make the initial effort to reduce emission levels, beginning at the source. If you can't, try to solve the problem by altering the path of the emissions from the source to the affected equipment. If these efforts don't reduce the emissions sufficiently, then it's time to consider applying shields, filters, or grounding techniques.
When it's possible (and without significant expense), test a proposed solution in the field before purchasing it. You may find that simply relocating the EMI source or affected piece of equipment or operating it from a different branch circuit solves the problem. For large expensive remedies, you may be able to predict their effectiveness by performing mathematical modeling.
Official verification should take place after you've applied the solution and facility operations have returned to normal. Watch the performance of the installed solution and retake emissions measurements at the affected equipment. Include the “after” measurements in a final report. With the final report completed, you can put the matter to rest. Just be sure to save these steps so you can tackle your next EMI encounter with confidence.
Philip Keebler is a power quality engineer at EPRI PEAC, where he conducts research and manages a number of tasks for the company's System Compatibility Research Project. Keebler has BS and MS degrees in electrical engineering from the University of Tennessee, Knoxville.
Kermit Phipps is a senior power quality technician at EPRI PEAC. He is in charge of testing and evaluating equipment performance in accordance with various published standards. Phipps has an associate's degree in avionics systems technology from the Community College of the Air Force.
You'll get the most benefit from gathering background information when you ask the right questions. Use this guide to make sure you've covered all the bases.
For a well-developed plan, draft a document covering the following areas. Review it with the facility manager, area supervisor, facility engineer, or other appropriate personnel, then obtain approval from all interested parties.